Patent Number: 048291919
Section: description

DETAILED DESCRIPTION OF THE DRAWINGS Referring to FIG. 1, there is shown an elongated capsule 1 with hemispherical ends. Capsule 1 is cutaway to reveal its internal components. Capsule 1 is made of a durable material which allows passage of neutrons. Capsule 1 is also double walled and it should conform with requirements and practices for handling and transportation of radioactive material. In this embodiment, capsule 1 is also made of a non-magnetic material, such as stainless steel, to permit linkage by magnetic attraction through the walls of capsule 1 thus enabling capsule 1 to be completely sealed and airtight. In this embodiment, capsule 1 is approximately 30 cm long and approximately 9 cm in diameter although the size can be changed to adapt to different applications. Mounted in capsule 1 is rotatable shaft 2 engaged at one end by bearing 3. The opposite end of shaft 2 is affixed to armature 4, shown in cutaway, which conforms in shape to capsule 1. Armature 4 is made of a ferrous material capable of magnetic attraction. Ball bearings 5 are mounted in armature 4 to provide low friction contact between armature 4 and capsule 1. A series of pairs of circular disks 6 and 7 are provided inside capsule 1 and about shaft 2. Twenty identical pairs of disks are provided (not all shown in FIG. 1). More or less disks could be used depending on desired neutron source strength. Reference is made to only one pair of disks, 6 and 7, with the understanding that the other pairs are identical. Disk 6 is affixed upon shaft 2 by means of key 8 so that it will rotate with shaft 2. Adjacent disk 7 is affixed to capsule 1 by key 9 and does not engage shaft 2. Key 8 is mounted on shaft 2 and provides for fixed mounting and alignment of all identical disks 6. Key 9 is mounted on the interior of capsule 1 and provides for mounting and fixed alignment of all identical disks 7. In FIG. 2a is shown an embodiment of circular disk 6 having a notched aperture 11 in its center for affixing to shaft 2 by means of key 8. Disk 6 is divided into pie shaped sections alternately coated with an alpha emitter, such as .sup.238 plutonium, indicated by the symbol .alpha. and a target material such as beryllium indicated by the letter n that emits neutrons when struck by .alpha. particles. In this embodiment, a disk is divided into 6 sections, three of alpha emitters and three of targets. The disks are coated on both sides with like sections of alpha emitters exactly aligned on both sides and like sections of targets exactly aligned on both sides. Disk 7, shown in FIG. 2b, is affixed to capsule 1 by means of key 9 and has a similar construction as disk 6 with alternating sections of alpha emitters and targets on both sides of the disk. The dimensions of disk 7 may be slightly larger than disk 6 to accommodate affixing to capsule 1 and permit free rotation of shaft 2 inside its center aperture 12. Also, it is notched on its outer edge to engage key 9. However, in all other respects, disk 7 is identical to disk 6 as shown in FIG. 2a. The disks are made of a material inert with respect to the (alpha, n) reaction, such as tantalum. The alpha emitter coating may be applied by a suitable method, such as by "paint and bake" or vapor deposition. The target portion coating, such as of beryllium, may be applied by a sputtering process. In this embodiment, the alpha emitter portions and the target portions are applied to each disk so as to leave a narrow strip 10 of the inert material of the disk separating each section as indicated in FIGS. 2a and 2b. Inert strips 10 eliminate possible (alpha, n) reactions at the edges of the sections. Referring again to FIG. 1, the end of capsule 1 containing armature 4 engages bracket 13 shown in cutaway. Inside bracket 13 is a rotary solenoid 14 which drives an electromagnet 15, also shown in cutaway. Capsule 1 fits in bracket 13 so that the end of capsule 1 is close to, but does not touch, electromagnet 15. In the OFF state, like sections of adjacent disks are in alignment. No appreciable amount of neutrons are generated. To operate, rotary solenoid 14 is turned ON causing electromagnet 15 to rotate. Magnetic attraction between electromagnetic 15 and the armature 4 through non-magnetic walls of capsule 1 cause armature 4 and the disks 6 attached to it to rotate with the electromagnet 15. Rotation of disks 6 with respect to disks 7 brings alpha emitting portions of each disk into alignment with the target sections of the adjacent disks facing it. When a target such as beryllium comes in close proximity to an alpha emitter such as .sup.238 plutonium, the nucleus of the target absorbs an alpha particle whereupon neutrons are emitted. The neutrons are emitted in all directions. Shielding for humans is necessary when the source is in the ON state. The solenoid 14 can be devised for ON/OFF operation, pulsed operation, or series pulsed operation. A coil spring 16 in FIG. 1 is devised to assure that alignment of alpha emitters of adjacent plates in the ON/OFF mode always occurs when the solenoid is turned OFF, effectively stopping the generation of neutrons and shutting OFF the device. This above described arrangement utilizing the electromagnet 15 provides for a completely sealed capsule for containment of the radioactive materials within. Where such containment is not necessary other more direct rotation means can be used. Other disk arrangements can be utilized for different purposes such as rapidly pulsed sources. For example, in FIG. a is a disk having narrow sections of alpha emitters separated by larger sections 17 of inert material. FIG. 3b shows a similar arrangement of target portions on an adjacent disk. In this embodiment, the alpha emitters and targets are not both on the same side of each disk, but are on adjacent sides of adjacent disks (although both could be placed on the same side if desired). FIG. 3c graphically depicts the output of this type of disk arrangement. The number of neutrons, depicted on the vertical axis, is plotted versus time on the horizontal axis. The pulsed, peaked output results from rotation of the shaft. This invention provides substantial flexibility with respect to shape, arrangement, and alignment method for the disks or plates to suit particular applications. For example, a set of movable plates coated only with target material could be swung into alignment with stationary plates coated only with alpha-emitting material by means of a rotating shaft connected to the movable plates by an arm. Alternately, the disk assembly could be donut-shaped with a pipe of arbitrary diameter through the central hole for passing samples, the rotating disks being driven by gears contacting teeth on their circumferences. In this case, the plates might also be formed of concentric drums surrounding the pipe, rather than donut-shaped circular disks and translation, rather than rotation, could be used to align the plates. The elements used as target material on the disks for receiving the alpha particles and emitting neutrons should be chosen for relatively high yield of neutrons in the energy range of interest, such as beryllium, boron, and lithium. Beryllium provides the highest neutron yield and highest neutron energies. It is thus preferable when such factors are important as for inelastic scattering and fast neutron activation. Boron is intermediate in yield and energy and may be chosen when simulation of the neutron fission spectrum is desired. Lithium gives relatively low yield in energy and it is a candidate target when thermal neutrons are needed as for neutron capture and thermal activation. Although lithium neutrons are the most easily moderated, sufficient moderator can thermalize beryllium and boron neutrons also. However, residual high energy neutrons may sometimes excite reactions that interfere with detection of desired materials, particularly in the case of beryllium. Beryllium also produces a relatively intense field of penetrating gamma-rays along with the neutrons, although the gamma dose rate is substantially lower than that of the neutrons. This gamma field can saturate detectors. However, collimation of the source to provide a radiation beam confined to the test region, combined with off-beam detector location and low-energy gamma-ray absorbers in front of detectors if necessary, will substantially reduce the gamma energy and intensity. Boron produces a less intense lower energy gamma field per neutron. Gamma radiation from lithium is negligible. Suitable alpha-emitting isotopes will have the following properties (a) a half-life long enough that the source could be useful for at least six months, but short enough that the alpha intensity per gram is sufficiently high to produce enough neutron yield using a suitable target, (b) a small spontaneous neutron emission rate in comparison with the (alpha,n) neutron rate attainable for the alpha decay rate; (c) sufficiently few spontaneous gamma rays particularly of high energy; (d) daughter products that do not contribute much unwanted radiation; and (e) availability in gram quantities at a reasonable cost. Six candidate isotopes are shown on the following table along with their properties as disk layers in combination with beryllium, boron, and lithium target layers. They are listed in order of decreasing neutron yield. __________________________________________________________________________ PROPERTIES OF SELECTED (ALPHA, N) SOURCES __________________________________________________________________________ ALPHA EMITTER 228.sub.TH 242.sub.CM 210.sub.PO 227.sub.AC 238.sub.PU 241.sub.AM HALF-LIFE 1.91 Y 163 D 138 D 21.8 Y 87.7 Y 432 Y GAMMA-RAYS 80-2600 44 800 50-870 44 60 ENERGY, KEV MREM/HR-CM.sup.2 @ 1 M 17,000 0.73 0.45 230 0.003 0.63 NEUTRONS Be 2100 1900 1200 190 5.9 1.1 PER CM.sup.2 -S .times. B 600 610 470 55 2.2 1.1 10.sup.4 "ON" Li 100 42 10 8.4 0.64 0.012 NEUTRON Be HIGH 40 HIGH HIGH 1300 6 .times. 10.sup.5 SWITCHING B HIGH 13 HIGH HIGH 480 2 .times. 10.sup.5 RATIO Li HIGH 0.88 HIGH HIGH 14 6 .times. 10.sup.3 __________________________________________________________________________ The emitter layer thickness is assumed to be the range of the highest energy alpha, generally about 20 mg/cm.sup.2. The gamma ray doses are per unit area of emitter layer and are constant, i.e. they do not include the relatively small gamma dose from target (alpha, n) reactions, while the neutron yields are per unit overlap area of the emitter and target layers on adjacent disks (maximum overlap area being reached in the ON position). .sup.228 Th and .sup.227 Ac are assumed to be in equilibrium with their daughters and achieve high neutron yields despite relatively long half lives because of additional alphas from daughter decays, which unfortunately also give relatively high doses of penetrating gamma-rays that cannot be switched OFF. These two isotopes would be useful only in cases where their gamma fields are tolerable in the OFF and ON positions. Gamma doses from the other isotopes are small: .sup.210 Po has a gamma ray of moderate energy but low emission rate, and .sup.242 Cm, .sup.238 Pu and .sup.241 Am produce only very low-energy gammas that are easily shielded. The neutron switching ratio is defined as the ratio of neutrons per second in the ON position from the (alpha,n) reactions to the total neutrons per second in the OFF position. The OFF neutron rate is equated to the spontaneous fission rate for isotopes having that decay process and the switching ratio is listed as "high" for the other isotopes. (Alpha,n) reactions and spontaneous fissions in impurities and construction materials (from stray alphas) will determine the switching ratios where listed as "high" (on the order of a million or so) and will lower somewhat the estimated numerical ratios shown. As expected, the neutron yields and switching ratios are highest for beryllium, intermediate for boron and lowest for lithium targets. Yields were obtained from approximate calculations and do not include losses due to edge effects at disk layer boundaries or due to neutron attenuation in construction materials. With just 20 disks of 50 cm.sup.2 active area per side, over 10.sup.10 n/s might be achieved for a fresh Po-Be source. Therefore, high yields and high switching ratios are attainable. The depths of the alpha-emitter and target layers on the disks need be only slightly larger than the respective alpha particle ranges, as no further increase in neutron yield is provided by thicker layers. A coating over the alpha emitter layer prevents flaking and emission of recoiling nuclei, essential to avoid buildup of neutron background in the OFF position. This coating may be made of gold. The disk diameter and numbers of disks will depend on the source size and neutron yield requirements of specific applications. Similarly, the nominal capsule size may be increased or decreased in specific applications, depending on the neutron yield and NRC/DOT requirements. The capsule could be constructed so as to be easily carried by one person, if no shielding is needed. Four time-coded operational modes are identified for use in various detection techniques: steady-state (0.1 second switching time), intermittent, slow pulsing (up to 10 revs/s), and fast pulsing (hundreds of revs/s). For examination of these operational modes, consider the applicable gamma-ray detection methods. If neutron capture were to be exploited, the neutron source device could be operated in the steady-state mode. A hydrogenous moderator would be placed in front of the device to thermalize the neutrons, since the neutron capture cross section is much higher for thermal neutrons. In the case of explosives detection, the prominent 10.8 MeV gamma line from the .sup.14 N(n,gamma).sup.15 N reaction would be observed. More sensitivity might be obtained if such a setup were operated in a pulsed mode in case there is high background coincident with the neutron source (such as gamma rays produced in the (alpha,n) reactions). If the system produced 100-microsecond pulses separated by OFF times of 1 millisecond, the gamma detectors could be turned off during the pulse thus avoiding counts coincident with the neutron source. Then the detectors could be turned on to detect the gammas from thermalized neutrons delayed by slowing down in the object being examined. To detect gammas from inelastic scattering, relatively high energy neutrons are needed to excite the reaction and moderating material would be kept to a minimum to lessen background from capture. It would probably also be necessary to emit neutrons in a pulse train, with approximately 50 microsecond pulse length and approximately 500 microseconds between pulses to further permit capture gammas to die away between pulses. The detectors would be on during the pulse and, in the case of explosives detection, would detect the 2.3 MeV gamma characteristic of the .sup.14 N(n, n'gamma).sup.14 N reaction. A widely used technique is neutron activation. Gamma detectors would be turned on when the source is turned OFF to observe count rates and decay rates at energies characteristic of activation products. If thermal-neutron activation is desired, the neutron beam would be moderated by hydrogenous material. Another technique is fast-neutron activation analysis in which high energy threshold reactions like (n, 2n) and (n, p) are exploited. For activation half-lives more than a few seconds long, a steady-state switching mode would be used. If material that activated with a half-life on the order of a few seconds were of interest, the neutron source could be operated in an intermittent steady-state mode, going through multiple cycles, if necessary, to build up count statistics. For shorter half-lives, a slow pulsed mode might be useful to build up statistics. The estimated yearly dose rate for a switched .sup.238 Pu-Be source in the OFF position compares favorably with the 148 mrem average human dose per year, so shielding would not be required for comparable switched sources in the OFF position. The present invention has substantial potential advantages over present unswitched radioactive sources, such as portability in the OFF position without shielding, the extension of radioisotope sources to the realm of time-coded source techniques--for safeguards, antiterrorist, weapons treaty verification, medical, and space applications, and reduced personnel exposure (or the same exposure with less shielding). It also has significant potential advantages over accelerator-based neutron sources: no bulky ancillary equipment is required and only nominal electrical power is needed (as long as fast rotation is not required). Battery operation at remote sites and in space would be feasible. It would also provide much better reliability and maintainability than afforded by accelerators.