U.S. patent application number 14/966980 was filed with the patent office on 2016-08-25 for ion induced impact ionization detector and uses thereof.
The applicant listed for this patent is Loma Linda University Medical Center. Invention is credited to Vladimir Bashkirov, Reinhard W. Schulte.
Application Number | 20160245929 14/966980 |
Document ID | / |
Family ID | 43826685 |
Filed Date | 2016-08-25 |
United States Patent
Application |
20160245929 |
Kind Code |
A1 |
Bashkirov; Vladimir ; et
al. |
August 25, 2016 |
ION INDUCED IMPACT IONIZATION DETECTOR AND USES THEREOF
Abstract
Disclosed are systems, devices and methodologies relating to an
ion induced impact ionization detector and uses thereof. In certain
implementations, the detector can include a dielectric layer having
one or more wells. An anode layer defining apertures to accommodate
the openings of the wells can be disposed on one side of the
dielectric layer, and a cathode such as a solid resistive cathode
can be disposed on the other side so as to provide an electric
field in each of the wells. Various design parameters such as well
dimensions and operating parameters such as pressure and high
voltage are disclosed. In certain implementations, such an ion
detector can be coupled to a low pressure gas volume to detect
ionization products such as positive ions. Such a system can be
configured to provide single ion counting capability. Various
example applications where the ion detector can be implemented are
also disclosed.
Inventors: |
Bashkirov; Vladimir; (Loma
Linda, CA) ; Schulte; Reinhard W.; (Grand Terrace,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Loma Linda University Medical Center |
Loma Linda |
CA |
US |
|
|
Family ID: |
43826685 |
Appl. No.: |
14/966980 |
Filed: |
December 11, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14203124 |
Mar 10, 2014 |
9213107 |
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14966980 |
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12896671 |
Oct 1, 2010 |
8669533 |
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14203124 |
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61255053 |
Oct 26, 2009 |
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61247916 |
Oct 1, 2009 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01T 1/16 20130101; G01T
1/185 20130101; G06F 30/20 20200101; G16B 5/00 20190201 |
International
Class: |
G01T 1/185 20060101
G01T001/185 |
Claims
1. (canceled)
2. A device configured to detect ions induced by particle radiation
in a low-pressure gas, the device comprising: an enclosure forming
a gas volume; an ion detector disposed within the enclosure, the
ion detector comprising: a first electrode defining a plurality of
apertures; a second electrode; a dielectric layer positioned
between the first electrode and the second electrode; a plurality
of wells extending through the dielectric layer, the plurality of
wells arranged in a two-dimensional grid to form a plurality of
rows of wells and a plurality of columns of wells, individual wells
of the plurality of wells substantially aligned with individual
apertures of the plurality of apertures to provide a path between
the individual aperture and the corresponding individual well; a
first plurality of readout strips, individual readout strips of the
first plurality of readout strips aligned along individual rows of
the plurality of wells, the first plurality of readout strips
defining a plurality of apertures such that individual apertures
are substantially aligned with corresponding individual wells of
the plurality of wells; and a second plurality of readout strips,
individual readout strips of the second plurality of readout strips
aligned along individual columns of the plurality of wells, the
second plurality of readout strips defining a plurality of
apertures such that individual apertures are substantially aligned
with corresponding individual wells of the plurality of wells,
wherein application of a sufficient voltage difference between the
first electrode and to the second electrode induces charge
multiplication within individual wells of the plurality of wells,
wherein the plurality of wells are in communication with the gas
volume such that a pressure of gas within the plurality of wells is
substantially equal to a pressure of gas within the gas volume.
3. The device of claim 2, wherein the dielectric layer has a
thickness of greater than or equal to approximately 1 mm.
4. The device of claim 3, wherein the dielectric layer has a
thickness of greater than or equal to approximately 2 mm.
5. The device of claim 4, wherein the dielectric layer has a
thickness of less than or equal to approximately 50 mm.
6. The device of claim 2, wherein the dielectric layer has a
thickness of between approximately 2 mm and 5 mm.
7. The device of claim 2, wherein the first electrode comprises one
or more layers of conductive material.
8. The device of claim 2, wherein the second electrode comprises a
resistive cathode layer.
9. The device of claim 2, wherein the first electrode and the
second electrode are separated by a distance that is substantially
the same as the thickness of the dielectric layer.
10. The device of claim 2, wherein each of the plurality of wells
has a cylindrical shape with a diameter.
11. The device of claim 10, wherein the plurality of wells are
arranged so as to define a pitch distance between walls of two
neighboring wells, the pitch distance configured so that a ratio
between the pitch distance and the diameter is in a range of about
1 to 5.
12. An ion detector to detect ions induced by particle radiation in
a low-pressure gas, the device comprising: a first electrode
defining a plurality of apertures; a second electrode; a dielectric
layer positioned between the first electrode and the second
electrode; a plurality of wells extending through the dielectric
layer, individual wells of the plurality of wells substantially
aligned with individual apertures of the plurality of apertures to
provide a path between the individual aperture and the
corresponding individual well; a first plurality of readout strips,
individual readout strips of the first plurality of readout strips
aligned to be substantially parallel with one another in a first
direction, the first plurality of readout strips defining a
plurality of apertures such that individual apertures are
substantially aligned with corresponding individual wells of the
plurality of wells; and a second plurality of readout strips,
individual readout strips of the second plurality of readout strips
aligned to be substantially parallel with one another in a second
direction different from the first direction, the second plurality
of readout strips defining a plurality of apertures such that
individual apertures are substantially aligned with corresponding
individual wells of the plurality of wells, wherein charge
multiplication within individual wells of the plurality of wells is
induced through application of a voltage difference between the
first electrode and to the second electrode, wherein the plurality
of wells are in communication with the low-pressure gas such that a
pressure of the low-pressure gas within the plurality of wells is
substantially equal to a pressure of the low-pressure gas outside
of the plurality of wells.
13. The device of claim 12, wherein the dielectric layer has a
thickness of greater than or equal to approximately 1 mm.
14. The device of claim 13, wherein the dielectric layer has a
thickness of greater than or equal to approximately 2 mm.
15. The device of claim 14, wherein the dielectric layer has a
thickness of less than or equal to approximately 50 mm.
16. The device of claim 12, wherein the dielectric layer has a
thickness of between approximately 2 mm and 5 mm.
17. The device of claim 12, wherein the first electrode comprises
one or more layers of conductive material.
18. The device of claim 12, wherein the second electrode comprises
a resistive cathode layer.
19. The device of claim 12, wherein the first electrode and the
second electrode are separated by a distance that is substantially
the same as the thickness of the dielectric layer.
20. The device of claim 12, wherein each of the plurality of wells
has a cylindrical shape with a diameter.
21. The device of claim 20, wherein the plurality of wells are
arranged so as to define a pitch distance between walls of two
neighboring wells, the pitch distance configured so that a ratio
between the pitch distance and the diameter is in a range of about
1 to 5.
Description
PRIORITY CLAIM
[0001] This application is a continuation of U.S. patent
application Ser. No. 14/203,124 (set to issue as U.S. Pat. No.
9,213,107 on Dec. 15, 2015), entitled "ION INDUCED IMPACT
IONIZATION DETECTOR AND USES THEREOF," filed Mar. 10, 2014, which
is a divisional of U.S. patent application Ser. No. 12/896,671
(issued as U.S. Pat. No. 8,669,533 on Mar. 11, 2014), entitled "ION
INDUCED IMPACT IONIZATION DETECTOR AND USES THEREOF," filed Oct. 1,
2010, which claims the benefit of priority under 35 U.S.C.
.sctn.119(e) of U.S. Provisional Patent Application No. 61/247,916,
entitled "ION INDUCED IMPACT IONIZATION DETECTOR AND
HIGH-RESOLUTION IONIZING TRACK STRUCTURE IMAGING METHOD," filed
Oct. 1, 2009, and U.S. Provisional Patent Application No.
61/255,053, entitled "ION INDUCED IMPACT IONIZATION DETECTOR AND
USES THEREOF, filed Oct. 26, 2009, each of which is hereby
incorporated herein by reference in its entirety to be considered
part of this specification.
BACKGROUND
[0002] 1. Field
[0003] The present disclosure generally relates to the field of ion
detectors, and more particularly, to systems and methods for
detecting ions so as to allow characterization of interaction of
ionizing radiation in matter and for utilization in ion-detection
based analytic systems.
[0004] 2. Description of the Related Art
[0005] Some detection apparatus and methods rely on electron
detection by detecting current. Although it can be easier to
produce and measure electrons, these methods may not be
sufficiently sensitive for certain applications.
[0006] In certain applications, it may be more desirable to analyze
or characterize a sample or an interaction by detecting positive
ions. Such positive ions can be generated in a number of ways.
SUMMARY
[0007] In certain implementations, the present disclosure relates
to an ion induced impact ionization detector that includes an anode
having a first side and a second side, and defining a plurality of
apertures. The detector further includes a dielectric layer having
a first side and second side, where the first side of the
dielectric layer is positioned adjacent the first side of anode,
and the dielectric layer has a thickness of approximately 1-50 mm.
The detector further includes a plurality of wells extending
through the dielectric layer, with the plurality of apertures of
the anode positioned relative to the plurality of wells so as to
define openings between the plurality of wells and the second side
of the anode. The detector further includes a cathode positioned
adjacent the second side of the dielectric layer forming a bottom
of each of the plurality of wells.
[0008] In certain embodiments, the thickness is in a range of
approximately 1 to 5 mm. In certain embodiments, the thickness is
in a range of approximately 2 to 5 mm.
[0009] In certain embodiments, the anode includes one or more
layers of conductive material. In certain embodiments, the cathode
includes a resistive cathode layer.
[0010] In certain embodiments, the anode and the cathode are
separated by a distance that is substantially the same as the
thickness of the dielectric layer.
[0011] In certain embodiments, the ion induced impact ionization
detector is sensitive to a single ionization event in each of the
plurality of wells.
[0012] In certain embodiments, the ion induced impact ionization
detector further includes a first plurality of readout strips
configured in a first orientation and disposed on the first side of
the dielectric layer. The detector further includes a second
plurality of readout strips configured in a second orientation and
disposed on the first plurality of readout strips so as to allow
identification of a well that has detected an ion. In certain
embodiments, the first and second orientations are substantially
perpendicular so as to define X and Y orientations.
[0013] In certain embodiments, the ion induced impact ionization
detector further includes a third plurality of readout strips
configured in a third orientation and disposed on the second
plurality of readout strips so as to allow determination of more
than one wells that detected ions substantially simultaneously. In
certain embodiments, the first, second and third orientations
define X, U and V orientations.
[0014] In certain embodiments, each of the plurality of wells has a
cylindrical shape, and each of the openings defines a circle having
a diameter. In certain embodiments, the diameter is selected to be
in a range of about one-tenth of the thickness to one thickness. In
certain embodiments, the diameter is selected to be in, a range of
about one-fourth of the thickness to one-third of the
thickness.
[0015] In certain embodiments, the plurality of wells are arranged
in an array so as to define a pitch distance between the edges of
two neighboring openings, with the pitch selected so that a ratio
between the pitch and the diameter is in a range of about 1 to 5.
In certain embodiments, the ratio between the pitch and the
diameter is in a range of about 1.1 to 3.
[0016] In certain implementations, the present disclosure relates
to a detector system that includes an enclosure having a volume of
low pressure gas. The system further includes a drift anode
disposed within the volume of low pressure gas. The system further
includes the ion induced impact ionization detector as summarized
above disposed within the volume of low pressure gas so as to
define a detection gas volume between the drift anode and the anode
of the ion induced impact ionization detector, with the wells
having substantially the same low pressure as in the volume of low
pressure gas due to the openings. The system further includes an
electrical power supply coupled to the drift anode, and the anode
and cathode of the ion induced impact ionization detector. The
anode of the ion induced impact ionization detector is at a ground
potential, the drift anode is at a positive potential relative to
the ground, and the cathode is at a negative potential relative to
the ground. The positive potential selected to provide a first
electric field in the detection gas volume for drifting of a
positive ion towards the anode of the ion induced impact ionization
detector. The negative potential selected to provide a limited
Geiger avalanche in the well where the positive ion drifts
into.
[0017] In certain embodiments, the low pressure of the gas is in a
range of about 1 to 100 Torr. In certain embodiments, the low
pressure of the gas is in a range of about 1 to 10 Torr.
[0018] In certain embodiments, the negative potential is selected
such that an electric field strength within the well is greater
than a threshold value associated with breakdown of the gas in the
well. In certain embodiments, the negative potential is selected
such that the electric field strength within the well is less than
a threshold value associated with field emission breakdown at a
surface of the well.
[0019] In certain embodiments, the negative potential is selected
such that an electric field strength within the well puts the gas
within the well in a super-tensioned state.
[0020] In certain embodiments, one or more dimensions of the well,
spacing between the wells, and the negative potential is selected
such that an electric field formed within the well is capable of
changing as the ion induced avalanche progresses to direct another
incoming ion to another nearby well.
[0021] In certain implementations, the present disclosure relates
to a method of detecting particles. The method includes detecting
one or more positive ions using the ion induced impact ionization
detector summarized above.
[0022] In certain embodiments, the method further includes
subjecting the second side of the anode to a gas, and maintaining
the environment surrounding the ion induced impact ionization
detector at a pressure of less than about 10 Torr.
[0023] In certain embodiments, the method further includes applying
a negative voltage to the cathode, and maintaining the anode at
ground potential such that an avalanche breakdown of the gas in the
wells results when a positive ion enters the sell, with the
avalanche resulting in a detectable collection of charges.
[0024] In certain embodiments, the negative voltage applied is in a
range of about 600-900V, and the operating pressure is selected to
be less than about 10 Torr. In certain embodiments, the operating
pressure is selected to be less than about 2 Torr, and the negative
voltage is selected so as to yield a quantity of electric field
strength divided by pressure (E/p) has a value of about 2000V/(cm
Torr).
[0025] In certain implementations, the present disclosure relates
to a method for modeling a sample of condensed matter. The method
includes identifying ionization clusters responsible for local
damage to the condensed matter.
[0026] In certain embodiments, the condensed matter is selected
from the group consisting of cells, polymers, nanoelectronics and
nucleic acid molecules.
[0027] In certain embodiments, the method further includes
subjecting the condensed matter to ionizing radiation produced by
the ion induced impact ionization detector summarized above, where
the subjecting step induces an aberration in the condensed matter.
In certain embodiments, the method further includes assessing
effects of the ionizing radiation on the condensed matter. In
certain embodiments, the effects are selected from a DNA double
strand break, a central nervous system effect, and cancer
induction.
[0028] In certain implementations, the present disclosure relates
to a method of track ion detection. The method includes imaging a
spatial distribution of initial energy deposits in condensed matter
by detecting positive ions using the ion induced impact ionization
detector summarized above.
[0029] In certain embodiments, the method further includes
correlating measurements from the imaging step with radiation
effects in the condensed matter. In certain embodiments, the ion
induced impact ionization detector provides a substantially full
topology of the ionization pattern of track segments and resolves
single and clustered ionization events along the radiation track
over a length in condensed matter.
[0030] In certain implementations, the present disclosure relates
to a track ion detector having the ion induced impact ionization
detector summarized above.
[0031] In certain implementations, the present disclosure relates
to a mass spectrometer, an ion mobility spectrometer or a gas
chromatograph including the ion induced impact ionization detector
summarized above.
[0032] In certain implementations, the present disclosure relates
to a particle detector for detecting the presence and location of a
particle. The detector includes a first electrical plate, and a
second electrical plate. The first and second electrical plates are
biased with respect to each other so as to define an electric field
therebetween. The detector further includes an insulating layer
interposed between the first electrical plate and the second
electrical plate so as to be positioned within the electric field.
The insulating layer includes a plurality of wells that extend
therethrough, where the wells are spatially distributed so as to
receive particles. The wells include opening through which
particles can enter. The detector further includes at least one
sensor that is positioned with respect to the wells so as to
provide signals indicative of the presence of particles within the
wells. The electric field, the length of the wells and the
atmosphere within the wells are selected so that a charged particle
entering the wells results in a limited Geiger avalanche breakdown
within the wells thereby resulting in a detectable signal by the
sensor indicative of the particle entering the wells.
[0033] In certain embodiments, the ratio of the pitch to the well
diameter is about 1.1 to 3. In certain embodiments, the wells have
a length of approximately 2-5 mm and the atmosphere within the
wells is maintained at a pressure of approximately less than 10
Torr, the diameter of the wells is about 0.1-2 mm, and the pitch is
about 0.2-5 mm. In certain embodiments, the pressure is less than
about 2 Torr, the wells have a length of about 3.2 mm, the diameter
of the wells is about 0.8 mm, and the pitch is about 2 mm. In
certain embodiments, the detector includes about 1-10,000
wells.
[0034] In certain embodiments, the particle detector further
includes at least two readout strip layers configured to determine
the relative location of detected ions.
[0035] In certain implementations, the present disclosure relates
to a method for detecting the presence and location of a positive
ion. The method includes establishing an electric field in a low
pressure gaseous environment to generate ions, where the electric
field is sufficient to create a limited Geiger avalanche breakdown.
The method further includes detecting a signal produced by the
avalanche breakdown.
[0036] In certain implementations, the present disclosure relates
to a radiation dosimeter that includes a first electrode layer. The
dosimeter further includes a second electrode layer having first
and second sides, where the first side of the second electrode
layer and the first electrode layer define an interaction region
occupied by gas molecules at a pressure, and where the second
electrode layer defines a plurality of apertures to allow passage
of charged particles generated from ionization of the gas molecules
by radiation passing through the interaction region. The dosimeter
further includes a third electrode layer disposed on the second
side of the second electrode layer. The dosimeter further includes
an insulating layer interposed between the second electrode layer
and the third electrode layer, where the insulating layer defines a
plurality of wells open towards the interaction region. The wells
are spatially distributed so as to substantially match the
plurality of apertures of the second electrode layer and so as to
receive the charged particles passing therethrough. The dosimeter
further includes a voltage control circuitry configured to provide
the first, second, and third electrode layers with different
electrical potentials, such that the interaction region is provided
with a first electric field that allows drifting of the charged
particles towards the second electrode layer without charge
multiplication. The wells are provided with a second electric field
that results in charge multiplications in wells where the charged
particles enter. The dosimeter further includes a detection
circuitry in communication with at least one of the second and
third electrode layers and configured to detect the charge
multiplications in the wells.
[0037] In certain embodiments, the charged particles include
positive ions generated from the ionization. In certain
embodiments, the second electrode layer is electrically connected
to an electrical ground. In certain embodiments, the first
electrode layer is provided with a positive potential relative to
the electrical ground so as to allow drifting of the positive ions
towards the second electrode layer. In certain embodiments, the
third electrode layer is provided with a negative potential
relative to the electrical ground so as to allow acceleration of
the positive ions in the wells to induce the charge
multiplications.
[0038] In certain embodiments, the interaction region is
dimensioned and the pressure is selected such that an ionization
cross-section in the interaction region is similar to an ionization
cross-section of a nano-scale condensed matter object. In certain
embodiments, the pressure is selected to be less than approximately
10 Torr so as to allow relatively large expansion of the
interaction region dimension to approximate the ionization
cross-section of the nano-scale condensed matter object. In certain
embodiments, the wells' depth is selected based on the selected
pressure and its corresponding range of electric field strength per
pressure (E/p) values where the charge multiplication occurs. In
certain embodiments, the wells' depth is selected based on
probability of the charge multiplication yielding sufficient
detectable charge. In certain embodiments, the wells' depth is
selected to reduce likelihood of dielectric breakdown of the
insulating layer. In certain embodiments, the wells' depth is
selected to be about 2 mm or greater. In certain embodiments, the
wells' depth is selected to be between about 2 mm and 5 mm.
[0039] In certain implementations, the present disclosure relates
to a radiation dosimetry method that includes providing a gaseous
volume such that radiation passing through the volume has a
probability of ionization interaction with gas molecules at a
pressure that is similar to a probability of ionization interaction
of the radiation with a nano-scale condensed matter object. The
method further includes providing a first electric field to the
gaseous volume so as to induce drifting of one or more positive
ions generated from one or more ionized gas molecules to a first
side of the gaseous volume. The first electric field is selected
for the gas molecules at the pressure so as to result in the
drifting but not in charge multiplication from the one or more
positive ions or corresponding electrons. The method further
includes detecting the one or more drifting positive ions at the
first side so as to characterize the ionization interaction in the
gaseous volume. The pressure of the gas molecules and dimension of
the gaseous volume traveled by the radiation are selected such that
the characterization of the ionization interaction in the gaseous
volume approximates the ionization interaction of the radiation
with the nano-scale condensed matter object.
[0040] In certain implementations, the present disclosure relates
to an ion detector element that includes an anode and an insulator
layer having a first side and a second side. The first side of the
insulator layer, is disposed adjacent the anode, and the insulator
layer has a thickness in a range of approximately 1-5 mm. The ion
detector element further includes a cathode disposed adjacent the
second side of the insulator layer. The insulator layer defines a
well that extends between the first and second sides of the
insulator layer. The well is provided with a gas at a pressure of
approximately 1-10 Torr. The anode and cathode provided with an
electrical potential difference of approximately 600-900 V.
[0041] In certain implementations, the present disclosure relates
to a gas chromatograph having one or more of the ion detector
elements summarized above.
[0042] In certain implementations, the present disclosure relates
to an ion mobility spectrometer including one or more of the ion
detector elements summarized above.
[0043] In certain embodiments, the spectrometer is configured to
detect trace amounts of one or more chemicals associated with
explosives, drugs, and chemical weapons.
[0044] In certain implementations, the present disclosure relates
to an ion detector having a plurality of the detector elements
summarized above arranged in an array so as to allow spatial
determination of ions incident of the detector elements. In certain
implementations such an array can be part of a dosimeter. In
certain implementations such an array can be part of a mass
spectrometer.
[0045] For purposes of summarizing the disclosure, certain aspects,
advantages and novel features of the inventions have been described
herein. It is to be understood that not necessarily all such
advantages may be achieved in accordance with any particular
embodiment of the invention. Thus, the invention may be embodied or
carried out in a manner that achieves or optimizes one advantage or
group of advantages as taught herein without necessarily achieving
other advantages as may be taught or suggested herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0046] FIG. 1 schematically depicts an example of an ion-counting
system where ionization-induced ions are generated in a
low-pressure gas volume and extracted into an ion-counter in a
high-vacuum environment.
[0047] FIG. 2A show distributions of sizes of ion-clusters that can
be induced by protons, alpha-particles and carbon ions, in a gas
volume of propane at about 1 Torr.
[0048] FIG. 2B shows a comparison of Monte-Carlo model simulations
with the experimental data over 3.5 orders of magnitude.
[0049] FIG. 3 shows that in certain implementations, a
two-dimensional array of ion detectors can be configured to detect
ions such as those resulting from ionization in a gas volume, where
the ion detector array can be operated in a similar gas pressure
environments as that of the gas volume.
[0050] FIG. 4A depicts a portion of a two-dimensional ion detector
that can be used for a number of purposes including imaging, where
the detector can include an array of wells formed in a dielectric
layer.
[0051] FIG. 4B shows a more detailed view of one of the wells of
the detector in FIG. 4A.
[0052] FIG. 5 shows examples of calculated energy distributions of
radiation induced ions that are drifting in propane across a well
under a field of approximately 1000V per 0.1 mm.
[0053] FIGS. 6A and 6B show example electric field lines in a well
during and close to termination of a limited-Geiger avalanche
triggered by a positive ion.
[0054] FIG. 7 shows a portion of a detector apparatus having a
number of wells configured to facilitate ion induced impact
ionization.
[0055] FIG. 8 shows that in certain embodiments, the detector
apparatus of FIG. 7 can be fabricated on a printed circuit board
(PCB).
[0056] FIGS. 9A and 9B show examples or readout electrode strips
that can be implemented in the detector apparatus of FIG. 8.
[0057] FIG. 10A shows an experimental setup using alpha-particles
for testing the detector of FIG. 8.
[0058] FIG. 10B shows examples of signals that can be extracted
from the example setup of FIG. 10A.
[0059] FIG. 11A shows an example distribution of charge resulting
from the detector of FIG. 8 operating at approximately 800 V in
propane at approximately 3 Torr pressure.
[0060] FIG. 11B shows that in certain implementations, the mean
signal amplitude corresponding to the charge output of FIG. 11A can
depend on the operating voltage.
[0061] FIG. 12 shows example plots of lateral resolution (rms) in
tissue-equivalent (TE) units for ions following a drift path of
approximately 40 nm in approximately 1 Torr propane.
[0062] FIG. 13 schematically depicts an example application where
interactions of radiation in a volume of low pressure gas can
provide an estimate of an equivalent dose of the radiation
delivered to a nano-scale condensed matter.
[0063] FIG. 14 shows an example process that can be implemented to
facilitate the dosimetry system of FIG. 13.
[0064] FIG. 15 shows that in certain implementations a gas model
can simulate a condensed matter such as a DNA molecule, thereby
allowing characterization of its interaction with radiation at a
large expanded scale.
[0065] FIG. 16 shows an example configuration of an interaction
volume of gas where ionization products can be generated, collected
and detected.
[0066] FIG. 17 shows an example process that can be implemented to
facilitate detection of single ions resulting from events such as
the ionization occurring in the gas interaction volume of FIG.
17.
[0067] FIG. 18 shows an example configuration of a well based
detector that can achieve detection of single ions.
[0068] FIG. 19 schematically depicts an example of show data can be
acquired from detection of ions in an analytic system.
[0069] FIG. 20 schematically depicts various components that can be
provided to a system capable of facilitating the process of FIG.
19.
[0070] FIG. 21 schematically depicts an ion detector element
capable of detecting single ions.
[0071] FIG. 22 schematically depicts an array having a number of
the detector elements of FIG. 21.
[0072] FIG. 23 schematically depicts an analytic system having the
detector array of FIG. 22.
[0073] FIGS. 24A-24C show non-limiting examples of systems having
one or more of the detector element of FIG. 21.
DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS
[0074] The headings provided herein, if any, are for convenience
only and do not necessarily affect the scope or meaning of the
claimed invention.
[0075] In embodiments described herein, a gas-based detector can
address various issues associated with detector designs. As also
described herein, scaled dimension (e.g., nanometer scale)
equivalent resolution is achievable using one or more features of
the present disclosure. Such a resolution capability can be
implemented in a number of applications and provide various
benefits. For example, patients can benefit because treatment
planning of proton and heavy ion radiation therapy can be made more
precise by taking the varying biological effectiveness of these
particles into account. In another example, society at large can
benefit by allowing better definition of radiation exposure limits
and thereby protecting its members from unwanted side effects of
radiation such as cancer and genetic alterations.
[0076] In yet another example, high-resolution imaging of radiation
track structure can be achieved using one or more features of the
present disclosure. Such a capability can be relevant in many
technical fields related to radiation interaction with matter, such
as in medicine, physics, radiobiology, and engineering.
[0077] Examples for applications or potential applications of track
structure imaging in medicine can include optimization of treatment
planning for therapy with particles such as protons and heavy ions,
and evaluation of space radiation for cancer risk estimates.
Applications in other fields can include solar neutrino studies,
dark matter search, x-ray polarimetry in astrophysics, radiation
protection, nuclear waste management, radiation therapy planning
with charged particles, manned space missions and damage to micro-
and nano-electronic elements in intense radiation fields in
accelerator and outer space environments. In certain
implementations of the present disclosure, nanometer-equivalent
resolution capability can be applied to some or all of the
foregoing example applications.
[0078] An important field that can benefit from precise
track-imaging devices is the study of radiation damage to cells and
DNA. Current devices and methodologies for track-imaging, however,
are generally not suitable for track-structure studies on the
DNA-to-chromosome scales. In certain situations, study of
interactions at such nanometer-scales can be facilitated by
detectors and devices that are sometimes referred to as
nanodosimeters.
[0079] In certain implementations of the present disclosure, a
device (e.g., a nanodosimeter) can be configured to detect positive
ions which are induced by particle radiation in a low-pressure gas
(e.g., propane of about 1 Torr). The use of the low gas pressure
allows scaling down a millimeter-size gas volume from which ions
are collected to an equivalent unit density volume of nanometer
size. In this particular example, the scaling is by a factor of
about 10.sup.-6. The nanodosimeter thus can simulate a short
segment of DNA of about 20 nm in length and 2-4 nm in diameter.
[0080] In certain implementations, the foregoing example
nanodosimeter can be configured to a frequency of ion clusters of
different sizes formed in such a volume. Such information can be of
interest for biomedical applications due to, for example, a
hypothesis that large clusters, despite being relatively rare, are
mainly responsible for irreparable DNA damage in a living cell.
[0081] There are a number of ways for detecting ionization products
such as positive ions. For example, FIG. 1 schematically depicts an
embodiment of an ion-counting nanodosimeter 10. Ionization-induced
ions are shown to be deposited in a sensitive volume such as a
volume of low-pressure gas (e.g., 1 Torr propane). The ions formed
in such a volume, representing, for example, a DNA segment, are
shown to be extracted into a high-vacuum ion-counter, where they
are multiplied and individually detected and counted. The number of
ions formed in the sensitive volume can be proportional to the
deposited energy; and the detectable time delays along a pulse
trail can provide information about the interaction location along
the sensitive volume (DNA) axis.
[0082] In certain embodiments, information obtained from such ion
measurements can be used to characterize different types of
radiation at various energies. Such characterization of radiation
and its representative interaction with matter (e.g., nano-scale
condensed matter) can be used, for example, to refine simulation
models.
[0083] For example, FIG. 2A shows various ion-cluster size
distributions induced by protons, alpha particles and carbon ions,
in the example sensitive volume of the nanodosimeter described in
reference to FIG. 1 (operated at approximately 1 Torr of propane
gas). FIG. 2B shows a comparison of Monte-Carlo model simulations,
where there is substantial agreement with experimental data over
3.5 orders of magnitude, as well as divergence in other regions.
Such differences can be addressed in the model appropriately.
[0084] In the example of FIGS. 1 and 2, counting of ions is
performed at a selected location with respect to the sensitive
volume; and spatial information about the ionization events are
obtained from the ion pulse train.
[0085] In accordance with certain, implementations of the present
disclosure, an ion induced impact ionization detector described
herein can permit substantially full track structure imaging. In
certain embodiments, the ion induced impact ionization detector
described herein can have a sensitive volume with a simple
geometrical shape (e.g., a cylinder, a sphere or a box). In certain
implementations, the ion induced impact ionization detector
described herein may be used to measure a correlation of clustered
ionization events over chromosomal-equivalent dimensions.
[0086] In certain embodiments, there is no significant pressure
difference between the sensitive volume and the ion counter; thus
obviating a double differential pumping system (e.g., a system
having two turbomolecular pumps) generally associated with the
example device of FIG. 1. Therefore, the ion induced impact
ionization detector described herein can be relatively compact,
mobile and inexpensive. The lack of a significant pressure gradient
can also minimize the distortion of ionization clusters as the
clusters move through the gradient.
[0087] In certain implementations, one or more features of the
present disclosure can be related to nanodosimetry track-structure
imaging methods that can provide a substantially full topology of
the ionization pattern of track segments and efficiently resolve
single and clustered ionization events along the radiation track
over a length in condensed matter.
[0088] In certain implementations, a low-pressure (e.g., .about.1
Torr) gaseous detector can be provided. FIG. 3 schematically
depicts a gaseous detector 100 having a volume
(D1.times.D2.times.2D3 in lab frame, and
D1TE.times.D2TE.times.2D3TE in tissue-equivalent representation) of
gas (102) where ionizations are shown to be induced along a given
track segment of an ionizing particle. For example, each of D1TE
and D2TE can have a TE dimension of about 100 nm. Examples of D3TE
dimensions are discussed below.
[0089] Primary ionizations are indicated as 120; secondaries as
122. Cluster formations are indicated as 124. The ionization
products are depicted as drifting (arrows 130) towards a detection
plane 110. In certain embodiments, the detection plane 110 can
include a two-dimensional array of positive-ion detectors as
described herein.
[0090] The detection plane 110 shown in FIG. 3 can be positioned on
one side of the gas volume 102. In certain embodiments, the gas
volume 102 and/or the positioning of the detection plane 110 can be
selected so as to keep the drift distances of the induced ions to
the detection plane as 110 small as possible. For example,
according to a calculated estimate, a tissue-equivalent (TE)
cylinder (depicted by circles 104 at the entrance and exit of the
gas volume) of about 16 to 40 nm radius (D3TE in TE representation)
centered about the primary track contains about 95% of the
ionizations (primaries 120 and secondaries 122) induced by 1 to 100
MeV protons, respectively. In propane of 1 Torr, this corresponds
to radius range of about 6 to 15 mm (D3 in the lab frame).
[0091] Interaction of radiation with the detector gas can lead to a
trail of molecular excitations and ionizations. The latter, in the
form of electron-ion pairs, can be utilized in micro- and
nanodosimetry for the measurement of the deposited energy as
well.
[0092] In certain implementations, various detector parameters such
as the detector size, including the drift length in the laboratory
frame, l.sub.LAB, and the cell size can be based on the gas
pressure. Generally, the higher the pressure, the cell size and the
overall detector dimension can be smaller. For example, at 1 Torr
of propane (with a scale conversion of 1 mm gas being equivalent to
about 2.6 nm TE), the vertical size (representative of an upper
limit in drift length) of the detector can be about 6 and 15 mm for
1 and 100 MeV protons, respectively. In some embodiments, in order
to image tracks of about 1 .mu.m length, the imaging plane of the
detector can have a size of approximately 400.times.400 mm.sup.2.
The cell size in such a detector can be be about 0.2 mm.sup.2
(approximately 0.5 nm.sup.2 TE cells).
[0093] FIG. 4A shows a portion 160 of an array of detection cells
154. FIG. 4B shows an expanded view of one of the detection cells
of FIG. 4A.
[0094] In the example shown, a relatively large number of
independent well-based gaseous detectors 164 can be provided. In
certain embodiments, the detectors 164 can operate in a limited
Geiger mode (fired/not fired).
[0095] The use of a relatively large number of individual detecting
cells can provide high detection efficiency for single ions,
despite the relatively large dead time that may exist in individual
cells operating in Geiger mode. Due to the diffusion of the ions,
at low gas pressures, even ions originating from the same
deposition point will likely be registered in different cells.
Hence, the effective counting efficiency of some embodiments can be
very high.
[0096] In certain implementations, each gaseous detector 164 can be
configured to multiply an ion 176 incident therein. Individual
positive ions, originating from radiation-induced primary and
secondary (delta-electrons) ionizations along the track of a
charged particle, may drift under an electric field across-the
interaction volume to the ion-multiplier 164. In some embodiments,
the ion-multiplier 164 can have a hole-type detector structure such
as the ones shown in FIGS. 4A and 4B.
[0097] In certain embodiments, the ion-multiplier 164 can be
configured to have some similarities with devices such as what is
sometimes referred to as a micro-well detector (MWD). Unlike the
MWD device, however, certain embodiments of the ion-multiplier 164
can operate in a limited Geiger mode under reversed polarity.
[0098] More particularly, a typical MWD device is designed to be
triggered by an electron entering the micro-well. Accordingly, an
anode is typically placed at the bottom of the micro-well, and a
cathode at the entrance surface. Thus, electrons resulting from the
multiplication process in the micro-well are thus accelerated to
the anode, and charge signals can be collected therefrom.
[0099] As described herein, certain embodiments of the present
disclosure can include an arrangement of electrodes that are
substantially reversed from typical MWD devices. In such a
reverse-polarity arrangement, a cathode can be positioned at or
near the bottom of a well, and an anode (relative to the cathode)
at or near the entrance surface. Various non-limiting examples of
such a configuration--by itself or in combination with a detection
gas volume--are described herein in greater detail.
[0100] It is noted that some embodiments of an MWD device can
include a densely perforated holes formed on relatively thin
insulator layer (e.g., an approximately 50 .mu.m-thick Kapton layer
with a thin metal clad on each of both faces. Such an insulator
layer can include holes (e.g., 50 .mu.m diameter). In such a
configuration, and with the foregoing MWD polarity of electrodes,
radiation-induced electrons can be focused into the wells and be
multiplied therein.
[0101] In certain embodiments, the detector array 160 can be much
thicker than the foregoing MWD example. The insulator layer (162)
can have a thickness in a range of about 1-5 mm, and its upper
surface can be provided with thin metal strips as readout strips
166. Each of the holes can have a diameter in a range of about
0.5-1 mm; and the holes can be spaced by a distance in a range of
about 200-1000 .mu.m. In some embodiments, to obtain a Geiger mode
of operation, a highly resistive electrode layer (168) can replace
the conductive layer at the bottom of the well.
[0102] In certain embodiments, the focusing of an ion into the well
164 and the charge multiplication process within the well can be
controlled by an electric field across the well. Such an electric
field can be provided by applying a negative voltage to the lower
resistive electrode 168. The upper electrode 166 can be formed of
strips common to each row of holes, and be kept at a ground
potential.
[0103] It is noted that a well-based detector having a relatively
small separation distance between the electrodes can be provided
with an operating voltage difference between the electrodes.
Approximating the arrangement of such electrodes as parallel
plates, such a voltage difference (V) can be represented by the
electric field strength (E) times the separation distance (d). To
maintain the same electric field strength within the well (so as to
provide a similar detection process) for a relative thicker
dielectric configuration, the corresponding increase in separation
distance (d) makes it necessary to increase the voltage (in a
linear manner). Utilizing such a relatively thick detectors in
certain operating conditions can result in a large and potentially
harmful voltage difference.
[0104] An ion entering the cell 164 can undergo collisions with gas
molecules and/or positive-ion impact ionizations of the gas to
yield an electron avalanche 180. In addition, the ion can collide
with the walls or bottom electrode of the cell, resulting in
secondary electron emission (182 in FIG. 4B). Cross sections for
such ionizing ion-gas collisions can depend on the type of gas, gas
pressure and on the electric field strength associated with the
cell. Available data for positive ion impact ionizations are
relatively scarce; thus, available data for 0.sub.2-N.sub.2
collisions (J. B. Hasted, Physics of Atomic Collisions, London
Butterworths, 1964) were used to obtain some rough estimations
herein.
[0105] For example, referring to FIG. 5, energy distributions of
ions in a 0.1 mm thick well were obtained with an ion transport
model configured for calculations for embodiments of the ion
counting nanodosimeter described herein.
[0106] The resulting estimated probabilities for gas ionization by
positive ions are presented in Table 1. The values listed in Table
1 are approximate values. The listed probability for each gas
pressure is a probability for at least one gas ionization in a well
of approximately 0.1 mm depth under an electric field strength of
approximately 1,000V/0.1 mm
TABLE-US-00001 TABLE 1 Probability of at Pressure, Torr Mean energy
of ions, eV least one ionization 1 470 15% 10 50 2.5% 100 5
0.25%
[0107] The values presented in Table 1 assume that ionizations are
only caused by ion-molecule collisions. Charge-exchange collisions,
resulting in fast neutral molecules, can also induce ionizations
but with far lower probabilities. These rough estimations show that
collision ionizations in the gas phase have a very low probability
to create secondary electrons (at best 0.15 at 1 Torr), and will,
therefore, not provide a desired efficiency for detecting ions.
Therefore, micro-pattern detector thickness of about 0.1 mm is
generally too small for providing high efficiency of single
positive ion registration. Accordingly, a thicker (e.g., a few mm)
detector structures can be provided to yield high probability
(e.g., close to 100% probability) of ion impact ionization as the
ion travels in the cell.
[0108] The ionization probability can also be increased by
providing higher electric field strength. However, such a higher
electric field can result in field emission breakdown that can
permanently damage the detector structure.
[0109] In the limited Geiger mode, the electrons can initiate a
substantially unrestricted avalanche in the cell hole; and such an
avalanche can develop due to ionizing collisions of secondary
electrons with the gas molecules. The process, once started, can go
on until substantially all gas in the hole is ionized. A discharge
can develop, unless the electric field is restricted by space
charge effects and/or by an external device or circuit (e.g., a
resistor in the high-voltage bias chain), as in a Geiger counter.
In certain situations, the discharge can be stopped when the
voltage drops due to the high volume resistivity of the cell's
bottom negative electrode.
[0110] In the limited Geiger mode, each cell can operate as an
independent Geiger micro counter, and an output signal can
determined by the charge accumulated in the cell's capacitor. Such
a charge can be represented as Q.sub.CELL=C.sub.CELL V, where
C.sub.CELL is the capacitance and V is the operating voltage across
the cell. Depending on the cell size, C.sub.CELL can of the order
of tens of fF (femtofarad); thus for V in a range of about 500 to
1000V, Q.sub.CELL value in the tens of pC (picocoulomb) range can
be expected. In certain implementations, such a charge output can
be read out and processed.
[0111] Recovery time of the cell can depend on the charge
collection time and recharging time; and the former is expected to
be in a sub-.mu.s to .mu.s range. In embodiments where the gas
pressure is low (e.g., about 1 Torr), such a recovery time
generally should not affect the ion detection efficiency; and in
particular, at relatively low ionization densities. In such
ionization densities, more than one ion entering the same cell is
quite rare. Further, ions will likely arrive spaced in time due to
their relatively low drift velocities (e.g. about 0.05 cm/.mu.s in
1 Torr propane, at 100 V/(cm Torr)).
[0112] A detector operating as a micro-Geiger cell structure 232
can include a feature where the electric field 230 configuration
can change during the discharge development within a hole. It can
be a dynamic process that can be explained on the basis of the
electron trajectories shown in FIGS. 6A and 6B before, during, and
close to the end of the discharge associated with electrons 234. As
one can see, majority of secondary electrons 234 are collected on
the strips in an initial stage of the discharge (FIG. 6A), because
they are produced in the vicinity to the strips.
[0113] The cell field can drop due to the positive charge of the
ions screening the charge on the resistive electrode at the bottom
of the well. The field of non-active neighboring cells can expand
to the vacant region to trap remaining electrons which otherwise
would drift upwards to the ionization volume and deflecting any
subsequently arriving ion into a nearby vacant cell. The remaining
electrons can be slowly collected in the reduced field
configuration, again by the strips (FIG. 6B).
[0114] In certain situations, some electrons can escape the trap or
hit the cell's sidewalls. Such a dynamic process and estimating
effects such as electron escape and possible wall up-charging can
be difficult to simulate.
[0115] A detected signal can be induced by electrons extracted from
the cell and collected by grounded electrode strips. In the Geiger
mode, such readout strips can be deposited on the cell's anode.
Additional pick-up strips (e.g., 170 in FIG. 4A) on the lower side
of the negative electrode can provide a second coordinate.
Alternatively, a pixelized 2D readout circuit can be directly
coupled to the bottom of the device and measure avalanche-induced
signals. With such a readout system, the detector can provide a 3D
image of the track, where a strip or pad readout circuit can
provide the 2D position of the detected ion, and the pulse timing
signal can provide the third coordinate.
[0116] In certain embodiments, an optical readout technique can be
implemented. Such a configuration can include recording
avalanche-induced photons with a sensor such as an intensified CCD
camera system. The CCD camera can provide a 2D image of the track,
and the third coordinate can be provided by simultaneously
recording the avalanche-light flashes with photomultiplier tubes.
In the example detector configuration shown in FIG. 4A, an optical
readout component 174 can include such a sensor (e.g., CCD camera).
Further, an optical element 172 such as a lens can facilitate
delivery of light signals to such an optical readout component
174.
[0117] FIG. 7 depicts an example ion detector 351. It will be
understood that such a depiction is not necessarily to scale. The
detector 351 can include a number of cells formed on a dielectric
layer or an insulating layer 352. The dielectric layer 352 can
include but not limited to insulator materials such as ceramic,
silicon dioxide, oxidized surface, porcelain or the like with
negligible electrical conductivity. In certain embodiments, a
printed circuit board insulator (e.g., FR4) can be used as the
insulating layer 352. In certain embodiments, one side of the
dielectric layer 352 is adjacent an anode layer 354, and the other
side is adjacent a cathode layer 356 (e.g., a glass cathode layer).
In certain embodiments, the dielectric layer 352 can be directly
adjacent to the anode 354 and/or cathode layer 356, so that there
is no intervening layer.
[0118] In some embodiments, a distance between the two electrical
plates or electrodes 354, 356 defining an inter-electrode gap
distance can be similar to or approximately the same as the
thickness (353) of the dielectric layer 352. In certain
embodiments, such an inter-electrode gap distance can be in a range
of about 1 mm to few cm (e.g., 5 cm). In certain embodiments, the
gap distance can be in a range of about 1 mm to 5 mm. In certain
embodiments, the gap distance can be in a range of about 2 mm to 5
mm. In the example detector described in reference to FIGS. 7 and
8, the gap distance can be approximately 3.2 mm. In certain
embodiments, the gap distance between the electrodes 354, 356 may
or may not be the same as the thickness of the dielectric layer
352.
[0119] In certain implementations, an increase in operating gas
pressure can be accommodated with a decrease in the inter-electrode
gap distance. Thus, for example, for a gas pressure of about 10
Torr, a gap distance in a range of about 1-2 mm may be appropriate.
For a lower gas pressure of about 1 Torr, a gap distance of about 5
mm may be appropriate. In the example detector described in
reference to FIG. 8 where the inter-electrode gap distance is about
3.2 mm (e.g., a thickness of a PCB), a gas pressure of about 2 Torr
can be appropriate for ion measurements.
[0120] In certain implementations, the inter-electrode gap distance
and/or the gas pressure can be adjusted for different types of
gases.
[0121] In certain embodiments, the anode layer 354 may be formed
from conductive materials such as metal (e.g., silver or copper).
The anode layer 354 may be further coated with materials such as
inert metal (e.g., gold or palladium). The anode layer 354 may
include openings that substantially align with the openings of the
wells 355 in the dielectric layer 352.
[0122] In certain embodiments, the anode layer 354 can include
strips common to each row of wells 354. In certain embodiments, the
anode layer 354 can define openings corresponding to well-openings
but otherwise substantially cover one side of the dielectric layer
352. In certain embodiments, the anode layer 354 can be
substantially continuous or discontinuous, and can cover at least a
portion of the dielectric layer 352.
[0123] In certain implementations, the cathode layer 356 can
include an electrode on one side of the dielectric layer 352
opposite from the side where the anode layer 354 is positioned. In
certain embodiments, the cathode layer 356 can include a resistive
cathode that can be formed from optically transparent or
semitransparent materials such as conductive glass if optical
detector readout is desired or implemented. In certain embodiments,
the resistive cathode can include resistive Kapton, conductive
ceramic, or composite material such as ruthenium resistive paste
fired on appropriate substrate (e.g., silica, glass).
[0124] In certain embodiments, the resistive cathode 356 can be a
layer which forms the bottom (357) of the well 355 in the
dielectric layer 352. The resistive cathode 356 may or may not be
directly adjacent the dielectric layer 352. In certain embodiments,
there is no intervening layer such as metal between the resistive
cathode 356 and the dielectric layer 352.
[0125] The dielectric layer 352 can define wells 355 having
openings 362 formed on the side adjacent the anode layer 354. The
anode layer 354 can also define similarly dimensioned and
positioned opening. In certain implementations, such openings can
be formed by techniques such as reactive ion etching, printed
circuit board (PCB) technology, or other appropriate methods. In
the example shown in FIG. 8, the wells 355 extend to the upper
surface of the cathode 356 so that the cathode 356 defines a floor
357 of each well 355.
[0126] For the purpose of description of FIG. 8, the wells are
sometimes referred to as cells.
[0127] In certain embodiments, the openings 362 allow the wells 355
to be in communication with a gas volume (not shown) above the
detector 351. Thus, the wells 355 can be occupied by the same gas
at substantially the same pressure as that of the gas volume.
[0128] In the particular example 351 described in reference to FIG.
8, the dielectric layer 352 has a thickness (353) that is about 3.2
min thick. Each of the wells 355 is cylindrically shaped, and has a
diameter (364) of about 0.8 mm. The wells 355 are arranged in an
array with a pitch (363) of about 1 mm.
[0129] In certain implementations, the dielectric layer thickness
353 can be less than or equal to the gap distance between the
electrodes 354, 356. Accordingly, in certain embodiments, the
thickness 353 can be in a range of about 1 mm to few cm (e.g., 5
cm). In certain embodiments, the thickness 353 can be in a range of
about 1 mm to 5 mm. In certain embodiments, the thickness 353 can
be in a range of about 2 mm to 5 mm. In the example detector
described in reference to FIG. 8, the thickness 353 can be
approximately 3.2 mm.
[0130] In certain implementations, wells described herein can
include a plurality of holes 355 in the dielectric layer 352. The
holes may have a relatively simple geometric shape such as
cylindrical or rectangular shape. In certain embodiments, the well
opening 362 or the cross sections of the wells 355 can be
substantially round.
[0131] In certain implementations, diameter 364 of each hole can be
in a range of about 0.1-2 mm, 0.5-1.5 mm, 0.6-1 mm. In the example
detector of FIG. 8, the diameter is about 0.8 mm.
[0132] In certain implementations, the diameter 364 of each of each
well 355 can be selected based on the thickness 353 of the
dielectric layer 352. In certain embodiments, a ratio between the
diameter and thickness can be in a range of about 1/10 to 1/1. In
certain embodiments, the ratio can be in a range of about 1/4 to
1/3. In certain embodiments, the ratio can be about 1/3. Thus, in
certain embodiments, the diameter can be based on such ranges of
ratios based on the foregoing example ranges of dielectric layer
thicknesses.
[0133] In certain implementations, the pitch 363 or the spacing
between the edge of one well to the edge of a neighboring well can
be selected based on factors such as a desired density of wells
and/or the diameter of each well. In certain embodiments, the pitch
363 can be in a range of about 1-10 mm or about 2-5 mm.
[0134] In certain embodiments, a ratio between the pitch 363 and
the diameter 364 can be about 1 to 5 or about 1.1 to 3. For
example, in one embodiment, the diameter 364 can be about 0.8 mm;
thus, the pitch 613 can be about 2 mm.
[0135] In certain, implementations, a detection array can include
from about 50-10,000 wells 355. The number of wells can be greater
or lesser than such an example range.
[0136] An upper readout electrode strip 360 is provided to each row
of wells and held at ground potential. A lower readout electrode
strip 361 that extends perpendicular to the upper strip 360 is also
provided. The upper and lower readout strips 360, 361 provide a
2-dimensional readout capability for the wells 355.
[0137] In certain implementations, readout electrode strips can be
configured in a number of ways. FIG. 9A shows an example
configuration 500 that can be similar to the configuration
associated with FIG. 7, where a first set of readout electrode
strips 504 extend in a first direction, and a second set of strips
506 in a second direction (e.g., perpendicular to the first
direction). Each of the strips 504, 506 can include a number of
apertures for accommodating the openings 502 of the wells.
[0138] In certain situations, the readout configuration of FIG. 9A
can result in ambiguities in hit locations when more than one ion
arrives on the array close enough in time (e.g., substantially
simultaneously). To address such issues, certain embodiments of a
readout configuration 510 (FIG. 9B) can include three or more sets
of readout electrode strips. In the example configuration 510, a
first set is shown to have strips 514 that extend along a first
direction. Second and third sets of strips 516, 518 are depicted as
extending along two different directions (e.g, about 135 degrees
and 45 degrees relative to the first direction) other than the
first direction and a perpendicular of the first direction. Such
angled configurations are sometimes referred to as "U" and "V"
orientations (in the context of "X" and "Y" orientations of FIG.
9A) can facilitate resolving of the foregoing ambiguities. Such
resolving techniques and parameters such as angle selections can be
achieved in known manners.
[0139] Positive ions, produced in a low pressure (e.g., about 1-10
Torr) working gas above the detector plane, drift to the detector
plane under a relatively weak (e.g., about 10-100V/cm) electric
field provided by anode (relative to the electrode 354, and not
shown) connected to a positive power supply. Focusing of the ions
into the detector openings 362, their acceleration and following
charge multiplication process in the well gas can be controlled by
applying a negative voltage to the lower electrode (cathode 356)
which provides a very high reduced-electric field across the well.
To prevent damaging discharges and sparks, this electrode can be
made of highly resistive material (e.g., glass) and each detector
cell can operate under a voltage well below the field emission
breakdown threshold.
[0140] For the example 3.2 mm-thick dielectric layer 352 shown in
FIG. 8, and when the gas used is propane, air or water vapor at
about 2 Torr pressure, the voltage applied to the cathode 356 can
be in a range of about 650-850V. The resulting reduced-electric
field (E/p) in the well is about 2000V/(cm Torr) which is well
above charge multiplication thresholds for electrons and ions
(about 30 and 70 V/(cm Torr), respectively, in propane).
[0141] A restricted avalanche in the well hole can start by
positive ion impact ionization and can develop due to ionizing
collisions of secondary electrons and positive ions, with the
former being responsible for most of the charge multiplication.
Ionization cross sections for positive ion impact at low energy
(e.g., 10-1000 eV) are scarce; however, available data for light
ions (H, He) indicate that ionization cross section for positive
ions is about factor 2-10 smaller then for electrons of the same
energy. According to an estimate based on observation of ion
induced charge multiplication in low pressure gas, a probability
for at least one gas ionization on 0.1 mm ion path in 1 Torr
propane under an electric field of the order of 1000V/cm is below
10%.
[0142] Therefore a micro-pattern detector thickness on the order of
about 0.1 mm is likely too thin in some embodiments for high
efficiency of single positive ion registration. In certain
embodiments, a much thicker (e.g., a few mm) detector structure can
provide a high probability of ion impact ionization as an ion
passes through the cell.
[0143] It is also noted that attempts to increase the ion impact
ionization probability by providing higher electric fields can
result in field emission breakdowns that can permanently damage
detector structures.
[0144] After the ion induced ionization occurs, the secondary
electron(s) accelerating in the high electric field across the cell
can initiate an avalanche propagating to the top of the cell. In
such a high electric field, the process, once started, goes on
until substantially all of the gas in the cell is ionized. A
discharge can develop, unless the electric field is restricted by
space charge effects or by an external device or circuit (e.g., a
resistor in the HV bias chain). In certain embodiments, the
discharge stops when the voltage across the cell drops due to the
high volume-resistivity of the cathode 357. This effect is similar
to the limited streamer process occurring in, for example,
resistive plate chambers (RPC).
[0145] The discharge can be restricted not only in time but in
space, since it is substantially confined to the cell where it
started. Propagation of the discharge due to UV photon feedback can
be limited by the cell walls; and the detector's reverse polarity
prevents photoelectron emissions from the anode 354.
[0146] The avalanche electrons are cramped to the cell because the
detector's electric field configuration can change during the
discharge development within a hole. It can be a dynamic process
that can be characterized on the basis of the electron trajectories
described in reference to FIG. 7 before and close to the end of the
electron part of discharge. The fired cell field can change (e.g.,
drop) because the vast majority of secondary electrons produced at
the top of the cell (avalanche head) can be promptly collected on
the readout strips, and the non-compensated positive charge of the
avalanche ions can be screening the negative potential of the
cathode at the bottom of the cell. The field of non-fired
neighboring cells can expand to the vacant region, trapping
remaining electrons within the fired cell, and deflecting any
subsequently arriving ion into a nearby vacant cell. The remaining
electrons in the fired cell can be collected on the strips in the
weak field configuration or recombine with the ions slowly moving
down to cathode positive charge cloud.
[0147] A recovery time of the fired cell can depend on the charge
collection time and recharging time, and is estimated to be in the
sub-.mu.s to the us range. This fired cell recovery time should not
affect the ion detection efficiency because ions will likely arrive
sufficiently spaced in time, due to their relative low drift
velocities (e.g., about 1 mm/microsecond in 2 Torr propane, at 100
V/(cm Torr)). Further, as mentioned above, ions arriving during
cell discharge are likely deflected to the neighbor cells.
[0148] In certain embodiments, a signal from the readout strip can
be determined by the charge accumulated in the cell's capacitor.
Such a charge can be estimated as Q.sub.cell=C.sub.cell V, where C
is the capacitance and V is the operating voltage. Cell capacitance
in the example detector 351 is on the order of a few tens fF
(femtofarad). Thus, for V of about 700V, charge (Q.sub.cell) values
in the tens pC range can be expected.
[0149] FIG. 8 shows a detector 150 that includes the various
features described in reference to FIG. 8. The example detector 150
can be manufactured using multilayer printed circuit board (PCB)
technology. The example detector 150 includes an array 152 of holes
154 that form detection cells (355 in FIG. 8). As previously
mentioned, the example detector 150 has a thickness of about 3.2
mm. Further, the array 152 has an active detection area of
approximately 2 cm.times.5 cm. There are 576 holes (154) in the
array 152.
[0150] To verify the signal amplitude representative of the charge
(Q.sub.cell) output from a cell, the detector 150 of FIG. 8 was
used. More particularly, the detector PCB was mounted on a Teflon
base with embedded glass cathode, so the HV electrode surface was
exposed to working gas only through detector holes. The detector
assembly was then installed into a drift chamber enclosure
providing controllable gas environment of about 0.1-10 Torr
pressure and a drift field of up to 1000V/cm. The chamber was also
equipped with a collimated Am-241 alpha source and a Si detector
defining alpha particle beam of about 2 mm in diameter and
generally parallel to the detector plane and about 5 mm above it.
With this set-up (schematically depicted in FIG. 10A), signals from
the top electrode were acquired as negative pulses having about 200
ns duration with amplitude of about 10 mV on a 50-Ohm load. Such a
signal is representative of electron charge of about 20 pC, which
is generally consistent with the tens pC range expected. The
observed signal amplitude corresponds to avalanche charge on the
order of about 10.sup.8 electrons, which is well above the Raether
limit for breakdown in gases. That is, each cell can operate as an
independent Geiger micro counter.
[0151] Measuring average ion arrival time at different anode
voltages (hence different ion drift field and velocity), it was
verified that the registered ions came from the alpha particle
track. The ion drift time distributions measured at anode voltages
of 100V and 10V are shown in FIG. 10B. Also measured was the total
background rate of the detector resulting from sporadic discharges
and ions from cosmic and background radiation. For these
measurements a working gas volume (about 2.5.times.5.times.5
cm.sup.3 filled with propane at 2 Torr) was used. The background
rate did not exceed 1 Hz for all of the 576 cells. These results
show that the 2-dimensional detector of FIG. 8 can be utilized as a
planar ion detector in an ion time projection chamber (TPC).
[0152] FIGS. 11A and 11B show other examples of characterizing
signal amplitudes representative of the charge (Q.sub.cell) output
from a cell of the detector 150 (FIG. 8). The data 220 shown in
FIG. 11A were obtained with the detector operating at about 800V
and in propane held at about 3 Torr. The data 220 is fitted with a
Polya distribution 222 that is commonly used to characterize
single-charge initiated avalanche processes.
[0153] As shown in FIG. 11A, the fit distribution 222 yields a mean
charge amplitude of about 35 pC, which is also generally consistent
with the tens pC range expected. Such a consistency between the
estimated range (tens pC) and data continues at other operating
voltages as shown in FIG. 11B. A number of mean charge amplitude
values are plotted as against operating voltages. As shown, the
measured charge amplitudes are in a range of about 25 pC to 35 pC
for voltages in a range of about 600V to 800V. Further, the output
charge amplitude increases linearly as a function of voltage, also
as expected.
[0154] In certain implementations of the present disclosure,
radiation-induced ionization patterns in condensed matter and in
equivalent gas models and of transport and multiplication processes
within the different detectors can be simulated. Monte Carlo
radiation-transport codes can be applied for improving or
optimizing the detector design and for evaluating its performance
Monte Carlo track structure codes can be improved to simulate the
transport of ions and secondary electrons in gaseous and condensed
media. The validity of interaction cross sections that are used in
Monte Carlo codes can be tested using data from the detectors.
[0155] In certain implementations, the example ion induced impact
ionization detector described in reference to FIGS. 7 and 8 or a
similar detector device can be utilized to obtain experimental
track-structure data associated with, different radiation fields.
Such experimental data can be obtained at a number of facilities,
including but not limited to, Loma Linda University's proton
synchrotron (where protons up to 250 MeV can be provided), Crocker
Nuclear Laboratory at UC Davis (where protons and light ions of low
and intermediate energies can be provided from its 76 inch
isosynchronous cyclotron), and Brookhaven National Laboratory--NASA
Space Radiation Laboratory (where protons and heavy ions up to
several GeV can be provided from the AGS Booster).
[0156] In addition to the foregoing examples of charged particle
beams, data can be obtained with radioactive sources including
alpha, beta (electron) and gamma sources, representing a wide range
of linear energy transfer (LET) values. Nanodosimetry data from a
low-intensity radioactive neutron sources can also measured. Thus,
a nanodosimetric track-structure database for validations and
practical applications can be developed and/or maintained.
[0157] Track-structure data measured with the example ion induced
impact ionization detector described in reference to FIGS. 7 and 8
has been shown to be useful for estimating radiation effects
observed in DNA and producing meaningful quality factors for
radiation protection. Measurements were made for frequency
distributions of ionization event sizes within a nanometric
sensitive volume with rough dimensions of a DNA segment (about 4 nm
diameter, FWHM and about 20 or 47 base pairs long) under various
geometrical beam conditions. As described herein, FIGS. 2A and 2B
show examples of nanodosimetric event size distributions measured
for various primary ions and energies, and a comparison of
experimental and Monte-Carlo simulated distributions for protons.
There is a good agreement between the measurements and simulations,
down to frequencies of about 2.times.10.sup.-3 (which corresponds
to about 6 ions per cluster). For larger clusters, there appears to
be an excess of measured ions with respect to the simulation
results.
[0158] While it is not desired or intended to be bound by any
particular theory, some experimental results suggest that these
additional ions seen in the foregoing experimental distribution may
be caused by a rare gas multiplication process which takes place in
the intermediate vacuum of the ion acceleration channel below the
detection cell aperture.
[0159] Additional studies correlating nanodosimetric data of
various radiation fields with radiochemical and radiobiological
data can validate various methodologies and translate such
validated methodologies into practical applications. Utilizing one
or more of the foregoing radiation fields, data for a number of
more specific applications can be obtained; and such applications
can include radiochemical yields of clusters of hydrogen peroxide
or other stable radiolysis products in nanoparticles; DNA double
strand break and other complex damage yields in DNA model systems;
DNA double strand break and other complex damage yields in cells;
CNS effects in suitable animal models; Cancer induction in suitable
animal models. Additionally, existing in vitro and in vivo data can
be used to test nanodosimetric prediction models.
[0160] In certain implementations, one or more of the features
described herein allows imaging of a track passing through a volume
of gas, which in turn can be scaled into tissue-equivalent (TE)
scales and units. For such systems, resolution of positive ion
imaging can be estimated.
[0161] Constraints imposed by ion diffusion on a track imaging
detector can be represented in tissue-equivalent (TE) units defined
by the scaling factor kp, where p is the pressure and k is a
gas-dependent "dE/dx" scaling factor of an order of about
10.sup.-6-10.sup.-7. On the TE scale, the rms broadening due to a
drift distance l.sub.TE can be represented as:
.DELTA. x TE = k 2 l TE D / K E / P ##EQU00001##
where D, K, and E represent the ion diffusion, ion mobility and
electric field, respectively.
[0162] Based on average D/K values, the rms TE resolution,
.DELTA.x.sub.TE, can be estimated as a function of E/p. Such an
estimate is depicted in FIG. 12 for different gases. For a
reasonable E/p value (e.g., approximately 100 V/(cm Torr) used in
at least some of the configurations described herein), one can
expect a typical TE resolution of .ltoreq.1.4 nm (rms) when an
upper limit of TE drift distance is about 40 nm. Methane or water
vapor can yield even a better resolution, provided that a
sufficiently high reduced-field can be maintained in these
gases.
[0163] In a laboratory frame, gas pressure can define or influence
actual dimensions and resolution of a detector, including the upper
limit of drift length, l.sub.LAB, and the resolution
.DELTA.x.sub.LAB, which in turn can provide design parameters
(e.g., pitch and/or cell size) related to pixelization of a
detector's array of cells.
[0164] For example, at a pressure of about 1 Torr propane, the
resolution limit due to ion diffusion is estimated to be about 0.5
mm (rms) for a drift of about 15 mm, thus a cell size of about 0.2
mm.sup.2 can be adequate. In some situations, increasing the
pressure will not improve the resolution on the TE scale, but will
reduce it in the laboratory frame if the pixel size remains
unchanged. Consequently, a detector with smaller pixelization will
be needed in such a case to provide the same TE resolution. Also,
in some situations, the dimensions of the detector and the drift
length can scale with pressure, affecting the overall detector
design.
[0165] In certain implementations, the gas choice and the
reduced-electric field can define the ion diffusion and the
resolution .DELTA.x.sub.TE, on the TE scale for a given upper limit
of TE drift length l.sub.TE, which in turn can be defined by the
type of radiation to be imaged (e.g., l.sub.TE, of about 16 and 40
nm for 1 and 100 MeV protons, respectively). A typical value of the
rms TE resolution for ions drifting over the upper limit of TE
drift length of about 40 nm in propane can be about 1.4 nm or
better.
[0166] In certain implementations, the present disclosure relates
to systems and methods for characterizing radiation in a manner
that approximates interactions between radiation and nano-scale
condensed matter. A DNA molecule is an example of such a nano-scale
condensed matter. There are a number of other materials and
situations where one or more techniques of the present disclosure
can be applied.
[0167] FIG. 13 schematically depicts a radiation measurement
configuration 250 where ionizing radiation (arrow 256) passes
through a gaseous interaction region 252. For the purpose of
description, the interaction region 252 can include one or more
types of gases generally maintained at a selected pressure or
within a range of pressure so as to provide an interaction
cross-section .sigma..sub.2 representative of a probability of
ionizing interaction between the ionizing radiation 256 and the gas
molecules or particles. Also, the interaction region 252 is
depicted as having a dimension of D2 generally along the direction
of the ionizing radiation 256.
[0168] In certain embodiments, and as described herein by way of
examples, configuration of the gas and the interaction region
dimension D2 can be selected so as to approximate interaction of
the ionizing radiation 256 with a much smaller and denser material
such as a nano-scale condensed matter 254 (depicted as having an
interaction dimension of D1). In certain embodiments, dimension
and/or density scaling between the nano-scale interaction and the
measured interaction can be achieved by making equivalent the
interaction probabilities during passages through the nano-scale
condensed matter 254 (dependent on cross-section .sigma..sub.1) and
the gaseous region 252.
[0169] As described herein, such scaling can allow representative
measurements and characterization of nano-scale materials in more
manageable and/or convenient detection formats. For example, in
situations where detector elements having milli-meter range
dimensions are utilized to approximate and characterize
interactions in nano-meter range dimensions, there can be an
effective detection volume scaling expansion by a factor of about 1
million.
[0170] FIG. 14 shows that in certain implementations, a process 260
can be implemented to achieve such a scaled expansion of detection
volume. In a process block 262, gas environment and interaction
volume can be selected to approximate interaction of ionizing
radiation with a nano-scale condensed matter. In a process block
264, a detector can be provided and configured to detect ionization
products (e.g., positive ions) resulting from the interaction.
[0171] FIG. 15 schematically depicts an example of characterization
of a smaller-scale matter (e.g., a nano-scale condensed matter) to
a larger-scale detection volume. As shown, the example nano-scale
condensed matter can be a DNA strand 340, through which radiation
256 passes. Such an interaction of radiation 256 with the DNA
strand 340 is shown to be characterized by characterizing the
interaction of the same or similar radiation with a gas detector
volume. Detection of ionization products such as positive ions and
characterization of the radiation track through the gas detector
volume can provide a gas model representation of how radiation 256
interacts with the DNA molecule. Because the gas model can be based
on more practical detection parameters (e.g., mm range dimensions,
gas choice, gas pressure, modes of detecting ionization products,
etc.), modeling and characterization of radiation's interaction
with a nano-scale sized object such as a DNA strand can be achieved
in a more controlled and practical manner.
[0172] In certain embodiments, one or more features described
herein can be implemented in a detector capable of detecting
spatial information for incident radiation (e.g., charged
particles). FIG. 16 schematically depicts how a gaseous interaction
volume 278, representative of the volume 252 in FIG. 13, can be
configured so as to allow detection of ionization products (e.g.,
positive ions 290). In certain embodiments, an interaction
apparatus 270 can include first and second electrodes 272 and 274
arranged to allow passage of ionizing radiation 256 through the
gaseous volume 278 defined between the electrodes 272 and 274. The
electrodes 272 and 274 are shown to be held at a potential
difference of .DELTA.V.sub.1.
[0173] The interaction apparatus 270 is further shown to include a
detector layer 280 disposed between the second electrode 274 and a
third electrode 276. The detector layer 280 and the associated
electrodes 274, 276 can be configured in manners similar to those
described herein (e.g., FIGS. 7 and 8).
[0174] The second electrode 274 can act as a ground, and the first
electrode 272 can be held at a selected positive voltage
+.DELTA.V.sub.1 relative to the ground. Thus, for the gaseous
volume 278, the first electrode 272 can act as an anode, and the
second electrode 274 as a cathode.
[0175] The third electrode 276 can be held a selected negative
voltage (-.DELTA.V.sub.2) relative to the ground. Thus, for the
detector layer 280, the second electrode 274 can act as an anode,
and the third electrode 276 as a cathode.
[0176] The potential difference .DELTA.V.sub.1 for the gaseous
volume 278 can be selected so as to provide a relatively weak
electric field (e.g., about 10 to 100 V/cm) for a given gas type
and pressure. Such a relatively weak electric field can facilitate
drifting (depicted as arrows 292) of positive ions 290 generated
from ionizing interactions and/or secondary interactions, without
promoting charge multiplication processes within the volume
278.
[0177] The potential difference .DELTA.V.sub.2 for the detector
layer 280 can be selected to promote such charge multiplication
processes. Various design considerations associated with such
potential difference (such as gas type, gas pressure, spacing
between electrodes) are described herein in greater detail.
[0178] In certain situations, it may be desirable to be able to
detect single ionization events. In each of such events, a positive
ion and an electron are generated from ionization. As described
herein, such positive ions from ionized gas molecules can be
detected with single-ion resolution, so as to allow
characterization of radiation interaction effects such as formation
of large ionization clusters that contribute to damages to
materials such as DNA strands.
[0179] FIG. 17 shows a process 300 that can be implemented to
facilitate such single ionization event characterization. In block
302, an interaction volume can be configured as described herein.
In block 304, a detector (e.g., detector layer 280 and associated
electrodes in FIG. 16) as described herein can be configured to
allow detection of single ions generated in the interaction
volume.
[0180] FIG. 18 schematically depicts a portion of a detector 310
that can be configured to provide such a single-ion detection
capability. Further, the detector 310 can have an array of
detection elements to allow spatial characterization of the
ionizing interactions occurring in the interaction volume.
[0181] As shown, the detector 310 can include a plurality of wells
320 defined between two electrodes 274 and 276. In certain
embodiments, the electrodes 274 and 276 can be the second and third
electrodes, respectively, described herein in reference to FIG. 16.
Additional details about the electrodes 274 and 276 (including an
example of array readout scheme via the ground electrode 274) and
electric fields generated by the electrodes are described herein in
greater detail.
[0182] In certain embodiments, the wells 320 can be formed on an
insulating layer 314 such as a dielectric layer. The example wells
320 in FIG. 18 are depicted as having a depth "d," a width "w," and
an inter-well spacing "s." Selections of such dimensions are
described herein in greater detail. In certain embodiments, the
wells 320 can be open on the side facing the interaction volume 278
such that gas pressure in the wells 320 can be generally same as
that of the interaction volume 278. Such a feature can simplify the
design and operation of the detector 280.
[0183] In the example configuration shown in FIG. 18, a single ion
290 from an interaction (not shown) in the volume 278 is depicted
as drifting (dotted line 330) into the well 320b. Due to the
electric field E.sub.2 provided by the electrodes 274 and 276,
and/or the gas configuration, the entering ion 290 can accelerate
towards the cathode 276 and result in multiplication of charges
(e.g., avalanche 334) substantially in the well 320b. Details about
such electric field and gas configurations, as well as charge
multiplication processes and detection thereof, are described
herein in greater detail.
[0184] As also described herein in greater detail (e.g., in
reference to FIGS. 6A and 6B), electric field formed at or about a
given well opening can change dynamically as ion-induced charge
multiplication occurs in the well (e.g., in 320b). Such a dynamic
nature of the electric field can promote substantial containment of
generated charges in the well, and also promote deflection of
additional incident ion(s) to other unoccupied well(s). For
example, the charge multiplication process in well 170b can result
in an electric field change that promotes deflection of another
incident ion (depicted as dashed line 332) into another unoccupied
well (e.g., a nearby well 320a).
[0185] In certain implementations, an ion detector such as the
example detector 310 of FIG. 18 can be configured to be a part of
system. Such a system can be configured to detect ions (primary
and/or secondary) so as to allow characterization of interactions
and/or processes that generate such ions. By way of an example,
FIG. 19 shows a process 370 that can be configured to characterize
interactions of ionizing radiation with a volume of low pressure
gas by detecting ion products.
[0186] As described herein, such characterization of the
interactions in low pressure gas can be utilized for
track-structure study of a number of radiation-matter interaction
settings. Such a study is sometimes referred to as ionization
pattern imaging, and can be applied to characterization of ionizing
radiation with nano-scale condensed matter objects such as DNA
strands.
[0187] Referring to FIG. 19, the process 370 shows ionizing
radiation (arrow 372) entering a low pressure gas volume 374 and
interacting with the gas therein. Interaction products such as
primary and/or secondary ions (arrow 376) can be detected by an ion
detector 378. In certain embodiments, such interaction products can
be subjected to electrical and/or magnetic field to move (e.g.,
drift or accelerate) the ions to the detector. The detection of the
ions can yield analog signals (arrow 380) from the detector 378,
and such signals can be converted to digital signals 384 by an
analog-to-digital converter (ADC) 382. Such digitized signals can
be provided to a computing device 386 for further processing and/or
analysis.
[0188] In certain embodiments, a system 390 having a number of
components can be configured to facilitate the example process 370
of FIG. 19. The system 390 can include a power source 392 for
providing power to, for example, electrodes that define drift
electric field in the interaction gas volume and ion detecting
electric field in each of a number of detection cells. The system
390 can further include one or more housing components 394,
including, for example, a housing configured to provide the
interaction gas volume. The system 390 can further include a data
acquisition (DAQ) component 396 configured to provide, for example,
readout of signals from an array of detection cells and conversion
of such signals into representative digital signals for further
processing by a computing device. The system 390 can have a
detector component 398 configured to provide one or more detector
features and/or capabilities as described herein.
[0189] In certain implementations, the foregoing detector component
398 can be based on one or more ion detector elements that are
configured to allow efficient detection of single ions. FIG. 21
schematically depicts such a detector element 400.
[0190] In certain embodiments, a number of such detector elements
400 can be arranged in an array to allow spatial determination of
ions' incidence locations on the array. FIG. 22 schematically
depicts such an array 410 of detector elements 400.
[0191] In certain embodiments, such an array of ion detector
elements can be used in an analytic system having an ion detection
component. FIG. 23 schematically depicts such a system 420 having
an ion detector array 410.
[0192] There are a number of analytic systems where the ion
detector array 410 can be implemented. FIGS. 24A-24C show some
non-limiting examples of the analytic system of FIG. 23. For
example, FIG. 24A shows a dosimeter system 430 having the ion
detector array 410. Various configurations and operating parameters
are described herein in the context of such a dosimeter system.
However, similar detector elements and arrays thereof can also be
implemented in other analytic systems.
[0193] In certain embodiments, FIG. 24B shows that a mass
spectrometer 440 or a similar system can include the ion detector
array 410 for detecting ions. For example, ions that undergo mass
separation due to electric and/or magnetic field(s) can be detected
by the detector array 410 and spatial separation of the detected
ions can be analyzed for mass identification.
[0194] In certain embodiments, FIG. 24C shows that a gas
chromatograph 450 or a similar system can include the ion detector
array 410 for detecting ions. For example, ions that emerge from a
column exit can be detected by the detector array 410 for
analysis.
[0195] As used herein, mass spectrometry can include an analytical
technique for determining the elemental composition or structure of
a sample or molecule. Mass spectrometers can include an instrument
used to implement analytical techniques of mass spectrometry. As
used herein, gas chromatography can include an analytical technique
for separating compounds in a mixture, wherein the mixture is
vaporized but not decomposed. A gas chromatograph can include an
instrument used to implement analytical techniques of gas
chromatography.
[0196] The ion detector element and/or an array formed by such
elements can be utilized in other systems. For example, ion
detection configurations as described herein can be implemented in
systems for detecting very low concentrations of chemicals. An ion
mobility spectrometer is an example where trace concentrations of
chemicals such as explosives, drugs, and chemical weapons can be
detected. Such spectrometers can include one or more features of
the ion detectors as described herein to allow efficient and
accurate characterization of ions.
[0197] In some embodiments, the ion induced impact ionization
detector can be capable of providing new valuable data in the field
of detector physics, related to ion interactions with gases and
solids, light emission, multiplication processes in gas and
advanced radiological imaging techniques.
[0198] Some embodiments as described herein can be directed to a
high-resolution, high-sensitivity 3D imaging track structure
imaging devices operating with low-density gases. The ion induced
impact ionization detectors as described herein can facilitate
characterization of the interaction of ionizing radiation with
matter in the condensed phase. In one aspect, an approach to such a
characterization can be based on experimental techniques, such as
nanodosimetry, and Monte Carlo (MC) track structure simulations.
One can then, for example, relate the gas phase track structure and
condensed phase track structure. Such an approach can facilitate
finding of track structure characteristics on the micro- and
nanometer-scale that are common to both phases and are relevant for
the effects of ionizing radiation on living cells and DNA
nanostructures. In some implementations, experimental track
structure data may be used to benchmark MC codes.
[0199] As used herein, condensed matter can include cells, tissues,
polymers, nanoelectronics and nucleic acid molecules such as DNA,
in which an aberration may be induced. As used herein, aberration
can include local damage to condensed matter associated with
ionization clusters that have been identified. As used herein,
ionization clusters can include a plurality of ionization products,
such as about 2-20 ions per cluster. In some cases ionization
clusters can have about 6 ions per cluster.
[0200] An example application for radiation medicine and protection
can relate to performing high-resolution track-structure studies to
obtain an improved mechanistic understanding of radiation damage to
DNA and chromosomes. For this, ionization tracks can be recorded in
dilute gas with a precision of about one tissue equivalent nm to
study clustering of ionizations on the DNA scale and over a track
segment length of one tissue-equivalent .mu.m. Study of these
clustering effects for different types of radiation used in
radiotherapy (high energy electrons, photons, and light ions) and
their comparison with the prediction of Monte Carlo simulations can
be used to develop sophisticated mechanistic models of radiation
effect on DNA and other important biomolecules.
[0201] In some embodiments, a track structure-imaging system can be
configured to be capable of highly efficient and precise
localization of ionization patterns on these scales. In some
embodiments, ionization-induced positive ions can be recorded and
deposited in low-pressure gas.
[0202] In certain implementations, modeling tissue with a
low-pressure gas target can allow expanding tissue scales,
according to the "dE/dx ratio" scaling factor up to, for example,
about 10.sup.6 for 1 Torr. Such a scaling factor can be chosen
according to the dimensions accessible by available gaseous
detectors, and/or the scale of the condensed matter being studied.
In some embodiments, a ion induced impact ionization detector
configured to provide substantially full track structure and single
charge sensitivity can provide a resolution of sub-nm
tissue-equivalent precision over a micrometer tissue-equivalent
range. The same dimensions can also be relevant to non-biological
applications such as those relating to nanoelectronics devices.
[0203] As used herein, "nanometer equivalent resolution" can
include resolution of sub-nm tissue-equivalent precision over a
micrometer tissue-equivalent range and similar dimensions in
nanoelectronics devices. Nanometer equivalent resolution may relate
to a spatial separation of individual energy-deposition events
(e.g., electron-ion, electron-hole pairs) in condensed matter. In
some embodiments, the spatial separation can be of the same order
of magnitude as the lateral dimensions of a DNA molecule or strand
and of some elements in nanoelectronics, i.e., in the nanometer
domain. For example, nanometer equivalent resolution includes a
precision of about one tissue equivalent nm, where such precision
can be useful for study of clustering of ionizations on the DNA
scale and over a track segment length of one tissue-equivalent
.mu.m.
[0204] In certain implementations, the ion detector element 400 of
FIG. 21 can be configured to include one or more of the features
described herein. In certain embodiments, such features can include
those described in reference to FIGS. 7-9.
[0205] In certain implementations, such an ion detector element 400
can be operated in a limited Geiger mode (e.g., fired/not fired).
In certain implementations of such an operating mode, one or more
features associated with the ion detector element 400 can be
configured to provide a gas environment that under super-stress by
the electric field applied to the well of the detector element 400.
Such a state can be analogous to a super-cooled or super-heated
state of liquid, where the phase transition (freezing or boiling)
has not occurred even after the temperature has gone below the
freezing temperature or above the boiling temperature. In such a
state, a slight disturbance and/or a seed condition can trigger a
rapid phase transition.
[0206] In certain implementations, the super-stressed gas
environment of the ion detector element 400 can be achieved by
providing an electric field to the well so that the electric field
strength is higher than a threshold value or range of values
associated with breakdown of gas. Such a threshold value or range
can depend on factors such as type of gas, pressure and/or
temperature. In certain implementations, the electric field
strength can be selected to be also higher than a threshold value
or range of values associated with ion multiplication. Such a
threshold value or range is typically higher than the corresponding
gas breakdown threshold value or range.
[0207] In certain implementations, the electric field strength can
be selected based on one or more of the foregoing threshold values,
and to be lower than a threshold value or range of values
associated with field emission breakdown at a surface of the well.
Such a value or range can depend on the material associated with
the well surface.
[0208] Voltages associated with the foregoing electric field
strengths can depend on factors such as separation distance of the
electrodes and electrode geometries. Thus, an applied voltage can
be selected for a given well size (e.g., depth), gas type, gas
pressure and/or gas temperature so as to yield a desired electric
field strength having one or more of the foregoing properties.
[0209] For example, the example detector configuration of FIG. 8
can be supplied with a voltage of about 600 to 900 V between the
electrodes to yield a desired gas condition when the detection
configuration includes propane, air or water vapor at about 2 Torr
in a well having a depth of about 3.2 mm.
[0210] In certain detection situations, sensitivity of a detector
and its stability can be balanced. For example, a detector
configured to be highly sensitive can be triggered easily by a
particle (e.g., an electron) other than a desired particle (e.g.,
an ion). In the context of the super-tensioned gas environment, an
electron can easily trigger the limited Geiger process. In certain
implementations of the present disclosure, however, likelihood of
such electrons entering the well of an ion detector element (e.g.,
400 in FIG. 21) can be reduced by a reversed polarity of the
voltage biasing configuration.
[0211] For example, and referring to FIGS. 10A, 16 and 18, certain
embodiments of the present disclosure can be configured so that the
deep end of a well can be held at a negative potential relative the
well's opening. Further, an anode that facilitates drifting of
positive ions towards the detector can be held at a positive
potential relative to the well's opening. Accordingly, electrons in
the vicinity of the well will be subjected to a force that directs
electrons away from the well (towards the anode).
[0212] In certain embodiments, an ion detection cell having one or
more of the features described herein can operate with a relatively
large dead time typically associated with the Geiger operation
mode. Despite such a property, a high detection efficiency for
single ions can be provided by use of a large number of such cells
distributed and configured to reduce the likelihood of two or more
ions entering a single detection cell.
[0213] Further, due to the diffusion of ions generated in a
low-pressure gas volume, even ions originating from the same
deposition point are likely to be registered in different cells.
Hence, the effective efficiency can be relatively high.
[0214] As used herein, a gas can include multi-atomic gases and gas
mixtures to simulate biological and semiconductor media. For
example, a gas can include one or more of the following gasses:
propane, ambient air, and water vapor. In certain implemetations,
the gas can be ionized by initiating an avalanche breakdown of gas
in a well, whereby the ionizing induces charges that are multiplied
thereby forming a detectable signal.
[0215] As used herein, low pressure or low pressure gas can include
a gas at a pressure that is less than about 100 Torr. In certain
implementations, low pressure can include a pressure in a range of
about 1 to 10 Torr.
[0216] As used herein, a breakdown potential can include a point at
which non-conducting gas becomes conductive as governed by the pd
product (p=pressure, d=inter-electrode gap distance) and the
Townsend mechanism. In some embodiments, the breakdown potential
can occur when an, electric field exceeds a particular value,
wherein an electron avalanche starts, for example, due to
multiplication of some primary electrons in cascade ionization. In
some embodiments, the breakdown potential can be about 0.1-1.5
cmTorr at an operating pressure of about 1-10 Torr, and an
inter-electrode gap distance of between 2-5 mm. In some
embodiments, the breakdown potential can be at a pd value of about
0.6 cmTorr (about 2 Torr and about 3.2 mm gap), for example for
propane, air and water vapor, can be about 400-600V assuming a
substantially uniform electric field. In some embodiments, the
field emission breakdown potential can be above about 5 kV for a
detector, wherein no breakdown was observed at pd<0.00003 cmTorr
(better than 0.0001 Torr vacuum and 5 kV over 3.2 mm gap).
[0217] As used herein, E/p value (electric field strength divided
by pressure) as used herein is sometimes referred to as
"reduced-electric field." Thus, for example ranges of E and p of
about 10-100 V/cm and about 1-10 Torr, respectively, the
corresponding reduced-electric field (E/p) can have a value in a
range of about 1-100 V/(cm Torr).
[0218] The terms "approximately," "about," and "substantially" as
used herein represent an amount close to the stated amount that
still performs the desired function or achieves the desired result.
For example, the terms "approximately", "about", and
"substantially" may refer to an amount that is within less than 10%
of, within less than 5% of, within less than 1% of, within less
than 0.1% of, and within less than 0.01% of the stated amount. The
term "at least a portion of" as used herein represents an amount of
a whole that comprises an amount of the whole that may include the
whole. For example, the term "a portion of" may refer to an amount
that is greater than 0.01% of, greater than 0.1% of, greater than
1% of, greater than 10% of, greater than 20% of, greater than 30%
of, greater than 40% of, greater than 50% of, greater than 60%,
greater than 70% of, greater than 80% of, greater than 90% of,
greater than 95% of, greater than 99% of, and 100% of the
whole.
[0219] Unless the context clearly requires otherwise, throughout
the description and the claims, the words "comprise," "comprising,"
and the like are to be construed in an inclusive sense, as opposed
to an exclusive or exhaustive sense; that is to say, in the sense
of "including, but not limited to." The word "coupled", as
generally used herein, refers to two or more elements that may be
either directly connected, or connected by way of one or more
intermediate elements. Additionally, the words "herein," "above,"
"below," and words of similar import, when used, in this
application, shall refer to this application as a whole and not to
any particular portions of this application. Where the context
permits, words in the above Detailed Description using the singular
or plural number may also include the plural or singular number
respectively. The word "or" in reference to a list of two or more
items, that word covers all of the following interpretations of the
word: any of the items in the list, all of the items in the list,
and any combination of the items in the list.
[0220] The above detailed description of embodiments of the
invention is not intended to be exhaustive or to limit the
invention to the precise form disclosed above. While specific
embodiments of, and examples for, the invention are described above
for illustrative purposes, various equivalent modifications are
possible within the scope of the invention, as those skilled in the
relevant art will recognize. For example, while processes or blocks
are presented in a given order, alternative embodiments may perform
routines having steps, or employ systems having blocks, in a
different order, and some processes or blocks may be deleted,
moved, added, subdivided, combined, and/or modified. Each of these
processes or blocks may be implemented in a variety of different
ways. Also, while processes or blocks are at times shown as being
performed in series, these processes or blocks may instead be
performed in parallel, or may be performed at different times.
[0221] The teachings of the invention provided herein can be
applied to other systems, not necessarily the system described
above. The elements and acts of the various embodiments described
above can be combined to provide further embodiments.
[0222] While certain embodiments of the inventions have been
described, these embodiments have been presented by way of example
only, and are not intended to limit the scope of the disclosure.
Indeed, the novel methods and systems described herein may be
embodied in a variety of other forms; furthermore, various
omissions, substitutions and changes in the form of the methods and
systems described herein may be made without departing from the
spirit of the disclosure. The accompanying claims and their
equivalents are intended to cover such forms or modifications as
would fall within the scope and spirit of the disclosure.
* * * * *