Tissue spheres 0.25, 0.5, 1.0, 2.0, 4.0 and 8.0 μm in diameter were simulated using a wall-less spherical counter filled with propane-based tissue-equivalent gas. Microdosimetric spectra corresponding to these site sizes were measured for five neutron energies (0.22, 0.44, 1.5, 6 and 14 MeV) and the related mean values$\overline{y_{{\rm F}}}$ and$\overline{y_{{\rm D}}}$ were calculated for several site sizes and neutron energies. An elaborate calibration technique combining soft X rays, an55 Fe photon source and a collimated${}^{244}{\rm Cm}$ α-particle source was used throughout the measurements. The spectra and their mean values are compared with theoretically calculated values for ICRU standard tissue. The agreement between the calculated and the measured data is good in spite of a systematic discrepancy, which could be attributed, in part, to the difference in elemental composition between the tissue-equivalent gas and plastic used in the counter and the ICRU standard tissue used in the calculations.

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