Lifetimes in 15N from gamma-ray line shapes produced in the 2H(14N,p gamma ) and 14N(thermal n, gamma ) reactions.

Abstract

Mean lifetimes of bound states in [sup 15]N were inferred from the Doppler-shift-attenuation (DSA) of [gamma] rays produced in the inverse reaction [sup 2]H([sup 14]N,[ital p][gamma]) and from the primary gamma-ray-induced Doppler broadening (GRID) of secondary [gamma] rays in the thermal-neutron capture reaction [sup 14]N([ital n],[gamma]). Targets for the DSA measurements were prepared by implanting first neon into a gold backing and then deuterium into the same region such that deuterium was trapped at the neon precipitates. To find out experimentally the initial velocity distributions of the recoiling [sup 15]N nuclei, another target was prepared by implanting deuterium into silicon, which is a slow stopping-power medium. Computer simulations with the Monte Carlo method and experimental stopping powers were used in the DSA analysis of the [gamma]-ray line shapes. This analysis yielded the following lifetimes for eight bound levels in [sup 15]N: 43[plus minus]4 fs (5.30 MeV), [lt]12 fs (6.32 MeV), 11[plus minus]2 fs (7.16 MeV), [lt]3 fs (7.30 MeV), 129[plus minus]6 fs (7.57 MeV), [lt]3 fs (8.31 MeV), [lt]10 fs (8.57 MeV), and [lt]4 fs (9.05 MeV). GRID measurements were made with melamine (C[sub 3]H[sub 6]N[sub 6]) and with silicon nitride (Si[sub 3]N[sub 4]) as both targets and slowing-down media.more » Measurements were also made with air (80% nitrogen). The Doppler-broadened [gamma]-ray line shapes were analyzed by molecular-dynamics simulations of the slowing-down process to obtain the lifetime values of 40[plus minus]3 fs (5.30 MeV), [lt]2 fs (6.32 MeV), [lt]3 fs (7.30 MeV), and [lt]3 fs (8.31 MeV) for levels in [sup 15]N. The extent to which the mirror symmetry of levels in [sup 15]N and [sup 15]O is valid is examined in some detail.« less