The international reference system (SIR) is characterized by its high stability and its simplicity. Since it came into existence, about 900 measurements have been made with over 60 radionuclides – giving a total of about 650 independent results – and interest in the method remains high. The results of the measurements are published in the KCDB and are used to construct the efficiency curve of the ionization chamber (IC) as a function of  -ray energy.
To date, the following radionuclides have been measured in the SIR:
Recently, use of a liquid scintillation counter has allowed the SIR to be extended to include beta-ray emitters.
Information for NMIs wishing to participate in the SIR is provided here.
The electronics of the SIR have been updated and verification measurements are in progress.
The high stability of the SIR over 25 years
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Summary
