Measurement of the W-value of low energy electrons

The W-value of low energy electrons between 16eV and 5keV has been measured for methane and the tissue-equivalent gas (T.E. gas) of Rossi and Failla. For high energies (approximately 5 keV) W-values were observed of 27.2 eV for methane and 29.4 eV for T.E. gas. At lower energies the W-values were found to increase; in addition there was seen a relative maximum of the W-value at about 300 eV for methane and between 500-600 eV for TE gas. The differential W-value, ω=dE/dN, evaluated from W(E) curves, has corresponding peaks between 250 and 300 eV for both gases. The peak of the T.E. gas curve is however asymetric extending to higher energies. This asymetry could be explained by a second peak of minor height at about 500 eV. It is not impossible, therefore, that these peaks are related to the binding energy of the K-shell electrons of the various atoms comprising the gas molecules. Both curves of ω as a function of energy show a second peak at about 30 eV. A comparison is made with other measurements of W for electrons and photons and the difference between the measured W-values of photons and electrons is discussed