The excitation of isomeric states by accelerator neutrons from the 7Li(p, n)7Be reaction and their application in selective activation analysis

The 7Li(p, n)7Be reaction has proved to be useful in selective neutron activation analysis via the production of nuclear isomers. This is due to the relatively high yield and narrow energy spread of the neutrons that can be produced with a thin lithium target. By a suitable choice of proton energy and target thickness, neutrons have been produced that are below the energy region for the excitation of threshold reactions such as 28Si(n, p)28Al and above the region where neutron capture reactions like 27Al(n, γ)28Al have large cross sections. These neutrons have been used for the selective excitation of nuclear isomeric states by inelastic scattering. Numerical methods were used to predict the resultant neutron spectra for different values of the lithium target thickness at different proton energies. The isomeric state of gold was excited by neutrons from the above reaction at proton energies between 4 and 10 MeV. The possible interfering reactions from silicon and aluminium, present in many ores, silicates and rock samples, have also been examined. The variation of the normalised peak areas, as a function of the proton energy incident on lithium targets of thicknesses of 0.8 and 2.8 mm is also presented here. (orig.)