A broad excited state was observed in 6He with energy Ex=5+-1 MeV and width Gamma=3+-1 MeV, following the reaction 7Li(γ,p)6He. The state is consistent with a number of broad resonances predicted by recent cluster model calculations. The well-established reaction mechanism, combined with a simple and transparent analysis procedure confers considerable validity to this observation.

A broad excited state was observed in 6-He with energy E_x = 5 +/- 1 MeV and width Gamma = 3 +/- 1 MeV, following the reaction Li-7(gamma,p)He-6. The state is consistent with a number of broad resonances predicted by recent cluster model calculations. The well-established reaction mechanism, combined with a simple and transparent analysis procedure confers considerable validity to this observation.

A broad excited state was observed in 6-He with energy E_x = 5 +/- 1 MeV and width Gamma = 3 +/- 1 MeV, following the reaction Li-7(gamma,p)He-6. The state is consistent with a number of broad resonances predicted by recent cluster model calculations. The well-established reaction mechanism, combined with a simple and transparent analysis procedure confers considerable validity to this observation.

Using the corpus of data on the (gamma,p), (gamma, n), (gamma, 2n), and (gamma, n0) cross sections of nine light nuclei (C-13, C-14, N-15, O-17, O-18, Mg-25, Mg-26, Si-29, and Si-30), the T> and T< isospin components of the giant dipole resonance have been separated. The relative strengths of these components have been extracted, together with the energy differences between the centroids of the components. The ratio of the T> energy-weighted integrated cross section to the total cross section is somewhat better represented by the simple geometric factor 1/(T0 + 1), where T0 is the isospin of the ground state of the excited nucleus, than by a more complete expression which takes into account dynamical effects. If the energy difference between the centroids is represented by U*(T0 + 1)/A, the average value of U* for the three p shell nuclei is found to be 57 MeV, while that for the six s-d shell nuclei is 93 MeV.

A photon tagging facility has been constructed for use with the c.w. electron beam at the Saskatchewan Accelerator Laboratory (SAL). The facility consists of a “clam-shell” type magnetic spectrometer with a 62 channel plastic scintillator focal plane detector allowing a post-bremsstrahlung electron energy resolution of 1%. Specialized focal plane and coincidence electronics have been designed and constructed at SAL and allows experiments to be performed with average tagged photon rates of greater than 1 × 108/s.

The giant dipole resonance (GDR) in $^{17}\mathrm{O}$ has been studied with the reaction $^{17}\mathrm{O}$(\ensuremath{\gamma},p${)}^{16}$N from ${\mathit{E}}_{\ensuremath{\gamma}}$=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 \ensuremath{\sim}26.5 MeV. The intermediate structure in the main GDR region is remarkably similar to that observed in $^{16}\mathrm{O}$, indicating that the valence neutron outside the doubly magic $^{16}\mathrm{O}$ core perturbs the core-excited states minimally, in support of the weak-coupling hypothesis. We correlate the trends in GDR structure of $^{16,17,18}\mathrm{O}$ with changes in ground-state properties related to static deformation. The (\ensuremath{\gamma},p) reaction selects strength predominantly from two-particle--one-hole configurations formed via E1 transitions from the 1${\mathit{p}}_{1/2}$ subshell; comparison with other reactions (photoneutron and radiative capture) provides information on the microscopic structure of E1 states. The peak observed near threshold at 15.1 MeV is remarkably strong; we infer that it originates from photoexcitation of a few narrow T=3/2 states and that M1 transitions contribute to the measured strength. The total absorption cross section is approximated by summing the (\ensuremath{\gamma},p) cross section and the previously published photoneutron cross 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 $^{17}\mathrm{O}$. Systematics of the integrated cross sections for the carbon, nitrogen, and oxygen isotopes are delineated.

The C-14(gamma,p)B-13 reaction cross section has been measured from threshold to 29.1 MeV using bremsstrahlung photons. A contribution from the C-14(gamma,pn + d)B-12 reactions is included, and is significant only at the highest energies measured here. The main features of the cross section are a weak resonance at almost-equal-to 22.5 MeV and a dominant, broad resonance at almost-equal-to 25.6 MeV. The integrated cross section up to 29.1 MeV is 17.9 +/- 3.2 MeV mb. We deduce that essentially the entire cross section results from decay of T > dipole states. In combination with the previously reported photoneutron cross section an estimate of the total photoabsorption cross section for C-14 is obtained. The T < and T > components of the photoabsorption cross section (up to 30 MeV) are estimated to carry strengths of 88 +/- 12 MeV mb and 37 +/- 8 MeV mb, respectively. An isospin splitting of the giant dipole resonance of 8.4 +/- 0.5 MeV is obtained. Comparisons of several shell-model calculations are made with the data, and general agreement is found. A comparison of photoabsorption cross sections for C-12,C-13,C-14 and O-16,O-17,O-18 shows dramatic redistribution of dipole strength as neutrons are added to the core nuclei.

The $^{14}\mathrm{C}$(\ensuremath{\gamma},p${)}^{13}$B reaction cross section has been measured from threshold to 29.1 MeV using bremsstrahlung photons. A contribution from the $^{14}\mathrm{C}$(\ensuremath{\gamma},pn+d${)}^{12}$B reactions is included, and is significant only at the highest energies measured here. The main features of the cross section are a weak resonance at \ensuremath{\approxeq}22.5 MeV and a dominant, broad resonance at \ensuremath{\approxeq}25.6 MeV. The integrated cross section up to 29.1 MeV is 17.9\ifmmode\pm\else\textpm\fi{}3.2 MeV mb. We deduce that essentially the entire cross section results from decay of ${\mathit{T}}_{>}$ dipole states. In combination with the reported photoneutron cross section an estimate of the total photoabsorption cross section for $^{14}\mathrm{C}$ is obtained. The ${\mathit{T}}_{<}$ and ${\mathit{T}}_{>}$ components of the photoabsorption cross section (up to 30 MeV) are estimated to carry strengths of 92\ifmmode\pm\else\textpm\fi{}14 MeV mb and ${31}_{\mathrm{\ensuremath{-}}5}^{+9}$ MeV mb, respectively. An isospin splitting of the giant dipole resonance of 8.1\ifmmode\pm\else\textpm\fi{}0.2 MeV is obtained. Comparisons of several shell-model calculations are made with the data, and general agreement is found. A comparison of photoabsorption cross sections for $^{12,13,14}\mathrm{C}$ and $^{16,17,18}\mathrm{O}$ shows dramatic redistribution of dipole strength as neutrons are added to the core nuclei.

This Brief Report presents a reanalysis of the energy scale of published measurements by Johnson {ital et} {ital al}. and Jury {ital et} {ital al}. of the {sup 17}O({gamma},{ital n}{sub 0}) cross section which brings the two sets into good agreement. This leads directly to new ({ital T}{sub {lt}}) isospin assignments for levels previously reported by Ajzenberg-Selove at energies of 14.4, 15.2, and 15.6 MeV.

Photoneutron time-of-flight spectra from the reaction 19F(γ, n0)18F 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 a1 and a3 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 a2 coefficient showed that the likely reaction mechanisms are mainly s → p and d → p single-particle transitions of channel spin 12. A comparison of the present angleintegrated ground-state cross section with the (γ, ntot) work of Veyssière et al. indicates that decays to excited states in 18F are much preferred (typically by a factor of 5) over the ground-state channel. The 19F(γ, n0) cross section shows reasonable agreement in structure and magnitude with the 19F(γ, p0) cross section of Kerkhove et al. as well as with the 18O(γ, n0) data of Jury et al. (although some discrepancies are seen at 16 MeV and above 23 MeV).

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 2s12 → 2p32 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.

The /sup 29/Si(..gamma..,n) cross section has been measured from 9.6 to 18.8 MeV with monoenergetic photons. These new data resolve the discrepancies between the results of two previous measurements of this reaction cross section.

Differential cross sections have been measured for the reaction $^{18}\mathrm{O}$(\ensuremath{\gamma},${\mathrm{n}}_{0}$${)}^{17}$O over the region of excitation energy from 14 to 26 MeV. The angle-integrated cross section for the ground-state transition reveals that this channel accounts for less than 20% of the total photoneutron cross section in the structured pygmy resonance region (near 14 MeV) and is a small fraction (10--15 %) of the cross section in the region of the giant resonance (near 25 MeV). The values of angular distribution coefficients fitted to the data are consistent with a description of this reaction in which electric dipole excitations dominate the cross section in the pygmy resonance. Narrow regions exist near 15.0, 16.0, and 20.0 MeV where nonzero ${a}_{1}$ coefficients are observed indicating the absorption of non-E1 radiation. The measured cross section and ${a}_{2}$ coefficients are compared to a direct-semidirect calculation which gives reasonable agreement and suggests that f-wave neutron emission dominates the ground-state channel and that there is little justification for the introduction of E2 amplitudes other than a pure direct E2 term.