High-resolution study of Gamow-Teller excitations in the Ca 42 (He 3,t) Sc 42 reaction and the observation of a "low-energy super-Gamow-Teller state"

To study the Gamow-Teller (GT) transitions from the Tz=+1 nucleus Ca42 to the Tz=0 nucleus Sc42, where Tz is the z component of isospin T, we performed a (p,n)-type (He3,t) charge-exchange reaction at 140 MeV/nucleon and scattering angles around 0. With an energy resolution of 29 keV, states excited by GT transitions (GT states) could be studied accurately. The reduced GT transition strengths B(GT) were derived up to the excitation energy of 13 MeV, assuming the proportionality between the cross sections at 0 and B(GT) values. The main part of the observed GT transition strength is concentrated in the lowest 0.611-MeV, Jπ=1+ GT state. All the other states at higher energies are weakly excited. Shell-model calculations could reproduce the gross feature of the experimental B(GT) distribution, and random-phase-approximation calculations including an attractive isoscalar interaction showed that the 0.611-MeV state has a collective nature. It was found that this state has all of the properties of a "low-energy super-Gamow-Teller state." It is expected that low-lying Jπ=1+ GT states have T=0 in the Tz=0 nucleus Sc42. However, T=1 states are situated in a higher energy region. Assuming an isospin-analogous structure in A=42 isobars, analogous T=1, 1+ states are also expected in Ca42. Comparing the Ca42(He3,t)Sc42 and Ca42(p,p′) spectra measured at 0, candidates for T=1 GT states could be found in the 10-12-MeV region of Sc42. They were all weakly excited. The mass dependence of the GT strength distributions in Sc isotopes is also discussed.