Ground-state photoneutron reactions in 14C.

Abstract

Photoneutron angular distributions were measured by time-of-flight techniques for the reaction /sup 15/N(..gamma..,n/sub 0/) /sup 14/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/sup ..pi../ = (3/2)/sup +/, T = (1/2) states in /sup 15/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/sub 1/2/..-->..d/sub 3/2/ transition is consistent with an observed value for the a/sub 2//a/sub 0/ coefficient of -0.7 +- 0.2. The (..gamma..,n/sub 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 compositionmore » with a theoretical prediction based on a shell model calculation using a residual interaction with a Soper mixture of exchange forces.« less