Lifetime of the unpaired electron species in calf thymus DNA thin films induced by nitrogen and oxygen K-shell photoabsorption

Purpose: To study the lifetime of the short-lived unpaired electron species induced in a calf thymus DNA film by soft X-rays in the energy regions around nitrogen and oxygen K-edge. Materials and methods: Dry calf thymus DNA films were measured by electron paramagnetic resonance (EPR) spectrometer during the monochromatic soft X-ray irradiation provided from a synchrotron accelerator (SPring-8, Super Photon ring-8 GeV). Two operation modes of SPring-8 with different electron bunch structures were used to explore dependence of EPR intensity on pulse interval of soft X-rays. Results: Observed EPR spectra did not show significant difference, in the spectral shape, intensity, g-factor and photon energy dependence, between the two bunch modes. Conclusions: The total yield of the unpaired electron species were estimated by assuming the exponential relaxation of the unpaired electrons with two different bunch modes of the synchrotron. The lifetime of the unpaired electron species is estimated to be in the range of a few tens to several hundreds of μs.