Nuclear temperatures from the evaporation fragment spectra and observed anomalies

Abstract

The extreme back-angle evaporation spectra of α, Li, Be. B and C from different compound nuclei near A≈100 (Ex = 76-210MeV) have been studied and compared with the predictions of standard statistical model codes such as CASCADE and GEMINI. The temperatures of the ensembles of residual nuclei were extracted by fitting the back-angle spectra of the fragments emitted from 16O+89Y[E(16O)lab = 96MeV)], 16O+93Nb[E(16O)lab = 116MeV] and 3He+Ag[(E(3He)lab = 90and198MeV] reactions with the statistical evaporation formula given by Moretto [Nucl. Phys. A 247, 211 (1975] and similar analysis were also done for the corresponding calculated statistical spectra obtained from CASCADE and GEMINI codes. It was found that the shapes of the α-spectra agree well with the predictions of the Statistical Model. However, the spectra of Li, Be, B and C show significantly gentler slopes implying higher temperatures of the residual nuclei, even though the spectra satisfy all other empirical criteria of statistical emissions. The observed slope anomaly was found to be largest for Li and decreases at higher excitation energy. These results could not be understood by adjusting the parameters of the Statistical Models or by using any other nuclear reaction model and might imply initial slow non-exponential decay of the exit channel dinuclear states (resulting in a long survival time and statistical character) followed by exponential decay with large Brit-Wigner widths producing higher temperature of the residual nuclei.