ISOVECTOR STRETCHED-STATE EXCITATIONS IN THE NE-20,MG-24,SI-28, AND S-32(P,N) REACTIONS AT 136 MEV

Isovector stretched-state excitations were studied in the (p,n) reaction at 136 MeV on the self-conjugate s-d shell nuclei Ne-20, Mg-24, Si-28, and S-32. The measurements were performed in three separate experiments using the beam-swinger neutron time-of-flight facility at the Indiana University Cyclotron Facility. Neutrons were detected in large-volume plastic scintillation detectors located in three detector stations at 0-degrees, 24-degrees, and 45-degrees with respect to the undeflected beam line; the flight paths varied from 75 to 134 m. Overall time resolutions of about 825 ps provided energy resolutions of about 320 keV in the first two stations and about 425 keV in the third station. Both (d5/2,d5/2(-1))5+ 0HBAR-omega and (f7/2,d5/2(-1))6- 1HBAR-omega stretched-state excitations are observed in the (p,n) reaction on Ne-20, Mg-24, and S1-28. For the (p,n) reaction on S-32, only the 6- strength is observed. The states are identified by comparison of the extracted angular distributions with distorted-wave impulse-approximation calculations and by comparison with known analog states observed in inelastic scattering. In the Ne-20 and Mg-24(p,n) reactions, the majority of the 5+ strength is observed in single states at low excitation energy; the strengths are described well by full s-d shell-model calculations. A weak 5+ state is observed in the Si-28(p,n)P-28 reaction that corresponds to the strength expected if there is approximately 1/3 of a hole in the d5/2 proton orbital. The 6- 1HBAR-omega strength is fragmented in all four reactions. Less than 60% of the strength expected in the extreme single-particle-hole model is observed in each case. The observed 6- strengths and distributions in the Si-28 and S-32(p,n) reactions are described well by large-basis shell-model calculations.