Characterisation of a 137Cs monoenergetic gamma radiation source used for instrument calibration at the Environmental Protection Agency using Monte Carlo simulations in EGSnrc

The Environmental Protection Agency employs an instrument calibration service for the whole of Ireland which is regularly used to calibrate radiation monitoring instrumentation such as survey meters, contamination monitors and electronic personal dosimeters for customers. The calibration service is accredited by the Irish National Accreditation Board to ISO17025 and ISO4037 standards. The calibration laboratory primarily uses a Hopewell GC60 (instrument that houses radioactive sources used for calibration) with a monoenergetic 137Cs source for its calibrations. The aim of this research project was to characterise the monoenergetic 137Cs radiation beam employed for calibration using experimental methods and to develop and validate a Monte Carlo simulation model of the source and calibration laboratory. The monoenergetic radiation beam was experimentally characterised by carrying out dose rate measurements and beam profiles of the radiation beam using a 1-litre ionisation chamber at distances from 100 cm to 500 cm from the 137Cs source and, for each distance, laterally from 0 cm (central axis) to a distance of 23.4 cm. At selected distances, beam profiles were also examined using EBT3 GafchromicTM film. Scattered radiation was measured in the laboratory using a Thermo Scientific EPD MK2.5s. A model of the calibration laboratory was developed in the Monte Carlo simulation package EGSnrc. To this end, models of the Hopewell GC60 source container, the 137Cs source and the 1-litre ionisation chamber were made within the specifications provided by the manufacturers, while a model of the room was constructed based on measured dimensions and other available information. The experimental dose rate measurements at 100 cm to 500 cm from the 137Cs source and laterally from 0 cm to 23.4 cm were replicated in the EGSnrc simulation to compare the beam profile with the experimental measurements. The scattered radiation in the simulated calibration laboratory was estimated by removing objects and measuring the difference this removal made on the dose rate measured by the simulated 1-litre ionisation chamber. To compare the measurements made by the 1-litre ionisation chamber and the measurements in the EGSnrc simulation, the results were normalised to the measurements taken at the zero point, which was the initial calibration position when the ionisation chamber was perfectly aligned with the radiation beam. The Zeta results reflect the good agreement between the experimental and simulated measurements for the 1-litre ionisation chamber, with all Zeta test results showing values below 2. Although the EBT3 GafchromicTM film measurements show good agreement with the measurements taken with the ionisation chamber, the simulated EGSnrc measurements and the calculated theoretical beam profiles at most distances, there were some discrepancies between some of the measurements made between the GafchromicTM film and the ionisation chamber measurements. This could be attributed to the long exposure time that was required to reach the minimum dose for the EBT3 GafchromicTM film using the EPA’s 137Cs source.