The number of particles in the beta-ray spectra of Radium B and Radium C

Abstract

The method of analysing a beam of beta-particles by deflecting them with a magnetic field in a vacuum has long been known. The beta-rays of Radium B + C, examined by this magnetic focussing method give a ‘’continuous spectrum” of beta-particles over a wide range of velocities, on which are superposed lines which are ascribed to the conversion of gamma-rays into beta-rays in escaping from the radioactive atom. There has been considerable difference of opinion as to the origin of the continuous spectrum, whether it consists of the electrons ejected from the nucleus in its disintegration, or whether it is merely secondary in origin. For example, Pohlmeyer, after some experimental work, recently came to the conclusion that the continuous spectrum of Thorium B + C might be due merely to lack of resolution of the line spectrum. He argued that this was probably true also of the spectrum of Radium B + C, a view previously maintained by Meither. The relative velocity-distribution of the particles from Radium B + C was examined by Chadwick,! using an electrical counter, and by Chadwick and Ellis, using an ionisation-chamber. In the latter case it is necessary to know the law of the variation of ionising power with velocity of the particles, as well as to estimate the reflection from the walls of the ionisation-chamber it is difficult to determine these quantities with accuracy. In the measurements by W. Wilson the beta-particles were deflected by a magnetic field into a Faraday cylinder, and the rate of accumulation of charge was directly determined. In the course of experiments made in this laboratory last year, Mr. Curtiss showed that this method could be combined with good focussing if a sufficiently sensitive electrometer were used ; an account of the work has been given.