A Study of the Heavy Rhenium Isotopes

Abstract

<p>The structures of the transitional nuclei ¹⁸⁷Re , ¹⁸⁹Re and ¹⁹¹Re have been studied using the (t,α) proton pickup reaction. Beams of 15 MeV unpolarized tritons and 17 MeV polarized tritons were produced by the tandam Van de Graaff accelerator facility of the Los Alamos Scientific Laboratory. The reaction products were analyzed in a Q3D magnetic spectrometer and detected with a helical-cathode position-sensitive proportional counter. The unpolarized (t,α) reaction on thin targets of ¹⁸⁸,¹⁹⁰,¹⁹²Os produced spectra with energy resolutions of 8-12 keV (FWMH), from which level energies were accurately determined. Later, when the polarized-triton source became operational, the polarized (t,α) reaction on thicker osmium targets yielded angular distributions of analyzing powers and cross sections. Distorted-wave calculations were done to determine a set of optical model parameters which could reproduce the experimental results for known levels in ¹⁸⁷Re. The predictions of DWBA calculations were then used to assign spins and parities to other level in ¹⁸⁷Re, and to levels in ¹⁸⁹Re and ¹⁹¹Re. This is the first time the (t,α) reaction has been used to study deformed nuclei, and its usefulness for the purpose has been clearly demonstrated. The analyzing powers obtained were large, and their angular distributions were quite distinctive for different values of ℓ and j. This is also the first time level schemes have been assigned in the nuclei ¹⁸⁹Re and ¹⁹¹Re. The results have been interpreted within the framework of the Nilsson model, with corrections for pairing and Coriolis coupling included. The model has been found to provide a fairly good explanation for most of the experimental results. Some aspects of the data were better explained by including a hexadecapole component in the nuclear deformation. The hexadecapole deformation parameter winch gave the best agreement with the experimental results was ε₄ = 0.06 ± 0.01, which agrees well with theoretical predictions for nuclei in this mass region.</p>