Beam of electrically neutral particles that cause ionization by interacting with atoms in a material, producing electrically-charged particles that subsequently cause
direct ionization in the material.
Example:
Gamma-rays and X-rays, which produce electrons, and neutrons, which produce alpha and beta particles.
Source:
PAC, 2021, 93, 69. 'Vocabulary of radioanalytical methods (IUPAC Recommendations 2020)' on page 82 (https://doi.org/10.1515/pac-2019-0302)