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Fission-product decay energy-release rates were measured for thermal-neutron fission of /sup 235/U. Samples of mass 1 to 10 .mu.g were irradiated for 1 to 100 s by using the fast pneumatic-tube facility at the Oak Ridge Research Reactor. The resulting beta- and gamma-ray emissions were counted for times-after-fission between 2 and 14,000 s. The data were obtained for beta and gamma rays separately as spectral distributions, N(E/sub .gamma./) vs E/sub .gamma./ and N(E/sub .beta./) vs E/sub .beta./. For the gamma-ray data the spectra were obtained with a NaI detector, while for the beta-ray data the spectra were obtained by using an NE-110 detector with an anticoincidence mantle. The raw data were unfolded to provide spectral distributions of moderate resolution. These distributions are given in graphical and tabular form as differential cross-section values of d sigma/dE/fission for gamma-ray energy intervals ranging from 10 keV for E/sub .gamma./ less than 0.18 MeV to 100 keV for E/sub .gamma./ greater than 6.8 MeV, and beta-ray energy intervals ranging from 20 keV for E/sub .beta./ less than 0.25 MeV to 160 keV for E/sub .beta./ greater than 6.4 MeV. Counting-time intervals range from 1 s for times-after-fission (t/sub w/) less than 6 s to 4000 s for t/sub w/ equals 10/sup 4/ s. The graphical representations also include calculated spectra using summation methods and the ENDF/B-IV fission yield and decay scheme data base. 92 figures, 87 tables.
Since September 11, 2001, much effort has been devoted to the development of new and improved means for the detection and prevention of the clandestine transport of special nuclear material (SNM, i.e. 235U or 239Pu) and other materials for producing weapons of mass destruction. In a recent Brief Communication, Borozdin et al. showed that cosmic-ray muons could be used to image dense objects inside containers. Here we describe a method for unequivocally identifying SNM in large seagoing containers. Our method is based on the fact that neutron-induced fission of 235U or 239Pu is followed by [beta] decays of short-lived fission fragments during which large numbers of high-energy [gamma] rays (above 3000 keV) are emitted. These [gamma] rays have energies above those of natural [gamma] background, are emitted with significantly greater intensity per fission than [beta]-delayed neutrons, have much higher probabilities of escaping hydrogenous cargo loadings than neutrons, and their energy spectra and time dependencies provide a unique signature of SNM. To demonstrate the main properties of high-energy delayed [gamma] rays, we produced neutrons by bombarding a 1-inch thick water-cooled Be target with 16-MeV deuterons from Lawrence Berkeley National Laboratory's 88-Inch Cyclotron. Neutrons were moderated using steel and polyethylene. We employed a pneumatic transfer system to shuttle targets from the irradiation location inside the polyethylene moderator to a remote shielded counting station. We irradiated 235U (93% isotopic content), 239Pu (95% isotopic content), wood, polyethylene, aluminum, sandstone, and steel targets for 30 seconds (in a thermal-neutron flux of 1.5 x 106/cm2-sec) and acquired 10 sequential [gamma]-ray spectra, each of 3 sec. duration starting 3 sec. after the end of bombardment. We used an 80% relative efficiency coaxial germanium detector and a 30-cm x 30-cm x 10-cm plastic scintillator to detect [gamma] rays and acquired data using ORTEC PC-based electronics and software. The qualitative difference in the spectra from SNM versus that of any other material is illustrated in Figure 1, where we show the results obtained following the irradiations of 0.568 grams of 239Pu and 115 grams of steel. From the steel target, we observed a small number of low-energy [gamma] rays produced by the decays of long-lived isotopes such as {sup 56M}n (t12 = 2.58 hours). Similar results were obtained for all other non-SNM targets. However, we observed a large number of high-energy [gamma] rays produced by the decays of short-lived fission fragments from the 239Pu target. Thermal-neutron fission of 235U produces about 3 times as many delayed high-energy [gamma] rays as from 239Pu. We concluded that a sensitive method to identify SNM is simply to integrate the total number of events in a wide energy interval. The results of this type of analysis for two energy intervals, (3000-4000 keV) and (4000-8000 keV), are shown. The integrated numbers of events from irradiated SNM decay with a short effective half-life of approximately 25 seconds, whereas those from all other materials tested showed much longer decay times. These two features--large numbers of high-energy [gamma] rays decaying with a short effective half-life--provide a unique signature of SNM. Because of the high-density of [gamma]-ray lines produced by the decays of fission fragments, a practical system for interrogating large objects would not require high resolution detectors. In fact, we obtained the same results using the low-resolution plastic scintillator as we did with the germanium detector. Based on our measurements, we have estimated the response of a full-scale system employing a 14-MeV neutron generator producing 1011 neutrons per second and an array of scintillator detectors surrounding a standard cargo container in which a 5-cm diameter sphere of 239Pu was hidden inside a load of wood. Neutron irradiation of this container for 30 seconds would result in about 350 detected [gamma]-ray events above 3000 keV in 30 seconds (for 235U the detected yield would be approximately 1000 events). Thus an entire cargo container could be scanned for SNM in about 1 minute. This system could be combined with a radiographic imaging system for rapid identification of SNM in a wide range of applications.
Decay heat measurements following the fast fission of 238U are well underway. The He-jet system and spectrometers were moved to the 1 MW research reactor to gain sufficient fast neutron flux for these measurements. On the Van de Graaff accelerator, the He-jet capillary has been shortened so that beta and gamma measurements following the thermal neutron fission of 235U could be made down to delay times near 0.1 s. Gamma-ray response functions are now well characterized for gamma energies up to 1.5 MeV for our large Nal spectrometer. Such response functions out to high energies are needed to extract energy distributions of our measured gamma spectra. The response function unfolding program, FERD-PC, has been operated successfully with trial spectra. Comparisons of individual fission products for 235U(n{sub th}f) with ENDF/B-VI at short delay times suggest several improvements to the data base particularly in production probabilities. The new data acquisition and data analysis systems have arrived and will soon be brought on line extending considerably the capabilities of our research group.