The efficiency of the SIR ionization chamber versus photon energy has been obtained by an iterative method, starting from a few monoenergetic gamma-ray emitters. The relative standard uncertainty on the efficiency curve obtained is lower than 10(-2) above 65 keV. An efficiency curve for beta particles has been deduced in a similar manner, from measurements of (quasi-) pure beta emitters. The possible uses of the efficiency curve to identify inconsistent SIR data or discrepant decay scheme data are discussed.