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

In order to study the Gamow-Teller (GT) transitions from the T z = +1 nucleus 42 Ca to the T z = 0 nucleus 42 Sc, where T z is the z component of isospin T we performed a ( p, n )-type ( 3 He , 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, J π = 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 (RPA) calculations including an attractive isoscalar (IS) 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 (LESGT) state.” It is expected that low-lying J π = 1 + GT states have T = 0 in the T z = 0 nucleus 42 Sc. On the other hand, 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 42 Ca. Comparing the 42 Ca( 3 He , t ) 42 Sc and 42 Ca( p, p ′ ) spectra measured at 0 ◦ , candidates for T = 1 GT states could be found in the 10 − 12 MeV region of 42 Sc. They were all weakly excited. The mass dependence of the GT strength distributions in Sc isotopes is also discussed.