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$α$ knockout reaction as a new probe for $α$ formation in $α$-decay nuclei

The $α$-decay phenomennon has been studied for more than a century but the mechanism of the $α$ particle formation and its tunneling process have not yet been fully understood. As an alternative to the $α$-decay lifetime measurement, we propose the proton-induced $α$ knockout reaction, ($p$,$pα$), as a new probe for the surface $α$ formation probability of $α$-decay nuclei. The $^{210,212}$Po($p$,$pα$)$^{206,208}$Pb reaction is described by the distorted-wave impulse approximation framework. It shows that the $^{212}$Po/$^{210}$Po ratio of the $α$ knockout cross sections agrees with that of the surface $α$ formation probabilities determined by lifetime measurements. The result implies that the ($p$,$pα$) cross section is a direct probe for the surface $α$ formation probability, which is an essential quantity to lead us to a complete understanding of the $α$-decay phenomenon.