U.S. patent application number 12/504479 was filed with the patent office on 2010-11-18 for particle beam isotope generator apparatus, system and method.
Invention is credited to Glenn B. Rosenthal.
Application Number | 20100290575 12/504479 |
Document ID | / |
Family ID | 41202765 |
Filed Date | 2010-11-18 |
United States Patent
Application |
20100290575 |
Kind Code |
A1 |
Rosenthal; Glenn B. |
November 18, 2010 |
PARTICLE BEAM ISOTOPE GENERATOR APPARATUS, SYSTEM AND METHOD
Abstract
An isotope generation apparatus is disclosed including: an ion
beam source of any of the types described herein; an extractor for
extracting the ion beam from the confinement region, where the beam
includes a portion of multiply ionized ions in a selected final
ionization state; a target including a target material; and an
accelerator for accelerating the ion beam and directing the ion
beam to the target. The ion beam directed to the target transmutes
at least a portion of the target material to a radio-isotope in
response to a nuclear reaction between ions in the selected final
ion state and atoms of the target material.
Inventors: |
Rosenthal; Glenn B.; (Los
Angeles, CA) |
Correspondence
Address: |
FOLEY & LARDNER
555 South Flower Street, SUITE 3500
LOS ANGELES
CA
90071-2411
US
|
Family ID: |
41202765 |
Appl. No.: |
12/504479 |
Filed: |
July 16, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61178857 |
May 15, 2009 |
|
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|
Current U.S.
Class: |
376/112 |
Current CPC
Class: |
H05H 1/02 20130101; H05H
1/14 20130101; H01J 27/18 20130101; Y02E 30/30 20130101; Y02E 30/10
20130101; G21G 1/10 20130101; H01J 37/08 20130101 |
Class at
Publication: |
376/112 |
International
Class: |
G21B 1/05 20060101
G21B001/05 |
Claims
1. A method comprising: generating an ion beam, said generating
comprising the steps of: providing a chamber disposed about a
longitudinal axis and containing a gas; producing a magnetic field
in a confinement region within the chamber, wherein the confinement
region is disposed about the axis and extends along the axis from a
proximal end to a distal end, and wherein the magnetic field
comprises: a first magnetic mirror located at the proximal end of
the confinement region; a second magnetic mirror located at the
distal end of the confinement region; a substantially uniform
magnetic field disposed about and directed substantially parallel
to the longitudinal axis, the substantially uniform magnetic field
being located between the first and second magnetic mirrors;
producing a time varying electric field to drive the cyclotron
motion of electrons located within the confinement region; causing
said driven electrons interacting with the gas to form a confined
plasma; and confining the plasma in the confinement region such
that a portion of atoms in the plasma experience multiple ionizing
interactions with the driven electrons to form multiply ionized
ions having a selected final ionization state; directing the ion
beam to a target comprising a target material; and transmuting at
least a portion of the target material to a radio-isotope by a
nuclear reaction between ions in the selected final ion state and
atoms of the target material.
2. The method of claim 1, wherein the time varying electric field
has a frequency substantially tuned to the electron cyclotron
resonance frequency corresponding to the substantially uniform
magnetic field.
3. The method of claim 2, wherein the electron cyclotron resonance
driver drives the cyclotron motion of electrons located throughout
a volume surrounding the substantially uniform magnetic field.
4. The method of claim 1, wherein directing the ion beam comprises
accelerating the ion beam.
5. The method of claim 1, wherein the ion beam has a current of 1
mA or greater.
6. The method of claim 1, wherein the ion beam has a current of 10
mA or greater.
7. The method of claim 1, wherein the ion beam has a current of 20
mA or greater.
8. The method of claim 1, wherein the ion beam has a current of 50
mA or greater.
9. The method of claim 7, wherein at least 70% of the ions in the
selected final ionization state.
10. The method of claim 7, wherein at least 80% of the ions in the
beam are in the selected final ionization state.
11. The method of claim 7, wherein at least 90% of the ions in the
beam are in the selected final ionization state.
12. The method of claim 1, wherein the atoms of the target material
have a longer half life than the radio-isotope.
13. The method of claim 1, wherein the ions in the selected final
ion state comprise alpha particles or .sup.3He.sup.++ ions.
14. The method of claim 10, wherein the nuclear reaction between
ions in the selected final ion state and atoms of the target
material comprise at least one from the list consisting of:
.sup.96Zr(.alpha.,n).sup.99Mo, .sup.209Bi(.alpha.,2n).sup.211At,
.sup.144Sm(.alpha.,.gamma.).sup.148Gd, .sup.147Sm(.alpha.,3n),
.sup.148Gd, .sup.114Cd(.alpha.,n).sup.117mSn, and
.sup.116Cd(.alpha.,3n).sup.117mSn.
15. The method of claim 1, wherein the radio-isotope comprises
.sup.99Mo, said method further comprising: generating a diagnostic
or therapeutic effective dose of .sup.99mTc from the .sup.99Mo by
negative beta decay.
16. The method of claim 15, wherein the entire diagnostic or
therapeutic effective dose of 99m Tc is generated without the use
of a nuclear fission reactor.
17. The method of claim 1, wherein the radio-isotope comprises
.sup.111In, said method further comprising: generating a diagnostic
or therapeutic effective dose of .sup.111In.
18. The method of claim 17, wherein the entire diagnostic or
therapeutic effective dose of .sup.111In is generated without the
use of a nuclear fission reactor.
19. The method of claim 7, wherein the radio-isotope comprises at
least one selected from the list consisting of: .sup.18F,
.sup.123Xe, .sup.123I, .sup.67Ga, .sup.111In, .sup.131Ba,
.sup.68Ge, .sup.82Sr, .sup.82Rb, .sup.89Sr, .sup.153Sm, .sup.124I,
.sup.211At, .sup.148Gd, .sup.76Br, .sup.199Tl, .sup.100Pd,
.sup.128Ba, .sup.117mSn and .sup.229Th.
20. The method of claim 1, wherein the nuclear reaction comprises
fission of atoms in the target material stimulated by bombardment
with the ions in the selected final state.
21. The method of claim 1, wherein the target comprises a layer of
a first target material overlaying a second target material, the
method further comprising: directing the ion beam at a first energy
to the layer of first target material such that: a first portion of
the ions in the beam transmute a portion of the first target
material into a first radio-isotope by a first nuclear reaction
between the first portion of ions and atoms of the first target
material; a second portion of the ions in the beam interact with
the layer to be decelerated to a second energy, and the second
portion of the ions in the beam transmute a portion of the second
target material into a second radio-isotope by a second nuclear
reaction between the second portion of ions and atoms of the second
target material.
22. The method of claim 21, wherein the ions at the first energy
more preferentially drive the first nuclear reaction than the
second nuclear reaction, and the ions at the second energy more
preferentially drive the second nuclear reaction than the first
nuclear reaction.
23. The method of claim 22, wherein: the first target material
comprises .sup.109Ag, the second target material comprises
.sup.96Zr, the first nuclear reaction comprises
.sup.109Ag(.alpha.,2n).sup.111In, the second nuclear reaction
comprises .sup.96Zr(.alpha.,n).sup.99Mo, the first energy is about
28 MeV, and the second energy is about 14 MeV.
24. An isotope generation apparatus comprising: an ion beam source
which generates an ion beam, the source comprising: a chamber
disposed about a longitudinal axis and containing a gas; a magnetic
confinement system configured to produce a magnetic field in a
confinement region within the chamber, wherein the confinement
region is disposed about the axis and extends along the axis from a
proximal end to a distal end, and wherein the magnetic field
comprises: a first magnetic mirror located at the proximal end of
the confinement region; a second magnetic mirror located at the
distal end of the confinement region; a substantially uniform
magnetic field disposed about and directed substantially parallel
to the longitudinal axis, the substantially uniform magnetic field
being located between the first and second magnetic mirrors; and an
electron cyclotron resonance driver which produces a time varying
electric field which drives the cyclotron motion of electrons
located within the confinement region, said driven electrons
interacting with the gas to form a confined plasma, wherein: during
operation, the magnetic confinement system confines the plasma in
the confinement region such that a portion of atoms in the plasma
experience multiple ionizing interactions with the driven electrons
to form multiply ionized ions having a selected final ionization
state an extractor for extracting the ion beam from the confinement
region, wherein the beam comprises a portion of the multiply
ionized ions in the selected final ionization state; a target
comprising a target material; and an accelerator for accelerating
the ion beam and directing the ion beam to the target; wherein the
ion beam directed to the target transmutes at least a portion of
the target material to a radio-isotope in response to a nuclear
reaction between ions in the selected final ion state and atoms of
the target material.
25. The apparatus of claim 24, wherein the ion beam has a current
of 1 mA or greater.
26. The apparatus of claim 24, wherein the ion beam has a current
of 10 mA or greater.
27. The apparatus of claim 24, wherein the ion beam has a current
of 20 mA or greater.
28. The apparatus of claim 24, wherein the ion beam has a current
of 50 mA or greater.
29. The apparatus of claim 27, wherein at 60% of the ions in the
beam are in the selected final ionization state.
30. The ion source of claim 27, wherein at least 80% of the ions in
the beam are in the selected final ionization state.
31. The apparatus of claim 24, wherein the atoms of the target
material have a longer half life than the radio-isotope.
32. The apparatus of claim 24, wherein the ions in the selected
final ion state comprise alpha particles or .sup.3He.sup.++
ions.
33. The apparatus of claim 32, wherein the nuclear reaction between
ions in the selected final ion state and atoms of the target
material comprise at least one from the list consisting of:
.sup.96Zr(.alpha.,n).sup.99Mo, .sup.209Bi(.alpha.,2n).sup.211At,
.sup.144Sm(.alpha.,.gamma.).sup.148Gd,
.sup.147Sm(.alpha.,3n).sup.148Gd, .sup.114Cd(.alpha.,n).sup.117mSn,
and .sup.116Cd(.alpha.,3n).sup.117mSn.
34. The apparatus of claim 32, wherein the radio-isotope comprises
at least one selected from the list consisting of: .sup.18F,
.sup.123Xe, .sup.123I, .sup.67Ga, .sup.111In, .sup.131Ba,
.sup.68Ge, .sup.82Sr, .sup.82Rb, .sup.89Sr, .sup.153Sm, .sup.124I,
.sup.211At, .sup.148Gd, .sup.76Br, .sup.199Tl, .sup.100Pd,
.sup.128Ba, .sup.117mSn, and .sup.229Th.
35. The apparatus of claim 24, wherein: the electron cyclotron
resonance driver produces a time varying electric field having a
frequency substantially tuned to the electron cyclotron resonance
frequency corresponding to the substantially uniform magnetic
field; and the electron cyclotron resonance driver drives the
cyclotron motion of electrons located throughout a volume
containing the substantially uniform magnetic field.
36. The apparatus of claim 24, wherein the magnitude of the
substantially uniform magnetic field varies by less than 10% over a
region disposed about the longitudinal axis, said region located
midway between the first and second magnetic mirrors and extending
along the longitudinal axis over a distance equal to at least about
25% of the axial distance between the first and second magnetic
mirrors.
37. The apparatus of claim 24, wherein the magnitude of the
substantially uniform magnetic field varies by less than 5% over a
region extending at least 15 cm along the longitudinal axis.
38. The apparatus of claim 24, wherein the magnetic field is
azimuthally symmetric about the longitudinal axis throughout the
confinement region.
39. The apparatus of claim 24, wherein the target comprises a layer
of a first target material overlaying a second target material.
40. The apparatus of claim 39, wherein the accelerator directs the
ion beam at a first energy to the layer of first target material
such that: a first portion of the ions in the beam transmute a
portion of the first target material into a first radio-isotope by
a first nuclear reaction between the first portion of ions and
atoms of the first target material; a second portion of the ions in
the beam interact with the layer to be decelerated to a second
energy, and the second portion of the ions in the beam transmute a
portion of the second target material into a second radio-isotope
by a second nuclear reaction between the second portion of ions and
atoms of the second target material; wherein the ions at the first
energy more preferentially drive the first nuclear reaction than
the second nuclear reaction, and the ions at the second energy more
preferentially drive the second nuclear reaction than the first
nuclear reaction.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is related to and claims benefit of U.S.
Provisional Application 61/178,857 filed May 15, 2009, the contents
of which are incorporated by reference in their entirety.
BACKGROUND
[0002] This disclosure is related to ion sources, and more
particularly to high intensity ion sources.
[0003] Ion sources may be used to generate ion beams useful in a
number of applications. For example, the beams may be used to
bombard targets to drive nuclear reactions for the production of
isotopes.
[0004] Some ions sources ionize neutral targets via collisions with
energetic electrons.
[0005] Electron Cyclotron Resonant (ECR) plasma sources generate
energetic electrons by exciting the cyclotron motion of the
electrons within a magnetic field. The ECR plasma source is located
in a vacuum chamber to control the gas that is ionized, and to
reduce the pressure allowing the electrons to reach ionization
energy.
[0006] A charged particle placed in a uniform magnetic field will
gyrate around the magnetic field with a frequency given by the
electron cyclotron frequency.
.omega. ce = eB m ##EQU00001##
[0007] If the magnetic field is not uniform, the electron still
gyrates around the magnetic field, but the orbit and frequency
become somewhat more complicated. ECR ion sources do not require a
uniform magnetic field to operate. In fact, many of them operate in
highly non-uniform magnetic fields. By applying an oscillating
electric field, the cyclotron motion of the electrons can be
excited. If the oscillating electric field is resonant with the
electron cyclotron frequency and couples to the electron motion,
the electrons will gain energy. The highest coupling would be an
electric field that rotated about the magnetic field in the same
direction as the electrons, and at the same rate. This electric
field would look like a DC electric field in the frame of the
electrons. Good coupling can also be obtained with a linear
polarized electric field that oscillates perpendicular to the
magnetic field. As the electrons gain energy they will collide with
any gas within the source, ionizing the background gas. This forms
plasma and creates more electrons that can ionize more background
gas. This prior art process continues as the plasma density
increases until losses are balanced with production.
[0008] In prior art devices, the balance occurs long before a
powerful beam can be generated, and a powerful beam is what is
needed to strike and transmute target material into such things as
useful medical isotopes. Many isotopes are, in theory, thought to
be useful, but heretofore their small obtainable quantities and
short half lives, prevent their use.
SUMMARY
[0009] The inventors have realized that a high intensity source of
multiply charged ions in a selected ionization state may be
provided. For example, some embodiments of the devices, systems and
techniques described herein produce a high density beam of ions,
useful in producing long and/or short half-live isotopes or for use
directly (e.g. for the treatment of tumors). Some embodiments
produce a beam of multiply ionized particles in selected final
ionization state. Some embodiments produce a high density beam of
ions that are highly ionized. Some embodiments produce a beam of
multiply ionized He relatively economically.
[0010] Some embodiments use ion beams to transmute atoms and
isotopes into useful isotopes that can not be produced in quantity
by other means. Some embodiments enable production of isotopes,
heretofore not available in useful quantities.
[0011] Some embodiments use ion beams to transmute isotopes, such
as those produced in commercial nuclear power plants, into fuel
that can be recycled into the reactor. Some embodiments provide a
machine which can be sited adjacent a commercial nuclear power
plant to transmute long half life isotopes, such as those produced
in commercial nuclear power plants, into short half life isotopes
which quickly decay into stable atoms without the requirement of
transportation or burial.
[0012] In one aspect, an ion source is disclosed including: a
chamber disposed about a longitudinal axis and containing a gas.
The source includes a magnetic confinement system configured to
produce a magnetic field in a confinement region within the
chamber, where the confinement region is disposed about the axis
and extends along the axis from a proximal end to a distal end. The
magnetic field includes: a first magnetic mirror located at the
proximal end of the confinement region; a second magnetic mirror
located at the distal end of the confinement region; and a
substantially uniform magnetic field disposed about and directed
substantially parallel to the longitudinal axis, the substantially
uniform magnetic field being located between the first and second
magnetic mirrors. The system also includes an electron cyclotron
resonance driver which produces a time varying electric field which
drives the cyclotron motion of electrons located within the
confinement region, the driven electrons interacting with the gas
to form a confined plasma. During operation, the magnetic
confinement system confines the plasma in the confinement region
such that a portion of atoms in the plasma experience multiple
ionizing interactions with the driven electrons to form multiply
ionized ions having a selected final ionization state.
[0013] In some embodiments, the first and second magnetic mirrors
each include a non-uniform magnetic field, where the field: is
directed substantially along the longitudinal axis, and has a
magnitude which increases as a function of axial distance from the
substantially uniform magnetic field to a peak magnitude greater
than the magnitude of the substantially uniform magnetic field. In
some such embodiments, the peak magnitude of the first magnetic
mirror is greater than the peak magnitude of the second magnetic
mirror. In some embodiments, the peak magnitude of the mirrors may
be equal or substantially equal. In some embodiments, the peak
magnitude of each of the first and second magnetic mirrors is
greater than about twice the magnitude of the substantially uniform
magnetic field. In some embodiments the peak magnitude of each of
the first and second magnetic mirrors may take any other suitable
values, e.g., one and a half, three, four, five, or more times the
magnitude of the substantially uniform magnetic field.
[0014] In some embodiments, the magnitude of the substantially
uniform magnetic field is a local axial minimum of the magnetic
field in the confinement region.
[0015] Some embodiments include an extractor for extracting a beam
of ions from the confinement region, where the beam includes a
portion of the multiply ionized ions in the selected final
ionization state.
[0016] In some embodiments, the ion beam has a current of 1 mA or
greater, 10 mA or greater, 20 mA or greater, or even 50 mA or
greater.
[0017] In some embodiments, at least 50% of the ions in the beam
(as measured by particle fraction, or as a percentage of total beam
current) are in the selected final ionization state. In some
embodiments, at least 60%, 70%, 80%, or 90% of the ions in the beam
are in the selected final ionization state.
[0018] In some embodiments, the electron cyclotron resonance driver
produces a time varying electric field having a frequency
substantially tuned to the electron cyclotron resonance frequency
corresponding to the substantially uniform magnetic field
[0019] In some embodiments, the electron cyclotron resonance driver
drives the cyclotron motion of electrons located throughout a
volume containing the substantially uniform magnetic field.
[0020] In some embodiments, the magnitude of the substantially
uniform magnetic field varies by less than 1%, less than 5%, or
less than 10% over a region disposed about the longitudinal axis,
the region located between (e.g. midway between) the first and
second magnetic mirrors and extending along the longitudinal axis
over a distance equal to at least about 10%, 15%, 25%, or even more
of the axial distance between the first and second magnetic
mirrors.
[0021] In some embodiments, the magnitude of the substantially
uniform magnetic field varies by less than 1%, less than 5%, or
less than 10% over a region extending at least 5 cm, 10 cm, 15 cm,
or greater along the longitudinal axis.
[0022] In some embodiments, the magnetic field is azimuthally
symmetric about the longitudinal axis throughout the confinement
region.
[0023] In some embodiments, the electron cyclotron de-correlation
time for electrons driven by the electron cyclotron resonance
driver is at least on the order of an average confinement time for
a heated electron in the confinement region.
[0024] In some embodiments, the electron cyclotron resonance driver
drives at least a portion of the electrons in the volume to an
energy of about 200 eV or more, about 300 eV or more, or about 1
keV or more.
[0025] Some embodiments include an ion cyclotron driver, which
directs radiation to the confinement region to preferentially drive
the cyclotron motion of ions in the plasma having a selected driven
ionization state to increase the motional energy of the ions in
directions perpendicular to the longitudinal axis. In some
embodiments, the ion cyclotron driver preferentially increases the
confinement time in the confinement region of the ions having
selected driven ionization state, thereby increasing the number of
the ions undergoing further ionizing interactions with the
electrons in the containment region to form ions having the
selected final ionization state. In some embodiments, the ion
cyclotron driver directs radiation to the confinement region having
a frequency substantially tuned to the ion cyclotron frequency of
the ions having the selected driven ionization state in the
substantially uniform magnetic field. In some embodiments, the
selected driven ionization state is a singly ionized state. In some
embodiments, the selected driven ionization state is a multiply
ionized state having an ionization state less than the final
ionization state.
[0026] In some embodiments, the ion cyclotron driver directs
radiation to the confinement region at a plurality of frequencies
each substantially tuned to the ion cyclotron frequency of ions
having a respective selected driven ionization state in the
substantially uniform magnetic field,
[0027] In some embodiments, the ion cyclotron driver includes an
antenna such as a filar antenna (e.g. a single or bi-filar), a
capacitor plate, an untwisted bi-filar antenna or an untwisted
filar antenna, or combinations thereof.
[0028] In some embodiments, at least one magnetic mirror includes a
magnetic field extending outside of the confinement region. The
electron cyclotron driver is tuned to the electron cyclotron
frequency corresponding to a portion of the magnetic field
extending outside of the confinement region to drive the cyclotron
motion of unconfined electrons in the portion of the field, where
the unconfined electrons interact with the gas to form an
unconfined plasma. The ion source further includes a sputter target
located in the chamber and proximal to the portion of the magnetic
field, and biased to attract ions from the unconfined plasma. In
response to collisions with the attracted ions, the sputter source
emits neutral particles which form at least a portion of the gas of
atoms. In some embodiments, at least a portion of the emitted
neutral particles interact with the unconfined electrons to form
ions which are attracted back to the sputter source.
[0029] In some embodiments, the sputter target includes an annulus
of material disposed about the longitudinal axis, an annulus of
material disposed about the longitudinal axis and having a target
surface which is angled with respect to the longitudinal axis, or
target material positioned about and extending along the
longitudinal axis, or combinations thereof.
[0030] In some embodiments, the gas includes He atoms, and the
magnetic confinement system confines the plasma in the confinement
region such that a portion of He atoms in the plasma experience two
singly ionizing interactions with the driven electrons to form
alpha particles or .sup.3He.sup.++ ions. Some such embodiments
include an extractor for extracting abeam of He ions from the
confinement region, where the beam includes alpha particles and/or
.sup.3He.sup.++ ions.
[0031] In some embodiments, the beam of He ions has a current of 1
mA or greater, 10 mA, or 20 mA or greater. In some embodiments, at
least 50%, at least 70%, at least 80%, or at least 90 or more of
the ions in the beam are alpha particles and/or .sup.3He.sup.++
ions.
[0032] In some embodiments, the magnetic confinement system is
further configured to produce a radial confinement magnetic field
which confines the radial motion of the plasma away from the
longitudinal axis. The radial confinement magnetic field does not
substantially extend into the substantially uniform magnetic field.
In some embodiments, magnetic confinement system includes a
multipole radial confinement magnet disposed about the longitudinal
axis which produces a magnetic field directed azimuthally to the
longitudinal axis and having a magnitude which decreases radially
with increasing proximity to the axis, except along one or more
cusps. In some embodiments, the multipole magnet includes 8 or more
poles.
[0033] In some embodiments, the electron cyclotron resonance driver
produces a time varying electric field having a frequency
substantially de-tuned to the electron cyclotron resonance
frequency corresponding to the substantially uniform magnetic
field. In some embodiments, the electron cyclotron resonance driver
drives the cyclotron motion of electrons located in a first region
of non-uniform magnetic field distal the substantially uniform
magnetic field along the longitudinal axis and a second region of
non-uniform magnetic field proximal the substantially uniform
magnetic field along the longitudinal axis. In some embodiments,
each of the first and second regions of non-uniform magnetic field
include a surface of points characterized such that the frequency
of the time varying electric field is tuned to the electron
cyclotron resonance frequency of the non-uniform magnetic field at
the points.
[0034] In some embodiments, the substantially uniform magnetic
field has a magnitude of about 0.1 T or greater, 0.5 T or greater,
or 6 T or greater.
[0035] In some embodiments, the gas includes molecules, and the
driven electrons interact with the gas to disassociate the
molecules to form the confined plasma.
[0036] In another aspect, a method of generating an ion beam is
disclosed, including: providing a chamber disposed about a
longitudinal axis and containing a gas and producing a magnetic
field in a confinement region within the chamber, where the
confinement region is disposed about the axis and extends along the
axis from a proximal end to a distal end. The magnetic field
includes: a first magnetic mirror located at the proximal end of
the confinement region; a second magnetic mirror located at the
distal end of the confinement region; a substantially uniform
magnetic field disposed about and directed substantially parallel
to the longitudinal axis, the substantially uniform magnetic field
being located between the first and second magnetic mirrors. The
method further includes producing a time varying electric field to
drive the cyclotron motion of electrons located within the
confinement region; causing the driven electrons interacting with
the gas to form a confined plasma; and confining the plasma in the
confinement region such that a portion of atoms in the plasma
experience multiple ionizing interactions with the driven electrons
to form multiply ionized ions having a selected final ionization
state.
[0037] In some embodiments, the first and second magnetic mirrors
each include a non-uniform magnetic field, where the field is
directed substantially along the longitudinal axis and has a
magnitude which increases as a function of axial distance from the
substantially uniform magnetic field to a peak magnitude greater
than the magnitude of the substantially uniform magnetic field, In
some embodiments, the peak magnitude of the first magnetic mirror
is greater than the peak magnitude of the second magnetic mirror
(in other embodiments they may be equal or substantially
equal).
[0038] In some embodiments, a peak magnitude of each of the first
and second magnetic mirrors is greater than about twice (or 1.5, 3,
4, 5, etc times) the magnitude of the substantially uniform
magnetic field.
[0039] In some embodiments, the magnitude of the substantially
uniform magnetic field is a local axial minimum of the magnetic
field in the confinement region.
[0040] Some embodiments further include extracting the ion beam
from the confinement region, where the beam includes a portion of
the ions which are in the selected final ionization state. In some
embodiments, the ion beam has a current of 1 mA or greater, 10 mA
or greater, 20 mA or greater, or 50 mA or greater.
[0041] In some embodiments, at least 50%, 60%, 70%, 80% or 90% or
more of the ions are in the selected final ionization state.
[0042] In some embodiments, the time varying electric field has a
frequency substantially tuned to the electron cyclotron resonance
frequency corresponding to the substantially uniform magnetic
field. In some embodiments, the electron cyclotron resonance driver
drives the cyclotron motion of electrons located throughout a
volume surrounding the substantially uniform magnetic field.
[0043] In some embodiments, the magnitude of the substantially
uniform magnetic field varies by less than 1%, 5%, 10%, or 15% over
a region disposed about the longitudinal axis, the region located
midway between the first and second magnetic mirrors and extending
along the longitudinal axis over a distance equal to at least about
5%, 10%, 25%, 50%, or more of the axial distance between the first
and second magnetic mirrors.
[0044] In some embodiments, the magnitude of the substantially
uniform magnetic field varies by less than 1%, 2.5%, 5%, or 10%
over a region extending at least 1 cm, 2 cm, 5 cm, 10 cm, 15 cm, or
25 cm or more along the longitudinal axis.
[0045] In some embodiments, the magnetic field is azimuthally
symmetric about the longitudinal axis throughout the confinement
region.
[0046] In some embodiments, the electron cyclotron de-correlation
time for the driven electrons is at least on the order of an
average confinement time for a heated electron in the confinement
region.
[0047] Some embodiments include driving the cyclotron motion of
electrons located within the confinement region to produce an
electron energy of about 200 eV or more, 300 eV or more, or 1 keV
or more.
[0048] Some embodiments further include directing radiation to the
confinement region to preferentially drive the cyclotron motion of
ions in the plasma having a selected driven ionization state to
increase the motional energy of the ions in directions
perpendicular to the longitudinal axis. In some embodiments,
directing radiation to the confinement region to preferentially
drive the cyclotron motion of ions in the plasma having a selected
driven ionization state includes preferentially increasing the
confinement time in the confinement region of the ions having
selected driven ionization state, thereby increasing the number of
the atoms undergoing further ionizing interactions with the
electrons in the containment region. In some embodiments, directing
radiation to the confinement region to preferentially drive the
cyclotron motion of ions in the plasma having a selected driven
ionization state includes: directing radiation to the confinement
region having a frequency substantially tuned to the ion cyclotron
frequency of the ions having the selected driven ionization state
in the substantially uniform magnetic field. In some embodiments,
the selected driven ionization state is a singly ionized state. In
some embodiments, the selected driven ionization state is a
multiply ionized state having an ionization state less than the
final ionization state. Some embodiments include directing
radiation to the confinement region at a plurality of frequencies,
each frequency substantially tuned to the ion cyclotron, frequency
of ions having a respective selected driven ionization state in the
substantially uniform magnetic field,
[0049] In some embodiments, directing radiation to the confinement
region to preferentially drive the cyclotron motion of ions in the
plasma having a selected driven ionization state includes:
directing radiation from an antenna of the types disclosed
herein.
[0050] In some embodiments, at least one magnetic mirror includes a
magnetic field extending outside of the confinement region. The
frequency of the time varying electric field is tuned to the
electron cyclotron frequency corresponding to a portion of the
magnetic field extending outside of the confinement region to drive
the cyclotron motion of unconfined electrons in the portion of the
field, where the unconfined electrons interact with the gas to form
an unconfined plasma. The method may further include providing a
sputter target located in the chamber and proximal to the portion
of the magnetic field and biasing the sputter target to attract
ions from the unconfined plasma, such that, in response to
collisions with the attracted ions, the sputter source emits
neutral particles which form at least a portion of the gas of
atoms. In some embodiments, at least a portion of the emitted
neutral particles interact with the unconfined electrons to form
ions which are attracted back to the biased sputter source. In some
embodiments, the sputter target includes an annulus of material
disposed about the longitudinal axis; an annulus of material
disposed about the longitudinal axis and having a target surface
which is angled with respect to the longitudinal axis, or a target
material positioned about and extending along the longitudinal
axis, or combinations thereof.
[0051] In some embodiments, the gas includes He atoms, and the
method includes confining the plasma in the confinement region such
that a portion of the He atoms in the plasma experience two singly
ionizing interactions with the driven electrons to form alpha
particles or .sup.3He.sup.++.
[0052] Some embodiments include extracting a beam of He ions from
the confinement region, where the beam includes alpha particles or
.sup.3He.sup.++ ions. In some embodiments, the beam of He atoms has
a current of 1 mA or greater, or 20 mA or greater. In some
embodiments, at least 50%, 60%, 70%, 80%, or 90% or more of the
ions in the beam are alpha particles. In some embodiments at least
50%, 60%, 70%, 80%, or 90% or more of the ions in the beam are or
.sup.3He.sup.++.
[0053] Some embodiments further include producing a radial
confinement magnetic field which confines the plasma radially. The
radial confinement magnetic field does not substantially extend
into the substantially uniform magnetic field. In some embodiments,
producing a radial confinement magnetic field includes producing a
magnetic field directed azimuthally to the longitudinal axis and
having a magnitude which decreases radially with increasing
proximity to the axis, except along one or more cusps.
[0054] Some embodiments include producing a time varying electric
field having a frequency substantially de-tuned to the electron
cyclotron resonance frequency corresponding to the substantially
uniform magnetic field. Some embodiments include driving the
cyclotron motion of electrons located in a first region of
non-uniform magnetic field distal the substantially uniform
magnetic field along the longitudinal axis and a second region of
non-uniform magnetic field proximal the substantially uniform
magnetic field along the longitudinal axis. In some embodiments,
each of the first and second regions of non-uniform magnetic field
include a surface of points at which the frequency of the time
varying electric field is tuned to the electron cyclotron resonance
frequency of the non-uniform magnetic field at the points. Some
such embodiments include effecting stochastic heating of electrons
in the confinement region which pass through the first and second
regions multiple times.
[0055] In some embodiments, the substantially uniform magnetic
field has a magnitude of about 0.1 T or greater, 0.5 T or greater
or 0.6 T or greater.
[0056] In some embodiments, the gas includes molecules, and the
causing the driven electrons interacting with the gas to form a
confined plasma includes disassociating the molecules.
[0057] In another aspect, a method is disclosed including:
generating an ion beam using any of the devices and techniques
described herein, directing the ion beam to a target including a
target material; and transmuting at least a portion of the target
material to a radio-isotope by a nuclear reaction between ions in
the selected final ion state and atoms of the target material.
[0058] In some embodiments, the atoms of the target material have a
longer half life than the radio-isotope.
[0059] In some embodiments, the ions in the selected final ion
state include alpha particles or .sup.3He.sup.++ ions.
[0060] In some embodiments, the nuclear reaction between ions in
the selected final ion state and atoms of the target material
include at least one from the list consisting of:
.sup.96Zr(.alpha.,n).sup.99Mo, .sup.209Bi(.alpha.,2n).sup.211At,
.sup.144Sm(.alpha.,.gamma.).sup.148Gd,
.sup.116Cd(.alpha.,3n).sup.117mSn and
.sup.114Cd(.alpha.,n).sup.117mSn and
.sup.147Sm(.alpha.,3n).sup.148Gd.
[0061] In some embodiments, where the radio-isotope includes
.sup.99Mo, and the method further includes: generating a diagnostic
or therapeutic effective dose of .sup.99mTc from the .sup.99Mo by
negative beta decay. In some embodiments, the entire diagnostic or
therapeutic effective dose of 99m Tc is generated without the use
of a nuclear fission reactor.
[0062] In some embodiments, the radio-isotope includes .sup.111In,
and the method further includes: generating a diagnostic or
therapeutic effective dose of .sup.111In. In some embodiments, the
entire diagnostic or therapeutic effective dose of .sup.111In is
generated without the use of a nuclear fission reactor.
[0063] In some embodiments, the radio-isotope includes at least one
selected from the list consisting of: .sup.18F, .sub.123Xe,
.sup.123I, .sup.67Ga, .sup.111In, .sup.131Ba, .sup.68Ge, .sup.82Sr,
.sup.82Rb, .sup.89Sr, .sup.153Sm, .sup.124I, .sup.211At,
.sup.148Gd, .sup.76Br, .sup.199Tl, .sup.100Pd, .sup.128Ba,
.sup.117mSn, and .sup.229Th.
[0064] In some embodiments, the nuclear reaction includes fission
of atoms in the target material stimulated by bombardment with the
ions in the selected final state.
[0065] In some embodiments, the target includes a layer of a first
target material overlaying a second target material, The method
further includes: directing the ion beam at a first energy to the
layer of first target material such that a first portion of the
ions in the beam transmute a portion of the first target material
into a first radio-isotope by a first nuclear reaction between the
first portion of ions and atoms of the first target material; a
second portion of the ions in the beam interact with the layer to
be decelerated to a second energy, and the second portion of the
ions in the beam transmute a portion of the second target material
into a second radio-isotope by a second nuclear reaction between
the second portion of ions and atoms of the second target material.
In some embodiments, the ions at the first energy more
preferentially drive the first nuclear reaction than the second
nuclear reaction, and the ions at the second energy more
preferentially drive the second nuclear reaction than the first
nuclear reaction. In some embodiments, the first target material
includes .sup.109Ag, the second target material includes .sup.96Zr,
the first nuclear reaction includes
.sup.109Ag(.alpha.,2n).sup.111In, the second nuclear reaction
includes .sup.96Zr(.alpha.,n).sup.99Mo, the first energy is about
28 MeV, and the second energy is about 16 MeV.
[0066] In another aspect, an isotope generation apparatus is
disclosed including: an ion beam source of any of the types
described herein; an extractor for extracting the ion beam from the
confinement region, where the beam includes a portion of multiply
ionized ions in a selected final ionization state; a target
including a target material; and an accelerator for accelerating
the ion beam and directing the ion beam to the target. The ion beam
directed to the target transmutes at least a portion of the target
material to a radio-isotope in response to a nuclear reaction
between ions in the selected final ion state and atoms of the
target material.
[0067] In some embodiments, the atoms of the target material have a
longer half life than the radio-isotope.
[0068] In some embodiments, the ions in the selected final ion
state include alpha particles or .sup.3He.sup.++ ions.
[0069] In some embodiments, the nuclear reaction between ions in
the selected final ion state and atoms of the target material
include .sup.96Zr(.alpha.,n).sup.99Mo,
.sup.209Bi(.alpha.,2n).sup.211At,
.sup.144Sm(.alpha.,.gamma.).sup.148Gd, and/or
.sup.147Sm(.alpha.,3n).sup.148Gd.
[0070] In some embodiments, the radio-isotope includes .sup.18F,
.sup.123Xe, .sup.123I, .sup.67Ga, .sup.111In, .sup.131Ba,
.sup.68Ge, .sup.82Sr, .sup.82Rb, .sup.89Sr, .sup.153Sm, .sup.124I,
.sup.211At, .sup.148Gd, .sup.76Br, .sup.199Tl, .sup.100Pd,
.sup.128Ba, and/or .sup.229Th.
[0071] In some embodiments, the target includes a layer of a first
target material overlaying a second target material.
[0072] In some embodiments, the accelerator directs the ion beam at
a first energy to the layer of first target material such that: a
first portion of the ions in the beam transmute a portion of the
first target material into a first radio-isotope by a first nuclear
reaction between the first portion of ions and atoms of the first
target material; a second portion of the ions in the beam interact
with the layer to be decelerated to a second energy, and the second
portion of the ions in the beam transmute a portion of the second
target material into a second radio-isotope by a second nuclear
reaction between the second portion of ions and atoms of the second
target material. The ions at the first energy more preferentially
drive the first nuclear reaction than the second nuclear reaction,
and the ions at the second energy more preferentially drive the
second nuclear reaction than the first nuclear reaction.
[0073] Various embodiments may include any of the above described
features either alone or in any combination.
[0074] It is to be understood that as used herein, the term gas may
refer to a single component gas (e.g. .sup.4He gas), or a
multi-component gas (e.g. .sup.3He/.sup.4He gas mix, a He/Xe gas
mix, a He/O.sub.2 gas mix, etc.).
[0075] Scientific notation known in the art has been used herein to
describe various nuclear reactions. For a reaction described in the
form A(b,c)D, "A" is the target nucleus, or irradiated material,
"b" is a bombarding particle, "c" is an, emitted particle, and "D"
is the product or residual nucleus. For a reaction described in the
form A(b,c)D1(D2), D1 and D2 primary and secondary products of the
reaction.
[0076] These and other aspects of the present invention will become
apparent to those skilled in the art after considering the
following detailed specification along with the accompanying
drawings where:
BRIEF DESCRIPTION OF THE DRAWINGS
[0077] FIG. 1 is a block diagram of an ion beam system;
[0078] FIG. 1A is a block diagram of an ion beam system for making
useful isotopes;
[0079] FIG. 1B is a block diagram of an ion beam system adapted for
treatment of radio active waste from commercial nuclear power
plants;
[0080] FIG. 1C is a block diagram of an ion beam system adapted for
medical treatment, e.g. treatment of internal growths;
[0081] FIG. 2A is an operational schematic of an ion beam
system
[0082] FIG. 2B is an operational schematic of an ion beam system
showing modified insulators;
[0083] FIG. 2C is an operational schematic of an ion beam system
showing a grounded chamber;
[0084] FIG. 2D is an operational schematic of an ion beam system
showing external ICR antenna.
[0085] FIG. 3A is a graph of the cross-section for formation of
He.sup.+ and He.sup.++ electron impact on neutral He versus
electron energy;
[0086] FIG. 3B is a graph of electron impact ionization
cross-section for the first six ionization states of Xenon;
[0087] FIGS. 3C and 3D shows graphs of the cross-sections for
electron impact ionization as a function of electron energy, of
neutral He (He+e.sup.-.fwdarw.He.sup.+2e.sup.-) and He.sup.+
(He.sup.++e.sup.-.fwdarw.He.sup.++2e.sup.-);
[0088] FIG. 4 is a graph of magnetic field magnitude as a function
of axial position in an ion source;
[0089] FIG. 5 is a graphical view of the loss cone associated with
the magnetic fields of FIG. 4;
[0090] FIGS. 6A and 6B are schematic diagrams of capacitor plate
ICR antennae, FIG. 6A depicting a linear drive system and FIG. 6B
depicting a circular drive system with the horizontal plates driven
90.degree. out of phase with the vertical plates.
[0091] FIG. 7 is a schematic diagram of a split ring antenna system
for the ICR;
[0092] FIG. 8 is schematic diagram of a bi-filer antenna where the
to coils are driven 90.degree. out of phase to generate a rotating
electric field for the ICR;
[0093] FIG. 9 is schematic diagram of an untwisted bi-filer antenna
where the two coils are driven 90.degree. out of phase to generate
a rotating electric field for the ICR;
[0094] FIGS. 10A and 10B are graphical representations of the ECR
resonance zones for the magnetic confinement fields when the ECR is
operating in an on-resonant and an off-resonant mode,
respectively;
[0095] FIG. 11 is a simplified cross-sectional view of a sputter
source implemented with an axially located target, showing
different support and bias material, and sputter material;
[0096] FIG. 12 is a simplified cross-sectional view of a sputter
source implemented with an annular target;
[0097] FIG. 13 is a simplified cross-sectional view of a sputter
source implemented as a conical sputter target;
[0098] FIG. 14 shows a multipole radial confinement magnet.
[0099] FIG. 14A shows the magnetic field generated by a multipole
radial confinement magnet.
[0100] FIG. 15 is a plot of total extracted beam current as a
function of central magnetic field strength for an ion beam
system.
[0101] FIG. 16 shows time-of-flight mass spectrometer data for an
ion beam generated in the ECR resonant mode of operation.
[0102] FIGS. 17A-C illustrate an exemplary target and target feed
system.
DETAILED DESCRIPTION
[0103] Referring to FIG. 1, an Ion beam generator 11 generates an
ion beam and directs it to target 12 to cause a nuclear reaction,
e.g. to transmute atoms in target 12 to produce a desired isotope.
Ion beam generator 11 includes an ion source 10 of the type
described herein, which produces a beam (e.g. a high intensity
beam) including ions in a selected final ionization state. Ion beam
generator 11 also includes a beam accelerator 13, which accelerates
the beam from source 10, and directs it to target 12. Optionally,
ion beam generator 11 also includes a filter for filtering ions ion
the beam, e.g. based on the charge or mass of the ions. In some
embodiments, accelerator 13 acts as a filter. For example, a
cyclotron accelerator will naturally separate ions having different
ionization states.
[0104] Beam accelerator 13 may be any suitable accelerator known in
the art. In some embodiments, the accelerator system is a hybrid
RFQ-DTL (Radio Frequency Quadrapole-Drift Tube LINAC) system
available for modification from various vendors. Other accelerator
systems such as almost any LINAC or cyclotron could be used.
[0105] A number of coupling methods can be used between ion source
10 and accelerator 13. For high intensity beams, a magnetic lens
system is advantageous. For low intensity beams, an electrostatic
lens system is more economic, compact, and effective. Generally,
using a magnetic lens allows for an easy way to implement a
particle filter that will reject any He.sup.+ within the beam where
the selected final ion state is He.sup.++.
[0106] FIG. 1A shows a system 20 featuring ion beam generator 11.
Ion beam generator 11 bombards target 12 with ions to transmute
atoms in target 12 to produce a desired isotope. The transmuted
target undergoes chemical separation 15 to provide a pure sample of
product isotope 16. In some embodiments, product isotope 16 is an
intermediate product, and undergoes decay 17 resulting in final
product 18. In some embodiments, the intermediate product is a
relatively long lived radio-isotope in comparison to the final
product isotope. In some embodiments, final product 18 is an
isotope suitable for use in medical treatment or diagnostic
applications, research applications (e.g. radio-marking), energy
generation applications (e.g. as nuclear fuel), etc.
[0107] FIG. 1B shows a system 21 for treatment of nuclear waste
500. Nuclear waste (e.g. commercial nuclear waste from power
generation, research, or medical use) undergoes chemical separation
501. Some long half life waste 502 not suitable for treatment is
transferred to long term storage. Other waste is included in target
12, which is bombarded with ions from ion beam generator 11. In
some embodiments, waste in target 12 is transmuted into a
relatively short half life product 504 which undergoes decay 505 to
become a stable product 506, which may be easily disposed of. In
some embodiments, waste in target 12 is transmuted to usable fuel
507, and is thereby recycled.
[0108] FIG. 1C shows a system for medical treatment. Ion beam
generator 11 generates an ion beam which is directed by probe 600
to human or animal body 601 in order to treat tissue in the body.
For example, in some embodiments, the ion beam is directed to a
tumor 602 in body 601. Some embodiments include producing
accelerated particles useful in treating interior tumors and the
like wherein the particles (such as H.sup.+, He.sup.++, C.sup.+4,
C.sup.+6, or O.sup.+8) loose energy very slowly until at a certain
energy state, substantially all of the particle's energy is
transferred to the tumor.
[0109] FIGS. 2A, 2B, 2C, and 2D are operational diagrams of ion
producing systems using Helium (He) ions as an example. In order to
understand the operation of the devices described herein, it is
helpful to understand the issues in generating multiple charged
ions. Although the ionization of helium is discussed, the basic
conclusions apply to all atoms, even including hydrogen (H) and
deuterium (.sup.2H) where a single electron is striped from the
atom to produce a proton (p) or deuteron (d), respectively.
[0110] If an ion source uses electron impact to ionize neutral
atoms, the ratio of charge states formed can be determined by the
cross sections involved. In the case of helium, the cross section
for production of He.sup.++ is more than two orders of magnitude
below the cross section for the production of He.sup.+ (see FIG.
3A). In general, prior art ionization processes produce around 1%
He.sup.++ and 99% He.sup.+.
[0111] Changing the electron impact energy does not make a
significant change in the production of He.sup.++. The best overall
performance is obtained near the peak of the He.sup.++ cross
section, approximately 300 eV. Using saturation of states and some
other known tricks, it is possible to generate a few percent
He.sup.++ with prior art devices.
[0112] For generating highly ionized states of any other atom, the
problem is made more difficult, by introducing more charge states,
a greater variation in the cross sections, and more dependence of
the cross sections on the electron energy. This can be seen in FIG.
3B, where the electron impact ionization for the first seven states
of xenon (Xe) are shown.
[0113] It is difficult to obtain He.sup.++ with a single electron
collision. If more then one electron collision is used, the
fraction of He.sup.++ can be increased greatly because the cross
section for production of He.sup.++ from He.sup.+ is significantly
higher than for the direct production of He.sup.++ from neutral He
(see FIGS. 3C and 3D).
[0114] Table I summarizes the different reactions for helium.
TABLE-US-00001 TABLE I He + e.sup.- .fwdarw. He.sup.+ + 2e.sup.-
0.35 .times. 10.sup.-16 120 He.sup.+ + e.sup.- .fwdarw. He.sup.++ +
2e.sup.- 0.045 .times. 10.sup.-16 200 He + e.sup.- .fwdarw.
He.sup.++ + 3.sup.e- 0.001 .times. 10.sup.-16 300
[0115] In order to ionize atoms by multiple collisions to reach a
selected final ionization state, it is necessary to confine the
intermediate states long enough for them to undergo additional
electron collisions. In the case of helium, where He.sup.++ is the
selected final ionization state, it is necessary to confine the
He.sup.+ long enough for it to undergo a second ionizing collision
to form He.sup.++. In the case of other atoms, it is necessary to
retain the ion long enough for it to undergo several ionizing
collisions to reach the selected state of ionization.
[0116] The systems described herein use a confining magnetic field
to retain the ions for the time required to reach the selected
ionization state. In some embodiments, an azimuthally symmetric
axial minimum magnetic field configuration is used. In other
embodiments, a "true" minimum magnetic field configuration is used.
For production of many ion charge states (e.g. He.sup.++) the
simpler axial minimum field configuration seems to be adequate. For
production of highly charged states of some atoms, a "true" minimum
magnetic field may be necessary.
[0117] FIG. 2A shows an exemplary ion source 10. Ionization chamber
30 is arranged along longitudinal axis A. Within the ionization
chamber 30, a local axial minimum magnetic field is formed between
two magnetic mirror fields 32 and 34 (field lines indicated with
dashed lines), preferably generated by superconducting magnets 36
and 38. The local axial minimum field is formed as central region
of lower, substantially uniform magnetic field 40, (field lines
indicated with dashed lines). FIG. 4 shows a plot of the magnitude
of the fields 32, 34, and 40 as a function of position along axis A
for an exemplary field configuration.
[0118] As will be described in detail below, uniform field region
40 provides several beneficial effects which allow for the
efficient production of intense ion beams of ions in a selected,
multiply ionized state. In embodiments featuring axial minimum
configurations, mirror fields 32, 34 and central field 40 may each
be azimuthally symmetric about axis A.
[0119] Referring back to FIG. 2A, although superconducting magnets
are used in some embodiments, they are not required for production
of the mirror fields 32 and 34 in every instance. In some
embodiments, the coils of superconducting magnets are easier to
adjust as the diameter of the wire can be smaller, leaving more
room for field adjustment. Refrigeration systems, not shown, for
superconducting magnets are commercially available.
[0120] The central magnetic field 40 can be implemented using a
central magnet 42, which may include copper coils, superconducting
coils, and/or fixed magnets. Magnet 42 may include a multi-pole
magnet is used to generate a "true" minimum field configuration (as
described below). In some embodiments, this may be implemented
using fixed magnets, although electromagnets or other magnets could
be used. A superconducting central magnet 42 is convenient when
superconducting magnets 36 and 38 are used, since the area must be
cooled close to absolute zero.
[0121] Exciter system 44 excites the electron cyclotron resonance
of electrons in chamber 40. In some embodiments, to produce ECR
excitation, the exciter system 44 introduces microwave energy into
the ionization chamber 30 directly. In typical embodiments the
center field magnetic region 40 between the two magnetic mirror
fields 32 and 34 is substantially uniform (constant magnetic field)
although a non-uniform magnetic field will operate. Preferably, the
rear part 47 of the ionization chamber 30 includes a connection 48
to a gas source 50, when a gas to be ionized is introduced into the
system. In other embodiments, the gas can be introduced anywhere a
convenient connection can be made. A perforated plate 52 separates
the chamber 30 from a vacuum pump 54 to maintain a slight gas
pressure within the chamber 30 and produce a beam 56 of the desired
ions from the plasma 57 confined between mirror fields 32 and
34.
[0122] The ECR exciter system 44 provides an electric field that
couples to the electron cyclotron motion of electrons in chamber
30. Since the electron cyclotron frequency is generally high, the
ECR drive tends to be in the microwave frequencies. For these
frequencies, wave guides provide the most efficient coupling. ECR
coupling, however, is not limited to wave guides, and can be
accomplished in other ways, such as cavity mode excitation, optical
drive, etc.
[0123] If a gyrotron, or other circular wave guide device, is used
as an ECR source, good performance will typically be obtained when
the rotation direction of the wave within the wave guide is matched
to the rotation direction of the electrons in the magnetic field
40. The position of the ECR drive output 55 and gas feed 48 can be
optimized so that plasma is primarily produced between the two
magnetic mirror fields 32 and 34. In some embodiments, this
increases the overall efficiency and minimizes the plasma
production in the rear part 47 of the chamber 30, which can
simplify the microwave feed system.
[0124] The frequency and power in the ECR drive system 44 are
related to the plasma density produced. Higher frequency and power
both typically correspond to a higher plasma density. The optimal
power and type of ECR source may be adjusted for each type of
ion.
[0125] The substantially uniform central magnetic field 40 between
the two magnetic mirrors 32 and 34 will operate for a wide range of
lengths. The optimal length may be readily determined
experimentally for each ion, selected final ionization state, beam
intensity, beam pulse length, and ECR power. Increasing the length
of the central magnetic region 40 increases the region over which
hot electrons are confined, and thus increases the potential for
ionizing collisions. Making this region larger increases the
ionization, but at the expense of increasing the time it takes to
form a stable plasma. Optimally, a pair of superconducting magnets
36 and 38 are used. Such magnets may not allow for significant
variations in the length of the magnetic field 40, because they are
not easily relocated. For production of He.sup.++, this is
typically not an issue. But for production of other highly charged
ions, more easily movable magnets may be used to allow adjustment
the magnetic fields.
[0126] In some embodiments, the central magnetic field 40 is chosen
to match the ECR frequency of the microwave source 44. The exact
values of the mirror fields 32 and 34 are not critical to the
operation of the system 10, but may have a large impact on system
efficiency. In some embodiments the rear mirror field 32 is higher
then the front mirror field 34 (e.g. as shown in FIG. 4). A beam
may be formed by allowing ions to leak out of the mirror fields 32
and 34. By making the rear mirror field 32 higher than the front
mirror field 34, most of the ion losses, which form the multiply
ionized particle beam 56, are through the front mirror field 34. In
other embodiments, mirror fields 32 and 34 may be of equal or
substantially equal strength.
[0127] The exact value of the front magnetic minor field 34 can be
chosen from a wide range of values, and is generally close to a
value of twice the central field 40. In other embodiments it may be
three, four, five, or more times that of the central field. Using a
higher value of the front field 34 increases the confinement of the
ions and electrons, but can cause instabilities in the flow of the
plasma 57. Using a lower value field decreases the fraction of ions
trapped between the mirror fields 32 and 34. The optimum value
depends on the ion, charge state, gas pressure, beam intensity, and
ECR power. The exact values may be determined experimentally in
each instance.
[0128] In order to understand the operation of the system 10, it is
helpful to understand how a magnetic mirror 32 or 34 works.
Magnetic mirroring occurs when a charged particle moves from a
region of low magnetic field to a region of high magnetic field.
The magnetic moment of the particle is an adiabatic invariant of
the motion:
.mu. .ident. mv .perp. 2 2 B ##EQU00002##
An adiabatic invariant remains invariant so long as the rate of
change of the parameters is "slow". "Slow", means that the magnetic
field and the perpendicular velocity change slowly over one
cyclotron period. In typical embodiments of the magnetic minors
described herein, this is generally an excellent approximation,
except possibly during extraction of the beam 56. The invariance of
the magnetic moment indicates that if the particle moves from a
region of small magnetic field to a region of large magnetic field,
the perpendicular velocity must increase. By conservation of
energy, this means that the parallel velocity must decrease. Thus,
if a particle with a given perpendicular velocity moves from a
region of low magnetic field to a region of sufficiently high
magnetic field, the parallel (axial) velocity will go to zero,
thereby stopping the particle. As is familiar to those skilled in
the art, a more through analysis of the particle dynamics shows
that, in fact the particle will be reflected.
[0129] By combining the magnetic moment invariant with the
conservation of energy,
E = 1 2 mv .perp. 2 + 1 2 mv .parallel. 2 ##EQU00003##
a simple set of equations governing the flow of a particle from
magnetic field B.sub.1 to magnetic field B.sub.2 can be
derived:
v .perp. 2 2 = v .perp. 1 2 B 2 B 1 ##EQU00004## v .parallel. 2 2 =
v .parallel. 1 2 + v .perp. 1 2 ( 1 - B 2 B 1 ) ##EQU00004.2##
[0130] When a particle travels from a low magnetic field to a high
magnetic field, the second term in the parallel velocity equation
can be negative. If it is sufficiently negative the particle will
stop and be reflected. Therefore, if particles are moving from one
magnetic field to another, the reflection depends on the ratio of
the perpendicular to parallel velocity of the particle.
Specifically, the particle will be reflected at some point
when:
v .parallel. 1 2 v .perp. 1 2 < ( B 2 B 1 - 1 ) ##EQU00005##
the non-reflected particles can be visualized as a cone 58 in
velocity space, as shown in FIG. 5.
[0131] Generally magnetic minors fields, such as magnetic mirror
fields 32 and 34 leak. Particles in the loss cone 58 leak out of
the front mirror field 34 (or the back mirror field 32). Particles
that are not in the loss cone 58 are eventually scattered into the
loss cone 58. The systems described herein, in various embodiments,
rely on this scattering to move the particles into the loss cone 58
that will then pass through the front magnetic mirror field 34 and
form a source of ions.
[0132] Referring back to FIG. 2A, during source operation
microwaves are used to excite the ECR in the central region 40 in
chamber 30 of the source. This generates hot electrons that ionize
the background gas. For He.sup.++ production, the preferred
electron energy is around 200-300 eV. For other atoms, the
preferred electron energy may easily be experimentally determined
by the energy required to produce the desired charge state and/or
the peak of the cross sections involved.
[0133] Considering first the production of alpha particles
(He.sup.++) the hot electrons generate mostly He.sup.+ and some
small amount of He.sup.++. Some of the generated He.sup.+ is
trapped between the magnetic mirror fields 32 and 34 and undergoes
a second collision forming He.sup.++. Gradually, the He.sup.++ and
He.sup.+ are scattered into the loss cone 58 where they leak out,
primarily through the front mirror field 34, forming the beam 56 of
Ne and He.sup.++. Generally the description focuses around alpha
particles (.sup.4He.sup.++), but the same discussions apply equally
to helium-3 ions (.sup.3He.sup.++) or other doubly ionized
ions.
[0134] The plasma beam 56 of He.sup.+ and He.sup.++ in a ratio
(determined as the ratio of the beam current of He.sup.++ to total
beam current) of up to 84% He.sup.++ or more leaking from the front
mirror field 34 is producible using an ECR system 44 only, is
accelerated by a extractor 59 which may include a sequence of
electrodes and focusing elements to form the particle beam 56. In
cases where it is desirable to transmit the beam 56 to a magnetic
field free region, the extraction system can be designed to make
sure that particles remain adiabatic until the particles are out of
the magnetic field 34.
[0135] If the plasma flow out of the magnetic field zone remains
adiabatic, the mirror relations can be applied. As the particles
travel from the high magnetic field to the low magnetic field,
perpendicular energy is converted to axial (parallel) energy. This
keeps the emittance of the beam 56 low during the particle
extraction. A non-adiabatic extraction of the beam 56 results in
perpendicular energy of the beam 56 being retained and the
extracted beam 56 has larger emittance.
[0136] There are four known major sources of particle losses in the
plasma system 60 of the present invention. These are:
recombination, charge exchange, radial diffusion, and loss cone
scattering (axial diffusion).
[0137] In typical embodiments, Recombination is not a significant
effect. Generally, the electrons in the plasma system 57 are at a
very high energy. As a new electron is liberated in an ionizing
collision, it rapidly gains energy. Therefore, recombination
between the energetic electrons and the ions is not
significant.
[0138] Charge exchange affects only the multiply charged ions. If
an electron is transferred from He to He.sup.+, the result is the
same number and composition of ions, that is, the He becomes
He.sup.+ and the He.sup.+ becomes He. If an electron is transferred
from He to He.sup.++, however, the result is two He.sup.+ ions, so
a He.sup.++ ion was lost. In the case of the devices described
herein, the ion energy can be sufficiently low that charge exchange
is not a significant problem.
[0139] Radial diffusion is not an issue if the central magnetic
field 40 is large. If the cyclotron orbit of an ion is small
compared with the size of the ionization chamber 30, radial
diffusion is very slow and particles are scattered into the loss
cone 58 faster than they diffuse radially.
[0140] In typical embodiments, scattering into the loss cone 58 is
the primary loss method for particles. The system 10 requires this
scattering process in order to generate the beam 56 from the plasma
57. Loss cone scattering of He.sup.++ is desirable as this forms
the extracted ion beam 56. Loss cone scattering of He.sup.+,
however, is not desirable as it causes He.sup.+ to enter the beam
56, thus increasing the percentage amount of He.sup.+ in the beam
56. He.sup.+ ions in the beam 56 can be separated from the
He.sup.++ ions using a mass ion filter 61, but this decreases the
beam intensity, increases the emittance, and decreases the overall
beam efficiency.
[0141] The hot electrons generated by the ECR source 44 have a
non-isotropic distribution, with the perpendicular energy being
much larger then the axial energy. Ions generated by the collisions
with the electrons also have a non-isotropic distribution with more
of the energy perpendicular to the magnetic field 40 than parallel.
Given a mirror ratio near 2 (B.sub.mirror/B.sub.central), somewhat
more than half the formed He.sup.+ is confined by the mirror fields
32 and 34, and somewhat less than half the He.sup.+ is not confined
and leaves the mirror fields 32 and 34 on the first pass. Some of
the initially unconfined particles exit from the rear mirror field
32 and the rest exit from the front mirror field 34. Depending on
the particle densities and temperatures, geometric considerations,
and some other parameters, the produced beam 56 will typically be
between 50-90% He.sup.++ and 50-10% He.sup.+. The exact ratio of
He.sup.+ to He.sup.++ in the generated beam 59 depends on a
complicated set of relations between gas density, ECR power, source
geometry, and magnetic field geometry. The exact ratio can be
determined experimentally and optimized to produce a desired beam
ratio 56. As noted above, beam ratios of 84% or greater have been
experimentally demonstrated using devices of the types described
herein.
[0142] The ratio of highly multiply ionized particles can be
increased by using an ICR (Ion Cyclotron Resonance) exciter system
62, although for He.sup.++, it may not be needed. This system 62
uses antennae 68 in the chamber 30 between the mirror fields 32 and
34 to emit at the radiation proper frequency to couple energy into
the ion cyclotron resonance of He.sup.+ ions, thus coupling energy
into the perpendicular motion of the He.sup.+ ions. In general, the
axial (parallel) velocity of the particles remains unchanged by ICR
excitation. Increasing the perpendicular energy pushes the driven
particles out of the loss cone 58. In some embodiments, by using
ICR excitation, the scattering rate of He.sup.+ into the loss cone
58 can be reduced to nearly zero.
[0143] In reality, the He.sup.+ ions cannot be kept out of the loss
cone 58 indefinitely. Given sufficient time, particles will be
scattered into the loss cone 58. This scattering basically is
converting some of the particle's perpendicular energy into
parallel energy. The ICR excitation can then add more perpendicular
velocity and push the particle back out of the loss cone 58 again,
but eventually, the parallel energy will get sufficiently high that
the particle will escape the mirror fields 32 and 34.
[0144] The goal is not to keep ions out of the loss cone 58
indefinitely, but rather just long enough to undergo a multiple
ionizing collisions. When the system 10 is operated at a high
plasma density, these multiple ionizing collisions occur faster
than scattering into the loss cone 58.
[0145] When the ICR excitation is used, the system 10 operates as
follows. Microwaves excite the ECR in the central magnetic region
40 of the chamber 30, which generates hot electrons. These
electrons ionize the background helium gas producing mostly
He.sup.+ ions, but also some He.sup.++ ions. The ICR system 62
excites the He.sup.+ resonance in the central magnetic region 40.
The He.sup.+ ions are trapped between the two magnetic mirror
fields 32 and 34 where a second collision with an electron causes a
second ionization to produce He.sup.++. The ICR excitation does not
couple to the He.sup.++ ions, which have twice the cyclotron
frequency. The He.sup.++ ions are scattered into the loss cone 58
where they exit the chamber 30 as the ion beam 56. In some
embodiments, most of the He.sup.++ exits via the front mirror field
34, which is lower. In some embodiments, the height of the hack
mirror field 32 should be as high as possible, or at least higher
than the front mirror field 34. However, system 10 may operate even
when front mirror field 32 is equal to or substantially equal to
back mirror field 34
[0146] In some embodiments, the ICR excitation causes the density
of the plasma to increase until losses balance production. In the
limit, where the ICR exciter 62 can hold the He.sup.+ out of the
loss cone 58 indefinitely, the plasma density will increase until
the production of H.sup.+ is equal to production of He.sup.++,
which equals the rate that He.sup.++ is scattered into the loss
cone 58.
[0147] Most of the above examples have focused on helium. Helium
has only two charge states no it is easy to see how system 10 can
produce a high density ion beam 56 of He.sup.++. The same technique
can also be used to produce multiply ionized ions in a selected
final ionization state of almost any other atom, especially when
the ICR system 62 is used. However, when working with other atoms
(e.g. those with more than two ionization states), several issues
are notable.
[0148] First, depending on the ionization state and the atom
required, it may be necessary to increase the electron energy of
the ECR heated electrons. This can be done by increasing the power
in the ECR source 44 and/or by decreasing the cooling effects on
the electrons. Under the correct conditions one may obtain electron
energies as high as 600 to 1000 keV. In order to obtain very high
electron temperatures, it may be necessary or desirable to add a
radially confining multipole field to the center region 70 of the
magnet system, creating a "true" minimum magnetic field
configuration. In some embodiments, a radially confining multipole
magnet 72 (e.g. a hexapole, or higher order multipole magnet) may
be used. This may have some real advantages if an ICR exciter
system 62 is used. As described in more detail below, the higher
order multipole magnet 72 localizes the field effects to the outer
radial edge of the plasma 57 and leaves the central part of the
plasma 57 (located proximal to axis A) unaffected. The "true"
minimum magnetic field configuration provides a form of insulation
between the plasma 57 and the wall 76 of the chamber 30 and makes
it easier to obtain high temperature electrons.
[0149] Second, if the ICR exciter system 62 is used, there is a
question of what ICR frequency to use. When there are several
ionization charge states available, so there will be several
potential ICR exciter frequencies:
1 eB M , 2 eB M , 3 eB M , 4 eB M , ##EQU00006##
In some embodiments, one frequency is chosen for excitation.
Generally, exciting the lowest ionization state provides the most
increase in production of any higher states. It is also possible to
excite multiple states, or all states. For example if the desire
was to produce O.sup.+8, useful in treating inoperable tumors, an
ICR excitation system could be provided with multiple antennae that
excite the ICR resonances of O.sup.+, O.sup.++, O.sup.+3, O.sup.+4,
O.sup.+5, O.sup.+6, and O.sup.+7, thus preferentially holding all
oxygen ions other than O.sup.+8 within the chamber 30.
[0150] The system 10 includes a positive source 78 to bias the
chamber 30 at high potential in order to accelerate the ions
extracted. Therefore a number of insulators 80 are necessary to
allow the system 20 to be biased at different voltages and to
electrically isolate the chamber 30 to provide operator safety.
[0151] There are a number of different ways to implement insulator
systems. These include, but are not limited to the embodiments of
system 10 shown in of FIGS. 2A, 2B, 2C, and 2D. The configurations
shown in FIGS. 2A and 2B are just variations of each other with
insulators 80 used to electrically isolate the vacuum pump 54, the
ECR source 44 and the atom source 50. The system shown in FIG. 2C
is significantly different in that the entire outer wall 86 of the
chamber 30 is biased at ground. An internal bias liner 88 is used
to control the plasma bias and thus the extraction energy of the
ions. In terms of fabrication, in some embodiments this may be
somewhat more complicated, but may provide greater operator safety
in that it is more difficult for an operator to come in contact
with the high voltage. With an inner bias liner 88, the liner 88
could be placed either inside or outside of the ICR antenna 90. In
some cases, it may be possible to use the ICR antenna 90 itself as
a bias liner 88, but if the bias liner 88 is placed inside the ICR
antenna 90, it must contain at least one slit to prevent shorting
out the ICR drive.
[0152] FIG. 2D shows a variation of system 10 using a non-conducing
vacuum wall 80 where the ICR antenna 90 external to the vacuum
system. A Faraday shield 98 is used as a bias liner inside the
vacuum system.
[0153] An external antenna design can be generalized to work with
most antennas. In most cases an insulating chamber wall can be
used. The antenna system can be located external to the vacuum
system. A bias liner containing slit(s) can be used inside the
chamber to control the plasma bias without interfering with the ICR
excitation.
[0154] As shown in FIGS. 6A and 6B, an ICR exciter system 62
includes an RF drive system 96, an antenna 90, and a tuning system
100. The goal is to generate an electric field that will couple to
the ion cyclotron motion. An electric field rotating in the same
direction as the ions provides the highest coupling, but other
combinations such as linear or radial electric fields also work.
Generally the ICR frequencies are in the RF range and the antenna
90 is fed from coaxial lines 102. A capacitor/inductor network
normally is used within the tuning system 100 to match the antenna
90 with the driver system 96. In typical embodiments, the magnetic
field in the region where the ICR exciter is applied should be
relatively constant so that the cyclotron frequency of the ions is
well defined. Central uniform field region 40 meets this
requirement. Depending on how the antenna 90 is implemented and how
the insulator system is implemented, the antenna 90 may be at high
voltage. Coupling between the antenna 90 and RF drive system 96 can
be provided using a transformer or capacitors in order to provide
isolation between the systems.
[0155] A number of different antenna systems such as antennae 90,
can be used for the ICR exciter 62 These include, but are not
limited to: cavity excitation, capacitor plates, split-ring, and
filer. Cavity excitation simply couples to the cavity mode of the
chamber 30 using a loop (not shown) or some other type of
exciter.
[0156] The optimal antenna design may depend strongly on the
specific ion and charge state desired. For production of multiply
charged ions other than doubly charged ones, it may be desirable to
drive the antennae at multiple frequencies.
[0157] Capacitor plate antennas can be either linear (FIG. 6A) or
circular drive (FIG. 6B). In the case of linear drive the two
plates 104 and 106 are driven to generate an oscillating electric
field. In the case of a circular drive, vertical plates 108 and 110
are driven 90.degree. out of phase with the horizontal plates 104
and 106, generating a rotating electric field. In other
embodiments, other phase angles may be used, thereby generating
elliptical drive radiation. Note also that although the terms
vertical and horizontal are used here for convenience to describe
the relative positioning of the plates, the antenna plates may be
oriented arbitrarily in chamber 30. The exact structure of the
plates is unimportant, other than that they provide an electric
field perpendicular to the magnetic field 40. In some embodiments,
capacitor plate antenna (or other antenna type) may be used in
exciter system 62 to produce output at multiple frequencies
[0158] Referring to FIG. 7, a split-ring antenna 111 consists of a
coil of wire (or tubing to allow cooling) 112 wrapped around a
Faraday shield 114. The Faraday shield 114 prevents the helical
electric field of the coil 112 from reaching the plasma 57. The
lengthwise slit 116 in the metal of the Faraday shield 114 allows a
mostly linear component of the electric field to reach the plasma
57, which couples to the ion cyclotron resonance. The actual
electric field is more like radial arcs coming from the slit 116,
but this couples to ion motion with about the same efficiency as a
linear drive system.
[0159] In cases where an internal bias liner is used, e.g. as shown
in FIG. 2C, it is possible to use the Faraday shield 114 as a bias
liner inside the ICR exciter system 62. A variation of this
arrangement, as shown in FIG. 2D, is to use a non-conducing vacuum
wall insulator 80 and place the ICR antenna external to the chamber
30, and use the Faraday shield 98 as a bias liner inside the
chamber 30.
[0160] An external antenna design can be generalized to work in
most configurations. In most cases, the chamber 30 can include an
insulating chamber wall 80 so that the ICR antenna 68 can be
located in a region of ambient pressure, outside of the chamber 30.
A bias liner 98 containing one or more slits 116 (as shown in FIG.
7) can be used inside the chamber 30 to control the plasma bias
without interfering with the ICR excitation.
[0161] Referring to FIG. 8, a filer antenna 124 contains a number
of helical conductors, 126 and 128 shown, in which currents are
driven. Depending on the type of filer, either a linear drive field
or a rotating (i.e. circular or elliptical) drive field is
generated. For a rotating drive field, as shown, there are actually
two filers 130 and 132 that are constructed perpendicular to each
other having helical conductors 126 and 128, and 135 and 136. The
two filers are driven 90.degree. out of phase to generate a
rotating electric field. A simple filer antenna would have only one
of the two filers 130 or 132.
[0162] In addition to the other variations in the ICR antenna
systems 60 discussed above (cavity excitation, capacitor plates,
split-ring, and filer) an untwisted filer antenna 140 can be used.
As shown, untwisted filer antenna includes two filers 141 and 142.
In some embodiments a singe untwisted filer may be used. In this
case, one of filers 141 and 142 would be omitted.
[0163] The bi-filer antenna 124 is basically a twisted set of
current carrying wires. The wires on opposite sides of the plasma
57 carry current in opposite directions. These wires combine to
generate a magnetic field in the center of the plasma 57. As the
current oscillates, the generated magnetic field oscillates, and
induces an electric field in the center of the plasma 57. A single
filer antenna generates a linearly polarized excitation and the
bi-filer antenna typically generates a rotating excitation field
(circularly or elliptically polarized). Generally inductive antenna
systems (such as the filers) have advantages over electrostatic
antennas (such as capacitor plates). This is because the high
density plasma 57 tends to screen out the electric field generated
by electrostatic antennas, thus such antennae primarily couple to
the plasma 57 only at the outer edge of the plasma 57. Inductive
antennae drive though an induced electric field that is generated
by an oscillating magnetic field. The magnetic field can pass
through the plasma 57 and thus the drive field is located
everywhere in the plasma 57, and generally is peaked on the axis of
the chamber 30.
[0164] Typically a filer antenna 124 has a twist in it to prevent
charge separation in the plasma 57. In the present invention, the
twist is not needed. The short length of the source allows the
particles at the end of the plasma 57, where the drive field is
weak, to provide neutralization, thus charge separation is not a
large problem. Further neutralization is provided by plasma
rotation, which prevents the antenna, such as bi-filer 124, from
driving only over one part of the plasma 57. The plasma rotation is
due to the slight non-uniformity of the plasma 57 as ions and
electrons are lost at slightly different rates, resulting in a
slight charge imbalance that equalizes these two loss rates
(ambipolar potential). This charge imbalance gives rise to a radial
electric field, which combined with the axial magnetic field gives
rise to plasma rotation.
[0165] The untwisted-filer antenna 140 has advantages that it will
couple to the ions going both directions in the source. The
twisted-filter antenna imposes a Doppler shift on the ICR drive
field. This means the drive field couples to particles with a
cyclotron frequency given by .omega..+-.k.sub.zv.sub.z (where the
sign is determined by the direction of the antenna twist relative
to the direction of ion cyclotron rotation, and k.sub.z is
determined by the structure of the filer). The twisted-filer can be
matched to particles going one direction in the device (say
positive-z), but not the other direction, since the sign of the
axial velocity will change. The untwisted-filer has k.sub.z=0, thus
it will couple to particles going both directions in the device.
This allows the ICR exciter to couple to more of the plasma.
[0166] Referring to FIGS. 10A and 10B, system 10 may be operated in
resonant (FIG. 10A) or sub resonant (FIG. 10B) ECR modes. In the
resonant mode, ECR source 44 produces microwaves are resonant with
the ECR motion of electrons the uniform field 40 (resonant zone 200
located between the two mirrors fields 32 and 34 as shown in FIG.
10A). This zone corresponds to a cylindrical volume about axis A in
region 70 of chamber 30. This arrangement provides the highest
coupling between the electrons and the microwaves.
[0167] The general requirements on the uniformity of the central
field for the resonant mode can be expressed as a relation between
the electron confinement time and the electron de-correlation time.
If an electron is placed in a uniform magnetic field it will orbit
about the field with the cyclotron frequency given by
.omega. ce = eB m . ##EQU00007##
If the electron motion is driven by an ECR drive generating a
rotating electric field that rotates in the same direction as the
electron, the electron can gain energy. If the drive frequency
(.omega..sub.ECR) exactly matches the electron cyclotron frequency
the electron will continue to gain energy until relativistic
corrections to the electron motion become significant. In typical
embodiments this is not a significant issue as relativistic
corrections will be come significant when the electron energy
becomes a significant fraction of the electron rest energy. (The
electron rest energy is 512 keV and, for many embodiments, target
electron energies are 200-300 eV for alpha particle production, and
about 1 keV for production of C.sup.6+).
[0168] If the drive frequency does not exactly match the electron
cyclotron frequency, then the electron will first gain energy then
loose energy, and then gain it again. In fact, when the electron
looses energy it will return to the same initial conditions as
before it was accelerated by the ECR drive. The time scale of this
loss and gain of energy is given, in terms of the ECR frequency
.omega..sub.ECR by
.tau. DC = .pi. .omega. ce - .omega. ECR . ##EQU00008##
The gain (or loss) of energy of an electron in an off-resonant ECR
drive field is also limited by
.DELTA. K max .apprxeq. m .xi. 2 2 ( .omega. ce - .omega. ECR ) 2
##EQU00009##
where .xi. is the normalized electric field eE/m.
[0169] In some embodiments, the field is not completely uniform. In
such a case, the cyclotron frequency can be replaced with an
average cyclotron frequency experienced by the electron. Or more
accurately an average value over all the electrons of the average
cyclotron frequency experienced by each electron.
[0170] Given these conditions we can express a sufficient condition
on the uniformity of the field as the de-correlation time must be
longer or on the order of the electron confinement time in the
device. For the on resonant mode, when this condition is met, the
field may be considered to be substantially uniform.
[0171] In some embodiments field 40 is uniform to 1% or less, 5% or
less, 10% or less over a region extending axially 5 cm, 10 cm, 15
cm, 30 cm or greater, while the mirror-to-mirror distance is 60
cm.
[0172] In some cases, operation in the resonant mode may result in
too energetic electrons. For example, in the case of formation of
He.sup.++, the peak of the cross sections for all formation methods
is between 200-300 eV. Given sufficient power and coupling,
electron temperatures well above 10 keV can be generated. These
high energy electrons are not useful for the formation of the
He.sup.++, and tend to produce unwanted X-rays. The electron
temperature can be controlled by manipulating the microwave power,
gas pressure, the length of the resonant zone(s) and electron
confinement time.
[0173] An alternative system, as shown in FIG. 10B, for controlling
the electron temperature is to adjust the magnetic field in the
central region 40 such that the ECR drive is detuned from the
uniform field 40. This is referred to herein as the sub-resonant
mode. This mode restricts the microwave coupling to zones 180 and
182 localized on each end of the chamber 30 adjacent the mirror
field zones 32 and 34 (in contrast to the large cylindrical volume
in which coupling occurs in resonant mode operation). The lower the
central field 40, the smaller these zones 180 and 182 become. This
method has the advantage that high microwave power in this
configuration corresponds mostly to high electron (and plasma)
density and not to high electron temperature. This method can be
more efficient for production of low to moderate ionization states
(such as He.sup.++).
[0174] In this mode of operation there are two small ECR sections
180 and 182 located between the uniform field region and the
magnetic mirrors. Particles are not in these zones long enough for
de-correlation to be a significant issue. Electrons pass through
the ECR zone, and depending on the phase of the particle relative
to the ECR drive phase, the particle will gain or loose some energy
in the zone. Because the phase at which the vast majority of
electrons encounter both ECR zones is not correlated, they gain or
loose energy each time they pass through the ECR zones. This leads
to a stochastic heating process (similar to random walk) that leads
to the gradual gain of energy (i.e. gradual as comparison to the
resonant mode using similar operating parameters). For this process
directly there are not requirements on the field uniformity of the
central field, but the limitations on plasma stability still
apply.
[0175] In some embodiments with this configuration the relation
between the central field and the ECR frequency is significant. If
these two match too closely, then one cannot consider the electrons
to leave the ECR zone, and the entire central region appears to be
one large ECR zone that is off-resonance. If this is the case the
performance of the system is very poor, as the energy gained by the
electrons is limited by the relation given above. Further the
de-correlation time is short, so the electrons gain and loose
energy rapidly leading to poor confinement.
[0176] If the central region corresponds to a magnetic field that
is sufficiently far from the resonance condition, then the energy
that can be gained or lost by an electron crossing the zone is
limited by the relations above. Further, if the central region is
sufficiently far from the resonant condition, other effects
dominate the transit through the central region. This allows the
electron phase to be effectively randomized between the two ECR
zones. This reduces or eliminates the number of particles that are
consistently poorly phased matched between the two zones.
[0177] The ICR exciter system 62 can be used as there still will be
a uniform magnetic field 40 in the center of the chamber 30, but
the ECR zones will be located only adjacent the zones of the mirror
fields 32 and 34. In such cases, the ICR frequency is decreased
slightly to match the lower resonant field. An added advantage is
the mirror ratios on the front and back mirror fields 32 and 34 are
increased somewhat as the central field 40' is lowered.
[0178] FIG. 15 illustrates both resonant and sub resonant modes of
operation. FIG. 15 is a plot of total extracted beam current as a
function be central magnetic field (at constant ECR drive frequency
of 18 GHz) using small aperture. The sharp peak located at
approximately 0.638 T corresponds to resonant mode of operation.
The broad flat peak around 0.5 T corresponds to the sub-resonant
mode of operation.
[0179] Note from the graph it may appear that the sub-critical mode
of operation yields more current and thus may be better, but this
can be misleading. The data was taken by varying only the central
field, thus the mirror ratio, which is strongly tied to plasma
production, is not constant across the graph. Also the beam
composition is not plotted here so the ratio of He.sup.++ to
He.sup.+ also varies along the graph.
[0180] The region between 0.635 T and 0.56 T, neither mode fully
applies. Clearly close of 0.63 T the electron de-correlation time
is short, and the maximum energy obtained the electrons is too low
for the formation of He.sup.++. As one approaches the 0.6 T, the
ECR zones have moved out from the central region, toward the higher
field, but the zones may be too large and too well correlated to
lead to an efficient stochastic heating process.
[0181] FIG. 16 shows time-of-flight mass spectrometer data for the
resonant mode of operation. The horizontal axis represents time;
the data near 3.5 .mu.s corresponds to He.sup.++ and the data near
5.5 .mu.s corresponds to He.sup.+. The vertical axis indicates the
central magnetic field. Note the He.sup.++ production is flat
between 0.636 T and 0.639 T. This region corresponds to the
electron de-correlation time being longer then the electron
confinement time so electrons are lost before they can
decelerate.
[0182] The uniform field region also has another advantage other
then just coupling energy to the electrons or ions. In some
embodiments, mirror devices are unstable to a number of
instabilities driven by the curvature of the magnetic field. In
particular, the shape of the field in the region between the
uniform field and the magnetic mirror can be very unstable. In this
region the combined drift of the particles in the curved magnetic
field is given by
v -> = v -> R + v -> .gradient. B = m q R -> C .times.
B -> R C 2 B 2 ( v .perp. 2 + 1 2 v .parallel. 2 )
##EQU00010##
[0183] where R.sub.C is the radius of curvature of the magnetic
field. These drifts cause ions and electrons to drift in opposite
directions. The resulting electric field generated by the change
separation gives rise to a radial E.times.B drift
v -> E .times. B = E -> .times. B -> B 2 ##EQU00011##
that can lead to radial diffusion. When the magnetic field is
curved toward the plasma (concave) the resultant drift causes
outward transport and loss of the particles. This is often referred
to as "bad" curvature. On the other hand, when the magnetic field
is curved away from the plasma (convex) the resultant drift causes
inward transport and confinement of particles. This is often
referred to as "good" curvature.
[0184] In a simple magnetic mirror, the particles spend much more
of their time in the regions of "bad" curvature then in the regions
with "good" curvature and this leads to rapid radial loss of
particles.
[0185] In the systems described herein, the uniform field regions
greatly increase the stability of the plasma and provide for ion
confinement time that is comparable to the time required to remove
the second (or more) electron(s) to form ions in the selected final
state.
[0186] The increased stability is provided by a number of factors.
First, the uniform sections have essentially no curvature so they
decrease the overall time particles spend in the regions of "bad"
curvature. There are thresholds associated with the instabilities
that lead to transport. Clearly curved magnetic field driven
instabilities will not grow if the average magnitude of the drifts
are smaller then thermal transport drifts. Second, the uniform
field region contains a large fraction of the plasma. This region
provides a charge reservoir that damps out the change separation
caused by the curved magnetic field drifts. Basically this allows
charge to flow along the axis and neutralize the change separation
caused by the "bad" curvature region. Third, the uniform region
disconnects the two regions of "bad" curvature. Thus instabilities
growing on one end of the device generally cannot couple to
instabilities growing on the other side of the device due to
inconsistent phase shifting by particles traveling from one side of
the device to the other.
[0187] In some embodiments, requirements on the field are on the
order of 5-10% uniformity over the majority of region 70. Stability
may be more of an issue in the sub-critical mode because of the
potentially higher mirror fields which reduces the threshold for
instabilities.
[0188] Referring to FIGS. 14 and 14A, in some embodiments, (e.g.
for generation of very highly ionized ion species and/or for
generation of very intense ion beams) it may be helpful to add a
radial surface confinement magnetic 76 to the system 10.
Confinement magnet 76 produces radial confinement field 184 turns
the axial minimum field configuration into a more complete minimum
field confutation, by adding a generally increasing magnetic field
as a function of radius. Note that a true minimum magnetic field
configuration cannot be generated (such a static configuration
would violate Maxwell's laws). Configurations referenced to in the
art as "true" magnetic minimum configurations are just partial
minimum magnetic fields. All of these types of configurations
contain cusp regions where the field does not increase as a
function of radius. The size, shape, and location of these regions
distinguish these different minimum magnetic field
configurations.
[0189] The surface confinement magnetic field increases the
confinement time of the electrons in the system 30. This increases
the electron density in the system 30, as well as the electron
temperature (energy), as the electrons are in the ECR drive field
longer. Higher electron density corresponds to more ionizing
collisions with neutrals and ions. Higher electron energy
corresponds to the ability to generate higher ionization states.
This is not useful for He.sup.++, but could used for creating
highly ionized states of other atoms.
[0190] In some embodiments, a hexapole fixed magnet system (not
shown) is used to generate a "true" minimum magnetic field
configuration. The hexapole magnets of alternating polarities
create a field that penetrates radially well into the plasma 57,
and limits the microwave coupling region, as well as preventing the
use of an ICR exciter system, because of the lack of a uniform
magnetic field region.
[0191] As shown in FIGS. 14 and 14A, a higher order multipole field
is used to produce a surface confinement field 184 that would only
affect the edges 138 of the plasma 57, allowing for a broad ECR
coupling region and use of an ICR exciter. A higher order multipole
field can be introduced using fixed magnets, but it can also be
introduced using a series of current carrying wires 154, located
around the source area In such a case, each wire 154 would carry
current in the opposite direction to the adjacent wire 154, giving
rise to an adjustable multipole magnetic field 184. The strength of
the current in the wires 154 determines the multipole field 184,
which can be quite strong very close to the wires 154, e.g. in
region 185. The wires 154 used to generate the multipole field 184
can be twisted or straight (as shown). The same wires 154 can be
used to produce the multipole field 148 and act as the ICR antenna
for the "twisted" or "untwisted" filers. In such an implementation,
the wires 154 would carry a DC and an AC component to the current
and thus establish both DC and AC components of the field 148. In
some embodiments, the number of wires 154 used for a multipole may
be high (more than the eight pairs shown), new phases could be
introduced in the ICR drive to increase the coupling efficiency, or
some arrangement like every other wire 154 could carry an ICR drive
signal.
[0192] Referring again to FIG. 2A, system 10 uses a microwave horn
55 to direct the microwaves for the ECR plasma production. In some
embodiments, a higher efficiency can be obtained using a cavity
system (not shown). The cavity both localizes the microwaves and
the gas used in the system 10. Localizing the microwaves is good in
that it increases the microwave power density and thus the plasma
density. Increasing the plasma density increase the number of
ionizing collisions that can occur. It also reduces shielding and
other problems from the microwaves.
[0193] Localizing the gas decreases the amount of wasted feed gas,
generally not a large expense, but less waste is less waste and
some rare feed gases are relatively expensive. This also reduces
arcing problems caused by high gas pressures in the chamber 30. If
the feed gas is localized in the cavity for the most part, it will
not be in the antenna system, or the insulators. The lower the gas
pressure can be kept outside the gas generator, the better for
suppression of arcs.
[0194] It is not necessary for the cavity to be fully closed in a
gas tight sense. A number of holes can be placed in the wall as
long as they are small compared to the wavelength of the
microwaves. This will confine the microwaves, but allow evacuation
of the gas. Limiting the holes in the wall controls the conductance
to the rest of the vacuum system. This can be used to keep the gas
pressure high in the cavity but low in the rest of chamber 30.
[0195] The gas source 50 can be operated using a wide variety of
material feeds. In most cases these feed systems provide a neutral
stream of particles from which to make a plasma and thus a
beam.
[0196] In the case of alpha production, helium gas is used as a
working gas. In the case of most other gaseous materials they can
be used as a working gas for the system. It does not matter if they
are not simple atomic gasses. For example, if an oxygen ion source
is desired, molecular oxygen can be injected into the source. The
hot electrons will both dissociate the oxygen and ionize it.
[0197] For materials that are not normally a gas there are several
ways to inject the material into the source. One alternative is to
inject a gas containing the desired material. For example, carbon
could be injected using CO or CO.sub.2 as a working gas. In both
cases, the plasma generated will contain both carbon and oxygen
ions, thus the beam generated will contain both types of ions. The
source may be followed by a mass filter to eliminate the undesired
ions.
[0198] An alternative would be to vaporize the substance into
system 10. In a case such as calcium, it would be possible to
simply heat solid calcium to vaporize it using an oven. Materials
could also be vaporized by e-beam. This type of source is well
suited to production of materials with low melting points such as
barium and calcium.
[0199] A laser system can be used to oblate a target, giving rise
to neutral material from which the plasma is formed. Such a system
would fire a laser at a cooled target located in the chamber to
produce neutrals. The prime choice for the target location would be
in the region 70 between the two magnetic mirror fields 32 and 34.
In some embodiments, this location makes the optics some what
difficult so placement to the rear of the system, near the peak of
the rear mirror, has a number of advantages. Other placements are
workable, but may require more laser power to generate sufficient
neutral particles between the magnetic mirrors 32 and 34.
[0200] One way to inject material is to use sputtering. Here the
exiting ECR source and magnetic mirrors may be used to generate a
sputter source for almost any solid material.
[0201] Referring to FIGS. 10A and 10B the ECR source may be
adjusted to make unconfined hot electrons at resonant zones 210 and
211 on both sides of the rear mirror. The non-trapped hot electrons
generated on the front and back of the mirrors are not useful for
generating plasma 57. These electrons are not confined and
typically are quickly lost. Further resonance zones 210 and 211 are
typically very narrow so there is not sufficient time for the
electrons to gain a lot of energy. However, in embodiments
featuring a sputtering source, one can use the unconfined hot
electrons on the back side of the mirror to form a sputter source.
In such a case it would be desirable to tailor the magnetic field
so the gradient is not so steep at zone 210 such that the resonance
zone is sufficiently large to produce the desired plasma
density.
[0202] Referring to FIGS. 11, 12, and 13, a sputter source 212
works by allowing the hot electrons to ionize small amounts of gas
to form a plasma in the vicinity of rear resonance zone 210. The
sputter target is biased with a large negative bias (e.g. 1-4 kV).
This negative bias draws ions from plasma formed and accelerates
them to high energies. These ions then strike the sputter target
220 producing a number of neutral particles. These neutral
particles then pass through the hot electrons in resonant zone 210.
Some fraction of them are ionized, attracted by the negative
potential, impact on the target 220, and sputter off more neutral
particles. Some of the neutral particles pass through the hot
electrons and become ionized in the trapped resonance zone 200 (in
resonant mode) or zones 180 an 182 (in sub-resonant mode) and form
part of the particle beam.
[0203] The target material 220 does not have to be electrically
conducting in order to make a sputter target. For example, carbon
can be used as a target by simply placing it on a conducting
surface 230. The conductor 230 is biased to create the potential
necessary to accelerate ions, but it is not necessary to bias the
actual surface of target material 220. Conductor 230 my be isolated
from the walls of chamber 30 by an insulator 240.
[0204] In some embodiments, sputter source 212 may include fluid
cooling pipes. Any suitable cooling technique known in the art may
be employed.
[0205] In some cases the target material 220 does not sputter well.
This can be expressed as a sputter yield. The average number of
neutral particles sputtered from the surface per single ion hitting
the surface is the sputter yield. Materials that form good sputter
targets have a yield greater then 1 (often significantly greater).
If a material with a low sputter yield is desired, a carrier gas
can be used to increase the yield. Generally heavy gasses such as
xenon are desirable because they tend to have high sputter yields
on most targets and cause minimal problems in the system. Some of
the carrier gas will get ionized and find its way into the output
beam, but this can be minimized by correct choice of the gas,
geometrical arrangement, and/or filtered in the output beam.
[0206] In some embodiments, the geometry of a sputter source 212 is
important. It is necessary that ions from the plasma can be
accelerated into the sputter target 220, that neutral particles
have a clear path into the plasma 57, and that the source does not
block key things such as the microwave access to the resonant zone.
For example, sputter source 212 may fall into one of two
geometrical configurations. These are a sputter target 212 located
on axis A (as shown in FIG. 11) and one located in an annulus (as
shown in FIG. 12) or cone (as shown in FIG. 13) around axis A.
[0207] An ECR sputter source may be particularly well suited for
generation of multiply changed ion beams from solid materials. As a
practical example, there are applications for highly charged nickel
ion beams. This source is particularly well suited to production of
a nickel ion beam.
[0208] Referring to FIG. 11, in the case of an axially located
target 220, the sputter target covers a small area along the axis
of the ion source. The ECR microwaves are allowed to flow around
the target, and may even be injected off-axis to improve
access.
[0209] In some embodiments, one may alternatively generate a
sputter source using ions from the main confined plasma. Biasing is
used to attract ions from the confined plasma to a sputtering
target. For example, the ions may be drawn backwards into a plate
on the axis biased negative relative to the plasma. In such
embodiments, it is not necessary to create an unconfined plasma. In
some embodiments, this may have advantages for microwave access at
high plasma density.
[0210] The extraction system 59 passes the beam 56 through an
extraction limiter 65, forming the first electrode of the
extraction system. The beam 56 is allowed to follow the magnetic
field and expand as the magnetic field decreases. A focusing lens
64 is used to converge the beam into the final limiter, which forms
the boundary between the ion beam source system 10 and the
following systems (e.g. a beam filter or accelerator).
[0211] By allowing the beam 56 to initially expand into a low field
region then accelerating the beam to extraction energy, it is
possible to transition the beam from the high field region magnetic
field region 32, 34, and 40 to a low field region without
increasing the beam emittance.
[0212] In various embodiments, other suitable extraction systems
and techniques known in the art may be employed.
[0213] The vacuum system 54 can be implemented using almost any
standard pumping system. The high vacuum pumps should provide a
base pressure at least as low as 1.times.10.sup.-5 torr. During
operation the gas or vapor from the atom source 50 can be bled into
the system 10 at a higher pressure, providing high purity of the
working gas.
[0214] There are a number of potential applications for high energy
alpha (.sup.4He.sup.++) or helium-3 (.sup.3He.sup.++) beams. The
most immediate applications are the activation of target nuclei for
the production of radio-isotopes. Due to short half-life and/or low
production yield, it is necessary to use an intense beam in order
to produce reasonable quantities of many of these radio-isotopes.
Also, by using a target of very high purity, few undesirable
products are made so post processing (e.g. chemical separation) for
isotope purity is minimized.
[0215] The following list indicates some of the possible target
radio-isotopes that can be produced from various targets using
He.sup.++ beams: [0216] .sup.18F, .sup.123Xe/I.sup.123, .sup.67Ga,
.sup.111In, .sup.131Ba, .sup.68Ge, .sup.82Sr/Rb.sup.82, .sup.89Sr,
.sup.153Sm, .sup.124I, .sup.211At, .sup.148Gd, .sup.76Br,
.sup.199Tl, .sup.100Pd, .sup.128Ba, .sup.117mSn, and .sup.229Th
[0217] Many of these radio-isotopes either are not currently
produced or are only produced in very limited quantities because of
the difficulties in their production. Using a .sup.3He.sup.++ or
.sup.4He.sup.++ beam, however, many of these radio-isotopes can be
effectively produced. For example alpha (.sup.4He.sup.++) particles
can be used to produce desirable products as follows:
.sup.12C(.alpha.,n).sup.15O .sup.15N(.alpha.,n).sup.18F
.sup.79Br(.alpha.,n).sup.82Rb .sup.86Kr(.alpha.,n).sup.89Sr
.sup.96Zr(.alpha.,n).sup.99Mo .sup.100Ru(.alpha.,n).sup.103Pd
.sup.106Pd(.alpha.,n).sup.109Cd .sup.114Cd(.alpha.,n).sup.117mSn
.sup.121Sb(.alpha.,n).sup.124I
.sup.120Te(.alpha.,n).sup.123Xe(.sup.123I)
.sup.60Ni(.alpha.,2n).sup.68Zn(.sup.62Cu)
.sup.66Zn(.alpha.,2n).sup.68Ge(.sup.68Ga)
.sup.80Kr(.alpha.,2n).sup.82Sr(.sup.82Rb)
.sup.83Kr(.alpha.,2n).sup.85Sr .sup.109Ag(.alpha.,2n).sup.111In
.sup.121Sb(.alpha.,2n).sup.123I .sup.123Sb(.alpha.,2n).sup.125I
.sup.129Xe(.alpha.,2n).sup.131Ba(.sup.131Cs)
.sup.197Au(.alpha.,2n).sup.199Tl+2n
.sup.199HG(.alpha.,2n).sup.201Pb(.sup.201Tl)
.sup.206Pb(.alpha.,2n).sup.208Po .sup.209Bi(.alpha.,2n).sup.211At
.sup.61Ni(.alpha.,p).sup.67Cu .sup.64Ni(.alpha.,p).sup.67Cu
.sup.108Cd(.alpha.,p).sup.111In
.sup.114Cd(.alpha.,p).sup.117In(.sup.117mSn)
.sup.120Te(.alpha.,p).sup.123I .sup.122Te(.alpha.,p).sup.125I
.sup.163Dy(.alpha.,p).sup.166Ho .sup.174Yb(.alpha.,p).sup.177Lu
Other transmutations can occur to produce the same product, for
example: .sup.144Sm(.alpha.,gamma).sup.148Gd
.sup.147Sm(.alpha.,3n).sup.148Gd
.sup.147Sm(.sup.3He.sup.++,2n).sup.148Gd For some reactions,
production using alpha particles is greatly improved relative to
other methods. Examples include .sup.108Cd(.alpha.,p).sup.111In
.sup.121Sb(.alpha.,2n).sup.123I .sup.83Kr(.alpha.,2n).sup.85Sr
.sup.15N(.alpha.,n).sup.18F
.sup.66Zn(.alpha.,2n).sup.68Ge(.sup.68Ga)
.sup.150Nd(.alpha.,n).sup.153Sm .sup.96Zr(.alpha.,n).sup.99Mo
.sup.121Sb(.alpha.,n).sup.124I .sup.197Au(.alpha.,2n).sup.199Tl
Some isotopes that can be produced, e.g. in usable quantities,
essentially exclusively with alpha (or He-3) particles. Exemplary
reactions include .sup.209Bi(.alpha.,2n).sup.211At
.sup.121Sb(.alpha.,n).sup.124I .sup.144Sm(.alpha.,gamma).sup.148Gd
.sup.147Sm(.alpha.,3n).sup.148Gd .sup.147Sm(.sup.3He,2n).sup.148Gd
.sup.116Cd(.alpha.,3n).sup.117mSn In some embodiments, a beam of
deuterium ions may be used to drive the following reactions:
.sup.30Te(d,n).sup.131I+
.sup.200Hg(d,n).sup.201Tl
.sup.130Te(d,n).sup.131I
[0218] .sup.64Ni(d,2n).sup.64Cu .sup.15N(d,2n).sup.15O
.sup.132Xe(d,2n).sup.132Cs
.sup.110Cd(d,n).sup.111In
.sup.176Yb(d,n).sup.177Lu
[0219] In some embodiments, a beam of protons (Hydrogen ions) may
be used to drive the following reactions:
.sup.18O(p,n).sup.18F
.sup.124Te(p,n).sup.124I
[0220] .sup.124Te(p,2n).sup.123I .sup.85Rb(p,4n).sup.82Sr
.sup.201Hg(p,n).sup.201Tl
[0221] Ion sources of the types described herein may also drive
reactions such as: .sup.67Zn(.sup.3He,2n).sup.68Ga
.sup.199Hg(.sup.3He,n).sup.201Tl
.sup.16O(.sup.3He,p).sup.18F
[0222] .sup.61Ni(.alpha.,p).sup.64Cu .sup.64Ni(.alpha.,p).sup.67Cu
.sup.108Cd(.alpha.,p).sup.111In
.sup.114Cd(.alpha.,p).sup.117mIn(.sup.117mSn)
.sup.120Te(.alpha.,p).sup.123I .sup.122Te(.alpha.,p).sup.125I
.sup.163Dy(.alpha.,p).sup.166Ho .sup.174Yb(.alpha.,p).sup.177Lu
[0223] As noted above, for treatment of radioactive waste, there
are at least two types of radioactive waste transmutation. The
first is the production of useful products from waste material.
Exemplary transmutations which may be accomplished using the
devices and techniques described herein include:
.sup.226Ra(.alpha.,n).sup.230Th+n. (.sup.230Th is a source for
.sup.213Bi.) .sup.232Th+.alpha..fwdarw..sup.235U which is nuclear
fuel. .sup.235U+.alpha..fwdarw..sup.238Pu which is used in nuclear
fuel cells as a heat source and is in short supply.
.sup.231Pa+.alpha..fwdarw..sup.233Np+n-->.sup.229Pa-->.sup.225Ac
which is a medical isotope. The .sup.225Ac decay path is
.sup.225Ac-.alpha.->.sup.221Fr-.alpha.->.sup.217At-.alpha.->.sup-
.213Bi-beta->.sup.213Po-.alpha.->.sup.209Pb-beta->.sup.209Bi.
[0224] Second, waste may be converted to a stable product.
Exemplary transmutations which may be accomplished using the
devices and techniques described herein include:
[0225] .sup.251Cf++.fwdarw..sup.253Fm+2n converts long lived Cf
isotopes to short lived (approximately 3 days) Fm isotope,
.sup.237Np+.alpha..fwdarw..sup.239Am+n.fwdarw..sup.235Np.fwdarw..sup.231P-
a.fwdarw. . . . .fwdarw.Pb.
[0226] Another other issue in treatment of nuclear waste is
stimulated fission reactions. In this case heavy radioactive
nuclides are bombarded with alpha particles from a beam source of
the type described herein. These nuclides undergo a stimulated
fission reaction and fall apart into a number of lighter fragments.
Generally the fragments are still radioactive, but have short
half-lives and quickly decay to stable elements. Unlike many of the
heavy nuclei, which are alpha and neutron emitters, these lighter
ones are more likely beta and position emitters, which are easier
to deal with. Intense (very intense) alpha beams are ideal for this
type of application.
[0227] .sup.99mTc is a metastable nuclear isomer of .sup.99Tc,
indicated by the "m". "Metastable" means that .sup.99mTc does not
change into another element upon its decay. Instead .sup.99mTc
emits a 140 keV gamma ray that medical equipment can detect from a
body into which it has been injected. Accordingly .sup.99mTc is
well suited for the role of a medical tracer because it emits
readily detectable 140 keV gamma rays, and its half-life for gamma
emission is 6.01 hours. Over 93% of it decays to .sup.99Tc in 24
hours. This short half life of the .sup.99mTc allows for scanning
procedures which collect data rapidly, but keep total patient
radiation exposure low. .sup.99Tc is the ground state of .sup.99mTc
that eventually (half-life of 213 thousand years) emits a beta
particle and decays to .sup.99Ru, which is stable.
[0228] .sup.99mTc usually is extracted from so called "moo cows",
.sup.99mTc generators which contain .sup.99Mo. The majority of
.sup.99Mo heretofore produced for .sup.99mTc medical use comes from
fission in nuclear reactors, and must be processed carefully to
remove nuclear contaminates.
[0229] 20 million diagnostic nuclear medical procedures every year
use .sup.99mTc, approximately 85% of diagnostic imaging procedures
in nuclear medicine use this isotope. Depending on the type of
nuclear medicine procedure, the .sup.99mTc is bound to different
pharmaceutical that transports it to the required location.
.sup.99mTc is chemically bound to Sestamibi when it is used to
image the blood flow, or lack thereof, in the heart. Since
Exametazime, is able to cross the blood brain barrier, .sup.99mTc
is used with Exametazime so the .sup.99mTc flows through the
vessels in the brain to image cerebral blood flow. Imaging of renal
function is accomplished by tagging .sup.99mTc to Mercapto Acetyl
Tri Glycine.
[0230] Similarly, .sup.111In is a radionuclide with a half-life of
2.8049 days and with gamma ray emissions of 171.2 and 245.3 keV. In
a chloride form, it is used as a bone marrow and tumor-localizing
tracer; in a chelate form, as a cerebrospinal fluid tracer and in a
trichloride form, is used in electron microscopy to stain nucleic
acids in thin tissue sections. It decays by electron capture:
.sup.111In.fwdarw..sup.111Cd+.gamma.(171.2 keV)+.gamma.(245.3
keV)
Electron capture decay can be though of as a positron decay where
the positron is destroyed in the nucleus by an captured electron.
In this case the decay produces the gamma ray emissions of 171.2
and 245.3 keV sensed in the PET scan.
[0231] For many applications, due to short half-life and/or low
production yield, it is necessary to use an intense alpha particle
beam in order to produce reasonable quantities of .sup.99Mo or
.sup.111In. Particles beams of the type described herein may be
used to produce diagnostic or therapeutically effective doses of
these materials. As used herein, the phrase diagnostic or
therapeutically effective dose is to be understood to mean a dose
sufficient to perform at least one diagnostic or therapeutic
procedure is a human or animal patient.
[0232] Advantageously, the devices and techniques described herein
may be used to produce usable quantities of .sup.99Mo or .sup.111In
without the use a nuclear fission reactor. Accordingly, production
of dangerous radioactive byproducts may be reduced or eliminated.
Also, by using beam targets of very high purity, few undesirable
products are made so post processing for isotope purity is
improved.
[0233] Referring to FIGS. 17A, 17B, and 17C one exemplary
production process is to form a target 301 by placing a layer 190
of .sup.96Zr atoms on a copper strip 192 and then place a thin
layer 194 of .sup.109Ag atoms on the layer 190 of .sup.96Zr atoms.
The strip 192 is shown extending between a supply roll 196 and a
take-up roll 198.
[0234] A beam from ion source 300 is accelerated by accelerator 310
to target 301. In some embodiments, accelerators 310 functions in a
pulse mode so after the beam is spread into a rectangular shape by
the beam spreader 320, either a pulse or more of alpha particles is
directed at a rectangular area 330 of the strip 192 (in other
embodiments, other beam shapes may be used). The target strip 301
is then indexed or moved continuously with respect to the beam. A
cooling system can be used to cool the target 301. For example
coolant may be circulated via ports 350. Any suitable cooling
technique known in the art may be used.
[0235] In some embodiments, alpha particles are accelerated to the
target 301 at an energy of approximately 28 MeV to transmute a
portion of the .sup.109Ag atoms of layer 194 into .sup.111In. Alpha
particles passing through layer 194 will lose some energy in the
process, and thereafter, a portion will impinge on layer 190 at an
energy of approximately 16 MeV, transmuting a portion of the
.sup.96Zr atoms or layer 190 into .sup.99Mo. The copper,
.sup.109Ag, .sup.111In, .sup.96Zr, and .sup.99Mo are easy to
separate chemically so the copper, .sup.109Ag and .sup.96Zr
remaining are recycled, and the .sup.99Mo and .sup.111In are
chemically separated and used for medical isotope purposes.
[0236] Similar techniques may be used for targets with more that
two layers of different target materials, or with a single layer of
target material.
[0237] Although embodiments are shown above with target 301 mounted
on a roll, and other suitable target mount may be used. In some
embodiments, target 301 may mounted on a plate (not shown). The
target plate is placed in front of the beam to undergo reactions,
after which the plate may be changed. In some embodiments, a target
station would support more then one plate at a time. This allows a
first plate to be exposed to the beam while another plate is
changed out and a new plate made ready during the exposure of the
first plate. This reduces beam down time. Further, using plates in
this manner allows the exposed product, which may have a short
half-life, to be taken out quickly.
[0238] Note that in various embodiments, the systems and techniques
described herein may also be used to produce beams of singly
ionized ions. We can also use the source for production of singly
charged ions. In some embodiments the system may selectively
operable in several modes corresponding to different ion types and
charge state outputs. In some embodiments, a single ion source may
be used. Some embodiments may feature multiple ion sources. In some
embodiments, multiple beams of various types may be generated from
a single source, e.g. using charge or mass filtering
techniques.
[0239] While particular embodiments of the present invention have
been shown and described, it will be obvious to those skilled in
the art that changes and modifications may be made without
departing from the invention in its broader aspects and, therefore,
the appended claims are to encompass within their scope all such
changes and modifications as fall within the spirit and scope of
this invention. All patents, published applications and articles
mentioned herein are incorporated by reference in their
entirety.
* * * * *