Patent Number: 044977689
Section: summary

BACKGROUND OF THE INVENTION The present invention relates generally to the quantitative evaluation of total fissile and total fertile nuclide content in samples, and more particularly to simultaneous photon and neutron interrogation of a sample coupled with the measurement of resulting prompt and delayed neutron emission as the basis for analysis of the totality of the fissile material present in the form of .sup.233 U, .sup.235 U or .sup.239 Pu, and the totality of fertile material present in the form of .sup.232 Th and .sup.238 U in the sample under investigation. Current U.S. Department of Energy guidelines for the management of transuranic waste have created a need for instrumentation to monitor such wastes at the 10 nCi/g level of fissile nuclides in the presence of fertile material. Solely passive systems which rely on the detection of gamma rays or neutrons from the decay or spontaneous fission of transuranic wastes are generally not suitable because of (1) the attenuation of high density matrix material with the attendant loss of sensitivity, and (2) interference from gamma and alpha emitting contaminants in the matrices. Active interrogation methods, wherein the waste is probed by an externally generated neutron or photon pulse, which cause fission in many of the nuclides present, or a combination of active and passive methods, overcome many of the problems associated with the passive methods alone. Two currently used active methods are: (1) photofission where a high energy photon beam induces fission in the waste sample and fission and delayed neutrons thereby produced are detected, and (2) thermal neutron fission where fast neutrons from a pulse source, after moderation, induce fissions in those nuclides present in the sample which are fissile. Active assay systems based on these methods have been used but neither approach itself is entirely adequate. Photofission offers good sensitivity for a large number of transuranic waste samples, but because of similarity of photofission cross-sections, identification of specific nuclides or classes of nuclides is difficult. For example, the important fissile and fertile groups cannot readily be distinguished. Thermal neutrons, on the other hand, offer very high sensitivity for fissile elements but essentially none for fertile elements. A combination of the two methods would be most desirable but technical complexities such as the need for two pulsed sources, together with longer assay times, have made such an analytical system impracticable. A combination of neutron and photon interrogation offers several distinct advantages over either applied alone, including a direct and unequivocable separation of fissile and fertile nuclides within the sample under investigation. The method and apparatus of the instant invention demonstrates that dual interrogation can be achieved using an electron linear accelerator (LINAC) as a pulsed source for both photons and neutrons. Moreover, both interrogations are initiated during each pulse of the LINAC, and the resulting prompt and delayed neutrons can be monitored with the same detection system. It is known in the art that high energy gamma radiation is produced as a result of bremsstrahlung in a heavy-metal target placed in the path of a high energy electron beam. The production of neutrons through the use of electron beams is also known, and occurs when the high energy gamma photons subsequently pass through additional layers of a target causing neutrons to be emitted in (.gamma.,n) processes. "Efficient Neutron Production Using Low-Energy Electron Beams," by C. D. Bowman, Nucl. Sci. Eng., 75, 12 (1980). However, the combination of neutrons and gamma radiation produced from a single source and used for analysis of transuranic waste samples for total fissile and total fertile nuclides present has not been reported. Most of the photons will pass into the volume of the waste sample where some will cause photofission. Prompt neutrons emitted from the photofission of either fissile or fertile nuclides will not be distinguishable from photoneutrons that are formed in the materials of the chamber containing the sample under investigation and/or the matrix materials which contain these fissile and fertile nuclides. However, delayed neutrons from photofission will be emitted on a continual basis during the whole period between LINAC pulses. Photoneutrons and prompt photofission neutrons will thermalize in a few tens of microseconds and will persist as thermals for hundreds of microseconds, during which time they will generate thermal neutron fissions among the fissile transuranic nuclides that may be present. Therefore, fission neutrons from thermal fission are separated in time from the photoneutrons, and can serve, along with delayed neutrons, as a quantitative signature. Essentially then, after an initial burst of photoneutrons and neutrons from photofission, the bulk of the fast prompt neutrons derive from thermal fission of fissile materials. Subsequent to the emission of these prompt neutrons are the emission of delayed neutrons which derive from both photofission of fertile and fissile material as well as delayed neutrons from thermal neutron fission of fissile nuclides. If the latter contribution to the total delayed neutron flux is made small, the delayed neutron emission is representative of the photofission events only. To achieve this result, some iteration of the interrogation neutron flux may be necessary when analyzing samples of completely unknown fissile content. This flux can be varied by choosing different target materials for the (.gamma.,n) source. Thus, in the instant invention, the events detected following a single LINAC pulse are separable into neutron fission (prompt fission neutrons) and photofission (delayed neutrons) events. This data can then be analyzed to yield the individual quantities of fertile and fissile isotopes present. SUMMARY OF THE INVENTION An object of the apparatus and method of the instant invention is to quantitatively evaluate total fissile and total fertile nuclide content in samples. Another object of our invention is to screen transuranic wastes for storage arrangements at the 10 nCi/g level. Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. To achieve the foregoing and other objects in accordance with the purpose of the present invention, as embodied and broadly described herein, the method of this invention may comprise generating repetitively pulsed gamma radiation and neutrons, using these radiations to interrogate the sample of interest, and detecting the emitted prompts and delayed fast neutron flux. High energy repetitively pulsed gamma radiation is generated and directed onto a partially transparent target from which photoneutrons are generated while allowing a substantial portion of the gamma radiation to pass through. The transmitted gamma radiation and generated photoneutrons are then allowed to simultaneously impinge on a chamber which surrounds and contains the sample under investigation. The photoneutrons are then thermalized by collisions within the walls of the chamber resulting in an enhanced probability that any fissile isotopes present within the sample will undergo a fission reaction. The high energy gamma radiation causes photofission in both the fertile and fissile materials, while the thermalized neutrons produce fission in only the fissile material present. From the actual fission processes, and from the fragments produced therefrom, both prompt and delayed fast neutrons are emitted. These emitted neutrons, along with the thermalized photoneutron flux, are measured in between the gamma radiation pulses. A plurality of such measurements are accumulated until a statistically significant signal is obtained, and the results, normalized by the accumulated thermalized neutron flux and gamma radiation photon flux, are related to the total fissile and the total fertile nuclide concentration contained in the sample to be quantitatively assayed, using known gamma and neutron cross-sections for these nuclides. Preferably, the pulsed gamma radiation has a photon energy in excess of about 10 MeV in order to produce significant numbers of photoneutrons. Preferably, also, the photoneutrons are thermalized within about 0.5 ms after the termination of a particular gamma radiation pulse. Since the fissile nuclides undergo both photofission and thermal neutron fission, the delayed neutron emission is composed of contributions from both the fertile nuclides and the fissile nuclides. Therefore, the gamma radiation generated neutron flux is adjusted such that the emitted delayed neutron flux is comprised principally of neutrons from only the photofission reactions, while allowing sufficient prompt fast neutron emission from the thermal neutron fission process in the fissile nuclides to enable statistically significant neutron measurements to be obtained in a practical accumulation time period. This is possible because of the high sensitivity of the method for prompt fast neutron emission, derived from the temporal and energy selectivity of the .sup.3 He-proportional counting system utilized. Preferably, prompt fast neutron measurements are made between about 0.5 and and 2.5 ms after the termination of a particular gamma radiation pulse, and this measurement determines the number of thermal-neutron-induced fissions. The delayed fast neutron measurements are performed between about 5.5 ms after the termination of a particular gamma radiation pulse and the commencement of the following gamma radiation pulse. Adjustment of the thermal-neutron interrogating flux, if necessary, is performed by allowing larger or smaller amounts of photoneutron emitters to be placed in the gamma radiation beam pathway. It is also preferred that the gamma radiation is derived from an electron accelerator, the high energy electrons produced therefrom, being caused to impinge upon a metal target thereby producing the desired radiation by a bremsstrahlung process. It is finally preferred that the photoneutron emitting target include beryllium. In a further aspect of the present invention, in accordance with its objects and purposes, the apparatus hereof may also comprise means for simultaneously generating repetitively pulsed gamma radiation and photoneutrons produced therefrom, a chamber which surrounds and contains the sample under investigation and into which the neutrons and gamma radiation are directed, means for thermalizing these photoneutrons to increase their fission reaction effectiveness for fissile nuclides present in the sample, means for measuring the thermalized photoneutron flux in the vicinity of the sample and for measuring prompt and delayed fast neutron emission from the sample as a result of photofission of fertile and fissile nuclides interacting with the gamma radiation, and from thermal neutron fission of fissile nuclides capturing thermalized photoneutrons during a time interval after the termination of the gamma radiation pulse, and the means for accumulating a plurality of such neutron measurements until a statistically significant signal is obtained and means for recording these accumulated measurements. Preferably, the thermalization of the photoneutrons occurs within the walls of the chamber. It is also preferred that the thermalized photoneutron flux is measured using at least one bare low partial pressure .sup.3 He-proportional counter located inside the chamber in the vicinity of the sample, and the prompt and delayed fast neutron emission measurements are made using at least one high pressure .sup.3 He-proportional counter surrounded by polyethylene which is further surrounded by cadmium foil and located within the chamber in the vicinity of the sample. Such shielded detectors block the large thermalized photoneutron background while allowing very sensitive prompt fission neutron detection. Preferably also, the gamma radiation is produced by means of an electron accelerator capable of providing greater than about 1 ma of electron beam current in short duration pulses at a repetition rate between 1 and 60 Hz and at energies in excess of about 10 MeV, said electrons impinging upon a heavy metal target producing high-energy gamma radiation by means of a bremsstrahlung process. Preferably, the pulsed gamma radiation is filtered to remove photons of low energy to reduce the gamma radiation background, and to remove background neutrons from the bremsstrahlung target using a polyethylene slab placed in between the bremsstrahlung target and the chamber. The polyethylene slab often provides sufficient photoneutrons for thermal fission interrogation without the need for the presence of beryllium which is sometimes inserted in the polyethylene slab in small quantities in the path of the gamma radiation. It is also preferred that the chamber wall be constructed of a thick polyethylene and graphite inner wall surrounded by a layer of cadmium foil which is further surrounded by a thick outer wall of borated polyethylene in order to reduce the effect of stray neutrons generated by the electron accelerator away from the direction of the electron beam, since it is desired that the interrogation neutrons originate in the chamber, and to thermalize the photoneutrons incident on this chamber. Finally, it is preferred that the number of neutrons generated from the pulsed gamma radiation is controlled by selection of a suitable target material which for low neutron flux requirements is simply the chamber wall, and for high neutron flux requirements may include a beryllium sheet placed in between the gamma radiation filter and the chamber. In summary, the apparatus and method of the instant invention allows the distinguishing of fissile and fertile nuclides in a sample under investigation, which is not available from separate interrogation by either neutrons or gamma radiation alone. For example, thermal neutron interrogation offers very high sensitivity for fissile elements but essentially none for fertile nuclides. It is important that the fertile elements be measured since a particular assay for fissile nuclides made alone cannot identify the material as being of low enrichment (small fissile-to-fertile ratio) or high enrichment (large fissile-to-fertile ratio). For proper disposal and handling a fissile-to-fertile ratio is required. The simultaneous production of pulsed gamma radiation and thermal neutrons from a single source provides great simplification for the experimental procedures necessary to do the assay of samples, and also greatly reduces the time involved in performing such assays.