arXiv subjects

K. Muto

Publications and source records attributed to K. Muto.

4 recordsLinked to original sources

arXiv2019

First observation of 20B and 21B

The most neutron-rich boron isotopes 20B and 21B have been observed for the first time following proton removal from 22N and 22C at energies around 230 MeV/nucleon. Both nuclei were found to exist as resonances which were detected through their decay into 19B and one or two neutrons. Two-proton removal from 22N populated a prominent resonance-like structure in 20B at around 2.5 MeV above the one-neutron decay threshold, which is interpreted as arising from the closely spaced 1-,2- ground-state doublet predicted by the shell model. In the case of proton removal from 22C, the 19B plus one- and two-neutron channels were consistent with the population of a resonance in 21B 2.47+-0.19 MeV above the two-neutron decay threshold, which is found to exhibit direct two-neutron decay. The ground-state mass excesses determined for 20,21B are found to be in agreement with mass surface extrapolations derived within the latest atomic-mass evaluations.

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arXiv2017

Single-neutron knockout from $^{20}\textrm{C}$ and the structure of $^{19}\textrm{C}$

The low-lying unbound level structure of the halo nucleus $^{19}\textrm{C}$ has been investigated using single-neutron knockout from $^{20}\textrm{C}$ on a carbon target at 280 MeV/nucleon. The invariant mass spectrum, derived from the momenta of the forward going beam velocity $^{18}\textrm{C}$ fragment and neutrons, was found to be dominated by a very narrow near threshold ($E_\textrm{rel}$ = 0.036(1) MeV) peak. Two less strongly populated resonance-like features were also observed at $E_\textrm{rel}$ = 0.84(4) and 2.31(3) MeV, both of which exhibit characteristics consistent with neutron $p$-shell hole states. Comparisons of the energies, measured cross sections and parallel momentum distributions to the results of shell-model and eikonal reaction calculations lead to spin-parity assignments of $5/2^+_1$ and $1/2^-_1$ for the levels at $E_x$ = 0.62(9) and 2.89(10) MeV with $S_n$ = 0.58(9) MeV. Spectroscopic factors were also deduced and found to be in reasonable accord with shell-model calculations. The valence neutron configuration of the $^{20}\textrm{C}$ ground state is thus seen to include, in addition to the known $1s^2_{1/2}$ component, a significant $0d^2_{5/2}$ contribution. The level scheme of $^{19}\textrm{C}$, including significantly the $1/2^-_1$ cross-shell state, is well accounted for by the YSOX shell-model interaction developed from the monopole-based universal interaction.

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arXiv1996

Realistic shell model calculation of $2νββ$ nuclear matrix elements and role of shell structure in intermediate states

We discuss two conditions needed for correct computation of $2νββ$ nuclear matrix-elements within the realistic shell-model framework. An algorithm in which intermediate states are treated based on Whitehead's moment method is inspected, by taking examples of the double GT$^+$ transitions $\mbox{$^{36}$Ar}\rightarrow\mbox{$^{36}$S}$, $\mbox{$^{54}$Fe}\rightarrow\mbox{$^{54}$Cr}$ and $\mbox{$^{58}$Ni} \rightarrow\mbox{$^{58}$Fe}$. This algorithm yields rapid convergence on the $2νββ$ matrix-elements, even when neither relevant GT$^+$ nor GT$^-$ strength distribution is convergent. A significant role of the shell structure is pointed out, which makes the $2νββ$ matrix-elements highly dominated by the low-lying intermediate states. Experimental information of the low-lying GT$^\pm$ strengths is strongly desired. Half-lives of $T^{2ν}_{1/2}({\rm EC}/{\rm EC}; \mbox{$^{36}$Ar}\rightarrow\mbox{$^{36}$S})=1.7\times 10^{29}\mbox{yr}$, $T^{2ν}_{1/2}({\rm EC}/{\rm EC};\mbox{$^{54}$Fe}\rightarrow \mbox{$^{54}$Cr})=1.5\times 10^{27}\mbox{yr}$,$T^{2ν}_{1/2}({\rm EC} /{\rm EC};\mbox{$^{58}$Ni}\rightarrow\mbox{$^{58}$Fe})=6.1\times 10^{24}\mbox{yr}$and $T^{2ν}_{1/2}(β^+/{\rm EC};\mbox{$^{58}$Ni} \rightarrow\mbox{$^{58}$Fe})=8.6\times 10^{25}\mbox{yr}$ are obtained from the present realistic shell-model calculation of the nuclear matrix-elements.

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arXiv1995

Up-Down Quark Mass Difference Effect in Nuclear Many-Body Systems

A charge-symmetry-breaking nucleon-nucleon force due to the up-down quark mass difference is evaluated in the quark cluster model. It is applied to the shell-model calculation for the isovector mass shifts of isospin multiplets and the isospin-mixing matrix elements in 1s0d-shell nuclei. We find that the contribution of the quark mass difference effect is large and agrees with experiment. This contribution may explain the Okamoto-Nolen-Schiffer anomaly, alternatively to the meson-mixing contribution, which is recently predicted to be reduced by the large off-shell correction.

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