Determination of double-layer gamma build-up factor using Monte Carlo code, FLUKA: Development of new empirical formula

Abstract

Designing shield for coupled neutron and gamma radiation field is a complex problem as it involves combination of high-Z and low-Z materials in composite or laminar form with optimized thickness. In the present work we have utilized general purpose open-source Monte Carlo code, FLUKA for estimation of gamma exposure build-up factors (EBF) of single and multilayer shields. Excellent agreement of FLUKA calculated results of EBF for iron, water and poly-methyl methacrylate (PMMA) (for different energy point-isotropic gamma source) with standard codes/database like EGS4 and ANSI, demonstrates the validity and reliability of this code for the present purpose. Subsequently, the code is utilized for determination of double-layer exposure build-up factor (DEBF) of iron/PMMA shield. In this regard, a new empirical formula is constructed for predicting DEBF of any arbitrary combination of layer materials, applicable over wide range of layer thicknesses and gamma energies. The coefficients of thus constructed empirical formula have been associated with radiation interaction parameters of individual components. Further, the proposed formula is shown to predict DEBF of iron/water shield with good accuracy, thereby ascertaining the robustness of the formula. The quantitative accuracy with which the proposed formula is able to predict DEBF for both Iron/PMMA and Iron/Water shield lies within 12% of FLUKA calculated data.