A technique to compensate for geometry-induced errors in a photon irradiator calibration

The calibration of the exposure rate instrumentation used at Hanford necessitates the exposure of the instruments to photon beams at various radiation intensities. To simulate the wide variety of photon energies encountered, half a gram of 226Ra + daughters is used. So that a minimum exposure to Calibrations personnel is assured, the source is contained in an open-topped concrete well about 8 m deep. Instruments to be calibrated are placed in a holder at the top of the well and the source is raised to the prescribed position that gives the desired exposure rate. The primary calibration of the well poses unique problems because the exposure rates vary from place to place across the top surface of the well. Therefore, a transfer standard, being a different volume and shape from the instruments being calibrated, responds differently to a source position than does a service instrument. To compensate for this variation, a service instrument has been modified to accommodate an integration device and a digital read-out. The instrument is first calibrated, free-in-air, by comparison to the transfer standard, and is then placed over the well in exactly the same position at which all instruments of that type are calibrated. The exposure rate is integrated over a sufficiently long time to eliminate the measurement errors resulting from a rate-only read-out. The calibration accuracy is discussed in this paper and an analysis of the error contributions at each step is provided. (author)