Contribution to inclusive (p,α) reactions from (p,pα) knockout at incident energies near 100 MeV

Abstract

Background: The extent to which knockout, as opposed to a pickup reaction mechanism, contributes in pre-equilibrium $(p,\ensuremath{\alpha})$ reactions is still not clear. Even with exclusive $(p,\ensuremath{\alpha})$ reactions at conveniently low incident energies to well-defined final states, the issue often appears to be ambiguous. Recently Uozumi et al. [Y. Uozumi, Y. Fukuda, Y. Yamaguchi, G. Watanabe, and M. Nakano, Phys. Rev. C 102, 014604 (2020)] used the intranuclear cascade (INC) theory to study pre-equilibrium $(p,\ensuremath{\alpha})$ reactions in a range of incident energies from 42 to 300 MeV. They conclude that, below a projectile energy of about 120 MeV, knockout contributes almost nothing to the upper half of the pre-equilibrium $(p,\ensuremath{\alpha})$ yield. This result is in disagreement with several other studies.Purpose: We investigate whether existing $(p,p\ensuremath{\alpha})$ quasifree knockout results support the conclusion from the INC study.Method: The distorted-wave impulse approximation (DWIA) is used. The theory is known to give a good reproduction of experimental energy distributions and angular distributions, as well as absolute cross sections of $(p,p\ensuremath{\alpha})$ reactions at incident energies at and above 100 MeV. The DWIA is simply exploited as a convenient way of interpolation and extrapolation of the available experimental distributions.Results: The experimental cross-section distributions of the coincidence knockout reactions predict contributions to the pre-equilibrium yield much higher than those estimated by the INC study. At small scattering angles of only ${30}^{\ensuremath{\circ}}$ to ${40}^{\ensuremath{\circ}}$ the knockout yield is already quite substantial. At ${60}^{\ensuremath{\circ}}$ it accounts for approximately all of the pre-equilibrium yield.Conclusions: Even a lower-limit estimate of the coincident contribution at forward angles to the cross section of pre-equilibrium reactions is already sizable at low excitation. The cross section is expected to increase very rapidly at higher excitation energies (equivalently, lower $\ensuremath{\alpha}$-particle energy).