J. W. Jury

Photoneutron angular distribution of 19F

Abstract Photoneutron time-of-flight spectra from the reaction 19 F(γ, n 0 ) 18 F were measured between 48° and 139° using 10 m flight paths over the excitation energy range from 15–25 MeV. The measured values of the normalized Legendre a 1 and a 3 coefficients are very small or close to zero over the energy region studied, indicating dominance of E1 absorption in this region. A simple modeldependent analysis of the a 2 coefficient showed that the likely reaction mechanisms are mainly s → p and d → p single-particle transitions of channel spin 1 2 . A comparison of the present angleintegrated ground-state cross section with the (γ, n tot ) work of Veyssiere et al . indicates that decays to excited states in 18 F are much preferred (typically by a factor of 5) over the ground-state channel. The 19 F(γ, n 0 ) cross section shows reasonable agreement in structure and magnitude with the 19 F(γ, p 0 ) cross section of Kerkhove et al . as well as with the 18 O(γ, n 0 ) data of Jury et al . (although some discrepancies are seen at 16 MeV and above 23 MeV).

Photoneutron angular distributions for 32S

Abstract Angular distributions of the energy spectra of photoneutrons emitted in the reaction 32S(γ, n0)31S were measured over the range of excitation energies from 18 to 29 MeV. Coefficients of Legendre polynomials fitted to the data gave no indication of photon absorption other than through electric dipole transitions, except at energies above 27 MeV. The measured value of the normalized Legendre coefficient a2 is − 1.0 ± 0.1 across most of the giant resonance region supports a simple shell model interpretation that the 2 s 1 2 → 2 p 3 2 single-particle transition (of channel spin 0) is the dominant mechanism for photoabsorption followed by neutron decay to the ground state of 31S. Comparison of the present results for the ground state channel with the total photoneutron cross section shows that up to an excitation energy of 18.5 MeV nearly all neutron-emitting transitions proceed to the ground state of 31S even though many channels for excited states are open. The ground state photoneutron channel exhausts about 6% of the TRK sum rule.

{sup 13}C({ital e},{ital p}{sub 0,1}){sup 12}B differential cross section

The differential cross sections for the reactions {sup 13}C ({gamma},{ital p}{sub 0}) and {sup 13}C ({gamma},{ital p}{sub 1}) have been measured in the energy range 21--26 MeV. Analysis of the present data and the {sup 13}C ({gamma},{ital p}) cross section reveals decay of the giant dipole resonance at 23 MeV, via the {ital p}{sub 4} channel. The valence neutron does not appear to interact strongly with the {sup 12}C core. Isospin splitting effects are observed.

Giant dipole resonance in sup 17 O observed with the (. gamma. , p ) reaction

The giant dipole resonance (GDR) in {sup 17}O has been studied with the reaction {sup 17}O({gamma},{ital p}){sup 16}N from {ital E}{sub {gamma}}=13.50 to 43.15 MeV using quasimonoenergetic photons. The measured cross section shows major peaks at 15.1, 18.1, 19.3, 20.3, 22.2, 23.1, 24.4, and {similar to}26.5 MeV. The intermediate structure in the main GDR region is remarkably similar to that observed in {sup 16}O, indicating that the valence neutron outside the doubly magic {sup 16}O core perturbs the core-excited states minimally, in support of the weak-coupling hypothesis. We correlate the trends in GDR structure of {sup 16,17,18}O with changes in ground-state properties related to static deformation. The ({gamma},{ital p}) reaction selects strength predominantly from two-particle--one-hole configurations formed via {ital E}1 transitions from the 1{ital p}{sub 1/2} subshell; comparison with other reactions (photoneutron and radiative capture) provides information on the microscopic structure of {ital E}1 states. The peak observed near threshold at 15.1 MeV is remarkably strong; we infer that it originates from photoexcitation of a few narrow {ital T}=3/2 states and that {ital M}1 transitions contribute to the measured strength. The total absorption cross section is approximated by summing the ({gamma},{ital p}) cross section and the previously published photoneutron crossmorexa0» section; comparison with particle-hole shell-model calculations shows that the main cross-section features, including isospin distribution, are well predicted. Evidence is found for isospin splitting in {sup 17}O. Systematics of the integrated cross sections for the carbon, nitrogen, and oxygen isotopes are delineated.«xa0less