Microdosimetric spectra and radial dose distribution of 252 Cf a particles were measured by a large volume cylindrical flowing tissue-equivalent proportional counter（TEPC）. Because a particles enter into the TEPC without a window

the probability of energy deposition per ion Rb is 1. The measured dose-mean specific energy z1D（Gy） from single-event varies with b’ the radial distance from the track core

according to the formula z1D=ab-x. where a = 0.328 and x=1.7 when b<0.6482 μm. and a = 0.288 and x = 2 when 6≥0.6482μm. The method to determine the maximum radial distance （penumbra） is given.