On Possible Limitations of Experimental Nanodosimetry

Experimental microdosimetry is based on the assumption that energy deposited in a simulated volume can be approximated by the product of the number of ionisations detected and the W value. Energy deposited, however, is a stochastic variable representing the energy transfer expended in ionisation and excitation events. The proportionality between energy transfer and ionisation is only valid for large numbers of interactions per event, a condition usually satisfied in microdosimetry but not necessarily in nanodosimetry. Monte Carlo computer simulations of particle tracks in water vapour show that minimum ionising electrons traversing 1 nm spheres produce on average, 1.4 energy transfers per event. Simulated proportional chamber measurements of single event spectra are dominated by ion counting statistics rather than true energy deposition, with subsequent determination of average lineal energy being more than two times the true value. The interpretation of such data is discussed, as are conditions for which meaningful experimental data can be obtained.