GAMOW-TELLER STRENGTH IN THE (P,N) REACTION AT 136 MEV ON NE-20, MG-24, AND SI-28

Gamow-Teller (GT) strength distributions were studied in the (p,n) reaction at 136 MeV on the self-conjugate s-d shell nuclei Ne-20, Mg-24, and Si-28. 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 delta-l = 0 contributions in transitions to discrete final states and also in the background and continuum. The 0-degrees, 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 Mg-24 is described well, but the distributions for Ne-20 and Si-28 are described poorly. The total B(GT) strength observed in discrete states (up to 12 MeV of excitation) for each reaction is 65 +/- 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 approximately 20 MeV), the entire amount predicted may be observed. These results are consistent with that observed in other light nuclei.