Survivability of Fullerenes under Γ-irradiation in the Interstellar Medium

Introduction: Although the presence of fullere-nes in the interstellar medium remains a subject for considerable discussion and waits for an unambiguous experimental confirmation, it is generally assumed that the large spherical carbon clusters can be generated within circumstellar envelopes of mass-losing carbon-rich stars [1-14]. The detection of fullerenes in carbo-naceous chondrites, in the geological strata of the Cretaceous-Tertiary and the Permian-Triassic boundary layers, associated with bolide impacts, can be considered as an indirect evidence of their formation in circumstellar envelopes [15-19]. Besides that, the presence of fullerene species in the extraterrestrial samples found on Earth implies that these compounds are able to survive very long passages through diverse space environments. Some of them (especially those in close proximity to the stars where the fullerenes can be produced) are characterized by strong ionizing radiation. One can expect that any kind of radiation (UV, proton, γ-irradiation, etc.) decomposes the newly formed fullerene (as any other carbon-containing) molecules, thus rising a question on the fate of fullere-nes in harsh radiation environments. In other words, the existence of the large spherical carbon clusters must finally depend on competition between the rates of their formation and radiation decomposition. Despite fullerene chemistry as a whole remains a subject for numerous studies, there is an obvious lack of experimental data in this area. We tried to roughly estimate the capability of C 60 , the most common member of fullerene family, to withstand prolonged γ-irradiation. We exposed C 60 to very high irradiation doses exceeding 6 MGy, and analyzed the irradiated samples by high-performance liquid chromatography (HPLC) and infrared (IR) spectroscopy. Experimental: Crystalline fullerene C 60 (from MER Corporation, 99.5%+ purity), toluene (Aldrich, 99.8%, HPLC grade) and methanol (EM Science, HPLC grade) were used as received. C 60 was finely ground, and two sample series were prepared: (1) dry samples, 50 mg C 60 each (in vials open to air); and (2) 50 mg C 60 mixed under ultrasonication with 0.5 ml of distilled deionized water, sealed in glass vials. The irradiation experiments were performed in a Cobalt-60 source facility at ICN UNAM at room temperature. The samples were placed as close as possible to the radiation source, at dose rate of 9.7 kGy per hour. Irradiation periods were 255, 534 and 640 h, correspond