title: Phenomenological explanation of cell inactivation cross section in terms of direct and indirect action.
abstract: OBJECTIVE
  The aim of this study was to use survival curves data for the inactivation of V79 cells and CHO-K1 cells by protons, neutrons, C12 ions and He3 ions to study the role of direct and indirect action in cell inactivation.
  
  
  METHODS
  A large number of survival curves for the inactivation of V79 cells by protons, neutrons, and C12 ions and for CHO-K1 cells inactivated by He3 ions over a wide energy range were taken from published references. Experimental data points were extracted from the published survival curves using MATLAB (Version 7.0) and fitted to the linear quadratic equation. The fit parameters were used to calculate the inactivation cross section (σ) at the initial slope, the 2Gy dose and at 10% survival for each particle type separately.
  
  
  RESULTS
  The results, in general, showed that the inactivation cross section decreases nearly exponentially when increasing the mean free path for primary ionisation (λ), except in the case of protons, and to some extent neutrons, where the cross section takes a constant value at specific λ values. The cross section increased with increasing linear energy transfer (LET) and also became independent of LET at specific LET values.
  
  
  CONCLUSIONS
  The results indicate that the cell damage due to the double strand breaks of DNA caused by indirect action is much larger than that caused by the direct action.
authors.affiliation: University of Mosul, College of Education, Department of Physics, Mosul, Iraq.
authors.forename: Faika A
authors.initials: FA
authors.lastname: Azooz
authors.forename: Suaad A
authors.initials: SA
authors.lastname: Meerkhan