The photon W-value of dry air in the energy range of 2.2-3.9 keV has been measured with relative expanded uncertainty of 0.84% (coverage factor k = 2). Monochromatized synchrotron radiation was used for both measurements of the radiant power by means of a cryogenic radiometer and of photoion currents for ion-pair production by means of a multi-electrode ion chamber technique. The evaluated dependence of the photon W-value for dry air is consistent with the previously reported data for electrons and seems to show a small change at the Ar 1s ionization threshold. These findings were well reproduced with a model calculation using atomic photoionization data. Mass attenuation coefficients for air have been observed in the present soft X-ray region with a small energy step, which are close to data previously reported.
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