Ground-state photoneutron reactions inN15

Photoneutron angular distributions were measured by time-of-flight techniques for the reaction ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}(\gamma , n_0){}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ 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 $J^{\pi }={\frac{3}{2}}^{+}$, $T=\frac{1}{2}$ 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 $p_{\frac{1}{2}}\to d_{\frac{3}{2}}$ transition is consistent with an observed value for the $\frac{a_2}{a_0}$ coefficient of -0.7±0.2. The ($\gamma , n_0$) 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.