Cross-sections for the inactivation of ribonuclease by slow heavy ions

Average cross-sections for inactivation of dry ribonuclease by H, He and N projectiles with energies between 100 eV and 10 keV are presented. The trend of the damage cross-sections with particle energy is consistent with the trend of the molecular quasi-elastic scattering and ionization cross-sections. A tentative radiation damage model is proposed in which single hit damage attributed to direct physical interactions by the primary and secondary radiations is supplemented by chemical action of the elastically scattered knock-on atoms which inactivate additional targets with an efficiency of 50-100%. Further experiments at energies below 100 eV are required for positive confirmation of the chemical efficiency. Free electrons (sub-ionizing) are chemically relatively innocuous (less than 10% efficiency). Saturation effects are satisfactorily accounted for in this model. Inactivation penetration depths measured in the microcrystalline enzyme agree with the theoretical predictions if a residual range of about 8 mug cm-2 is included. The mean energy expenditure to produce inactivation of a ribonuclease molecule is found to vary from 30 eV for 100 eV protons to 220 eV He and N ions.