Scintillation Counter for measuring Radioactivity of Vapours

Journal/Book: Reprinted from Nature. Vol. 182 pp. 133 134 July 12 1958. 1958;

Abstract: Medical Research Council Experimental Radiopathology Research Unit Hammersmith Hospital London W.12. May 14. THE gas chromatographic technique of James and Martin1 2 is often used to fractionate substances biosynthesized from precursors labelled with carbon-14. For the assay of carbon-14 in the various fractions it is necessary at present to collect the fractions individually and then transfer them to a counting device. Direct measurement of the radioactivity of vapours as they leave the chromatographic column would save much time and labour and improve accuracy. We have devised an Instrument which counts the carbon-14 content of vapours emerging from a gas chromatographic apparatus parallel with the analytical chromatographic record. In principle the chromatographic vapours are condensed into a circulating liquid phosphor : a solution of diphenyl oxazole in xylene. The scintillations excited in the phosphor by the carbon-14 are detected with a photomultiplier. Fig. 1 (without Fig.) illustrates the instrument diagrammatically. The vapours and carrier gas (nitrogen) flow through capillary tube A which is heated electrically to the temperature of the column ; then they bubble into the liquid phosphor which is kept in vigorous circulation around the circuit BCDE by the bubbles of carrier gas rising in tube B-C. As the vapours are soluble in xylene and have high boiling points they are stripped from the carrier gas an entering the liquid phosphor. Jacket F cools the xylene an its path from A to the counting cell S. This cell is a cylindrical cavity (1 cm. deep 16 ml. capacity) formed by pressing a polyethylene cup over the window-end of a photomultiplier. Thus the scintillations produced in the phosphor are 'seen' directly by the photomultiplier there being no lose of light due to interfaces between the phosphor and the photomultiplier. The phosphor is introduced and removed through a thin polyethylene tube inserted into opening X and extending to point E. The opening X also allows the carrier gas to escape. A combined support and lead shield holds the photomultiplier horizontally the whole assembly being placed in a lagged and light-tight box. Cooling liquid at about 0° C. is pumped through tubes in the side of the box and circulated through the jacket F and the photomultiplier support. ... ___MH

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