Finite and infinite system gamma ray buildup factor calculations with detailed physics Haluk Atak, Osman Şahin Çelikten, Mehmet Tombakoğlu n Hacettepe University, Department of Nuclear Engineering, Beytepe, Ankara 06800, Turkey HIGHLIGHTS  Gamma ray buildup factors have been generated both for finite and infinite ordinary water systems.  The importance of detailed physics calculations is emphasized for low energy gamma rays.  Results of finite and infinite calculations are compared. article info Article history: Received 25 September 2013 Received in revised form 14 April 2015 Accepted 17 July 2015 Available online 20 July 2015 Keywords: Buildup factor Buildup Radiation shielding Coherent scattering Monte Carlo method MCNP abstract Examination of physical interactions of photons in materials is a significant subject for buildup factor studies. In most of the buildup calculations, by default, coherent (Rayleigh) scattering is ignored and the Compton scattering is modeled by free-electron Klein–Nishina formula with “simple physics” treatment. In this work, photon buildup factors are calculated for many different cases including “detailed physics” by taking into account coherent and bound-electron Compton scatterings with the Monte Carlo code, MCNP5, and the results are compared with the literature values. They are computed for point isotropic photon sources up to depths of 20 mean free paths and at the three photon energies most widely used (0.06, 0.6 and 6 MeV). Calculations are made for both finite and infinite homogeneous ordinary water media. It is concluded that Coherent scattering is very dominant at low energies and for deep pene- trations and assumed physical approximation (simple/detailed, finite/infinite) is the critical point for determining shielding material dimensions. After all, it can be stated that all parametric assumptions should be clearly given and indicated in the tabulation of photon buildup factors. & 2015 Elsevier Ltd. All rights reserved. 1. Introduction Shielding design calculations are carried out in order to protect the biological tissues and also the materials from the hazards of ionizing radiation. However, interaction of photons in the shield- ing medium occurs in a complex manner producing new (secondary) and scattered photons with different energy and directions. Hence, physicists and engineers should make some corrections to their calculations considering these secondary and scattered radiations. For this purpose, “the buildup factor” is used as a multiplier to the uncollided fraction of the incident photon beam response in the detecting material after passing the shield- ing. The response (or the physical quantity of interest) can be number and energy flux density, absorbed dose in water and tissue or exposure. It is of prime importance, yet not the general case in the lit- erature, to state explicitly which quantity of interest is used in the calculations. In addition, since the buildup factors are traditionally given for infinite medium calculations in the literature (Goldstein and Wilkins, 1954; Chibani, 2001; Harima et al., 1991; Hirayama, 1995), their utilization is not suitable for shields of finite thickness. In most of the studies, calculations of the interactions of photons within the medium are performed by using “simple physics” treatment in which coherent scattering is ignored and the Compton scattering is modeled using free-electron Klein–Nishina formula. In addition, no assumptions about the geometry of the system and the physical interaction mechanisms are given in the buildup factor calculations in some studies (Sardari et al., 2009, 2011). All these factors undoubtedly introduce errors to dose cal- culations depending on the type and thickness of the material and energy of the photon source. In this work, the main goal is to show the importance of using the proper definition of buildup factor in dose calculations. For this Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/apradiso Applied Radiation and Isotopes http://dx.doi.org/10.1016/j.apradiso.2015.07.019 0969-8043/& 2015 Elsevier Ltd. All rights reserved. n Corresponding author. E-mail addresses: haluk.atak@hacettepe.edu.tr (H. Atak), osc@hacettepe.edu.tr (O.Ş. Çelikten), mtombak@hacettepe.edu.tr (M. Tombakoğlu). Applied Radiation and Isotopes 105 (2015) 11–14