Preliminary insight into dose deposition processes in sedimentary media on a scale of single grains: Monte Carlo modelling of the effect of water on the gamma dose rate Guillaume Guérin * , Norbert Mercier Institut de Recherche sur les Archéomatériaux, UMR 5060 CNRS e Université de Bordeaux, Centre de Recherche en Physique Appliquée à l’Archéologie (CRP2A), Maison de l’archéologie, 33607 Pessac cedex, France highlights < Monte Carlo simulations allowed us to study dose deposition processes in sediments. < Simulation tools are presented and their performance is discussed. < Beyond matrix effects, geometry parameters are of paramount importance. < Grain/pore size is the most important parameter in such grain scale effects. article info Article history: Received 2 February 2011 Received in revised form 26 April 2012 Accepted 2 May 2012 Keywords: Gamma dose rate Monte Carlo simulations GEANT4 Moisture effect abstract Water in soil affects the dose rate in sedimentary media. First it dilutes the radioelements, and second the mass stopping power as well as photon interaction cross sections are greater in water than in typical sediment constituents (Aitken, 1985). Here the effect of moisture on the gamma dose rate is investigated using Monte Carlo simulations based on the GEANT4 toolkit. Dose deposition processes are studied on the scale of individual grains in modelled well-sorted sediments: detailed tracking of gamma rays and secondary electrons shed light on the relative importance of the different interaction modes at stake, as a function of grain size, porosity and fraction of saturation of the sediments, as well as the energy of gamma rays. A description of the specifically designed GEANT4 codes is provided, followed by a comprehensive analysis of the phenomena. It is shown that geometry effects, specifically grain size and compactness, have a great impact on gamma dose rates received by sediment grains and that these can be quantified in the case of very simple grain assemblages. For a better accuracy in retrospective dosimetry dating methods, morphological features of the sediments that influence dose rates should be characterized and their influence on dose rate studied; Monte Carlo transport codes seem to be the favoured tools for that purpose. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction In retrospective dosimetry dating methods such as lumines- cence, the dose rate is affected by moisture in the environment. Zimmerman (1971) evaluated the effect of moisture content on alpha, beta and gamma dose rates for the case of pottery dating with an approach based on what Aitken (1985) called “the infinite matrix assumption”. In that case radioactivity measurements are performed on dry bulk samples, and dose rates are calculated using the deduced radioisotopic contents multiplied by conversion factors (Guérin et al., 2011). The measured or estimated water content is then taken into account: not only does water dilute the radioisotope concentration in sediments, but also the mass stop- ping power and attenuation cross sections of water are higher than those of typical sediments. Zimmerman (1971), followed by Aitken (1985), proposed the following equation to calculate the annual gamma dose in a moist medium: D w ¼ D d 1 þ xWF (1) where D d is the dose in the medium when dry, W is the saturation water content expressed as (weight of water/dry weight), F is the fraction of pore space occupied by water and x is the correction * Corresponding author. Tel.: þ33 5 57 12 10 84; fax: þ33 5 57 12 45 50. E-mail address: guillaumeguerin@ymail.com (G. Guérin). Contents lists available at SciVerse ScienceDirect Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas 1350-4487/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2012.05.004 Radiation Measurements xxx (2012) 1e7 Please cite this article in press as: Guérin, G., Mercier, N., Preliminary insight into dose deposition processes in sedimentary media on a scale of single grains: Monte Carlo modelling of the effect of water on the gamma dose rate, Radiation Measurements (2012), doi:10.1016/ j.radmeas.2012.05.004