Extracting GT matrix elements from (p,n) cross sections

Abstract

Nuclear transition matrix elements for the spin-isospin (Gamow-Teller) transition operator have been of considerable interest for several decades as indicators of nuclear structure. Knowledge of these matrix elements is also of vital importance in astrophysical calculations in connection with the general problem of nucleosynthesis, and, more specifically, in connection with the evolution of supernovae. Furthermore, knowledge of these matrix elements is required for the calculation of neutrino absorption cross sections for nuclei used as neutrino detectors. The most precise measures of spin-isospin matrix elements come from beta-decay lifetimes and endpoints with additional information, in some cases of mixed Fermi-GT transitions, from angular correlation measurements. Unfortunately, most transitions of special interest are not energetically accessible to beta decay. It is also well known that the nuclear shell model fails badly for calculating these matrix elements. The best alternative we have is to use the (p,n) reaction which was suggested at least 30 years ago. [1] The (p,n) reaction connects exactly the same nuclear states as beta decay, but to some extent different operators can come into play so that (p,n) cross sections are not exactly proportional to beta decay matrix elements. Considerable experimental effort has now been put into the problem of determining how accurately GT matrix elements can be deduced from (p,n) reaction data which in some cases includes polarization transfer measurements. The body of systematic measurements shows a good correlation between beta decay and (p,n) determinations of matrix elements for transitions between spin-0 and spin-1 states, but problems still exist for odd-A nuclei where the initial and final states necessarily have non-zero spins. Additional measurements are still in progress.