The ultrafast transient (10-14 to 10-12s) thermal and mechanical response of water subject to ionizing radiations of different linear energy transfers has been investigated in order to understand the initial events which lead to cell mutation and lethality. Based on computational fluid dynamics, the production of a 'thermal spike' around the trajectory of a charged particle and subsequent diffusion of deposited heat are calculated for particles with linear energy transfer (LET) of 4, 40, and 400 keV/μm. A radiation damage region (that is, the so-called 'thermal core') is identified, and the transient behavior of the thermal core is studied. The local and transient environment has a dimension of nanometers, a scale which is of critical interest in understanding mechanisms of radiation damage in cells. The radius of the thermal core, D(d), at temperatures (or internal energy density) of up to 1000 K, is observed to increase with LET, L, as D(d) (in nanometers) = C4 · L (in keV/μm)0.6, where, for example, C4 = 0.50 for T = 800°C.
