Range of products withA=131–136 in the reactor neutron fission of237Np,243Am by and thermal neutron fission of233U,235U, and239Pu

Abstract

The kinetic energy of fission fragments of nuclei is measured by using semiconductor detectors or the time-of-flight technique [1][2][3][4] as well as by determining their mean free paths in absorbers [5][6][7][8]. The last approach is most suited for determining the kinetic energy of the fragments formed during symmetric fission of heavy nuclei, when the number of such events is small in comparison with asymmetric fission.For the most part prior to these studies reliable measurements had been made of the range of radioactive products that had an acceptable period and well known decay scheme and, moreover, were not screened by long-lived or stable precursor nuclides. In addition, no studies have been done on measuring the mean free path of the products of mass chains with A = 134 and 136. When z~5 U was fissioned by thermal neutrons the range of ~Y'Cs and *SRb in metallic uranium was found to be anomalously low (10-12% less than that of the products of closest mass) [17]. That anomaly, however, was not confirmed when the kinetic energy of the fragments was measured by instrumental methods [1, 4].The interest in investigating the properties of fragment in this region of mass stems from the fact that for most even-even and some odd fLssile nuclei at low and intermediate excitation energy the yield is higher for A = 134 and lower for A = 136, which is manifested as a fine structure by the mass distribution curve [9, 10].The technique of thin multilayer targets, similar to that described in [5], was used to obtain experimental information about fission product ranges. The three-layer target consisted of an aluminum substrate with a layer of fissile material of thickness (0.7-1.7). 10 -4 g/era 2, on which an aluminum filter of thickness 1.5, 2/2. or 2.8 g/cm 2 was deposited. The filter, in turn, was coated with aluminum foil of thickness 2.8-3 g/era 2. The targets were rolled into a cylinder and put into quartz cells, which were then vacuum-sealed.The targets were irradiated in the irradiation holes of the WRM reactor at the Institute of Nuclear Physics. Targets with ~3U, ~sU, and z39Pu were irradiated in holes with a cadmium ratio of -50 at a thermal neutron flux density of 5.10 t~ sec-t-cm -2. Containers with targets containing 237Np and 243Am were placed in cadmium cans with 0.5-mm thick walls and were irradiated in low-cadmium-ratio cans. After irradiation and cooling, which is necessary for accumulating the final decay products, the target was separated into its constituent parts (substrate, filter, and aluminum foil), each of which was vacuumsealed in a quartz cell. Stable xenon isotopes formed in the cell when the cell was heated to the aluminum melting point.The isotopic composition of xenon was analyzed on an MI-1201M mass spectrometer. The error of measurement of the relative isotope abundance ranged from 0.1 to 0.7%, depending on the amount of material analyzed.The main objective of the study was to estimate the mean free path and kinetic energy of the fission products with A = 131-136. The results were obt...