Evaluation of the shadow shield technique for the measurement of scattered radiation

ISO standard 4037 specifies the characteristics and production methods of x-ray and gamma-ray reference radiation for calibrating protection-level dosimeters and rate dosimeters. The standard limits scattered radiation to 5%. The shadow shield technique is a widely accepted test used to determine scatter contribution. Using an MCNP model, an analysis of the shadow shield technique was performed. It was found that the shadow shield technique is accurate in predicting the scatter due to the walls and floor for source-to-detector distances (R) less than 6 m. For R greater than 6 m for the modeled irradiator and calibration range, the shadow shield blocks photons that scatter upbeam from the detector, which causes the shadow shield method to under-predict the scatter percentage. Moreover, the shadow shield blocks scatter that originates from the irradiator, which causes the shadow shield method to under-predict the scatter percentage by as much as 10 percentage units for the irradiator and calibration range that were modeled.