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The analysis of nuclear evaporation spectra: Charged particle emission

1968; Elsevier BV; Volume: 107; Issue: 3 Linguagem: Inglês

10.1016/0375-9474(68)90783-5

1873-1554

D. C. Williams, T. Darrah Thomas,

Quantum, superfluid, helium dynamics

Abstract It is shown that erroneous values of the nuclear level density parameter a may be obtained when nuclear evaporation spectra are analysed by the usual formalism that does not explicitly take into account the spin dependence of the level density of the residual nucleus. The value obtained from nucleon spectra will usually be too large, and the apparent value will depend upon both the mode of formation of the compound nucleus and the type of particle emitted. The emission ratios for competing particles can depend significantly upon the compound nucleus angular momentum and therefore, upon how the compound nucleus is formed. The effect of spin fractionation is important in many cases. None of the factors taken into account can explain the decrease in a with increasing bombarding energy that has been reported for some neutron evaporation spectra, although it is predicted that alpha-particle spectra will show this effect in some cases if the nuclear moment of inertia is small enough. In general, the calculated spectra depend upon the magnitude of the spin-cut-off parameter as well as upon the level-density parameter, and the behavior of the spin-cut-off parameter as a function of excitation energy is also shown to be important. It will therefore be difficult to extract meaningful values of the level density parameter from the analysis of a single evaporation spectrum. On the other hand, it may prove possible to obtain information on both the energy dependence and the spin dependence of the nuclear level density through the study of several suitably chosen evaporation spectra that all involve the same residual nucleus.