Energy and angular distributions of the alpha particles from 14.1 MeV neutron reactions in Al 27 and Ni 58

Abstract

A chamber with one CsI(Tl)-scintillation counter has been used for the investigation of fast neutron reactions in which charged particles are emitted. Different types of radiation are distinguished by applying the pulse-shape discrimination method. Angular distributions can by measured by successively setting up the targets at different places around the crystal the position of which is fixed relative to the neutron source. The results obtained for the energy and angular distributions of the alpha particles from 14.1 MeV neutron reactions in Al 27 and Ni 58 are presented. A statistical theory analysis leads to the conclusion that the alpha particles predominantly are evaporated in compound nucleus reactions. Reasonable values for the level density parameter a, the nuclear temperature T and the spin cut-off parameter σ are derived from the data. The alpha particles from Al 27 can be attributed entirely to the reaction Al 27(n, α)Na 24 for which a cross section of 119±10 mb is measured. The total alpha particle yield from Ni 58 is the sum of the contributions from the reactions Ni 58(n, α)Fe 55, Ni 58(n, αn′)Fe 54 and Ni 58(n, n′α)Fe 54. The portion of alpha particles from the latter reaction is separated by means of the statistical theory, and a cross section of 30±6 mb is determined for this process. For the sum of the other two reactions a cross section of 125±16 mb is obtained. Arguments are given for the statement that at least ninety per cent of this cross section may be attributed to the reaction Ni 58(n, α)Fe 55. As far as possible the results are compared with corresponding ones of other authors.