Determination of scattered gamma radiation in the calibration of environmental dose rate meters

Practical free-field and shadow-shield calibration techniques using a variety of environmental dose rate meters were studied, and experimental and theoretical determinations were made of the contribution of scattered photons to the air kerma rate from certificated Cs-137, Co-60 and Ra-226 gamma sources in different calibration geometries.

Scattered air kerma rates from surrounding media were calculated with a Monte Carlo code and matched closely the experimental results. Calibration experiments comprised measurements in both open-field and garden environments to see how nearby buildings affected the detector responses. Insignificant differences of the order of 1 % between the results for the two geometries were found both experimentally and theoretically.

It is thus concluded that the scattered radiation from surrounding buildings farther away than around 15 m from a calibration set-up contributes negligibly to the detector response relative to that from ground and air. Shadow-shield measurements were used to deduce the contribution to the response from the scattered radiation in free-field geometries and the experimentally obtained results were found to agree agree extremely well with those calculated.

The Monte Carlo code used enables the scatter components from ground and air to be separated. Calculated relative air kerma rates scattered from ground and air for the radionuclides Cs-137, Co-60 and Ra-226 are listed with the aim of recommending these values in practical free-field calibrations of environmental dose rate meters at source-to-detector distances of 0.5 to 50 m.