Purpose: To add further support to the designation of hard gamma‐rays, such as those emitted by ⁶⁰Co or ¹³⁷Cs, as the common reference radiation, as recommended by the International Commission on Radiological Protection (ICRP) Task Group 37 in the recent ICRP Publication 92.

Materials and methods: The study examined the microdosimetric properties of seven commonly used reference radiations and quantified differences in radiation quality among them. The principal tool is the proximity function, which is derived from the spatial distributions of energy deposits calculated by Monte Carlo simulations.

Results: The microdosimetric properties of the seven commonly used reference radiations were significantly different. However, there were no significant differences in the spectra of energy deposition in microscopic regions between hard γ‐rays over a wide range of distances of biological interest.

Conclusions: From a microdosimetric point of view, γ‐rays from ⁶⁰Co or ¹³⁷Cs are the most suitable reference radiations for specification of the relative biological effectiveness.