α-decay branching ratios for high - and low -spin isomers inHo151,152,153,154

Abstract

$\alpha$-decay branching ratios were obtained for high- and low-spin isomers in ${}_{}{}^{151,152,153,154}{}_{}{}^{}\mathrm{Ho}$. These activities were produced by bombarding ${}_{}{}^{144,147}{}_{}{}^{}\mathrm{Sm}$ with ${}_{}{}^{11,10}{}_{}{}^{}\mathrm{B}$ ions, respectively, accelerated in the Oak Ridge isochronous cyclotron. With the use of a gas-jet-capillary transport system it was possible to do $\alpha$-particle, x-ray, and $\gamma$-ray counting simultaneously. Branching ratios were deduced in the main by determining the number of $K{\alpha }_1$ x rays emitted and then applying appropriate correction factors to obtain the total number of electron-capture and positron decays. For ${}_{}{}^{152,154}{}_{}{}^{}\mathrm{Ho}$, where the $\gamma$-ray spectral measurements could be used to extract information concerning their decay to ${}_{}{}^{152,154}{}_{}{}^{}\mathrm{Dy}$, it was possible to deduce alternate, and thus independent, $\alpha$-decay branching ratios. (Incidentally, our results confirm those of Ward and Neiman for the decay of the high-spin ${}_{}{}^{154}{}_{}{}^{}\mathrm{Ho}$ isomer to levels in ${}_{}{}^{154}{}_{}{}^{}\mathrm{Dy}$.) Within error limits the two sets of ratios for ${}_{}{}^{152,154}{}_{}{}^{}\mathrm{Ho}$ were found to be in agreement. For the two cases, i.e., 35.6-sec ${}_{}{}^{151}{}_{}{}^{}\mathrm{Ho}$ and 9.3-min ${}_{}{}^{153}{}_{}{}^{}\mathrm{Ho}$, where experimental $\alpha$-decay branches had been previously measured our data agree with the earlier results. $\alpha$-decay reduced widths were calculated by using the formalism developed by Rasmussen and compared with values for neighboring even-even nuclei, which can be taken to represent unhindered $\alpha$ decay. The present study is the first systematic investigation of $\alpha$-decay rates for a series of isotopes of an odd-$Z$ element in the rare earth region. It was found that, as for odd-$A$ isotopes in the heavy-element region, the $\alpha$-decay reduced widths range from those of even-even nuclei down to much smaller values.