αDecay from High-Spin Isomers inHo153andHo154

Abstract

$\alpha$-emitting high-spin isomeric states in ${}_{}{}^{153}{}_{}{}^{}\mathrm{Ho}$ and ${}_{}{}^{154}{}_{}{}^{}\mathrm{Ho}$ were found by bombarding targets of ${}_{}{}^{142}{}_{}{}^{}\mathrm{Nd}$ and ${}_{}{}^{144}{}_{}{}^{}\mathrm{Nd}$ with nitrogen ions accelerated in the Oak Ridge isochronous cyclotron. In the experiments, recoil nuclei ejected from the thin targets were stopped in helium gas, swept through an orifice, and deposited on the surface of a wheel. After bombardment, the wheel was turned by remote control to place the collected activity in front of an $\alpha$-particle Si(Au) spectrometer for assay. The previously unreported high-spin isomer in ${}_{}{}^{153}{}_{}{}^{}\mathrm{Ho}$ was found to have a half-life of 2.0 ± 0.1 min and an $\alpha$-decay energy of 3.91 ± 0.01 MeV. The $\beta$ decay of the 3.25-min ${}_{}{}^{154}{}_{}{}^{}\mathrm{Ho}$ high-spin isomer had been investigated earlier by Ward and Neiman. In the present study it was found that the isomer also emits $\alpha$ particles with an energy of 3.72 ± 0.01 MeV. The nuclidic assignments of these two new $\alpha$ emitters were based primarily on (1) a comparison of yield curves for their production with yield curves for the production of the known pairs of $\alpha$-emitting isomers in ${}_{}{}^{151}{}_{}{}^{}\mathrm{Ho}$ and ${}_{}{}^{152}{}_{}{}^{}\mathrm{Ho}$, and (2) cross bombardments which demonstrated that the $\alpha$ groups could not be due to decay from previously unknown isomers in dysprosium and terbium nuclei in the investigated mass region.