Analysis of experimental cross-section for (α , n) reactions in odd A and odd Z heavy nuclei: A systematics on pre-compound emission

Aiming to develop a systematics in the pre-compound emission process and to get its driving parameters, a sensitive analysis of the measured excitation functions for (α, n) reaction channel on target nuclei viz., La, Tb, Ta, Au and Tl has been performed with codes PACE4 and ALICE at low projectile energies, where the compound nucleus process is dominant. The code PACE4 considers only compound nucleus emission, while the ALICE code takes into account both compound and pre-compound emissions. The observed enhancement in experimental excitation functions in the tail portion of energies as compared to calculations performed with code PACE4 may be attributed to the contribution from pre-compound emission. Further, the calculations performed with the ALICE code confirm the significant contribution of the pre-compound emission. The effect of various pre-compound parameters on measured excitation functions has also been studied. The energy dependent yield of the pre-compound contribution over the compound nucleus emission for neutron emission channels is deduced in terms of pre-compound fraction “F” which gives a good systematics on such process. The developed systematics in α-induced reactions on target nuclei reflects that the pre-compound process is governed by the excitation energy available with the nucleons at the surface of the composite systems. Furthermore, the mass number of the target nuclei may also play an important role in the pre-compound process at low projectile energies.