Development of beta-gamma spectroscopy for precise measurements of beta-decay branching ratios

The $ft$ values for superallowed beta decays yield the most precise value of $V_{ud}$, the up-down element of CKM matrix. Recently, the $ft$ values for the mirror decays with $T$ = 1/2 can be used to extract precise weak interaction information. In these studies, the precise values of branching ratio as well as $Q_{EC}$ and half -life are required. However, it is very challenging to determine the precise branching ratio for the beta-decay feeding to not only excited states but also the ground state. In this determination, the knowledge of a very precise absolute gamma-ray detection efficiency and the counting system of total number of beta decays are required. Therefore, we developed a new system for beta-gamma spectroscopy with high-energy secondary beam. The high-energy secondary beam including beta emitters was implanted in the center of a 6-mm-thick GSO scintillator. The beta rays were measured by the GSO scintillator, which ensured the high beta-ray detection efficiency of nearly 100\%. The gamma rays were measured by four clover type germanium detectors installed around the GSO scintillator. The absolute gamma-ray detection efficiency was determined by using other superallowed beta emitters with known gamma-ray intensity in the same condition as the nucleus of interest.