Cross section for fissioning by thermal neutrons and the resonance integral of fission of238Pu

Abstract

At the present time 238pu is finding application as a source of energy in medicine and space technology. To determine the rate of accumulation of 238pu in the reactor fuel and to take account of burnup, it is necessary to know, besides the cross sections of the reactions (n, 3') and (n, 2n), the fission cross section of 238pu in the thermal and resonance neutron-energy ranges. The fission cross section of 238pu on a thermal spectrum (trf) and the resonance integral (If) were measured in Refs. 1-5. The data from ~ese works are presented in Table i. As one can see from the table, af and If exhibit a large variance, and in addition If exhibits large measurement errors.In the present work 238pu was obtained from the c~-decay of 242Cm, from which plutonium was removed beforehand. The target, prepared by spraying in vacuum, contained 2.10 + 0.01 /zg of 238pu and 10.6 + 0.5 ng of 239pu. The impurity 239pu was determined from the area of the resonance at 0.3 eV in measurements, performed with this target, of the energy dependence of the fission cross section using a spectrometer according to the moderation time of the neutrons in lead [7]. The 238pu target was irradiated in the thermal column of a F-1 reactor at the Russian Science Center "Kurchatov Institute." The neutron flux density was equal to 1.6.107 sec-l.cm -2 with a cadmium ratio of 600 with respect to 235U (n, f). The fission resonance integral was measured in the horizontal channel of the F-1 reactor with flux density (I/E) of the resonance neutrons equal to 1.6.107 sec-l.cm -2 with a cadmium ratio of 14 with respect to 55Mn and 63Cu and 30 with respect to 235U (n, 0.The cadmium screen was I mm thick. We employed as reference targets a 238pu target with a mass of 38.3 5:03 ng and a 235U target with a mass of 260 + 13 ng. The mass of the 235U target was determined by the method of backscattering of c~-particles. The fission fragments were detected with mica detectors [8]. The reference values of af and If for 239pu and 235U were taken from Ref. 9: the fission cross section for thermal neutrons with a Maxwellian spectrum is trf(239pu) = af~ = 780.8 + 1.8, where trf is the fission cross section for neutrons with E n = 0.0253 eV; go is the Westcott factor at 290 K; If(235U) = 280b barns. Finally, we obtained for 238pu af = 16.7 + 0.8 barns and If = 26.3 + 1.5 barns (the cutoff energy is 0.5 eV). As one can see by comparing the measured values with the tabulated data, in the present work the error in the determination of lf(238pu) has been substantially reduced.