High-energy photon conversion of a proton into a neutron (such as via photoproduction processes like gamma + p -> n + pi+) is a standard physical process established in nuclear and particle physics, requiring no citation to verify.
The claim describes a fundamental particle physics interaction (e.g., charged pion photoproduction off a proton, where a high-energy photon interacts with a proton to produce a neutron and a positively charged pion). This is a well-established physical phenomenon taught in standard nuclear physics and constitutes common knowledge within the field, meaning specific paper citations are unnecessary for verification.