Investigation of $\alpha$ clustering with knockout reactions

TL;DR

This paper investigates how alpha knockout reactions can be used to probe alpha cluster amplitudes in nuclei, showing that such reactions primarily measure surface-region clusters and depend on reaction conditions like incident energy.

Contribution

The study introduces a masking function within the DWIA framework to identify the nuclear surface region probed by alpha knockout reactions and analyzes energy dependence of this effect.

Findings

Alpha knockout reactions probe the nuclear surface region.

The masking function reveals suppression of interior reaction amplitudes.

Proper kinematics allow surface-focused alpha cluster measurements.

Abstract

Our goal is to reveal how the $\alpha$ cluster amplitude is probed through $\alpha$ knockout reactions depending on reaction conditions, e.g., the incident energy. We consider $^{20}$Ne($p$,$p\alpha$)$^{16}$O and $^{120}$Sn($p$,$p\alpha$)$^{116}$Cd 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 $\alpha$ cluster wave function. It is clearly shown by means of the masking function that the $\alpha$ knockout reaction probes the $\alpha$ cluster amplitude in the nuclear surface region, which is the direct measure of well-developed $\alpha$ cluster states. A simplified form of the masking function is introduced and the incident energy dependence of the masking effect is investigated. $\alpha$ knockout reaction can probe the $\alpha$ 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 $\alpha$ cluster states are spatially developed.