Branching-Ratio Measurements for Superallowed β Emitters at NIRS-HIMAC*

The ft values for the superallowed  0⁺ → 0⁺ Fermi β  decays provide valuable information on the CVC hypothesis, the unitarity of CKM matrix, and the isospin-symmetry breaking. The  ft values for the 13 superallowed β emitters have been already determined very precisely. However, the emitters of ¹⁸Ne and ³⁰S  have not been determined yet due to large uncertainties of the branching ratios. Because they have the β feeding to both of  the excited states including the 0⁺ state and the ground state, we have to measure not relative gamma intensities but absolute ones. The new  precise branching-ratio measurement system is being developed. Our test measurement was carried out at NIRS-HIMAC. The secondary beam including ¹⁸Ne(³⁰S) was produced with the projectile fragmentation reaction of ²⁰Ne(³⁶Ar) on a polyethylene target. The beam was separated and identified by passing through the secondary beam line and was implanted in a 6-mm-thick GSO active stopper surrounded by four clover Ge detectors. The β particles and γ rays were detected by the GSO stopper and the Ge detectors, respectively. We will report the current status of our results and a future plan to improve performance with movable active stoppers.