Photonuclear and atomic cross sections of 27Al between 3 and 38 MeV.

Abstract

The total cross section for photon absorption of Al was measured for energies between 3 and 38 MeV with a el-ray spectrometer consisting of a liquid deuterium target viewed by a photoneutron time-of-flight detector. Corrections to the observed total cross section for background electromagnetic processes in the absorber were computed by a Monte Carlo method. Between 5 and 8 MeV where the photonuclear cross section sigma/sub N/ is negligible and the uncertainty in the measurements is small, values of the atomic cross section sigma/sub Z/ calculated by others systematically exceed the measured values by 0.7%. In this energy interval sigma/sub Z/ is mainly due to incoherent photon scattering by atomic electrons. Nonrelativistic atomic form factors and estimated radiative corrections used in calculating the tabulated incoherent scattering cross sections introduced enough uncertainty to justify decreasing sigma/sub incoh/ by 0.87% to bring sigma/sub Z/ into agreement with experiment. Correcting this discrepancy causes the integral between 10 and 38 MeV of sigma/sub N/, which has a maximum value of about 5% of sigma/sub Z/, to increase by 18% to 500 MeV mb. We conclude that the giant dipole resonance of /sup 27/Al has a strength of 1.24 classical sum rule units.