Yes, That is correct. At calibration labs, the source(usually or exclusively Cs-137 at varying activity levels), or the beam is usually measured with a very quality directly NIST traceable Ion Chamber and a good calibrated quality accurate multimeter. The Ion chamber response is not photon energy dependant at 662 kev range interval. This calibrated beam is then used to measure and calibrate GM and other instrument's response in mR/hour, R/hour CGray/hour, etc. GM type instruments are not usually used to calibrate a radiation beam or a photon field. Good Luck at your studies.