Analogous Gamow-Teller and M1 transitions in ²⁶Mg, ²⁶Al, and ²⁶Si

Under the assumption that isospin T is a good quantum number, isobaric analog states and various analogous transitions among these states are expected in isobars with mass number A. Strengths of analogous Gamow-Teller (GT) and M1 transitions have been compared for an A = 26 isobar triplet with T_z= +1, 0, and -1, where T_z is defined by T_z = (N-Z)/2. The T_z= + 1 → 0 GT transitions from the J^π = 0 ⁺ground state of ²⁶Mg to excited J^π= 1 ⁺ states in ²⁶Al were studied up to the excitation energy (E_x) of 9 MeV by using a high energy-resolution (³He,t) reaction. The distribution of observed GT strengths was well reproduced in a shell-model calculation. The isospin symmetric T_z= - 1 → 0 GT transitions can be studied in the ²⁶Si β decay. The GT strengths from the (³He,t) reaction were in good agreement with the β-decay values evaluated up to E_x = 2.7-MeV states in ²⁶Al. The GT strengths were further compared with the strengths of analogous M1 γ transitions in ²⁶Al, i.e., the M1 transitions from the excited 1⁺ states to the isobaric analog state of the ²⁶Mg ground state in ²⁶Al. Through this comparison, contributions of spin and orbital terms in these Ml transitions were studied. The GT and M1 strengths as well as spin and orbital contributions in M1 strengths are interpreted by both the shell model and the particle-rotor model assuming a correlated proton and neutron pair around a ²⁶Mg core. The isospin T of each excited 1⁺ state in ²⁶Al was also studied by examining the existence or the nonexistence of the analog 1 ⁺ state in the T_z = 1 ²⁶Mg nucleus.