A proportional counter for measurement of the bio-effectiveness of ionising radiations at the DNA level

Abstract

A description is given of a novel single grid, parallel-plate proportional counter, operated at low pressures (1 Torr) of propane gas for use as a nano-dosimeter. The sensitive volume has a mean chord dimension equivalent to 1.85 nm at unit density and a response function simulating that of a double-stranded segment of the DNA. The detector must be capable of extracting, detecting and resolving single primary electron charges released by ionising radiation traversing the sensitive volume. This is achieved by operating the detector with large internal amplification (∼10 6), minimising electronic `noise', reducing background signals and by optimising sensitivity. Observed charge spectra were processed using a deconvolution procedure coupled with a least square minimisation to determine the mean number of primary charges per event and the frequency distributions of the component integer charge spectra. Detailed results in the form of damage efficiency factors are presented for irradiation of a single target volume by alpha particles (Am-241; efficiency 70%), fission neutrons (Cf-252; 36%), and low energy photons (Co-57; 9.5%). As a test, the experimentally derived damage efficiency factors quoted above are compared with effect cross-sections for radiobiological damage in mammalian cells. The results are satisfactory but further improvement in the signal-to-noise ratio for single charges is required for sparsely ionising radiation.