On the conversion of charge-exchange cross sections to “GT” strength: Anomalies in odd- A nuclei

Abstract

Gamow–Teller (GT) transitions are the most common weak interaction processes of spin–isospin (στ) type in atomic nuclei. They are of interest not only in nuclear physics but also in astrophysics; they play an important role in supernovae explosions and nucleosynthesis. The direct study of weak decay processes, however, gives relatively limited information about GT transitions and the states excited via GT transitions (GT states); β decay can only access states at excitation energies lower than the decay Q-value, and neutrino-induced reactions have very small cross-sections. However, one should note that β decay has a direct access to the absolute GT transition strengths B(GT) from a study of half-lives, Qβ-values and branching ratios. They also provide information on GT transitions in nuclei far-from-stability. Studies of M1γ transitions provide similar information. In contrast, the complementary charge-exchange (CE) reactions, such as the (p,n) or (3He, t) reactions at intermediate beam energies and 0°, can selectively excite GT states up to high excitation energies in the final nucleus. It has been found empirically that there is a close proportionality between the cross-sections at 0° and the transition strengths B(GT) in these CE reactions. Therefore, CE reactions are useful tools to study the relative values of B(GT) strengths up to high excitation energies. In recent (3He, t) measurements, one order-of-magnitude improvement in the energy resolution has been achieved. This has made it possible to make one-to-one comparisons of GT transitions studied in CE reactions and β decays. Thus GT strengths in (3He, t) reactions can be normalised by the β-decay values. In addition, comparisons with closely related M1 transitions studied in γ decay or electron inelastic scattering [(e,e′)], and furthermore with “spin” M1 transitions that can be studied by proton inelastic scattering [(p,p′)] have now been made possible. In these comparisons, the isospin quantum number T and associated symmetry structure in the same mass A nuclei (isobars) play a key role. Isospin symmetry can extend our scope even to the structures of unstable nuclei that are far from reach at present unstable beam factories.