To further the understanding of the biological effects of radiation a knowledge of the spatial distribution of ionizations in small volumes is required. A cloud chamber capable of resolving the droplets formed on individual ions in the tracks of low-energy electrons has been constructed. It is made to high-vacuum specifications and contains a mixture of permanent gases and vapours, unsaturated before expansion, at a total pressure of 10 kPa. Condensation efficiencies close to 100% are obtained without significant background from condensation on uncharged particles and molecular aggregates.This paper describes the chamber, associated equipment and method of operation and discusses the performance of the system. Photographs of the droplets produced from the interaction of low-energy x-rays in the chamber gas for various modes of operation are presented. The mean energy loss per ion pair (W) for electrons produced by the interaction of A1 x-rays in the chamber gas (8130 Pa Hz, 700 Pa CzHjOH, 690 Pa HzO, 400 Pa He, 70 Pa air) has been measured as 29.8 0.7 eV per ion pair compared with a calculated value of 29.6 k0.4 eV per ion pair.The measurement and analysis of the spatial distribution of droplets will be presented elsewhere.
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