Calibration instrumentation used by the AAPM Radiological Physics Center

The Keithley model 602 electrometer-Farmer ionization chamber combination has served as the standard dosimetry system for calibration measurements made by the AAPM Radiological Physics Center at 156 different institutions. The constancy of this system is compared to that for the Baldwin-Farmer Secondary Standard Dosemeter type 2502, and is based on measurements made over a 3-yr period on a Cobalt-60 gamma-ray source. A Strontium-90 check source is also shown to be a reliable standard, and frequent checks with such a source can prevent serious calibration errors.