Spectroscopic studies of two $\beta$ rays from ${}^{100}\mathrm{Mo}$ are shown to be of potential interest for investigating both the Majorana $\nu$ mass by neutrinoless double $\beta$ decay $\left(0\nu \beta \beta \right)$ and low energy solar $\nu$'s by inverse $\beta$ decay. With a multiton ${}^{100}\mathrm{Mo}$ detector, coincidence studies of correlated $\beta \beta$ from $0\nu \beta \beta$, together with the large $Q$ value ( $Q_{\beta \beta }$), permit identification of the $\nu$-mass term with a sensitivity of $\sim 0.03\mathrm{eV}$. Correlation studies of the inverse $\beta$ decay and the successive $\beta$ decay of ${}^{100}\mathrm{Tc}$, together with the large capture rates for low energy solar $\nu$'s, make it possible to detect, in real time, individual low energy solar $\nu$ in the same detector.

Application of a large-volume NaI scintillator to search for the cold dark matter (DM) in the Universe is discussed. A low-noise (∼5 keV) and low-background (∼5/keV kg day) were achieved even with a large (36.5 kg) NaI scintillator. Light output efficiencies for ${}_{}{}^{23}{}_{}{}^{}\mathrm{Na}$ and ${}_{}{}^{127}{}_{}{}^{}\mathrm{I}$ were measured as 0.4±0.2 and 0.05±0.02, in the recoil energy range of 5–100 keV and 40–300 keV, respectively. NaI is shown to have merits of a large volume, large scattering cross sections for dark matter, and of 100% abundance of finite-spin nucleus, but to have the demerits of a small form factor due to the finite-size effect of ${}_{}{}^{127}{}_{}{}^{}\mathrm{I}$ at the presently measured energy region.

Spin-dipole (SD) nuclear matrix elements (NMEs) M±(SD2) for unique first forbidden β±2−→0+ ground-state-to-ground-state transitions are studied by using effective microscopic two-nucleon interactions in realistic single-particle model spaces. The observed values of the NMEs Mexp±(SD2) are compared with the values of the single-quasiparticle NMEs Mqp±(SD2) without nucleon spin–isospin (στ) correlation and the QRPA NMEs MQRPA±(SD2) with the στ correlation. The observed SD matrix elements are found to be reduced by the factor k≈0.2 with respect to Mqp±(SD2) and by the factor kNM≈0.5 with respect to MQRPA±(SD2). We then infer that the SD NME is reduced considerably partly by the nucleon στ correlations and partly by other non-nucleonic and nucleonic correlations which are not explicitly included in the QRPA. Impact of the found reduction factors on the magnitudes of the NMEs involved in neutrino-less double beta decays and astro-neutrino interactions are discussed

The Gamow-Teller (GT) strength distribution has been investigated in a high-resolution (${}^3$He,$t$) charge-exchange experiment on the double-beta ($\beta \beta$) decaying nucleus ${}^{100}$Mo. The experiment was carried out at the Osaka University Research Center for Nuclear Physics (RCNP) with a 420 MeV incident ${}^3$He beam and the Grand Raiden magnetic spectrometer. A final-state energy resolution of 33 keV was achieved. It was found that nearly the entire low-energy (up to $\approx 4$ MeV) ${\mathrm{GT}}^{-}$ strength is concentrated in the ground-state transition to ${}^{100}$Tc, which is interpreted as a result of the particular neutron and proton sub-shell occupancies. The impact on the matrix elements for the two-neutrino double-beta ($2\nu \beta \beta$) decay and the use of ${}^{100}$Mo as a neutrino detection material is discussed.

Ordinary muon capture (OMC) on enriched ${}^{100}\mathrm{Mo}$ isotopes was studied for the first time to investigate neutrino nuclear responses for neutrinoless double-$\beta$ decays and supernova neutrino nuclear interactions. Muon capture on ${}^{100}\mathrm{Mo}$ proceeds mainly as ${}^{100}\mathrm{Mo}(\mu ,xn){}^{100-x}\mathrm{Nb}$ with $x$ being the number of neutrons emitted from negative muon capture. The Nb isotope mass distribution was obtained by measuring delayed $\gamma$ rays from radioactive ${}^{100-x}\mathrm{Nb}$. By using the neutron emission model after the muon capture, the neutrino response (the strength distribution) for OMC was derived. The OMC strength distribution shows a giant resonance (GR) at the peak energy around 11–14 MeV, suggesting the concentration of the OMC strength at the muon GR region.

Nuclear matrix elements (NMEs) for double beta decays (DBDs) are crucial for studying neutrino mass and neutrino properties of particle physics interest. The major components of the DBD NMEs are axial-vector NMEs associated with spin isospin transition operators. In this work it is discussed how absolute values for the NMEs are reduced (quenched) with respect to those for the model NMEs. The reduction is discussed in terms of the renormalized (quenched) axial-vector coupling $g_A^{eff}$ in unit of g_A for free nucleon. Recent experimental studies at RCNP on single and double β NMEs relevant to DBDs are briefly discussed. The axial-vector single β NMEs are shown to be reduced by the renormalization (quenching factor) factor $g_A^{eff}/g_A\approx$ 0.5–0.6 with respect to the QRPA (Quasi particle Random Phase Approximation) model NMEs due to non-nucleonic and nuclear medium effects which are not explicitly included in the model. Two-neutrino axial-vector DBD NMEs are reduced much with respect to the QP (Quasi Particle) NMEs, and are reproduced by the FSQP (Fermi Surface Quasi Particle) model NMEs. Impact of the reduction of the axial-vector NMEs on the DBD NMEs and the DBD experiments is briefly discussed.