Ground-state photoneutron reactions inN15

Abstract

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.