Microdosimetric Measurements of Ionization by Monoenergetic Photons

Microdosimetric event distributions for monoenergetic photons at eight primary photon energies between 12 and 1250 keV have been determined from measurements of ionization with a wall-less proportional counter. Event spectra were taken at simulated cavity diameters of 0.24, 0.48, 0.96, 1.9, 3.9, and 7.7 ..mu..m. Distributions of dose in lineal energy density, the microdosimetric equivalent of LET, are displayed as functions of site diameter and photon energy. The frequency mean and dose mean of lineal energy density are calculated and their behavior as a function of primary photon energy is discussed and compared with elementary theoretical predictions. Our results are consistent with previous microdosimetric measurements of photon radiation, and in agreement with the limited theoretical analysis available.