Contents lists available at ScienceDirect Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem Modeling secondary particle tracks generated by intermediate- and low- energy protons in water with the Low-Energy Particle Track Simulation code Alexey Verkhovtsev a, ⁎ , Ali Traore a , Antonio Muñoz b , Francisco Blanco c , Gustavo García a a Instituto de Física Fundamental, Consejo Superior de Investigaciones Científicas (CSIC), Serrano 113-bis, 28006 Madrid, Spain b Scientific Computing Unit, Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT), Av. Complutense 40, 28040 Madrid, Spain c Departamento de Física Atómica, Molecular y Nuclear, Universidad Complutense de Madrid, Plaza de Ciencias 1, 28040 Madrid, Spain ARTICLE INFO Keywords: Track structure Monte Carlo simulations Nanodosimetry Low-energy electrons ABSTRACT Using a recent extension of the Low-Energy Particle Track Simulation (LEPTS) Monte Carlo code, we model the slowing-down of heavy charged particles propagating in water, combined with an explicit molecular-level description of radiation effects due to the formation of secondary electrons, their propagation through the medium, and electron-induced molecular dissociations. As a case study, we consider the transport of protons with the initial energy of 1 MeV until their thermalization, so that we cover the energy range that contributes mainly to the energy deposition in the Bragg peak region. In order to include protons into the simulation procedure, a comprehensive dataset of integral and differential cross sections of elastic and inelastic scattering of intermediate- and low-energy protons from water molecules is created. Experimental and theoretical cross sections available in the literature are carefully examined, compared and verified. The ionization cross section by protons includes recent experimental measurements of the production of different charged fragments. 1. Introduction Understanding radiation effects produced by charged projectiles traversing biological media is of great interest in radiation biology, radiation therapy, and environmental radiation protection. Advantages of ion-beam therapy (Schardt et al., 2010; Durante and Loeffler, 2010; Surdutovich and Solov'yov, 2014) over conventional radiotherapy with photons result from a characteristic energy deposition profile as a function of the traversed distance and, as a consequence, from higher relative biological effectiveness of ions as compared to other radiation modalities. The depth-dose profile for ions is characterized by the so- called Bragg peak positioned closer to the end of the ion's trajectory, where a significant amount of energy is deposited into a medium. An important feature of interaction of ionizing radiation with biological systems is the complexity of biodamage (García Gomez- Tejedor and Fuss, 2012). A thorough understanding of radiation therapy requires evaluation of molecular level effects related to dose deposition on the nanoscale (Blanco et al., 2013; Baccarelli et al., 2010). For that purpose, deep knowledge of numerous interactions induced by charged particles traversing living matter is strongly essential (Surdutovich et al., 2016). It is well understood nowadays that ions traversing a biological medium deposit their kinetic energy by ionizing or exciting molecules of the medium. Secondary electrons and other reactive species which are formed as a result of these processes may interact with biomole- cules and produce damage to them (Surdutovich and Solov'yov, 2014; García Gomez-Tejedor and Fuss, 2012). One of the commonly used methods to study these effects in detail is based on Monte Carlo simulations performed by the track structure codes (Blanco et al., 2013; Muñoz et al., 2005; Krämer and Kraft, 1994; Friedland et al., 2003; Nikjoo et al., 2006; Incerti et al., 2010). By sampling a sufficiently large number of tracks and averaging over the ensemble obtained, a Monte Carlo simulation can provide, to a high level of accuracy, insights into the mechanisms of the interaction of radiation with matter (García Gomez-Tejedor and Fuss, 2012). A Monte Carlo approach aims at the detailed simulation of trajectories of single particles in a medium, i.e. the complete track structure of the projectile and all secondary particles generated in the medium (Arce et al., 2015). Thus, a good quantification of interaction parameters in a wide energy range is required. A common way to precisely determine the physical and chemical events occurring on the nanoscale is to utilize models that can describe energy-loss processes in the medium in terms of interaction cross sections. Being the primary input for track structure codes, such data should include appropriate http://dx.doi.org/10.1016/j.radphyschem.2016.09.021 Received 31 May 2016; Received in revised form 9 September 2016; Accepted 17 September 2016 ⁎ Corresponding author. E-mail address: verkhovtsev@iff.csic.es (A. Verkhovtsev). Radiation Physics and Chemistry 130 (2017) 371–378 0969-806X/ © 2016 Elsevier Ltd. All rights reserved. Available online 20 September 2016 crossmark