Microdosimetric Properties of Electron Tracks Measured in a Low-Pressure Cloud Chamber

Abstract

A cloud chamber which can make visible individual ions in the tracks of low-energy electrons and ..cap alpha.. particles is available at Harwell. Work on the theory of droplet formation and growth has enabled a tissue-equivalent gas with a density of 8.8 X 10/sup -3/ kg m/sup -3/ to be used in the chamber. This is half the density of previous mixtures. This has enabled ions in particle tracks as close as a distance equivalent to 0.5 nm in water at 1000 kg m/sup -3/ to be individually resolved. Extensive spatial analysis was carried out on electrons from aluminum X rays, and the results were compared with those previously obtained in an earlier gas mixture. The accuracy of these results is limited mainly by diffusion of droplets during growth (previously calculated to be equivalent to (2.8 +/- 1.5) nm in water at 1000 kg m/sup -3/). Track lengths and mean LET values were derived and shown to be in agreement with other work. In order to provide more detailed imformation on track structure, Rossi ''Y'' distrbutions were calculated from target sizes of 10 and 50 nm in water. Interdroplet distance distributions were also derived. These distributions were compared with Montemore » Carlo calculations and the agreement was found to be within 10%. This increases confidence in both the cloud-chamber analysis and the Monte Carlo calculations.« less