THE β- AND γ-RAYS OF ACTINIUM B AND ACTINIUM C″

Abstract

The absorption method of finding the end-point of a β-ray spectrum has been made more precise and free of errors that are due to straggling and the γ-ray background. The absorption curves of the β-rays of radium E and uranium X2 were taken as standard, since, from experiments with the magnetic spectrograph, their end-points are known to be 1.15 and 2.32 Mev respectively. The extrapolated ranges of these fastest β-rays are, from experiments with homogeneous beams, 0.51 and 1.12 gm. per cm.2 of aluminium. In general the extrapolated range R can be expressed in terms of the energy E in Mev by the equation:[Formula: see text]for E > 0.6 Mev. It is shown that the final portions of the absorption curves of the β-rays from radium E and uranium X2 have the same shape when plotted against m/R, where m is the mass in gm. per cm.2 of absorber and R is the range of the fastest β-rays in the spectrum. The range and end-point of another β-spectrum can be found by fitting its absorption curve to the standard one.The absorption curve of the β-rays of actinium B was obtained separately from that of actinium C″ for the first time. Growth curves were plotted from measurements of the β-activity of the active deposit obtained by exposing aluminium foils to filtered actinon for a few minutes. The β-activity was measured through thickness of aluminium up to 0.32 gm. per cm.2 From the initial portions of the growth curves and from the absorption curve of the β-rays of actinium (B + C) in equilibrium, separate absorption curves were plotted which indicated ranges of 0.64 and 0.68 gm. per cm.2 of aluminium for the β-rays of actinium B and actinium C″ respectively. The corresponding end-points are 1.39 and 1.47 Mev.There is some evidence that the β-spectrum of actinium B consists of two primary components, namely, a strong one having an end-point at 1.39 Mev and a weak one having an end-point at about 0.5 Mev. The difference between the two end-points is approximately equal to the energy (0.83 Mev) of a γ-ray emitted in the transformation actinium B.C. The ratio of the numbers of β-particles in the two partial spectra is approximately 1:7. This gives an excitation probability of 0.12 for the 0.83 Mev level of the actinium C nucleus, which is of the same order of magnitude as that obtained from the intensities of the γ-rays.