Investigation of Thulium α Emitters; New Isotopes, 155Tm and 156Tm

In the present investigation the α decay of the previously unknown isotopes ¹⁵⁵Tm and ¹⁵⁶Tm was observed. These thulium nuclides were produced by bombarding targets of ¹⁴⁴Sm and ¹⁴⁷Sm with 103-MeV ¹⁴N ions accelerated in the Oak Ridge isochronous cyclotron. Supplementary data were also obtained by irradiating ¹⁶²Er with 110-MeV ³He 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 α activity. Three new α 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) ¹⁵⁵Tm, E_α=4.45+/-0.01 MeV, T₁₂=39+/-3 sec; (2) ¹⁵⁶Tm (low-spin isomer), E_α=4.23+/-0.01 MeV, T₁₂=80+/-3 sec; and (3) ¹⁵⁶Tm (high-spin isomer), E_α=4.46+/-0.01 MeV, T₁₂=19+/-3 sec.