Suppression of Gamow-Teller and M1 transitions in deformed mirror nuclei 25Mg and 25Al

The mirror nuclei ²⁵Mg and ²⁵Al are expected to have very similar structures. The Gamow-Teller (GT) transitions from the ground state of ²⁵Mg to the excited states in ²⁵Al were studied by high-resolution measurements of the ²⁵Mg( ) charge-exchange reaction at and at 140 MeV/nucleon. Assuming the usual selection rule for the spin-isospin-type GT transitions, the states with , and 7/2 ⁺ should be excited. However, of the more than ten states with these values below 6 MeV excitation energy, only the 5/2 ⁺ ground state and the 7/2 ⁺ , 1.613 MeV state in ²⁵Al were strongly populated, while all other states were strongly suppressed. The analysis of M1 transitions in ²⁵Mg also suggested a very similar feature for the analogous M1 transitions. Both ²⁵Mg and ²⁵Al are known to be largely deformed, and most low-lying states can be interpreted in terms of one-particle quantum numbers in the deformed potential and the associated rotational spectra. The observed suppression can be explained in terms of the K quantum number selection rules that are inherent to axially deformed nuclei.-1Received: 18 July 2003, Published online: 18 December 2003PACS: 21.10.Re Collective levels - 21.60.Ev Collective models - 25.55.Kr Charge-exchange reactions - 27.30. + t H. Fujita: Present address: Research Center for Nuclear Physics, Osaka University, Ibaraki, Osaka 567-0047, Japan.J. Kamiya: Present address: Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195, Japan.T. Wakasa: Present address: Department of Physics, Kyushu University, Higashi, Fukuoka 812-8581, Japan.