Advances in Cloud-Chamber Techniques and Measurements of W Value in a Tissue-Equivalent Gas

Abstract

A cloud chamber which can make visible individual ions in the tracks of low-energy electrons and ..cap alpha.. particles in available at Harwell. Work on the theory of droplet formation and growth has enabled a tissue-equivalent gas of lower density than previously possible to be used in the chamber. This has enabled even the closest ions in particle tracks to be individually resolved. Techniques have been developed to provide more accurate assessment of the gas and vapor pressures of the various components in the gas mixture. An x-ray set has been built to allow electrons produced from carbon-characteristic x rays (0.28 keV) to be studied. W values have been calculated and results of 35.9 +- 0.4 and 31.0 +- 0.6 eV (ion pair)ka have been obtained for electrons from carbon and aluminum x rays, respectively. Fano factors have been found to be in the region of 0.25 for carbon x rays and 0.4 for aluminum x rays. A /sup 244/Cm ..cap alpha.. source with suitable filtration has been used to study ..cap alpha..-track segments at energies of 5.0, 3.5, and 1.1 MeV. W values of 32.5 +- 3.5, 30.2 +- 3.5, and 33.7 +- 3.5 eV (ion pair)ka, respectively, havemore » been obtained.« less