Effect of density on the total ionization yields in X-irradiated argon, krypton, and xenon

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Abstract
The amounts of ionization produced by absorption of X radiation in the liquids xenon, krypton, and argon are respectively 1.5, 1.4, and 1.2 times greater than those produced by the absorption of the same energy in the corresponding low density gases. The yields of free ions in the irradiated liquids were measured at applied electric fields 1–40 kV/cm and extrapolated to infinite field strength. The total number of ionizations per 100 eV absorbed was Gtot = 6.6 in xenon, 6.0 in krypton, and 4.5 in argon. The reason that the ionization yield is larger in the liquid than in the corresponding low density gas is partly that the energy gap Eg between the top of the valence band and the bottom of the conduction band in the liquid is smaller than the gas phase ionization potential IP. The ratio IP/Eg = 1.36 for xenon (Roberts and Wilson), 1.27 for krypton, and 1.17 for argon (from data of Jortner et al.). An extra source of ionization in the liquid might be reaction of the higher excited states, M* + M → [M2+ + e−].

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