Measurement of the Delayed Photon Yield of the Photofission Reaction as a Method for Identifying Fissile Materials
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asurement of the Delayed Photon Yield of the Photofission Reaction as a Method for Identifying Fissile Materials S. V. Zuyeva, E. S. Konobeevskia, *, M. V. Mordovskoya, V. G. Nedorezova, V. N. Ponomareva, and G. V. Solodukhova aInstitute
for Nuclear Research, Russian Academy of Sciences, Moscow, 117312 Russia *e-mail: [email protected]
Received March 4, 2019; revised March 20, 2019; accepted March 29, 2019
Abstract—The results of measuring the photofission reaction yield for actinide nuclei in the near-threshold energy region are presented. At the LUE-8-5 accelerator of the Institute for Nuclear Research of the Russian Academy of Sciences, the energy and time spectra of the delayed gamma quanta were obtained by irradiating fissile isotopes by bremsstrahlung photons. The conclusion is drawn about the possibility of identifying fissile isotopes by the behavior of the photofission cross section in the near-threshold region. DOI: 10.1134/S1063779619050265
INTRODUCTION A photofission reaction has not only purely scientific but also applied significance for determination of the presence of fissile materials in samples. The reaction thresholds for different fissile materials are different; however, all of them are within a range of 6 ± 1 MeV. Knowing thresholds for different elements, it is possible not only to determine the presence of fissile material, but also to find a particular isotope. The reaction yield can be determined from the yield of delayed gamma quanta, which are radiated within an interval of 10−3−103 s after the act of fission. The total energy of gamma radiation per act of fission is 6– 8 MeV, while the mean energy of gamma quanta is ~1 MeV. The energy spectrum may be very complicated and include up to 200 γ -lines, corresponding to a decay of different fission products. Although there are many different fission products and relevant decay times, they are grouped into several groups with average decay times from fractions of a second to thousands of seconds. EXPERIMENTAL SETUP In this work, the possibility to determine a threshold for the photofission reaction is verified for a particular element when irradiating it with f luxes of particles available at the LUE-8-5 accelerator. At the accelerator, there is a possibility of operation with the 4–9-MeV electron beam and with a beam of bremsstrahlung photons at the end-point energy of 4– 9 MeV. Moreover, the operation is possible with the
quasi-monoenergetic photon beam if the procedure of bremsstrahlung spectrum monochromatization [1] is applied. In the experiment, a beam of electrons with the energy Ee was deflected in the magnetic field of the magnetic analyzer by an angle of 270° and fell either on the sample or on the bremsstrahlung target. In the latter case, the sample is irradiated by a flux of bremsstrahlung photons with the end-point energy max E ph ≈ Ee . The data are obtained for two setups intended for the operation with delayed photons in different time domains. Setup 1 (BOVR) is intended for research of delayed photons with long decay times,
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