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

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Abstract
A cloud chamber which can make visible individual ions in the tracks of low-energy electrons and .alpha. particles is 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 were developed to provide more accurate assessment of the gas and vapor pressures of the various components in the gas mixture. An X-ray set was built to allow electrons produced from C-characteristic X-rays (0.28 keV) to be studied. W values [mean energy lost per ion pair] were calculated and results of 35.9 .+-. 0.4 and 31.0 .+-. 0.6 eV (ion pair)-1 were obtained for electrons from C and Al X-rays, respectively. Fano factors were found to be in the region of 0.25 for C X-rays and 0.4 Al X-rays. A 244Cm .alpha. source with suitable filtration was used to study .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)-1, respectively, were obtained.