Direct calibration of a reference chamber against the low air kerma rate primary standard at Ir-192 energy

Abstract

Using a 60 cc spherical graphite ionization chamber as the Primary Standard the HDR Ir-192 source was standardized at the hospital site in a minimum scatter geometry, in units of Air Kerma Strength (AKS). A 400 cc bakelite reference chamber was calibrated against the Primary Standard. It was seen that the Primary Standard and the Reference chamber, both being of low Z, showed roughly the same scatter response and yielded the same calibration factor for the 400 cc reference chamber, with or without scatter. The scatter correction, however, is required for the determination of the Air kerma strength. The theoretical Monte Carlo estimates of the scatter corrections, obtained by simulating the source and the room geometry, showed excellent agreement with the experimental values. This reference chamber can now be used at other hospital sites for determining the AKS of the source, by applying proper scatter corrections. However, any likelihood of change in the reference chamber calibration would necessitate the transport of the Primary Standard to the hospital site for recalibration. Frequent transport of the Primary Standard can lead to changes in its response due to movement or other extraneous causes. The calibration of the Reference chamber against the Primary Standard at the Standards Lab., for an industrial type Ir-192 source maintained at the lab., showed excellent agreement with the hospital calibration, making it possible to check the reference chamber calibration at the lab. itself. Further work is in progress.