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Qualitative and Quantitative Analyses of the Decomposition Products that Arise from the Exposure of Thymine to Monochromatic Ultrasoft X Rays and 60Co Gamma Rays in the Solid State

Abstract Akamatsu, K., Fujii, K. and Yokoya, A. Qualitative and Quantitative Analyses of the Decomposition Products that Arise from the Exposure of Thymine to Monochromatic Ultrasoft X Rays and 60Co Gamma Rays in the Solid State. Radiat. Res. 161, 442–450 (2004). HPLC analyses of condensed thymine irradiated with monochromatic synchrotron ultrasoft X rays in the energy region around nitrogen and oxygen K-shell edges were performed. Cobalt-60 γ rays were used as a reference radiation. The radiation chemical dose response of each separated thymine decomposition product was also determined. Uracil (U), 5-(hydroxymethyl)uracil (HMU), 5,6-dihydrothymine (DHT), 5-formyluracil (foU) and four main unknown products were found in the HPLC chromatogram of the sample irradiated with ultrasoft X rays in vacuo. Similar spectra of the products were also found in the γ-ray experiment; however, some unknown products that appeared after elution of the thymine peak were significantly larger than those in the ultrasoft X- ray experiment. This result indicates the difference in radiation quality. The G value of DHT produced by γ radiation was 10 times larger than those produced by the ultrasoft X- ray photons with energies of 395 and 407 eV corresponding to below and on the nitrogen K-shell edge, respectively. This result suggests that the differences in the photon energy and/ or in the energy spectra of the secondary electron between ultrasoft X rays and γ rays are causing differences in the process of the radiation chemistry. Moreover, the yields of all the thymine decomposition products induced by 538 eV photons (oxygen K-shell edge) were significantly smaller than those induced by photons around the nitrogen K-shell edge. The K-shell excitation of oxygen in thymine may efficiently promote the production of small thymine fragments susceptible to desorption from the sample.