High-resolution study of T-z =+1 -> 0 Gamow-Teller transitions in the Mg-26(He-3,t)Al-26 reaction

In order to study the T-z = +1 -> 0 Gamow-Teller (GT) transitions in the mass A = 26 system, a charge-exchange reaction Mg-26(He-3,t)Al-26 was performed at an incident energy of 140 MeV/nucleon and scattering angle at and near 0 degrees, where T-z is the z component of isospin T defined by (N - Z)/2. In this (p, n)-type reaction, it is expected that GT states with T = 0, 1, and 2 are excited. An energy resolution of Delta E = 23 keV allowed us to study many discrete states. Most of the prominent states showed 0 degrees-peaked angular distributions, which suggested that they are the states excited by Delta L = 0 GT transitions. Candidates of GT states were studied up to an excitation energy E-x = 18.5 MeV. The reduced GT transition strengths, B(GT), were derived assuming the proportionality between cross sections and B(GT) values. Standard B(GT) values were obtained form the Si-26 beta decay, where the mirror symmetry of T-z = +/- 1 -> 0 GT transitions was assumed. The GT strength, as a whole, is divided in two energy regions: the region of up to 8.5 MeV and the higher-energy region of 8.5-12.8MeV, where the strength in the latter region distributed like a resonance. The obtained GT strength distribution is compared with the results of random phase approximation calculations. The T = 2 GT states are expected in the region E-x >= 13.5 MeV. By comparing with the results of (n, p)-type Mg-26(d, He-2)Na-26 and Mg-26(t, He-3)Na-26 reactions, the isospin symmetry of T = 2 GT states is discussed. Owing to the high-energy resolution, we could study the decay widths Gamma for the states in the E-x > 9 MeV region. The T = 2 state at 13.592 MeV is not noticeably wider than the experimental energy resolution. The narrow width of the state is explained in terms of isospin selection rules.