Production, Extraction, and Identification

Preparation of the isotopes of astatine is more difficult than with most radionuclides, as they cannot be synthesized by neutron irradiation this precludes the use of a nuclear reactor. To date, the bulk of 

Numerous nuclear reactions have been employed to produce astatine. Three of these are particularly suited for routine preparation of the relatively long-lived isotopes with mass numbers 209, 210, and 211. The most frequently used is the ° Bi(a,xn) At (a = 1-4) reaction, in which bismuth 44, 74,120) or bismuth oxide (7,125) is bombarded by 21-to 40-MeV a-particles. The ° Bi(He, xn) At reaction can also be used to produce isotopes of astatine 152), the nuclear excitation functions (62) favor a predominant yield of ° At and °At. The routine preparation of astatine is most conveniently carried out through the ° Bi(a,xn) At nuclear reactions, from which a limited spectrum of astatine nuclides may be derived. The excitation functions for these nuclear reactions have been studied extensively (78, 89, 120). The 

Astatine can be readily and routinely obtained in an inorganic form suitable for chemical and biomedical application from the a-irradiated bismuth target by either extractive (7, 113, 116) or dry 2, 3, 48, 74, 93, 120) distillation techniques. Both methods have their relative merits (55.101). 

Meyer and Rossler 101) showed that the overall yields for wet and dry extractive procedures are comparable, being approximately 60% for Some workers have found that yields vary somewhat, due to adsorption of evaporated At onto vessel walls, and to the possibility of the retention of astatine within the target due to the formation of nonvolatile compounds 11). However, the dry evaporation method is more applicable to studies with high-activity targets it is rapid and lends itself to further development within the scope of remote handling techniques. Aspects of both extraction approaches have been discussed widely 2, 7, 33, 89,101,116,120,160). 

Identification and quantitation of the three main astatine isotopes, ° At, °At, and At, can be achieved by the appropriate measurement of a-, y-, or X-ray activity. X-rays and y-rays can be counted conveniently by well-type crystal counters, whereas a-counting requires that astatine samples be measured as infinitely thin or thick preparations. Astatine-211 can be measured by counting its 79- to 92-keV Po K-L,M,NX-rays, and, importantly, discriminated from At and ° At by their y-emissions (245, 1180 195, 545, and 782 keV, respectively). Identification and aspects of counting other astatine isotopes have been briefly discussed elsewhere (6,110). 

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