Gamow-Teller strength in the (p,n) reaction at 136 MeV on 20Ne, 24Mg, and 28Si.

Abstract

Gamow-Teller (GT) strength distributions were studied in the (p,n) reaction at 136 MeV on the self-conjugate s-d shell nuclei $^{20}\mathrm{Ne}$,${\mathrm{}}^{24}$Mg, and $^{28}\mathrm{Si}$. The measurements were performed in two separate experiments with the beam-swinger neutron time-of-flight facility at the Indiana University Cyclotron Facility. The flight paths were 91 and 131 m, respectively, for the two experiments. The neutrons were detected in large-volume plastic-scintillation detectors. The overall time resolutions were about 825 ps; this provided energy resolutions from 300 to 400 keV. GT strength was identified as \ensuremath{\Delta}l=0 contributions in transitions to discrete final states and also in the background and continuum. The 0\ifmmode^\circ\else\textdegree\fi{}, \ensuremath{\Delta}l=0 cross sections were converted to B(GT) units using a ``universal'' conversion formula calibrated to (p,n) reactions on other even-even s-d shell nuclei. The resulting B(GT) distributions were compared with full s-d shell-model predictions. The distribution for $^{24}\mathrm{Mg}$ is described well, but the distributions for $^{20}\mathrm{Ne}$ and $^{28}\mathrm{Si}$ are described poorly. The total B(GT) strength observed in discrete states (up to 12 MeV of excitation) for each reaction is 65\ifmmode\pm\else\textpm\fi{}10 % of that predicted. If one considers B(GT) strength observed in the continuum above a calculated quasi-free-scattering background, the strength increases to 70--100 % of that predicted. If one considers B(GT) strength in an analysis of the full continuum (up to \ensuremath{\sim}20 MeV), the entire amount predicted may be observed. These results are consistent with that observed in other light nuclei.