The mean energy required to form an ion pair (W) is measured for photons having energy 0.277, 1.49, and 5.89 keV in several polyatomic gases and two tissue-equivalent gas mixtures. An increase of W values with decreasing energy is observed. Generally, a continuous decrease of W with an increasing number of atoms in a series of organic compounds was reported (ICRU Report No. 31), while our measurements show an oscillation of W values in alkanes; the W value for an alkane with an even number of carbon atoms in the molecule (ethane and butane) is lower than that for the next higher alkane with an odd number of C atoms (propane and pentane, respectively). The W value for low-energy electrons was calculated from experimental photon W values. A good agreement between the calculated and the experimental W values in the low-energy region, and the high-energy W values is obtained. Our results fill the gap between the very low-energy region (below 300 eV) and the high-energy (above 10 keV) region.

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