Absolute Measurement of W air for Sulfur-35 Beta Rays

Abstract

Recently a source of high-specific-activity sulfur-35 provided by Dr. G. Failla of Columbia University was measured in the National Bureau of Standards Peltiereffect microcalorimeter (1, 2) and its rate of energy emission determined to within +0.3%. An accurately known fraction of this original source was retained, and the remainder was returned to Dr. Failla for a determination of Wtir. Dr. Failla's group used a parallel-plate extrapolation ionization chamber for the determination of the rate of ion production, making corrections for backscattering and relative stopping powers (3). It was considered worth while to make independent current measurements at the National Bureau of Standards, using instead of an extrapolation ionization chamber a free-air ionization chamber, thus obtaining directly the rate of ion production in absolute units. Since the rate of energy emission was also determined in absolute units, the value of Wair obtained is an absolute value, independent of uncertainties in the knowledge of the energy of disintegration, relative stopping powers, and backscattering. The free-air ionization chamber measurement of total current, in the case of the A-rays of sulfur-35, has associated with it three important sources of error: (1) source self-absorption, (2) film-mounting absorption of direct and scattered radiation, and (3) gas discharge and ion recombination effects in the saturation region. In this paper the techniques developed for the elimination or evaluation of these sources of errors are described, and a value of Wair for sulfur-35 0-rays is obtained which is considered more accurate than values previously reported in the literature.