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As72 - As78 These are, very broadly, the most useful of the arsenic isotopes. As - As are produced by (p, n) or (p, y) reactions on Ge; As - As by (n, p) or (y, p) reactions on Se;As and As are fission products. Note also that As is produced by the (n, 2n) reaction on stable As while As is the (n, y) product of stable As. As has a very hard 8+ and a very prominent y peak at 0.838 MeV together with a large annihilation radiation peak, and so is easily counted. As decays by electron capture and is rather difficult to count satisfactorily because its soft y-rays are highly converted. As emits 6+ and B~ and has a good y-ray at 0.596 MeV plus annihilation radiation; these are rather close but can be resolved by y-spectrometry. As emits a hard, '-'~particle, and so is readily counted; it also has a prominent y-ray at 0.559 MeV. As is rather more difficult, since its 8~ particles are on the soft side and its y-rays are weak in intensity, the most prominent being at 0.246 MeV. As is easily counted as it emits hard B~ particles, and has a well defined, fairly hard y-spectrum. Some workers have experienced trouble with contamination in these isotopes from Cu , a 12.8h positron emitter. The annihilation peak of this isotope is very prominent, and careful y-spectrometry is needed if arsenic isotopes are to be counted by scintillation methods when such contamination is present. Fortunately the half- life of Cu is sufficiently far from those of the arsenic isotopes that resolution of decay curves is reasonably easy. As79-As81 These are all short lived isotopes. They are easily counted, since they all emit hard B~ particles. In addition, As has a prominent y-ray at 0.66 MeV. XI. GENERAL REMARKS CONCERNING THE DETAILED RADIOCHEMICAL PROCEDURES OF SECTION XII. The 21 detailed procedures which follow in section XII fall into only a few main groups, classified according to the principal means of As purification. By far the commonest such means is distillation of either AsC1j or AsBrj. The principal inter- fering element is then Ge, but this is easily removed by distilling it off first, having previously oxidised the As to As(V). Having removed the Ge, the As is then reduced to As(III) and distilled in its turn. This is the main purification method in 13 pro- cedures, numbers 1, 2, 4, 5, 6, 7, 8, 9, 10, 12, 13, 15 and 17. Solvent extraction of Asl^ into CHC1j is used in procedure 14 and into C^H, in procedure 19. Anion exchange is used in procedure 16. Precipitation of As2Sc, with removal of Ge by distillation is used in procedure 3. The preferred means for the rapid separation of As from complex mixtures is evolution of AsH, used in procedures 11, 18, 20 and 21 - the last two taking only 10s and 45s respectively. The procedures use various physical and chemical means for removing other gaseous hydrides which are evolved, (germane stannane and stibine). 24