Optical potential for incident and emitted low-energy $\alpha$ particles. III. Non-statistical processes induced by neutrons on Zr, Nb, and Mo nuclei

Abstract

The reliability of a previous $\alpha$-particle optical-model potential (OMP) on nuclei with mass number 45$\leq$$A$$\leq$209 was proved for emitted $\alpha$ particles as well, for proton--induced reactions on Zn isotopes [Phys. Rev. C {\bf 91}, 064611 (2015), Paper I]. However, the same was not the case of neutrons on Zr stable isotopes [Phys. Rev. C {\bf 96}, 044610 (2017), Paper II]. A recent assessment of this potential also for nucleon--induced $\alpha$-emission on $A$$\sim$60 nuclei, including pickup direct reaction and eventual Giant Quadrupole Resonance (GQR) $\alpha$-emission, is completed for neutrons incident on Zr, Nb, and Mo stable isotopes. Consistent sets of input parameters, determined through analysis of independent data, is involved while no further empirical rescaling factors of the $\gamma$ and nucleon widths have been involved. A suitable account of all competitive reaction channels is confirmed by careful uncertainty analysis, to avoid parameter ambiguities and/or error compensation. Additional validation of this potential is also supported by recently measured $(\alpha,\gamma)$ and $(\alpha,n)$ cross--sections of Zr and Mo nuclei. An increase of the $\alpha$-emission beyond the statistical predictions, through consideration of additional reaction channels of the pickup direct interaction and {\it like}--GQR decay, makes possible the description of both absorption and emission of $\alpha$ particles by the same optical potential.