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.

Photoneutron time-of-flight spectra from the $^{14}\mathrm{C}$(\ensuremath{\gamma},${\mathrm{n}}_{0}$${)}^{13}$C reaction were measured as functions of laboratory angle over the excitation energy region from 10 to 28 MeV. Angular distribution coefficients and differential cross sections were extracted as functions of excitation energy between 10 and 23 MeV. The angle-integrated ground-state cross section indicates that ground state transitions dominate the ${T}_{<}$ giant dipole resonance region below 13 MeV, but only contribute about 50% of the strength in the neutron channel in the rest of the giant dipole resonance region. The results support a mechanism of dominant E1 absorption in the energy region from 13 to 23 MeV where an average value of ${a}_{2}$=-0.5 indicates ${p}_{1/2}$\ensuremath{\rightarrow}${d}_{3/2}$ single-particle neutron transitions. Angular distribution information suggests that much of a prominent resonance at 11.3 MeV (with an integrated cross section of about 1.03 MeV mb) is due to an M1 transition from the ground state of $^{14}\mathrm{C}$. If this is the case, there is little fragmentation of the M1 strength in $^{12}\mathrm{C}$ brought about by the presence of valence neutrons. When combined with the observation of the lack of a pygmy E1 resonance below the giant dipole resonance region, these results suggest that a model of $^{14}\mathrm{C}$ as a $^{12}\mathrm{C}$ ``core'' with two valence, weakly coupled, neutrons is inappropriate. Below an excitation energy of about 19 MeV, there is reasonably good quantitative and qualitative agreement between the present data and the results of a recent shell model calculation.

Photoneutron energy spectra from the $^{16}\mathrm{O}(\ensuremath{\gamma},{\mathrm{n}}_{0})^{15}\mathrm{O}$ reaction were measured as functions of laboratory angle over a range of excitation energies from 30 to 35 MeV. Angular distribution Legendre-polynomial coefficients were extracted up to third order as functions of excitation energy. Nonzero values of the coefficients ${a}_{1} (+0.25\ifmmode\pm\else\textpm\fi{}0.02)$ and ${a}_{3} (\ensuremath{-}0.2\ifmmode\pm\else\textpm\fi{}0.02)$ were observed over the energy region explored, indicating interference between states of opposite parity. These values can be accounted for in a simple model incorporating electric quadrupole absorption strength decaying via the ground-state photoneutron channel. When combined with the previously-determined amplitude ratio for the $E1$ $p\ensuremath{\rightarrow}s$ and $p\ensuremath{\rightarrow}d$ single-particle transitions, the present results suggest that about 4% of the isovector energy-weighted sum rule is found in the ($\ensuremath{\gamma}$,${\mathrm{n}}_{0}$) channel in the energy range studied. The value of the ($\ensuremath{\gamma}$,${\mathrm{n}}_{0}$) cross section was found to vary from 1.5\ifmmode\pm\else\textpm\fi{}0.1 mb at ${E}_{x}=30$ MeV to 0.8\ifmmode\pm\else\textpm\fi{}0.1 mb at 35 MeV. The average magnitude of the $E2$ contribution to this cross section was estimated to be 0.05\ifmmode\pm\else\textpm\fi{}0.02 mb. This is in reasonable agreement with a recent continuum randomphase approximation shell model calculation, but is in disagreement with a previous measurement.

Photoneutron angular distributions were measured by time-of-flight techniques for the reaction $^{15}\mathrm{N}(\ensuremath{\gamma}, {n}_{0})^{14}\mathrm{N}$ over the region of excitation energy from 15 to 25 MeV. Ground state cross sections were obtained by stepping the bremsstrahlung end point over the energy region of interest in 2 MeV intervals. By fitting the spectral data to a series of Legendre polynomials, angular distribution coefficients were extracted and interpreted on the basis of a simple single particle model. It appears that a large fraction of the photoabsorption strength leading to decays via the ground state channel is due to the formation of ${J}^{\ensuremath{\pi}}={\frac{3}{2}}^{+}$, $T=\frac{1}{2}$ states in $^{15}\mathrm{N}$ which decay by $d$-wave neutron emission. The data support an approximation of purely electric dipole absorption in the region measured. Some small amount of $s$-wave neutron emission interfering with the dominant ${p}_{\frac{1}{2}}\ensuremath{\rightarrow}{d}_{\frac{3}{2}}$ transition is consistent with an observed value for the $\frac{{a}_{2}}{{a}_{0}}$ coefficient of -0.7\ifmmode\pm\else\textpm\fi{}0.2. The ($\ensuremath{\gamma}, {n}_{0}$) cross section integrated between threshold and 30 MeV is estimated to represent about one-third of the total strength in the neutron channel. A state identified at 17.3 MeV is consistent in energy and composition with a theoretical prediction based on a shell model calculation using a residual interaction with a Soper mixture of exchange forces.NUCLEAR REACTIONS $^{15}\mathrm{N}(\ensuremath{\gamma}, {n}_{0})$, ${E}_{\ensuremath{\gamma}}=15\ensuremath{-}25$ MeV; measured differential cross sections as function of angle; extracted Legendre coefficients; estimated $\frac{s}{d}$ matrix-element ratio in dipole approximation.

In disagreement with recent suggestions that the beta spectrum of 32P displays a non-allowed shape with a discontinous slope to the shape factor, it has been found that this transition displays the statistical shape with W0 = 1697±2 keV. Non-statistical shapes can be simulated with inadequate attention to source quality or having a small error in the end-point energy W0. It is shown that shape factor coefficients are so extremely sensitive to W0 as to make their utility doubtful.