Production of neutrons in laminated barriers of radiotherapy rooms: comparison between the analytical methodology and Monte Carlo simulations.

Gabriel Fonseca Da Silva Rezende,Alessandro Facure,Luiz Antonio Ribeiro Da Rosa

doi:10.1120/jacmp.v15i6.5035

Gabriel Fonseca Da Silva Rezende, Alessandro Facure + Show 1 more

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https://doi.org/10.1120/jacmp.v15i6.5035

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Abstract

The necessity to build or adapt radiotherapy rooms in reduced areas leads to the search for unconventional solutions for shielding projects. In most cases, adding metals to the primary barriers is the best alternative to shield rooms properly. However, when photons with energies equal or higher than 10 MV interact with high atomic number nuclei, neutrons are ejected and may result in a radioprotection problem for both outside and inside the room. Currently, the most widely used mathematical model to estimate the neutron dose equivalents, beyond the barriers composed by concrete and metal, is applicable only in very specific conditions. Moreover, a validation work of this model had not yet been performed. In this work, the Monte Carlo code MCNPX was used to check the validity of the aforementioned mathematical model for cases of primary barriers containing steel or lead sheets, considering the existence of linear accelerators of 15 or 18 MV. The results of the study showed that over 80% of the values obtained by computational simulations revealed deviations above a factor of 2, when compared to the analytical formula. This led to the conclusion that the McGinley method cannot be considered an adequate mathematical model to describe the mentioned physical phenomenon.PACS numbers: 87.56.bd, 02.70.Uu.

Highlights

With the high demand for linear accelerators able to operate with high-energy photon beams, the necessity of a careful review of the shielding projects for radiotherapy rooms that will house such equipments arises
Validation of photoneutron emission with MCNPX code From the observation of data obtained by Facure et al[4] it can be concluded that there is a good agreement between the values obtained using the MCNP code and those obtained by measurements
Neutron ambient dose equivalent rates beyond laminated barriers The ambient dose equivalent rate for neutrons obtained by the McGinley method and by the MCNPX code can be found in Tables 3, 4, and 5

Summary

Introduction

With the high demand for linear accelerators able to operate with high-energy photon beams, the necessity of a careful review of the shielding projects for radiotherapy rooms that will house such equipments arises. A key point of the project is the study of what materials will be used in the construction of the rooms, so that they become viable both from an economic perspective and from the point of view of available space for the building. A widely used material for shielding of radiotherapy rooms is ordinary concrete. The preference for this material is due to its physical characteristics and relatively low cost. The exclusive use of ordinary concrete to shield rooms that will house high-energy accelerators can produce barriers thick enough to preclude adaptations of pre-existing rooms or even the construction of new rooms in reduced spaces. The reason for that is the large variation between tenth-value layers (TVL) of concrete for low and high-energy photons.[1]

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