The Experimental Determination by Ionization Methods of the Rate of Emission of Beta-and Gamma-Ray Energy by Radioactive Substances

The known relation between the ionization in cavity type ionization chambers and the γ-ray energy absorbed per unit mass of the wall material has been employed for the absolute determination of the rate of emission of γ-ray energy by samples of the isotopes Na24, Co60, and I131. When the rate of emission of quanta by the sample has been determined by counting procedures, the measured rate of emission of γ-ray energy agrees well with expectation based on the accepted decay schemes of Na24 and Co60, and the Metzger Deutsch scheme for I131. In principle an analogous method may be used to measure the absolute rate of emission of β-ray energy by a sample of isotope uniformly incorporated in the walls of a cavity type ionization chamber. Measurements of the rate of emission of β-ray energy by Na24, P32, Co60 and I131, have been made by means of ionization chambers lined with gelatine containing these isotopes. By comparing the results with the rate of emission of γ-ray energy from the same sample of isotope, the following values have been obtained for the ratio of the mean β-ray energy to the γ-ray energy emitted per disintegration: Na24 = 0·139 ± 0·007, Co60 = 0·043 ± 0·002, I131 = 0·505 ± 0·025. These figures are in satisfactory agreement with the accepted decay schemes for Na24 and Co60, and the Metzger Deutsch scheme for I131. The principles of the method and the factors which limit accuracy are discussed in some detail. In the case of the β-ray measurements, errors arising from the finite size of the chamber and the use of gelatine linings of thickness less than the maximum β-ray range of the isotope under investigation are evaluated experimentally. It is concluded that when all sources of error are taken into account absolute γ-ray energy measurements may be considered reliable to about 4 per cent., β-ray measurements to about 6 per cent., and the ratio of β- to γ-ray energy to about 5 per cent. for the isotopes studied.