Delayed proton emission of N=81 odd-odd precursors: 148Ho, 150Tm, and 152Lu.

Abstract

Beta-delayed proton decay was observed in $^{148}\mathrm{Ho}$, $^{150}\mathrm{Tm}$, and $^{152}\mathrm{Lu}$ for the first time. Contrary to neighboring even-Z N=81 isotones, the proton spectra appear structureless and statistical in nature. Proton branching ratios for the high-spin isomers in these isotopes, based on the intensity of \ensuremath{\gamma}-ray transitions in the intermediate nuclei, are (${8}_{\mathrm{\ensuremath{-}}2}^{+1}$)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$, (${1.2}_{\mathrm{\ensuremath{-}}0.4}^{+0.2}$)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}2}$, and (1.5\ifmmode\pm\else\textpm\fi{}0.7)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}1}$, respectively. The onset of proton emission in all three cases occurs at a proton to \ensuremath{\gamma} width ratio of about ${10}^{\mathrm{\ensuremath{-}}4}$. Protons were found to be in coincidence with x rays, \ensuremath{\gamma} rays, and positrons. Coincident K x rays served to identify the Z of the precursor, while the \ensuremath{\gamma} rays gave quantitative information about proton decay to excited states in the daughter nuclei. By comparing the final state branching ratios with statistical model calculations, it was concluded that $^{148}\mathrm{Ho}$ and $^{150}\mathrm{Tm}$ both have isomers with probable spin values of ${1}^{+}$ and ${6}^{\mathrm{\ensuremath{-}}}$. Statistical model calculations with ``standard'' prescriptions for level densities and \ensuremath{\gamma} widths, and with a reduced level density parameter are compared to experiment. To reproduce the shape of the proton spectrum it was necessary to assume Gaussian-shaped Gamow-Teller \ensuremath{\beta} strength functions centered near 8 MeV excitation energy. In the course of these experiments \ensuremath{\beta}-delayed proton emission in $^{148}\mathrm{Er}$ was also identified.