High-resolution 96Zr(3He,t) experiment and the matrix element for double-β decay

The Gamow-Teller (GT) strength distribution has been investigated in a high-resolution (3He,t) charge-exchange experiment on the double-beta (ββ) decaying nucleus 96Zr. The experiment was carried out at the Osaka University Research Center for Nuclear Physics (RCNP) with a 420 MeV incident 3He beam and the Grand Raiden magnetic spectrometer. A final-state energy resolution of 32 keV was achieved. A large fraction of the low-energy GT− strength is identified in a single state at 694 keV. A similar observation has been made in a 96Mo(d,2He)96Nb experiment at 120 keV resolution for the GT+ strength. It is found that this single state makes up nearly 100% of the nuclear matrix element for the 96Zr two-neutrino double-beta (2νββ) decay.