A Re-entrant Well Gas Flow Proportional Counter for Radioactivity Measurements of Solutions Containing 35S and other Radionuclides

Journal/Book: International Journal of Applied Radiation and Isotopes 1969 Vol. 20 pp. 130-133.. 1969;

Abstract: Received 28 August 1968 Research Establishment Australian Atomic Energy Commission Lucas Heights N.S.W. 2232 Australia CURRENTLY available absolute counting techniques permit the measurement of 35S activities with an accuracy close to ± 1 per cent(1 2) while some reported reproducibilities of relative measurements of 35S and a few other soft pure beta emitters are considerably better still.(3 4) These high reproducibilities were obtained an samples extracted from the solution to be assayed and measured in carefully defined conditions to ensure constancy of beta absorption. Such procedures are not intended for routine assays and do not satisfy the long standing demand for a method permitting activity measurements of soft pure emitters e.g. 35S and 45Ca with anything like the ease and speed attainable for hard beta and most gamma emitters.(4 5 6) The problem of measuring soft pure beta emitters and especially 35S in aqueous solution sealed in glass vials has been solved here by employing a multianode gas flow counter fitted with a re-entrant well and operated in the proportional region. Bremsstrahlung produced in the solution and the glass is then counted in quasi 4 geometry. For routine assays of 35S activity the reproducibility is about ± 6 per cent. Most measurements require only a few minutes and no speciaI precautions. With more care and only slightly modified conditions the reproducibility of measurements can be improved to ± 0·7 per cent. The Counter is shown diagrammatically in Fig. 1. The wall of the inner aluminium tube was turned down to 0·05 cm along its central portion to reduce absorption. Aluminium discs sealed to the ends of the tube each contain four symmetrically spaced Teflon insulators which support 0·0025 cm platinum anode wires. The structure was pushed into an outer brass tube and the whole assembly was screwed to a bare made from sheet brass. The resulting pressure against a neoprine gasket at the top and a similar gasket and an O-ring at the bottom ensured adequate gas tightness for gas flow operation. The vials holding the active solution fit snugly into the aluminium tube and are suspended from a spring clip whose position is secured by a grub screw. Using conventional electronics a 0·5 keV threshold and argon-10 per cent methane as the counting gas the bremsstrahlung plateau from 35S activity produced in the vials extends from about 1650V to 1800V with a slope less than 0·5 per cent per 100V. ... ___MH

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