THE EXCITATION OF ISOMERIC STATES BY ACCELERATOR NEUTRONS FROM THE LI-7(P, N)BE-7 REACTION AND THEIR APPLICATION IN SELECTIVE ACTIVATION-ANALYSIS

The Li-7(p, n)Be-7 reaction has proved to be useful in selective neutr on activation analysis via the production of nuclear isomers. This is due to the relatively high yield and narrow energy spread of the neutr ons that can be produced with a thin lithium target. By a suitable cho ice of proton energy and target thickness, neutrons have been produced that are below the energy region for the excitation of threshold reac tions such as Si-28(n, p)Al-28 and above the region where neutron capt ure reactions like Al-27(n, gamma)Al-28 have large cross sections. The se neutrons have been used for the selective excitation of nuclear iso meric states by inelastic scattering. Numerical methods were used to p redict the resultant neutron spectra for different values of the lithi um target thickness at different proton energies. The isomeric state o f gold was excited by neutrons from the above reaction at proton energ ies between 4 and 10 MeV. The possible interfering reactions from sili con and aluminium, present in many ores, silicates and rock samples, h ave also been examined. The variation of the normalised peak areas, as a function of the proton energy incident on lithium targets of thickn esses of 0.8 and 2.8 mm is also presented here. (C) 1998 Elsevier Scie nce B.V.