Monte-Carlo simulation of ion track structure in water: ionization clusters and biological effectiveness

The biological action of swift ions decreases for increasing stopping power (LET), and the loss of efficiency is particularly high for heavy ions when the LET reaches its maximum value. In one of the models developed in the l iterature 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 repro duced when clusters with E greater than or equal to 340 eV in spheres of ra dius R approximate to 4.54 nn are considered, with an efficiency approximat e to 4%, in very good agreement with the literature. However, these paramet ers cannot be applied to uranium ions (here 1.4 MeV/u U32+): the calculated number of clusters is approximate to 5 times larger than expected, a resul t which sets a limit to the cluster model. (C) 1998 Elsevier Science B.V. A ll rights reserved.