title: Low-energy Auger- and photo-electron effects on the degradation of thymine by ultrasoft X-irradiation.
abstract: OBJECTIVE
  To investigate quantitatively and qualitatively the production of thymine radicals produced by monochromatic ultrasoft X (USX) or (60)Co gamma-rays using electron paramagnetic resonance (EPR) spectroscopy.
  
  
  METHODS
  Thymine was chosen as the DNA component for the irradiation. The EPR experiments of irradiated thymine were performed using an X-band EPR device installed in a soft X-ray beamline (BL23SU) in SPring-8. Sample pellets were irradiated with USX photons in a microwave cavity in a vacuum chamber. EPR measurements of thymine powder pellets irradiated with USX photons at energies of 407 and 538eV were performed at 77 K or room temperature. For reference, (60)Co gamma-irradiation to a pellet was also performed at room temperature.
  
  
  RESULTS
  The following three features were found: (1) comparison between the two energies shows that the EPR dose-response curves are clearly distinguishable from each other: the curve for 407 eV saturated at a lower dose and spin number than that for 538 eV. (2) no evident qualitative difference between the radical species produced at the two energies was observed. (3) the EPR signal of the 538 eV USX-irradiated sample measured after annealing for 12 days is similar to that obtained with (60)Co gamma-irradiation.
  
  
  CONCLUSIONS
  The difference observed in the EPR dose-response relationship reflects the difference in the K-absorption cross-sections of carbon, nitrogen and oxygen in the thymine molecule which govern the photo-/Auger electron energy spectrum.
authors.affiliation: Risk Analysis Laboratory, Tokai, Jaeri, Japan. kakamatu@riskest.tokai.jaeri.go.jp
authors.forename: Ken
authors.initials: K
authors.lastname: Akamatsu
authors.forename: Kentaro
authors.initials: K
authors.lastname: Fujii
authors.forename: Akinari
authors.initials: A
authors.lastname: Yokoya
mesh_terms: D004307:Dose-Response Relationship, Radiation; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods*; D004583:Electrons; D018499:Linear Energy Transfer; D008968:Molecular Conformation / Q000528:radiation effects; D017785:Photons; D011829:Radiation Dosage; D013941:Thymine / Q000737:chemistry* / Q000528:radiation effects*; D014965:X-Rays
chemical_list: D013941:Thymine