Selective activation analysis with ion-beam-tailored neutron spectra — A comparison between the reactions 7 Li(p, n)7Be and 9Be(p, n)9B

Abstract

Abstract When endoergic (p, n) reactions are used for the production of neutrons, the energy distribution of the neutron flux depends on the energy of the protons. By changing the proton energy, certain neutron reactions are selectively excited. This effect can then be used to determine particular elements in matrices that contribute overwhelming interference when the conventional techniques of thermal- or fast-neutron activation analysis are applied. For example, reactions that produce isomeric states through inelastic scattering have maxima in their excitation functions at energies of a couple of MeV, while (n, p) and (n, α) reactions typically have thresholds above this region. The two targets for neutron production most appropriate for this method are lithium and beryllium. According to the literature, the reaction 7Li(p, n)7Be is a more prolific source of neutrons at proton energies below 6 MeV. On the other hand, high beam powers are necessary for practical applications and lithium metal has a melting point of 179° C in contrast with 1278°C in the case of beryllium. In this work, the response of the inelastic scattering reaction 197Au(n, n′)197mAu as well as interfering threshold and neutron capture reactions in a silicate matrix have been studied for neutrons produced by proton reactions on a lithium target and compared with work, reported by us previously, for a beryllium target. The results indicate that, for thick targets, the advantages of using beryllium outweigh the disadvantages and that a suitable high-current accelerator equipped with a beryllium target would be able to determine gold rapidly at the low ppm level in a silicate matrix. The ultimate limit of detection for elements in a matrix containing aluminium, such as a rock, is dependent on interference produced by the neutron capture reaction 27Al(n, γ)28Al. Ways of reducing this interference are discussed.