A code for calculations of electron, ion and photon radiation action on tissue-equivalent matter (water vapor) by the Monte Carlo technique is presented. The new "fluctuation detector" method is efficient in evaluating of probability distributions and moments of absorbed energy and number of ionizations in small sites. Spatial and energy distributions of particles fluences and fluctuation characteristics of radiation action on spherical and thread-like sites of nanometer diameter are compared with various experimental and theoretical data and discussed. Non-equivalence of energy absorption and ionization events and consequences of that non-equivalence are numerically analysed. As an example of radiobiological application the yield of single- and double-strand breaks of DNA is calculated in a threshold model.

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The paper is devoted to the ionisation method for determination of various characteristics of energy deposited in small sensitive sites of irradiated objects by ionising radiation. It is shown that traditional ionisation methods supposing the direct proportionality between energy deposited in a site and the number of ionisations in it fails to determine fluctuation characteristics in nanometre sized sites. The exact formulae connecting fluctuation characteristics of deposited energy and those of the number of ionisations, are proposed. The new radiation parameters are defined, in particular, fluctuation generalisations of W value; the most important of them are calculated for radiations of different quality using Monte Carlo techniques. The way to correct theoretical and experimental results obtained by a traditional ionisation method is proposed.