Evolution over the past century of quantities and units in radiation dosimetry

Following the discovery of x-rays in November 1895, of radioactivity in February 1896 and of radium in December 1898 a large number of proposals were made for x-ray units and for radium units. These were based on various radiation effects, including blackening of photographic film, chemical effects, fluorescence and skin erythema. Some were also based on the ionisation effect, and it was this effect which eventually, in 1937, became the basis for all future radiological units of measurement, without any differentiation between x-rays and radium gamma-rays. During this period the International Commission on Radiation Units and Measurement (ICRU) and the International Commission on Radiological Protection (ICRP) were set up by the International Congress of Radiology (ICR), as well as some national committees on this topic. This led to a series of reports in respect of (1) fundamental quantities and units for general use produced by the ICRU and (2) quantities and units for use in radiation protection produced by the ICRU and ICRP. In regard to the latter series, alternating contributions from the two commissions were made over the years, but these have resulted in a dual approach to the subject: (a) the ICRP's mean-value quantities for dose-limitation purposes and (b) the ICRU's point-value quantities for measurement purposes, with conversion factors between them. This review summarises this historical evolution over the past century.