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

Background: The extent to which knockout, as opposed to a pickup reaction mechanism, contributes in pre-equilibrium (p, alpha) reactions is still not clear. Even with exclusive (p, 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, 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, alpha) yield. This result is in disagreement with several other studies. Purpose: We investigate whether existing (p, p 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 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 degrees to 40 degrees the knockout yield is already quite substantial. At 60 degrees 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 alpha-particle energy).

Automated summary

The study compared the contributions of knockout and pickup reaction mechanisms to pre-equilibrium (p, alpha) reactions. Even at low excitation conditions, the experimental results show that coincident knockout reactions contribute significantly more to the pre-equilibrium yield than previously estimated by the INC study.