3.1 Entrance channel effects of light charged particle emission from the ¹⁵⁶Er compound nucleus

J.D. Bierman, P. Chan, M.P. Kelly, J.F. Liang, A.A. Sonzogni, R. Vandenbosch and J.P.S. van Schagen

As reported in last year's Annual Report,¹ the light charged particle decay of the ¹⁵⁶Er compound nucleus was measured for ¹²C + ¹⁴⁴Sm and ⁶⁰Ni + ⁹⁶Zr reactions. The excitation energy and the spin distribution for the compound nucleus formation are matched in both systems. Comparisons of the light charged particle spectra, in coincidence with evaporation residues, show that the spectral shape of the ¹²C induced reaction is harder than the ⁶⁰Ni induced reaction.

The experiment was repeated by varying the geometry of the electrostatic deflectors and detectors to look for possible bias in the measurement. The differences in the particle spectra between the two systems persist. Since this is not expected by the theory of compound nucleus decay, dynamical effects were investigated to try to resolve the discrepancies.

The amalgamation time for ⁶⁰Ni + ⁹⁶Zr is three to four times longer than that for ¹²C + ¹⁴⁴Sm, according to calculations performed by the one body dissipation model code HICOL.² Because of the high excitation energy of the reactions, the lifetime of the composite system ⁶⁰Ni + ⁹⁶Zr can be shorter than the amalgamation time. This may lead to particle and -ray emissions during formation which would remove energy from the system. Thus, the final excitation energy reached would be less than that of the ¹²C + ¹⁴⁴Sm system. Statistical model calculations taking into account particle emission during formation were carried out. Fig. 3.1-1 presents the results of EVAP³ calculations for both reactions using an amalgamation time estimated at spin l = 30. The solid curves are results including particle emission during formation and the dashed curves are results of no pre-compound (p.c.) emission. For the ¹²C + ¹⁴⁴Sm reaction, the effect of pre-compound emission hardly changes the high energy slope of the particle spectrum. However, the inclusion of particle emissions during formation can almost account for the softer spectral shape of the ⁶⁰Ni + ⁹⁶Zr reaction. It should be pointed out that HICOL predicts that the formation time as a function of spin is not linear. The amalgamation time becomes significantly longer for higher spins.

Fig. 3.1-1. Energy spectra of protons and particles emitted from the decay of the ¹⁵⁶Er compound nucleus. See text for the explanations of curves in the figure.