primary beam

https://doi.org/10.1351/goldbook.09270
Directed flux of beam particles or photons incident on a sample.
Source:
PAC, 2020, 92, 1781. 'Glossary of methods and terms used in surface chemical analysis (IUPAC Recommendations 2020)' on page 1810 (https://doi.org/10.1515/pac-2019-0404)