Monte-Carlo simulation of ion track structure in water: ionization clusters and biological eectiveness C. Champion, A. L'Hoir * , M.F. Politis, A. Chetioui, B. Fayard, A. Touati Groupe de Physique des Solides, Universit es Paris 7 et Paris 6, CNRS UMR 75-88, Tour 23, 2 place Jussieu, 75251Paris Cedex 05, France Abstract The biological action of swift ions decreases for increasing stopping power (LET), and the loss of eciency is particularly high for heavy ions when the LET reaches its maximum value. In one of the models developed in the literature to account for this behavior, the biological action of light ions (and X rays) is attributed to clusters of energy deposition by low energy electrons. We have performed Monte-Carlo simulations to test this model for heavy and light ions (here uranium and helium ions). For helium ions, the experimental inactivation cross sections variations with LET are well reproduced when clusters with E P 340 eV in spheres of radius R  4.54 nm are considered, with an eciency 4%, in very good agreement with the literature. However, these parameters cannot be applied to uranium ions (here 1.4 MeV/u U 32 ): the calculated number of clusters is 5 times larger than expected, a result which sets a limit to the cluster model. Ó 1998 Elsevier Science B.V. All rights reserved. PACS: 87.50.GI Keywords: Cell inactivation; Clusters of ionization; Heavy ions 1. Introduction One major aim of radiation biophysics is the understanding of mechanisms leading to a bio- logical endpoint such as chromosome aberrations, cell inactivation or cell transformation. Parameters such as the dose, the stopping power (or linear energy transfer, LET) and the relative biological eciency (RBE: c rays of 60 Co taken as a refer- ence) are mean values which have been proved to be inappropriate for understanding radiation damage. The objective is hence to develop mech- anistic models based on the identi®cation of the primary events which lead to the biological eects. From experimental and theoretical arguments, it has been soon realized that spatial distribution of the energy deposition plays an important role in determining radiation eectiveness. In particular, clusters of damages occurring at clusters of energy deposition are thought to be less reparable lesions. Two main approaches are being used for biologi- cal damage: one consists in looking for particular clustering properties of the track; the other ex- plicitly takes into account the nature and ar- rangement of biological targets. In the critical lesion approach to interpreting track-structure, it is assumed that a single radia- Nuclear Instruments and Methods in Physics Research B 146 (1998) 533±540 * Corresponding author. Tel.: 33 1 44274674; fax: 33 1 44277309; e-mail: lhoir@gps.jussieu.fr 0168-583X/98/$ ± see front matter Ó 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 4 3 8 - 8