Thermal-neutron fission cross section of²³⁶Pu

n,r, the impurity 2~gPu (2-3%) was established by the mass-spectrometric analysis of the isotopic composition of the plutonium. In addition, comparing the measurements of (i), performed 2 and and 250 days and longer after the chemical separation of plutonium from uranium, shows that short-lived 2a6Np (s), T1/2 = 22.5 h, and therefore the long-lived 236Np (£), TI/2 = 1.55-i0 s yr, both isomers of 2~SNp with a large thermal neutron fission cross section (3, 4), could have been present in the target with 236Pu. Since the ratio of the cross sections for the formation of 2a6Np (s) and 236Np (£) in the reaction 23sU (d, n) 2~6Np is equal to approxi- mately seven (5), it follows from the data of (i) that the value presented for the cross section oth~(236Pu) could have been 15% too high owing to the presence of 236Np (£) in the n,r ~ plutonium target and 35% too high in the case of the measurements two days after the 236Pu was separated from the uranium. Thus the value oth~(236Pu) = 165 + 20 b (6) recommended n,l~ based on the results of (i, 2) is unreliable and requires further refinement. In this work 236Pu was obtained by the reaction 238U (p, N) 2~6Np ~-236Pu (7). It was separated from the layers of uranium with the highest content by ion-exchange chromatography followed by electrolytic deposition on a substrate. The residual mixture of uranium in the separated plutonium preparation was determined by the x-ray fluorescence and luminescence methods. The content of 23sU in the target was equal to 0.3% of the mass of 2~6pu. The preparation contained, aside from 236Pu, 238Pu and 2agPu, which accumulated after ~- decay of the corresponding isotopes of neptunium, forming in the reactions 2~eU (p, n), 238U (p, y), 23sU (n, y). The content of plutonium isotopes in the target, determined by ~ and mass-spectrometric analyses, was equal to 28 ± 0.14 ng 236pu, 21.9 i 0.7 ng 238pu, 10.9 ± 0.3 ng 2~SPu. The admixture of 2~2U, accumulated in the target as a result of a decay of 2~Pu (T~/2 = 2.85 ± 0.01 yr (8)), reached during the experiment 8.57 ± 0.05 ng. The thermal-neutron fission cross section of 2~6pu was measured with respect to 2SgPu (n, f) in the thermal column of the F-1 reactor at the I. V. Kurchatov Institute of Atomic Energy. The Maxwell±an temperature of the neutrons was equal to 290 K, the neutron flux was equal to 1.6"10 ~ cm-='sec -~, and the cadmium ratio was equal to 600 with respect to 2~sU (n, f). The ~a~Pu and =~gPu (49.9 ± 0.9 ng ~S(Pu) targets were placed in a thin-walled duraluminum con- tainer flush against one another. The plutonium fission fragments were recorded with the help of mica track detectors (9). As a result of the experiment 2,694 fission-fragment tracks were recorded from the target with ~3~Pu and 7,763 tracks were recorded from the target with 2~SPu. The thermal-neutron fission cross section of 2~6Pu was calculated using the values of the reference fission cross sections of o0 for E n = 0.0253 eV and the Westcott factors g presented in (i0); the main values are o 0 (=agPu) = 744.4 ± 1.7 b, o 0 (=a~Pu) = 16.5 ± 0.5 b, o 0 (2~2pu) = 74 ± 3 b, g (2~gpu) = 1.049. The thermal-neutron fission cross section of 2a6Pu, taking into account the 3% correction for the contribution of =~6Pu fissions by supercadmium neutrons, was equal