| Research Abstract |
Using monoenergetic X-rays from irradiation systems at the Phorom Factory, National Laboraotry for High Energy Physics, Tsukuba, absorption specturm of DNA film and action spectrum of DNA strand-break induction were measured around K-absotption edge of phosphorus (2.143 keV, 5.787 A). The main results are summatized as follws:
Absorption aspectrum of DNA: Calf thymus DNA (Na-salt, Sigma type I) was dissolved in distilled water, spreaded over a plastic sheet and dried to form DNA film The absorption spectrum had a large resonance peak at 2.153 keV, a small peak at 2.167 keV and a minimum between them at 2.159 keV. The irradiations, therefore, were performed at the peak energy and at lower and higher energies, 2.147 and 2.160 keV.
DNA strand-breaks: Closed circular (Form I) pBR322 DNA was irradiated in vacuum (10-6 Torr), redissolved, and separated by agarose electrophoresis from form II (open circular) and III (Iinear). Survival curve of the form I remaining fraction and induction curvo of form III fraction were obtained. Form those curves fosed (D0) giving on strand-break (single-and double-strand) were calculated. Exposure dose (R) mcasured by an ionization chamber was converted to absorbed dose nGy) by calcuated conversion factors, 0.84, 1.86 and 1.02 cGy/r at 2.147, 2.153 and 2.160 keV, respectively. For single-strand breaks, D0 were almost same, 6.6-7.3 kGy, at 3 energies. For double-strand breaks, D0 at the peak energy was 103 kGy, which was somewhat lower than those at other enetgies.
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