U.S. patent application number 11/462999 was filed with the patent office on 2009-09-17 for gamma source for active interrogation.
Invention is credited to William A. Barletta, Ka-Ngo Leung, Tak Pui Lou.
Application Number | 20090230314 11/462999 |
Document ID | / |
Family ID | 41061993 |
Filed Date | 2009-09-17 |
United States Patent
Application |
20090230314 |
Kind Code |
A1 |
Leung; Ka-Ngo ; et
al. |
September 17, 2009 |
GAMMA SOURCE FOR ACTIVE INTERROGATION
Abstract
A cylindrical gamma generator includes a coaxial RF-driven
plasma ion source and target. A hydrogen plasma is produced by RF
excitation in a cylindrical plasma ion generator using an RF
antenna. A cylindrical gamma generating target is coaxial with the
ion generator, separated by plasma and extraction electrodes which
has many openings. The plasma generator emanates ions radially over
360.degree. and the cylindrical target is thus irradiated by ions
over its entire circumference. The plasma generator and target may
be as long as desired.
Inventors: |
Leung; Ka-Ngo; (Hercules,
CA) ; Lou; Tak Pui; (Berkeley, CA) ; Barletta;
William A.; (Oakland, CA) |
Correspondence
Address: |
FULBRIGHT AND JAWORSKI LLP
555 S. FLOWER STREET, 41ST FLOOR
LOS ANGELES
CA
90071
US
|
Family ID: |
41061993 |
Appl. No.: |
11/462999 |
Filed: |
August 7, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60705763 |
Aug 5, 2005 |
|
|
|
Current U.S.
Class: |
250/390.01 ;
378/121; 378/141 |
Current CPC
Class: |
G01V 5/0091 20130101;
Y02E 30/10 20130101; H05G 2/00 20130101 |
Class at
Publication: |
250/390.01 ;
378/141; 378/121 |
International
Class: |
G01T 3/00 20060101
G01T003/00; H01J 35/12 20060101 H01J035/12; H01J 35/00 20060101
H01J035/00 |
Goverment Interests
STATEMENT OF GOVERNMENTAL SUPPORT
[0002] The invention described and claimed herein was made in part
utilizing funds supplied by the U.S. Department of Energy under
Contract No. DE-AC03-76SF00098. The government has certain rights
in this invention.
Claims
1. A cylindrical gamma ray generator, comprising: a cylindrical
RF-driven plasma ion source for producing a hydrogen ion containing
plasma: a hydrogen gas source in fluid communication with said
plasma ion source whereby hydrogen gas can be introduced into said
ion source; a cylindrically-shaped radial ion extractor system, the
system disposed coaxially about the ion source for extracting
hydrogen ions radially from the ion source; and a cylindrical
target disposed coaxially outside and spaced from the ion source to
receive hydrogen ions extracted from the source by the cylindrical
extractor system, said target comprising a LaB.sub.6 target
material, which target material is capable of undergoing
proton/gamma (p..gamma.) reactions when irradiated by hydrogen ions
having an energy below 675 keV emitted from said source to produce
gamma rays, wherein said produced gamma rays have an energy level
above 6 MeV.
2. The generator of claim 1, further comprising an RF antenna
disposed within the ion source.
3. The generator of claim 2, further comprising water cooling
within the RF antenna.
4. (canceled)
5. The generator of claim 1 wherein the cylindrically-shaped radial
ion extractor system comprises a plurality of axially extending
slots.
6. (canceled)
7. (canceled)
8. The generator of claim 1 further comprising a vacuum chamber
disposed to contain the target.
9. (canceled)
10. (canceled)
11. (canceled)
12. (canceled)
13. (canceled)
14. The generator of claim 1, wherein the hydrogen ions have an
energy of approximately 163 keV.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Patent Provisional
Application 60/705,763, filed Aug. 5, 2005, which is incorporated
by reference herein.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] This invention relates generally to detection of special
nuclear materials, and, more specifically, to a compact, low-cost
gamma ray generator to aid in such detection.
[0005] 2. Background
[0006] Many non-intrusive active interrogation techniques utilize
neutrons or gamma rays to detect special nuclear material (SNM)
concealed in cargo. For active interrogation systems with neutron
sources, neutron induced gamma rays are detected and, sometimes,
transmitted neutrons are measured. Neutron induced gamma spectra of
different materials are used as the fingerprints for them. Fast
neutrons are often in use to obtain a deep penetration into large
inspected objects and, thus, generate a very high background from
surrounding materials. While this high background restricts the
maximum screening speed of many neutron-based systems, neutrons
also tend to activate the surrounding materials after an extensive
long period of operation.
[0007] On the other hand, gamma-based systems detect neutrons
produced from photonuclear reactions or transmitted gamma rays.
Because the neutron production cross sections of many special
nuclear materials due to photofission are much higher than that of
most common materials, the neutron background in gamma-based
interrogation techniques is fairly low. Furthermore, the induced
radioactivity of surrounding materials due to gamma rays of less
than 16 MeV is rather small due to the high threshold energy of
photonuclear reactions. However, most existing gamma-based
interrogation systems use electron linacs and microtrons to
generate the gamma beams; thus, the deployment of these systems is
limited by their size, complexity and high cost of ownership. Thus
there is a need for low-cost, portable gamma sources to use in
active interrogation systems to detect SNM.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] The foregoing aspects and others will be readily appreciated
by the skilled artisan from the following description of
illustrative embodiments when read in conjunction with the
accompanying drawings.
[0009] FIG. 1 is a graph showing photoneutron production
cross-sections of .sup.235U (.gamma.,n) as a function of
energy.
[0010] FIG. 2 is a graph showing photoneutron production
cross-sections of .sup.56Fe (.gamma.,n) as a function of
energy.
[0011] FIG. 3 shows cross-section view of a gamma source coaxial
geometry according to an embodiment of the invention.
[0012] FIG. 4 is a schematic drawing of a coaxial gamma source.
[0013] FIG. 5 a conceptual design of a simple axial gamma
source.
[0014] FIG. 6 shows gamma ray spectra were collected from LiF,
Teflon, B.sub.4C, and Mg bombarded with a continuous beam of
protons.
[0015] FIG. 7 is a schematic drawing a method to detect special
nuclear materials according to an embodiment of the invention.
DETAILED DESCRIPTION
[0016] In general, cylindrical gamma generators can be designed
using a coaxial RF-driven plasma ion source, as has been done
earlier in U.S. Pat. No. 6,907,097 for neutron generators and is
included by reference herein. A plasma is produced by RF excitation
in a plasma ion generator using an RF antenna. A cylindrical
gamma-generating target is coaxial with, or concentrically arranged
around, the ion generator and is separated therefrom by plasma and
extraction electrodes which can contain many slots. The plasma
generator emanates ions radially over 360.degree., and the
cylindrical target is thus irradiated by ions over its entire inner
surface area. The plasma generator and target can be made as long
as desired.
[0017] A co-axial gamma-tube design has several advantages that
would carry over from the neutron tube system. The advantages
include (i) high beam current, (ii) good cooling, (iii) simple
design, (iv) compactness, and (v) spatially uniform photon flux.
FIGS. 3 and 4 show different schematic views of a coaxial type
gamma source that is very similar to the neutron tube design.
[0018] For the (p,.gamma.) target material, Table 1 lists four
possible low-energy nuclear reactions that produce gamma-rays with
energies greater than 6-MeV (the photofission threshold energy is
approximately 5.5 MeV). Of these, the 163-keV .sup.11B and 203-keV
.sup.27Al reactions may be the simplest to work with to create a
gamma tube system through modification of co-axial neutron
generator technology. Suitable target materials for these reactions
include LaB.sub.6 or B.sub.4C (for p-B) and Al (for p-Al), which
are easy to fabricate and also have good thermal, electrical, and
mechanical properties.
TABLE-US-00001 TABLE 1 Four promising (p, .gamma.) reactions for
high energy gamma (6 to 18 MeV) production Gamma Cross Proton
Energy Section energy Target E.gamma. .sigma. E.sub.p Fabri- (MeV)
(mb) (keV) cation .sup.11B(p, .gamma.).sup.12C 16.1, 11.7, 4.4 0.16
160 Easy .sup.27Al(p, .gamma.).sup.28Si 11.5, 9.8, 1.8 <0.03
202.8 Easy 120~180 632.2 .sup.19F(p, .alpha..gamma.).sup.16O 6.1,
6.92, 7.12 160 340 Difficult .sup.7Li(p, .gamma.).sup.8Be 12.24,
14.74, 17.64 6 441 Moderate
[0019] The p-B based system is particularly suitable for special
nuclear material (SNM) detection. More than 90% of the excited
.sup.12C* produced from a 160 keV proton beam hitting on a B target
decays directly to its ground state. Therefore, a p-B gamma
generator can produce an intense 16.1 MeV gamma beam. Many SNMs
have a much higher photoneutron production cross-section at 16.1
MeV gamma energy compared to other common materials. For example,
the photoneutron production cross-sections of .sup.235U at 16 MeV
is .about.0.7 b as shown in FIG. 1, while the photoneutron
production cross-section of .sup.56Fe at the same energy is
.about.0.01 b as shown in FIG. 2. The p-B based system can use a
high current, low energy coaxial accelerator system because of its
relatively small (p,.gamma.) cross section.
[0020] Lanthanum hexaboride (LaB.sub.6) is a rigid ceramic with
good thermal shock resistance and good chemical and oxidation
resistance. LaB.sub.6 also has high electron emissivity and good
electrical conductivity. Similarly, boron carbide (B.sub.4C) is one
of the hardest materials known, ranking third behind diamond and
cubic boron nitride. B.sub.4C has very good chemical resistance,
good nuclear properties (commonly used as a neutron absorber in
reactors), and has low density (2.52 g/cm.sup.3). B.sub.4C can be
formed as a coating on a suitable substrate by vapor phase reaction
techniques, e.g., using boron halides or di-borane with methane or
another chemical carbon source.
[0021] The p-Al system reaction is also capable of detecting SNM
and other contrabands because its branching ratios to different
excited states are comparable to each other. Varying the proton
beam energy can also change the energy level of .sup.28Si* and,
thus, the branching ratios. There are six resonances for the p-Al
reaction between 500 to 680 keV. Gamma ray transmission
spectroscopy can be used to detect elements besides SNM while
neutron detectors can be used to monitor the presence of SNM. On
the other hand, the system based on p-Al uses a modest-energy axial
accelerator. Other possible materials to use as targets include
LaB.sub.6, B.sub.4C, Al, LiF, Teflon.TM., and Mg
[0022] The main drawback with both the p-B and p-Al reactions is
their low cross sections which necessitate operating the gamma tube
at a high proton current to increase the source output. For
example, in a boron-based interrogation system, a co-axial source
producing an ampere of proton current at the 163-keV reaction
resonance will only generate about 6.times.10% gammas/sec. The next
boron resonance occurs at a higher energy (675 keV) and its cross
section is even smaller (0.05 mb). Similarly, the resonant nuclear
reaction for aluminum at a proton energy of 203-keV has a cross
section of less than 0.03 mb.
[0023] The other (p, .gamma.) reactions in Table 1 have
significantly larger reaction cross sections, but require scaling
the gamma tube source voltage to higher energies. For the
production of multiple discrete high-energy gammas, a beam of
protons with energy greater than 340 keV are required. However, it
is difficult to scale the coaxial tube design to these higher
proton voltages. To achieve these higher energies, a simple axial
accelerator concept can be used, as will be discussed later.
[0024] FIG. 3 shows cross-section view of a gamma source geometry
according to an embodiment of the invention. Gamma generator 10 has
a cylindrical plasma ion source 12 at its center. There is a
cylindrical gamma generating target 22 disposed around and spaced
apart from the cylindrical plasma ion source 12. The principles of
plasma ion sources are well known in the art. Conventional
multicusp ion sources are illustrated by U.S. Pat. Nos. 4,793,961;
4,447,732; 5,198,677; 6,094,012, which are herein incorporated by
reference.
[0025] The ion source 12 includes an RF antenna (induction coil) 14
for producing an ion plasma 20 from hydrogen gas which is
introduced into the ion source 12. Antenna 14 is typically made of
copper tubing, which may be water cooled. For gamma generation, the
plasma 20 is preferably a hydrogen ion plasma. The ion source 12
can also include a pair of spaced electrodes, plasma electrode 16
and extraction electrode 18, along its outer circumference. The
electrodes 16, 18 control the passage of ions electrostatically
from the plasma 20. The electrodes 16, 18 can contain many
longitudinal slots 19 along their circumferences so that ions
radiate out in a full 360.degree. radial pattern. In an alternative
embodiment (not shown), the electrodes 16, 18 can be grids.
[0026] Coaxially or concentrically surrounding ion source 12 and
spaced therefrom is the cylindrical target 22. The target 22 is the
gamma generating element. Ions from the plasma source 12 pass
through the slots 19 in the electrodes 16, 18 and impinge on the
target 22, typically with energy of 120 keV to 150 keV. The target
22 may be made of any of the materials listed in Table 1, or
others. In one embodiment, the target 22 is made of LaB.sub.6 or
B.sub.4C. In another embodiment, the target 22 is made of aluminum.
Gamma rays are produced in the target 22 as the result of ion
induced (p,.gamma.) reactions. Outer cylinder 24 defines the vacuum
chamber in which the entire assembly 10 is enclosed.
[0027] The extraction apertures in electrodes 16, 18 can be in the
form of slots 19 whose length can be extended to any desired value.
The hydrogen ion beam hits the target 22 in 360.degree. and
therefore the target area is very large. By making the gamma
generator as long as practical in the axial or longitudinal
direction, a high gamma flux can be obtained. For p-B gamma-based
interrogation system, a long co-axial source that can produce
ampere(s) of current is useful.
[0028] FIG. 4 is a schematic cutaway drawing of a coaxial type
gamma source 11, which is very similar to a coaxial neutron
generator design. A cylindrical ion source is located at the center
of the gamma generator. Hydrogen plasma is formed by RF induction
discharge. An antenna 14 can be water-cooled copper tubing enclosed
inside a quartz tube. It has been demonstrated that RF discharge
plasma is capable of generating atomic hydrogen ion species higher
than 90%. An extraction grid 17 controls the passage of ions
electrostatically from the plasma. The ions are accelerated across
a gap and impinge on a target 22 with full 160 keV energy.
Permanent magnets 30 are in a regular arrangement around the plasma
source and running longitudinally to form a magnetic cusp plasma
ion source. The principles of magnetic cusp plasma ion sources are
well known in the art, as cited above.
[0029] To ensure reliable high voltage operation the gamma-ray
generator 11 can also be vacuum pumped. With reasonable pumping,
the pressure can drop to the 10.sup.-4 Torr range, which allows
trouble free high voltage operation. The ion source can protected
from the secondary electrons with a filter rod structure (not
shown); this prevents high-energy electrons from accelerating back
to the source and potentially over-heating it. The protection from
the secondary electrons is especially important when generating
gammas. Due to the fairly small cross-section of some of the
nuclear reactions, the generator run at fairly high current, which
can cause the ion beam power at the target to be on the order of
200 kW. Although the large surface area of the target helps to
dissipate the thermal load, higher power operation of the gamma
source may be more successful with appropriate target cooling
systems, as have been used for neutron generators.
[0030] FIG. 6 shows gamma ray spectra were collected from LiF,
Teflon.TM., B.sub.4C, and Mg bombarded with a continuous beam of
protons. Each spectrum was collected with a 5-inch NaI detector and
normalized to I-.mu.C of charge. The (p,.gamma.) target to detector
distance was set at 7 cm. Both boron carbide and magnesium have
rather low gamma-ray yield which is consistent with the reported
.sup.11B cross section value given in Table 1. Magnesium was tested
because it had been reported that a 6.19-MeV gamma-ray (in addition
to 4.86-MeV and 0.82-MeV gammas) is produced corresponding to the
317-keV resonance of the .sup.25Mg(p..gamma.).sup.26Al reaction.
The spectra clearly show the 6.19-MeV gamma-ray and also gammas
that arise from higher energy (4-MeV) branching channels that can
occur for the .sup.25Mg(p,.gamma.).sup.26Al reaction. The LiF and
Teflon.TM. spectra are dominated by the characteristic 6.13-MeV
fluorine gamma-ray which was even observed for 250-keV protons from
the accelerator (fluorine has a small resonant cross section of
-0.2 mb at 224-keV proton energy). As indicated in Table 1, the
resonant reaction for lithium occurs at 441-keV which accounts for
the significant jump in the measured yield between the 350-keV and
450-keV spectra. The Li reaction is of interest because it produces
17.64-MeV (63% emission/reaction) and 14.74-MeV (37%
emission/reaction) gamma-rays which coincide well with the peak of
the photofission cross section. There also appears to be an
unidentified, weak low-energy nuclear reaction in Teflon that
produces 12-MeV gammas and may be due to a trace impurity in the
material.
[0031] As mentioned above, it is difficult to scale the coaxial
tube design to proton voltages with energies greater than 340 keV,
as are used for the larger cross section reaction shown in Table 1.
To achieve these higher energies, a simple axial accelerator 40, as
shown in FIG. 5, can be used. In this system, the protons are first
produced in an rf-driven plasma source. The rf antenna is shown as
44. The protons are then extracted and accelerated to their full
energy using a simple electrostatic accelerator column 45. The
accelerated protons then impinge on a water-cooled, V-shaped target
42 (rather than a cylindrical target as in the coaxial design). The
chamber 46 is vacuum pumped through a pumping port 48 to minimize
the electrons produced by ionizing the gas in the beam path.
[0032] A significant advantage of the source designs is its
potential to scale to almost any length by stacking together
individual base units. For example, the coaxial gamma tube can be
taken to an order of magnitude higher power level by stacking ten
of the 1-Amp systems together. In the base units, the lower vacuum
plate is at ground potential and the upper one is at the target
potential (e.g., .about.165 kV for the p-B reaction). These sources
can be stacked on top of each other in a sequence, where two high
voltage flanges are shared in one end of the two generators and, on
the other end, the pumping chamber is shared with another
generator. In this exemplary embodiment, the stack of ten
generators can be operated with only five high voltage feeds, five
vacuum pumps and five rf-systems.
[0033] Another embodiment of the invention integrates gamma-ray and
neutron generators to produce a new active interrogation source.
Owing to its linear scalability, the dual source may be useful for
many diverse applications ranging from very large fixed site
interrogation systems to intermediate-size mobile or remote
inspection systems to compact systems for assaying the internal
contents of hazardous waste drum containers.
[0034] While a simple, compact, and low-cost gamma source design is
important for the wide deployment of these gamma-based
interrogation systems, a sophisticated detection system and a
contraband database are also desirable in order to make the best
use of these systems. FIG. 7 shows a conceptual drawing of an
exemplary embodiment for an integrated system design. A gamma
source 50 is located in the ground as an easy way to shield
inspection workers from the radiation. An array of detectors (one
neutron detector is shown as 54) is positioned around a cargo
container 58 to monitor neutrons and gammas coming out of the
container 58 for signals that indicate the presence of SNM. Some of
the detectors are sensitive to both gammas and neutrons, as fission
also produces a significant amount of prompt gammas. It would be
useful to have a database of induced gamma/neutron ratios for
various combinations of materials and packaging.
[0035] As discussed above for the p-Al based system, gammas of
different discrete energies are produced by the gamma generator.
Thus gamma detectors can be set on the top of the cargo container
58 for gamma transmission spectroscopy to identify other hazardous
materials.
[0036] This invention has been described herein in considerable
detail to provide those skilled in the art with information
relevant to apply the novel principles and to construct and use
such specialized components as are required. However, it is to be
understood that the invention can be carried out by different
equipment, materials and devices, and that various modifications,
both as to the equipment and operating procedures, can be
accomplished without departing from the scope of the invention
itself.
* * * * *