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Beyond the Modified Kersey Method

Abstract of the technical paper/presentation presented at:
Canadian Organization of Medical Physicists Annual Scientific Meeting
July 13, 2017

Prepared by:
Adam Dodd and Kevin Shimotakahara
Canadian Nuclear Safety Commission



The purpose of this study was to calculate dose rates from scattered neutrons and capture gamma rays beyond the maze entrance of a generic linac vault, because the National Council on Radiation Protection and Measurements Report No. 151 “Structural Shielding Design and Evaluation for Megavoltage X- and Gamma-Ray Radiotherapy Facilities” provides no information on this.


The Monte Carlo code MCNP6 was run in coupled neutron-photon mode, treating the linac as a point source of neutrons to calculate effective doses. The sensitivity of the results to the vault dimensions, neutron source energies, types of concrete, and the position of the X-ray target was investigated. The results were compared with experimental results for the neutron dose rate and energy spectra within a vault housing a Varian 21EX at the Montreal General Hospital.

Results and discussions

The Monte Carlo simulation reproduced the experimental results taken at the Montreal General Hospital reasonably well. Photon and neutron fluences, energy spectra and effective doses are given for different points both inside and outside a range of generic vaults.


The results show that the dose rates away from the maze entrance fall exponentially, with an exponent that depends on the distance between the maze entrance and the opposite shielding wall.

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