The long-lived236Np isomer's fission cross section for thermal neutrons

Abstract

In nuclear reactors (especially the fast ones), the 237Np (n, 2n) reaction is the starting point for the buildup of the long-lived 23~Np(1) isomer with a TI/2 = 1.55.105 yr, which has the large fission cross section for thermal neutrons of 2500 • 150 b [i] (i b = i0 -2e m2). Recently wE measured the fission cross section of the short-lived ~S~Np(s) isomer and found that it equalled 2740 ~ 140 b [2]. As one can see from the comparison, the values of the fission cross sections-of the short-lived and long-lived 2S6Np isomers (spins i-and 6-, respectively) are similar. This conflicts with the findings in [3] on the correlations between fission cross section and target-nucleus spin for several nuclei, viz., a large cross section corresponds to a high spin. In order to verify such a correlation in the work concerned we repeated the measurement of the familiar fission cross section of 2S6Np (Z) by using analogous methods of chemical purification of the substance and identification of the fragments as were used in the case of 2~Np(s).Organization of the Experiment.The 2~Np(1) isotope was produced by the =SSU(p, 3n) reaction which gives amaximum yield [4][5][6]. The 238U target itself was in the formof a uranium metal disk with a diameter of 20 mm and a thickness of 2mm. Irradiation was carried out by protons with an energy of 30 MeV and an average flux of 8 ~A for 60 h in the U-150 cyclotron at the Institute of Nuclear Physics of the Academy of Sciences of the Kazakh Soviet Socialist Republic.After 6 months in storage the target registered an activity of 12 mCi (i Ci = 3.7.10 :~ Bq). The irradiated uranium metal was machined down layer-by-layer in increments of 0.i mm. Each layer was dissolved separately in concentrated nitric acid. The chemical separation of the 2S6Np(l ) was carried out from the layers of uranium with the maximum ~-activitv of the ~6Pu accumulating as a result of the decay of the 2S6Np(s). To purify the =36Np(1) from U, fission products, and 2S6Pu, a chromatography extraction method was used. As stationary phase, trioctylmethylammonium nitrate was chosen [2]. Unlike the conditions chosen on the previous occasion [2], sorption of the neptunium was carried out from 4 M HN0s in order to increase the coefficient of decontamination from the fission products, especially from the Zr and Nb. A threefold purification by chromatography made it possible to attain decontamination factors for the Np from U, Pu, and fission fragments of ~3.107, ~I06, and I0~-I05, respectively. The purified Np was separated by an electrolytic method onto a stainless-steel support. As tracers for monitoring the =36Np(1) yield in the chemical separation process we used the 235Np formed in the 238U(p, 4n) reaction as well as known quatity of 239Np which was added to an aliquot of the original solution with a high y-activity due to fission products. The resultant Np yield from the chemical purification process was 95%.Previously in all the 236Np(I) studies the mass of the substance was calculated from the mass-spectrometer analysis[deter...