An Absolute Method for Measuring the Activity of Radioactive Isotopes

Abstract

RADIOACTIVE isotopes are, at the present time, being used in a wide variety of scientific investigations. Thus the development of accurate methods for the assay of these substances is of considerable importance. The three methods in common use involve the Geiger counter, the ionization chamber and the micro-calorimeter; and, although simple in principle, they are quite complicated in practice. It therefore seemed worth while to investigate the possibilities of a new method based on the fact that a flow of β-particles is equivalent to a current. In the case of a pure β-emitting isotope, 3.7 × 10⁷ β-particles are emitted per second per millicurie (by definition), and this is equivalent to a current of 5.93 × 10⁻¹² amp. If the isotope is deposited on a thin foil, mounted in a vacuum to eliminate gas ionization, and an electric field is applied to prevent slow secondary electrons from escaping, then the current flowing from the foil should be a direct measure of the total activity. The accurate measurement of a current in the region of 10⁻¹² amp. is quite feasible, so that in principle it should be possible to measure radioactivity in this way. A preliminary investigation of the method has been completed, and the present communication gives a brief account of the results so far obtained, with particular reference to the effects caused by secondary emission and by ionization of the residual gas.