röntgen

https://doi.org/10.1351/goldbook.R05404
Non-SI unit of exposure to X- or γ-radiation: R = 2.58 x 10E-4 C kg.
Sources:
Green Book, 2nd ed., p. 113 (https://dev.goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 991 (https://doi.org/10.1351/pac199668040957)