Additional experimental evidence is presented in support of the recent hypothesis that a possible solar influence could explain fluctuations observed in the measured decay rates of some isotopes. These data were obtained during routine weekly calibrations of an instrument used for radiological safety at The Ohio State University Research Reactor using 36 Cl. The detector system used was based on a Geiger-Müller gas detector, which is a robust detector system with very low susceptibility to environmental changes. A clear annual variation is evident in the data, with a maximum relative count rate observed in January/February, and a minimum relative count rate observed in July/August, for seven successive years from July 2005 to June 2011. This annual variation is not likely to have arisen from changes in the detector surroundings, as we show here.

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.

ISO standard 4037 specifies that for calibrating protection level dosimeters, scattered radiation should contribute less than 5% of the exposure. In previous work, the authors reported the results of an MCNP analysis of the shadow shield technique that was performed for a calibration range with a Cs irradiator. This paper examines the energy distribution of the photons contributing to the exposure percent scatter (S%) and the detailed origin of the scatter that originates in the irradiator. In summary, it reports that: 1) the majority of S% is due to photons with energies that are significantly below the source energy, 2) a significant percentage of S% is due to photons that scatter within the source and source capsule walls, and 3) S% due to scatter within the irradiator is even more significant than previously reported.

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