Patent Number: 044977689
Section: description

DETAILED DESCRIPTION OF THE INVENTION Reference will now be made to the present preferred embodiment of the invention, an example of which is illustrated in the accompanying drawings. FIG. 1 is a schematic representation of the apparatus of the instant invention. Gamma radiation photons are produced in a bremsstrahlung target 2 affixed to the LINAC beam tube terminus. The photons then pass through a polyethylene slab 3 which hardens the photon spectrum by preferentially filtering out photons of energy less than about 2 MeV. Some portion of the higher energy photons which pass through the filter (those above various reaction threshold energies) will produce photoneutrons in the neutron source 4. As will be discussed below, it is often unnecessary to provide a separate target to produce neutrons as photoneutrons in sufficient quantities may be produced in the walls of the chamber. Most of the photons will pass into the volume of the chamber 22 which contains the waste sample 5 where some will cause photofissions. Prompt photofission neutrons, emitted from the interaction of the gamma ray photons with matter in their path, will not be distinguishable from photoneutrons that are formed in the materials comprising the sample chamber. They therefore contribute to the overall flux of thermal neutrons. However, delayed neutrons from photofission processes are emitted continuously during the entire period between LINAC pulses, and therefore their measurement provides important information as to the contents of the sample. Photoneutrons and prompt photofission neutrons will thermalize in a few tens of microseconds and will persist as thermal neutrons for hundreds of microseconds, during which time they will generate thermal-neutron-fissions among fissile nuclides present in the sample. Prompt fission neutrons emitted from thermal fission are thus separable from the photoneutrons by use of time and energy discrimination and can serve along with the delayed neutrons from the photofission as an important quantitative analytical measurement. Means are provided for detection of both the thermalized neutron flux and the fast prompt and delayed neutron flux. Prompt and delayed fast neutron emission can be measured during the thermal die-away time using a fast neutron sensitive moderated .sup.3 He-proportional counter 23, which is shielded from the thermal neutron flux by cadmium 11, in the preferred embodiment of the instant invention. This detector also provides discrimination against any gamma radiation emissions during the counting period since such gammas generate much lower pulse heights in the detector and can be removed electronically from the signal of interest. Means are provided for generating an intense electron beam with maximum electron energies in excess of 10 MeV, since the photons to be generated therefrom must have at least this energy in order to produce significant photofissions and photoneutrons in their interaction with the materials upon which they impinge. Preferably, electron energies of about 12 MeV are provided with a pulse width of about 4 .mu.s. It is also preferred that the repetition rate lie between 1 and 60 Hz and the peak current between approximately 1 and 200 ma. Bremsstrahlung radiation can be generated in heavy metal targets 2 attached to the terminus of the electron accelerator beam tube 1. In a preferred embodiment of the apparatus of the instant invention an EG&G LINAC was employed. A 10 cm thick polyethylene slab 3 was placed in front of the target to harden the photon spectrum as mentioned above. The samples under investigation 5 and the neutron detectors 12, 23, were contained within a polyethylene enclosure 8 with internal dimensions 35.times.38.times.61 cm and 10 cm wall thickness. The outside of the enclosure was covered with 0.6 mm cadmium sheet 7 and a 10 cm thick layer of borated polyethylene 6 to reduce the effects of neutrons generated elsewhere in the concrete irradiation zone 20. In a preferred embodiment of the apparatus of the instant invention, a single 5 cm diameter.times.34 cm long proportional counter 9 filled to three atmospheres of .sup.3 He served as the primary fast neutron detector. The tube was encased in 1.25 cm of polyethylene 10 which was in turn wrapped with 1.7 mm of cadmium 11. This thickness of cadmium provides an attenuation factor for thermal neutrons of approximately 10.sup.8. A 2.5 cm diameter.times.51 cm long bare tube 12 containing 1% .sup.3 He (99% .sup.4 He) at low pressure was used as a neutron flux monitor. The LINAC beam current was monitored at the target 2 to provide normalization for the photofission yields. The proportional counter outputs were fed to singly-differentiating preamplifiers 15, which were in turn fed to linear amplifiers 16 and then to single-channel analyzers 17. The detected pulses were then directed to scalers 18, 19 outside of the irradiation cell. The scalers were gated on 21 after each LINAC burst by a pulse from the LINAC injector. Signals from both the primary detector and the flux monitor were summed during the irradiation, as was the LINAC beam current. FIG. 2 depicts the time sequence of emitted neutrons after an interrogation pulse of gamma radiation and photoneutrons. The first few hundred microseconds following the LINAC pulse are dominated by photoneutrons and prompt neutrons from photofission. Detection of these neutrons is complicated by residual effects of the intense gamma flux and electromagnetic noise on the counting system. The remainder of the first half millisecond is typically a period during which the initially fast neutrons are thermalized. The next two milliseconds (0.5-2.5 ms) is the period in which fast prompt neutrons from thermal-neutron-induced fissions are measured with the cadmium-wrapped detector 23. A second period, after the thermal interrogating flux has subsided, includes most of the remaining time before the next LINAC pulse (5.5-25.5 ms). During this period there is an approximately constant level of fast-neutron signal arising from delayed neutron emission. The first counting segment then, is generally dominated by counts from fissile nuclides present, and the second segment by counts from both fertile and fissile nuclides which have undergone photofission. It is arranged such that this later signal is comprised principally of neutrons from photofission by reducing the number of thermally-induced-fissions occuring while still maintaining a sufficiently strong prompt neutron signal to enable good signal statistics in a practical accumulating time period. This is achieved by controlling the number of thermal neutrons in the chamber. FIG. 3, curves a and b, display the time history of the observed neutron count rate arising from the simultaneous photon and neutron interrogation of 1 g of .sup.233 U which is a fissile nuclide. Curve a shows the .sup.233 U thermal-neutron-fission reaction neutrons, while Curve b shows the delayed neutron emission, 97% of which is from .sup.233 U photofission reactions. A LINAC beam energy of 12 MeV, a pulse width of 4 .mu.s, and a repetition rate of 30 Hz were used to obtain the data in FIG. 3. The peak beam current was about 200 ma and about 20,000 interrogating pulses were accumulated. Samples 5 were positioned at the center of the chamber 22 along the beam line at a point approximately 1 m from the bremsstrahlung target 2. Background count rates were obtained by irradiating with the samples removed. Net prompt neutron counts were normalized to the neutron flux monitor counts, and the delayed neutrons to the electron beam current monitor counts. FIG. 4, Curves a and b, display the time history of the neutron count rate arising from the simultaneous photon and neutron interrogation of 1 g of .sup.239 Pu. Here the contribution of each of the interrogating fluxes to the prompt and delayed neutron counts was investigated by comparing the detector response to a plutonium sample covered with an about 1.8 mm thick cadmium cover (Curve b), and to one without such a cover (Curve a). It is seen that the delayed neutrons are only weakly affected by the cadmium whereas the thermally-induced-fission, prompt neutrons are essentially absent with the cadmium present. This means that the photofission process (where the cadmium cladding should have a negligible effect) is the primary source of the delayed neutrons. In other words, by wrapping the plutonium sample, in the form of PuO.sub.2, thermal fission neutrons are prevented from reaching the fissile nuclides thereby preventing any thermal-neutron-fission neutron emision from contributing to the delayed neutron flux. Gamma radiation, on the other hand, easily passes through such a thin sheet of cadmium, and since they are of substantially greater energy than that required to cause photofission in plutonium, a substantial number of photofission neutrons are produced. The LINAC conditions were identical to those used in FIG. 3. To illustrate the method of this invention, the following examples are presented. The results of irradiating .sup.239 PuO.sub.2 with masses of 1, 0.2, and 0.05 g appear in lines 1-3 of Table 1. The background during both the thermal-fission prompt neutron region (0.5-2.5 ms) and the delayed neutron region (5.5-25.5 ms) was about 1% of the signal for the 1 g sample (line 8). This implies a similar sensitivity for the two counting periods. The net delayed neutron counts are approximately proportional to the quantities of the three masses investigated. However, the net prompt neutron counts were found to be reduced because of the self-masking effect most notably in samples of plutonium. That is, for compact samples (each sample was oxide powder in a doubly-encased, stainless steel cylinder about 3 cm long, 1 cm in diameter, with a total wall thickness of about 1.8 mm), the larger masses absorbed a sufficient number of thermal neutrons in their outer layers to lower the apparent effective mass of the entire sample. This is not a problem for delayed neutrons arising from photofission events. A useful feature of the simultaneous interrogation method of the instant invention is that the presence of photofission neutrons can, in certain cases, provide an internal measure of the masking effect. Self-masking is not a serious problem for waste samples since the fissile material density is generally low. TABLE 1 __________________________________________________________________________ Prompt neutron region Delayed neutron region (0.5-2.5 ms) (5.5-25.5 ms) Net Net Line Gross normalized Gross normalized Delayed Linac number Sample counts counts counts counts Prompt pulses __________________________________________________________________________ 1 .sup.239 Pu (1 g) 11081 1.418 1850 0.114 0.080 20,006 2 .sup.239 Pu (0.2 g) 2140 0.554 190 0.020 0.036 10,008 3 .sup.239 Pu (0.05 g) 1904 0.188 153 0.007 0.037 20,001 4 .sup.233 U (1 g) 19570 2.218 2987 0.150 0.068 20,010 5 .sup.235 U (0.19 g) 4999 0.501 946 0.047 0.094 20,004 6 .sup.238 U (1.5 g) 412 0.036 3939 0.196 5.44 20,003 7 .sup.238 U (1.5 g) 11855 1.40 6739 0.356 0.254 20,007 plus .sup.239 Pu (1 g) 8 background 100 0.012 13 0.0006 -- 20,001 __________________________________________________________________________ Yields from 1 g of .sup.233 U, 0.20 g of .sup.235 U and 1.5 g of .sup.238 U are given in Table 1, lines 4, 5, 6, respectively. The potential of the apparatus and method of the instant invention for distinguishing between fertile and fissile components of transuranic waste is illustrated by the results for .sup.238 U (line 6). That is, the lack of a significant thermal fission cross-section in .sup.238 U relative to the fissile nuclides is clearly shown by the high delayed-to-prompt-neutron ratio for .sup.238 U. Although the depleted uranium used did contain a small amount of .sup.235 U which contributed a small thermal fission yield, the presence of a more significant fissile nuclide concentration is easily handled by reducing the number of thermal neutrons available for capture by such fissile nuclides such that the fissile nuclide contribution to the delayed neutron flux is minimized while sufficient prompt neutrons exist for their determination. Even when this is not done, as for the results of a mixture of .sup.238 U and .sup.239 Pu, illustrated in line 7 of Table 1, it is very easy to tell that there are both fissile and fertile nuclides present. The net normalized prompt neutron flux is clearly the result of about 1 g of .sup.239 Pu as can be derived from line 1 of the table, while the net normalized delayed neutron flux can be seen to be the result of about 1 g of .sup.239 Pu and about 1.5 g of .sup.238 U (lines 1 and 6). Moreover, the delayed-to-prompt ratio has a value of 0.25, so that even where a substantial fissile nuclide concentration exists in the sample, it is easily observed that there must be a substantial fertile nuclide concentration by simply observing this ratio. The sensitivity of a preferred embodiment of our invention was estimated using a 3.sigma. criterion wherein the minimum detectable signal is taken to be three times the square root of the background. Background counts were typically 100 and 13 in the prompt- and delayed-neutron measurement intervals, respectively, as is seen in line 8 of Table 1. In the case of 1 g .sup.239 Pu, the net delayed neutrons are measured to be about 170 times the 3.sigma. value of 11, which gives a lower limit of detectability of about 6 mg. A similar treatment of the prompt-neutron yield of 1 g .sup.239 Pu gives a lower limit of about 3 mg. No corrections were made for matrix effects, and the detection system had only one .sup.3 He detection tube. Further, counting times were limited to about 11 min. By adding detectors to the system, increasing the counting time, or increasing the beam current, which in turn would increase the number of interrogation photons, the system sensitivity can be markedly improved. The quoted sensitivities can be converted to units which the United States Department of Energy uses, which translates to the fact that our invention can be used to assay plutonium contents of less than 10 nCi/g in 208 l (55 gal.) barrels. In summary, the apparatus and method of the instant invention can be used to determine the total fissile nuclide concentration and the total fertile nuclide concentration in a sample of transuranic waste by the use of simultaneous neutron and gamma radiation interrogation of the sample. An electron linear accelerator through the bremsstrahlung effect produces sufficient photons and photoneutrons in each electron pulse to perform sample analyses with high sensitivity. In so doing, the advantages of simultaneous photon interrogation and neutron interrogation can be realized along with the simplicity of a single source- and sample-handling system. Moreover, the prompt and delayed neutrons produced in the induced fissions can be counted using the same detection system and the same geometry, and since they are clearly separable temporally, the thermal neutron and photofission contributions to this emitted fission neutron flux are easily and quantitatively discernible. The foregoing description of the preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. For example, any electron accelerator providing greater than about 1 ma of beam current at electron energies in excess of about 10 MeV, and capable of operation at pulse rates between about 1 and 50 Hz can be used to provide an appropriate source of photons and photoneutrons for the sample interrogation. The embodiment was chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.