Background: Nuclear clustering has been one of the main interests in nuclear physics. In order to probe the α clustering through reaction observables, α transfer and α knockout reactions have been studied. It is very important to probe the α cluster amplitude at nuclear surface since the α spectroscopic factor is not necessarily a direct measure of the α clustering. Purpose: Our goal is to reveal how the α cluster amplitude is probed through α knockout reactions depending on reaction conditions, e.g., the incident energy. Method: We consider Ne20(p,pα)O16 and Sn120(p,pα)Cd116 at 100-400 MeV within the distorted wave impulse approximation (DWIA) framework. We introduce a masking function, which shows how the reaction amplitude in the nuclear interior is suppressed and defines the probed region of the α cluster wave function. Results: It is clearly shown by means of the masking function that the α knockout reaction probes the α cluster amplitude in the nuclear surface region, which is the direct measure of well-developed α cluster states. The incident energy dependence of the masking effect is investigated, using a simplified form of the masking function. Conclusions: The α knockout reaction can probe the α cluster amplitude in the nuclear surface region by choosing proper kinematics owing to the masking effect originated from absorptions of distorting potentials, and is a suitable method to investigate how α cluster states are spatially developed.
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