Track theory rested on the foundation of the radial distribution of dose from δ rays as the central contribution of atomic physics to heavy ion radiobiology. Here, a new calculation of the radial distribution of dose is applied, in which the classical angular distribution of dose of delta rays and a logarithmic polynomial representation of the electron range-energy relation are used, to form the basis of the present thindown calculation. Calculations of inactivation cross sections for heavy ions in the track width regime displaying thindown for E. Coli B/r and Bs-1, and for Bacillus Subtilus are straightforward for these are 1-hit detectors, Calculations for V-79 hamster cells are more complex. They follow the original development of this model for eucaryotic cells, and make use of the cross sections calculated for hypothetical internal targets which are then asserted to be proportional to the measured cellular inactivation cross sections. The results are in reasonable agreement with experimental data.