In the present investigation the $\alpha$ decay of the previously unknown isotopes ${}_{}{}^{155}{}_{}{}^{}\mathrm{Tm}$ and ${}_{}{}^{156}{}_{}{}^{}\mathrm{Tm}$ was observed. These thulium nuclides were produced by bombarding targets of ${}_{}{}^{144}{}_{}{}^{}\mathrm{Sm}$ and ${}_{}{}^{147}{}_{}{}^{}\mathrm{Sm}$ with 103-MeV ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ ions accelerated in the Oak Ridge isochronous cyclotron. Supplementary data were also obtained by irradiating ${}_{}{}^{162}{}_{}{}^{}\mathrm{Er}$ with 110-MeV ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ particles. The experimental apparatus used in this study is based on the idea of stopping recoil products in helium gas. The stopped recoils are then swept out through a small orifice and deposited on a wheel which, after bombardment, conveys the collected radioactivity to a position in front of a Si(Au) detector for the assay of $\alpha$ activity. Three new $\alpha$ emitters were observed. Their decay characteristics and mass assignments (made on the basis of yield-curve measurements and parent-daughter relationships) are as follows: (1) ${}_{}{}^{155}{}_{}{}^{}\mathrm{Tm}$, $E_{\alpha }=4.45\mathrm{}\pm 0.01$ MeV, $T_{\frac{1}{2}}=39\mathrm{}\pm 3$ sec; (2) ${}_{}{}^{156}{}_{}{}^{}\mathrm{Tm}$ (low-spin isomer), $E_{\alpha }=4.23\mathrm{}\pm 0.01$ MeV, $T_{\frac{1}{2}}=80\mathrm{}\pm 3$ sec; and (3) ${}_{}{}^{156}{}_{}{}^{}\mathrm{Tm}$ (high-spin isomer), $E_{\alpha }=4.46\mathrm{}\pm 0.01$ MeV, $T_{\frac{1}{2}}=19\mathrm{}\pm 3$ sec.

• Received 19 August 1971

DOI:https://doi.org/10.1103/PhysRevC.4.2223