Improved measurement of the half-life of the Jπ=8- nuclear isomer Eu 152m2

The standard γ-ray energy calibration source Eu152 is well known based on the 13.5 y decay of its ground state. However, in addition to this decay Eu152 also has two relatively long-lived isomeric states: a 9 h Jπ=0- state at E∗=46 keV and a 96 min Jπ=8- state at E∗=148 keV. Here we report a new measurement of the half-lives of both of these isomeric states. Excited states in Eu152 were populated following the Sm154(p,3n) reaction using a 25 MeV proton beam from the K-150 cyclotron at the Cyclotron Institute of Texas A&M University. Post irradiation, γ rays from the de-excitation of the long lived isomeric states were measured using the six BGO shielded high-purity germanium (HPGe) clover detectors that are part of the STARLiTeR array. The half-life of the Jπ=8- isomer Eu152m2 was obtained by measuring the decrease in intensity of the 90 keV γ ray from the cascade to the ground state. The half-life of this state was measured to be 95.8(4) min which is in agreement with and significantly more precise than the previously measured value of 96(1) min. In a manner similar to the ground state the second long-lived isomer Eu151m1, the Jπ=0- state at 46 keV, β decays to excited states in Gd152 and Sm152. The half-life of this state was measured to be 9.39(7) h using five γ-ray transitions.