Consistent optical potential for incident and emitted low-energy α particles. II. α emission in fast-neutron-induced reactions on Zr isotopes

Background: Challenging questions of the alpha-particle optical-model potential (OMP) are still pointed out by recent high-precision measurements of alpha-induced reaction data below the Coulomb barrier. Moreover, the reliability of a previous OMP for alpha particles on nuclei within the mass number range 45 <= A <= 209 has been recently proved for emitted alpha particles as well, but only in the case of proton-induced reactions on Zn isotopes [Phys. Rev. C 91, 064611 (2015) (Paper I)]. Purpose: Analysis of most recent (alpha,gamma) reaction data for Ge and Zr isotopes, which provides an additional validation of the above-mentioned potential, is related to a further account of alpha-particle emission in neutron-induced reactions on Zr isotopes, at the same time with a suitable description of all competitive processes. Methods: A consistent parameter set, established or validated by independent analysis of recent various data, particularly gamma-ray strength functions, has been involved within model calculation of the (alpha,gamma) as well as (n,alpha) reaction cross sections. The latter are part of the whole analysis of the neutron activation of Zr isotopes, in order to avoid any error compensation or latent ambiguity. Results: The aforesaid potential provides a consistent description of recent alpha-induced reaction data with no empirical rescaling factors of the gamma and/or nucleon widths. On the other hand, its use leads to underestimated predictions of the pre-equilibrium emission and statistical models for the (n,alpha) reaction cross sections. Conclusions: An optical potential with a volume imaginary component seems to be needed to describe the low-energy alpha-particle evaporation, while only surface absorption occurs in alpha-induced reactions at similar energies.