Patent Grant
8000447
This disclosure relates to high energy gamma ray (photon) production, and more particularly to production of high energy gamma rays from low energy nuclear reactions.
Detecting and characterizing shielded fissionable material (e.g., Special Nuclear Material (SNM)) is a difficult problem. Conventional passive methods rely on spectroscopy of low-energy (less than 500 keV) gamma rays (i.e., photons) from natural decay, but this approach is not suitable when thick shielding may be present. For example, the attenuation of 500 keV gamma rays is such that only about 20% penetrate 2.54 cm of steel shielding, and only about 1% penetrate the same thickness of lead. Higher energy gamma rays can more easily penetrate shielding and, furthermore, increase the detectability of uranium and plutonium isotopes by detecting reaction products of photofission. A means capable of generating a sufficient intensity of high energy photons is needed.
High-energy gamma rays can be produced via (relatively) low energy nuclear reactions. Table 1 summarizes various possible reactions, indicating the minimum incident proton kinetic energy required to initiate the nuclear reaction, the specific reaction concerned, the energy of the emitted gamma ray obtained from the reaction, and the cross-section of the reaction (specified in millibarns). In particular, two reactions for proton induced gamma ray generation are considered (indicated in bold type). One reaction involves 11B(p, γ)12C, i.e., the conversion of boron-11 to carbon-12 by nuclear absorption of a proton, emitting a gamma (γ) ray. At proton energy of 163 keV, the reaction produces an exit gamma ray of 11.7 MeV, with a cross-section of 0.157 millibarns (mb), which is a measure of the “efficiency” of producing the high energy gamma rays. This reaction is nominally referred to as a “12 MeV” reaction, and protons may conventionally be accelerated to approximately 180 keV or higher to assure a substantial production of 12 MeV gammas rays. A second reaction of interest (19F(p, αγ)16O) requires a higher proton energy of 340 keV incident on fluorine-19 to emit an alpha particle (α) and a 6.1 MeV gamma ray, converting the fluorine-19 to oxygen-16, with a reaction cross section of 160 mb. This reaction is nominally referred to as a “6 MeV” reaction, and protons may conventionally be accelerated to approximately 360 keV or higher to assure a sufficient production of 6 MeV gamma rays.
163
11
B(p, γ)
12
C
11.7
0.157
19F(p, αγ)16O
9Be(p, γ)10B
340
19
F(p, αγ)
16
O
160
7Li(p, γ)8Be
A first vacuum pump 115, coupled to the plasma generation chamber 105 via a gas How limiting plasma aperture 111, maintains a suitable plasma generation pressure at the first pressure in the plasma generator 105 due to gas inflow at the gas line 106 while lowering the pressure at the intermediate second pressure chamber 112. The intermediate second pressure decreases the ion losses due to ion neutralization allowing highly efficient ion transport to the accelerator chamber 130. A second vacuum pump 135 coupled to the accelerator chamber 130 maintains a further lower pressure to provide a satisfactory mean-free-path for the protons to reach the target without substantial loss of energy through collisions along the acceleration path with neutral gas molecules and ensures corona and arcing free operation of the accelerator column 140.
Proton production may be obtained by plasma generation in a working gas of hydrogen in an RF-induction cavity, i.e., the plasma generator section 105 of
Once in the accelerator chamber 130, the required kinetic energy for the desired nuclear reaction must be achieved. In the case of the boron reaction 11B(p, γ)12C a single pair of electrodes forming an axial beam of protons may be sufficient to obtain the ˜180 keV required to produce 12 MeV gamma rays, however the current may not be sufficient to produce gamma rays in sufficient quantity, as determined by the cross-section. In addition, any effort to make the system more compact makes a single pair electrode beam accelerator more difficult to achieve, as the higher electric field gradient under the existing pressure conditions may not be stable, leading to arc discharge. A conventional target material source of boron nuclei is LaB6. For the 19F(p, αγ)16O reaction, which requires 340 keV, this is correspondingly more difficult. Since fluorine occurs naturally as a gas, a solid fluorinated compound must be used.
It would therefore be advantageous to provide an ion accelerator that reduces the electric field gradient to prevent arc discharge in the accelerator region while providing the kinetic energy needed to produce the desired nuclear reaction.
A gamma ray generator includes a source of ionized gas in a first chamber maintained at a first pressure, provided by a gas inflow from a gas line at the plasma generator. A second chamber maintained at a lower pressure is configured co-axially to surround the first chamber. A third chamber with a third gas pressure is situated co-axially to surround the second chamber. A puller electrode between the second chamber and the third chamber draws ions from the plasma generator, wherein the puller electrode is configured in a concentric arrangement around the ion source to separate the second chamber from the third chamber and having at least one channel aperture to provide for a restricted passage of ions and gas from the second chamber to the third chamber. A plurality of accelerator electrodes are placed in the third chamber as co-axial rings surrounding the second chamber. The accelerator electrodes have at least one channel corresponding to the at least one puller electrode aperture to provide for the passage of a beam of ions passing from the puller electrode channel through the plurality of accelerator electrodes.
The puller electrode can be used to pulse the ion beam by applying a sufficiently negative pulsed potential to the puller electrode. The puller electrode functions in this manner as a gate electrode between the plasma source and the accelerator column. The puller electrode can be operated at substantially lower potential than the full accelerating voltage of the accelerator column, thus allowing for beam pulsing with a relatively lower potential gating voltage. The accelerator electrodes increase the energy of the ions to a selected energy in successive stages by application of an accelerating voltage to each of the successive pairs of accelerator electrodes. A target disposed co-axially around the plurality of accelerator electrodes receives the at least one beam of accelerated ions of the selected energy. The target material is selected to produce gamma rays resulting from a nuclear reaction with the incident ions.
A method and apparatus for generating high energy monochromatic gamma rays includes a compact low energy proton accelerator and a nuclear target source for generation of high energy gamma rays.
As indicated in Table 1, the cross-section for the boron target reaction is 0.157 mb, a factor of about 1000 less than the cross-section for the fluorine target reaction. To increase the gamma ray production, a design for the proton generator and accelerator is disclosed that provides an increased proton current and corresponding gamma ray production rate. The disclosure provides a structure and method for gamma ray production from either the boron-based or fluorine-based target individually, or simultaneously. The disclosure further provides a structure for generating the proton current and acceleration under differential pressure conditions. The disclosure further provides for a suitable target composition of boron and/or fluorine to enable suitable electrical conditions for the protons to interact effectively with the target. Furthermore, the disclosure provides structures for configuring the boron-based target individually, the fluorine-based target individually, or both target materials together. The disclosure further provides for a differential pumping of two or more chambers concentrically arranged. The disclosure further provides for a pulsed ion beam generation by using a pulsed voltage applied to the puller electrode. The disclosure pertains to other nuclear gamma ray photo-production reactions by suitable modifications and variations of features of the disclosure as may be understood by those skilled in the art.
Referring to Table 1, a trade-off between the two reactions of interest is apparent: a lower proton energy of 163 keV is easier to achieve in a compact system, but the cross-section of the boron reaction (11B(p, γ)12C) is low compared to the fluorine reaction (19F(p, αγ)16O), i.e., approximately 1/1000th of the latter. To achieve comparable gamma ray flux would require about 1000 times the proton current. On the other hand, accelerating protons to more than 100 keV in a single electrode pair stage may result in arcing if pressure in the accelerator region is not kept low enough. Achieving a higher acceleration energy of 340 keV, as required for the fluorine reaction, is a significant challenge in a compact system.
In one embodiment a multi-beam, coaxial, high ion beam current 11B(p, γ)12C gamma tube (GT) generating protons in excess of 163 keV for generating 11.7 MeV gamma rays is disclosed. In another embodiment, a lower ion beam current 19F(p, αγ)16O GT capable of generating protons in excess of 340 keV for generating 6.1 MeV gamma rays is disclosed.
In a third embodiment, a single device encompassing both gamma ray sources (e.g., boron and fluorine), i.e., a “6/12-MeV” GT, is disclosed.
The prior art implementation of
The vacuum pumping in the intermediate pressure second chamber 330 and the third chamber 335 introduces a gradient in pressure across the plasma aperture 311 and the puller electrode 320, while maintaining high gas pressure in the plasma generator 305 for efficient ion production and very low gas pressure in the accelerator region of the third chamber 335 for reliable high voltage operation. The combination of slot aperture sizes and pumping rates may be optimized to achieve the desired pressures in each region in order to maximize the ion current while providing a satisfactory mean-free-path for the protons to reach the target 350 without substantial loss of energy through collisions along the acceleration paths with neutral gas molecules or by contacting the accelerator electrodes 340. The pressure in the first chamber 325 may be about 5-10×10−3 Torr to support plasma generation. Pressure in the third chamber 335 containing the accelerator electrode rings 340 and target 350 may be about 4×10−6 to 10−7 Torr, or less. Pressure in the intermediate second chamber 330 may be about 5×10−5 to 10−4 Torr.
Another aspect of the disclosure is the nature of the target. Conventional GTs, as described above with reference to
An embodiment in accordance with this disclosure is to use pure boron as a target material for the gamma generation. There are two advantages of pure boron over LaB6: first, the pure boron is substantially 100% 11B, which enhances the gamma yield, secondly the boron crystal is weakly conducting. In normal circumstances the weakly conducting target material would be a problem, but in the case of a gamma generator, this property leads to self biasing of the target. Depending on the temperature of the boron target, the positive potential on the target surface can be from a few tens of volts to hundreds of volts. This self-biasing nature of the target surface renders secondary electron filtering unnecessary, thus simplifying the electrode structure of the gamma generator. Self biasing occurs when a positive ion beam strikes the target surface, and charges up the surface more positively in comparison to the surrounding electrodes. This positive target surface will attract the secondary electrons back to the target, thus suppressing the electron emission. An insulating material would be unsuitable, due to a too large voltage drop across the target surface. However, the weakly conducting boron (at elevated temperature) is very suitable for self-biasing and secondary electron filtering from the target.
A similar challenge arises in the case of fluorine. However, a target material with similar electrical properties could be fluorite (CaF2). This material is also weakly conducting, with a dielectric constant of ˜6. It would also self-bias in a manner similar to pure boron, and the melting temperature is high at ˜1300 C, so that it can withstand heating from the proton beam.
Another aspect of the disclosure is the ability to combine both target materials in one target ring to obtain both 6 MeV and 12 MeV photoproduction. One embodiment of the combination target may be an inner layer/outer layer, where one target material is disposed as a “jacket” around the other. In one embodiment, it may occur that a portion of 360 keV protons transiting first a fluorine-based target scatters and loses energy without being absorbed, making these scattered protons better suited to interact with boron. In this embodiment, it may be preferable to have the boron-based target as the outer target and the fluorine-based target as the inner target. The thickness of each layer may be selected to provide the desired ratio of 6 MeV/12 MeV gamma rays, taking into consideration the cross-section of each reaction, and the currents generated in the accelerator rings. In other embodiments, the reverse order of target materials may be preferred.
In another embodiment, the puller electrodes and accelerator rings may be adapted (e.g., segmented) to provide proton beams selected to have different energies, in which case the target may be segmented around the circumference to provide target material appropriate to the energy of the beam. That is, a subset of ˜180 keV beams may be directed at boron-based portions of the target and the complement or another subset of ˜360 keV beams may be directed at fluorine-based portions of the target.
In another embodiment the puller electrode can be used to pulse the ion beam by applying a sufficient pulsed negative potential to the puller electrode, thus controlling the extraction of the positive ions from the plasma generator. The puller electrode functions in this manner as a gate electrode between the plasma source and the accelerator column. The puller electrode can be operated at substantially lower potential than the lull energy at the accelerator column, thus allowing for high energy beam pulsing with a lower puller electrode potential.
In another embodiment, the proton beams, or a subset of the beams, may be shaped substantially as “ribbons” by design of the aperture channels, and the targets may be formed as a layered set of rings stacked in the direction defined by the common axis of the accelerator rings, e.g., like a layer cake with the accelerator rings in the center, so that a ribbon beams may intersect and irradiate layers of each type of target.
In another embodiment, the target may have a single composite composition of boron and fluorine compounds, a gradient composite of compounds, or a composite of tiles of the two compounds, in various spatial arrangements, where the mass fractions of boron and fluorine are selected to substantially obtain a desired ratio of 6 MeV/12 MeV gamma rays.
In another embodiment, referring to Table 1, the target may comprise Li-7, and be irradiated by proton beams accelerated to energies of at least 441 keV to produce gamma rays with energies of 12.1, 14.7, and 17.6 MeV. The structure of the generator tube may be substantially the same, with the number and spacing of concentric accelerator electrodes 340 needed to reach at least 441 keV being determined by the voltage steps between the successive pairs of accelerator electrodes 340. Additionally, the target may comprise any combinational mixture of target materials selected from among materials including boron-11, fluorine-19 and Li-7 in the various ways described above to obtain a desired intensity ratio of the various gamma ray energies available from the proton-nucleon reactions.
It may be appreciated that other target materials may be adapted and used in a modification of the gamma ray generator described above for generation of other energies of gamma rays for different spectroscopic analysis, for example, of other materials. Furthermore, the various target materials may be arranged in various spatial arrangements similar to configurations described above to achieve the same objective of high gamma ray photoproduction.
Although the present disclosure and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present disclosure.
This application claims priority to U.S. provisional patent application No. 61/085,722 filed Aug. 1, 2008, entitled “Co-Axial, High Energy Gamma Generator”, which is hereby expressly incorporated in its entirety by reference.
The invention described and claimed herein was made in part utilizing funds supplied by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. The government has certain rights in this invention.
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| Number | Date | Country | |
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| 20100111258 A1 | May 2010 | US |
| Number | Date | Country | |
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| 61085722 | Aug 2008 | US |
