Evaluation of the Shadow Shield Technique for the Measurement of Scattered Radiation

Abstract

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.

Development of 137Cs irradiation facility for metrological application

“…In this method, a lead cone is located between the source and the chamber to avoid the primary photons to reach the chamber. The chamber is completely shielded from the primary radiation and only the scattering radiation reaches of the chamber [15,16]. The lead cone thickness that used in this work can reduce the primary radiation intensity in the detector position less than 0.4% of its original value.…”

Evaluation of Scattered Radiation in a Calibration Range Using Exposure Rate Energy Spectra

“…These checks were: 1) comparing the specific gamma constant (⌫) for the source calculated with MCNP with accepted values, and 2) comparing the exposure rates calculated with MCNP with measurements of the exposure rate for points on the centerline of the beam. The results of the validation calculations are presented in Steele et al (2011).…”

“…The scatter percentage S % m was previously calculated (Steele et al 2011) for scatter from a variety of constituents (m) [such as the irradiator (I), walls (W), and floor (F)] using eqn (5) below:…”

“…The 137 Cs source and irradiator were modeled as they are described in Steele et al (2011). VISED drawings of the irradiator as it is modeled in MCNP are presented with varying levels of detail in Figs.…”

“…An International Organization for Standardization (ISO) requirement limits the percentage of scattered exposure (S % ), extraneous to that from the source capsule, [ISO] to less than 5%. The results of measurements using the shadow shield technique (Minniti and Seltzer 2007) showed that in order to satisfy the ISO requirements, use of the range should be restricted to source-to-ion chamber (IC) distances (R) less than 7 m. An MCNP model was constructed to identify the origins of the scattered component of the exposure (Steele et al 2011) in the hope that this component can be reduced if its origins are known. This paper reviews:…”