Compound nucleus evaporative decay as a probe for the isospin dependence of the level density

Abstract

The evaporative decay of the compound nucleus ¹³⁹Eu produced by the 180MeV ³²S + ¹⁰⁷Ag reaction was studied with the aim to test the empirical isospin expressions of the level density, recently appeared in the literature. We measured light charged particle spectra and angular correlations in coincidence with the evaporation residues and the invariant velocity distribution of the evaporation residues. In addition, an independent experiment was performed on the reaction ³²S + ¹⁰⁹Ag at the same incident energy. Evaporation residue angular distribution was measured and the fusion-evaporation cross-section was determined. All the measured quantities are compared with the predictions of different level density prescriptions: a) isospin independence, b) a dependence from N - Z and c) a dependence from Z-Z₀ as proposed by Al-Quraishi et al. Results show that the predictions of the Z-Z₀ dependence are far off the experimental data for all the measured observables. Regarding the isospin independent prescription and the N - Z dependence, although no great differences appear between their predictions the N - Z prescription seems to better describe the experimental data.