Yields and isomeric yield ratios of antimony isotopes from the interaction of 159 MeV to 18·2 GeV protons with uranium

Abstract

Relative cross-sections have been measured for the formation of antimony isotopes of A = 115–131 by bombarding uranium with protons of energies from 159 MeV to 18·2 GeV. Absolute cross-sections have been measured at 570 MeV and 18·2 GeV. The isotopic yield distributions show a broadening towards lower mass numbers when the bombarding energy increases, but also a remaining structure at the highest mass numbers A = 128–131. Isomeric yield ratios have also been determined at the same energies for the isomeric pairs of antimony at mass numbers 116, 120, 124, and 126. Each of these ratios remains constant throughout the energy range. Qualitative arguments are presented suggesting that structure observed in isotopic yield distributions in the mass region 115–140 from high-energy proton bombardments of uranium is due to a combined effect of the breadth of the spectrum of energies deposited in the target nucleus by the protons, and the large and rapid variations in the deformabilities of the nuclei when crossing the 82 neutron shell. The assumption that the fragments are “cold” in the scission configuration seems to be necessary. An attempt to calculate the angular momentum distribution of the primary fragments from knowledge of the isomeric yield ratios only leads to a demand for more, and accurate, recoil measurements giving kinetic energies and excitation energies of the fragments. In the course of this work, new determinations of the half-lives of seventeen nuclides from 115Sb to 131Sb were made on isotope-separated samples.