The β -ray spectrum of radium E

Abstract

The problem of the β -ray spectrum of Ra E is too familiar now to require more than brief introduction. Owing to the eminent experimental suitability of this substance, its β -ray spectrum has been investigated widely, and various workers have given summaries of the results up to date, e.g. O’Conor (1937) and Martin and Townsend (1939). Most work has been done on the upper limit or on the form of the intermediate part of the spectrum, with fairly concordant results. Only a scanty amount of work has been done on an accurate investigation of the lower regions of the spectrum, where the experimental difficulties are much greater on account of the necessity of avoiding scattering in the source and along the track of the particles in the spectrograph, and of reducing the absorption in the window of the counter. Richardson (1934) first demonstrated by cloud chamber measurements, using a satisfactorily thin source mounting, that there is a considerable intensity of slow electrons between 15 and 60 ekV, and that the ordinate of the energy distribution curve is about as high in this region as at the previously estimated maximum. However, on account of straggling and other causes, it would be extremely difficult to determine exactly the form of the energy spectrum below 100 ekV by cloud chamber methods, and most other work has been done by semicircular magnetic deflexion with Geiger counters for registration of the particles. The results of chief importance are those of Alichanian and Zavelsky (1937) and of Flammersfeld (1937, 1939). In the discussion below the conclusion is reached that in both these sets of investigations the source mountings were quite satisfactory even down to 30 or 20 ekV, but that in the former the thickness of the counter window (1 µ cellulose acetate) can scarcely be regarded as small enough for measurements below 50 ekV, whilst in the latter absorption effects in the 0.3 µ Zaponlak window must begin to enter between 30 and 20 ekV.* Moreover, in both cases the spectrographs are not sensibly free from scattering. The experiments to be described below were made in an attempt at a precise and direct observation of the undistorted spectrum, especially below 100 ekV, and it is believed that the results do represent a practical realization of this ideal, down to the region of 20 ekV at least, without the need for the application of corrections of any kind.