These results have been combined with the range measurements of Lewis and Wynn-Williams to give the relation between velocity and the range for a-particles of ranges betwreen 4 and 9 cm.It is found th at for ranges greater than 5 cm. this relation is accurately given by V2 3*26 -The corrections for loss of velocity due to absorption a t the source havebeen fully discussed.
Absorption curves in paper have been obtained for the β-rays of actinium (B + C) and actinium C′. Some inferences regarding the shape of their energy distribution curves have been drawn, and the ranges of the β-rays of actinium B and actinium C′ fix their maximum β-ray velocities at Hρ 3430 and 6140. The potency of the method is emphasized for elements which cannot be easily investigated directly in a magnetic spectrograph.The γ-ray ionizations of actinium B and actinium C′ have been compared under the same conditions with the β-ray ionization of actinium C′.In conclusion, the writer wishes to thank Professor Sir Ernest Rutherford for his kind interest in the work. Thanks are also due to Dr J. Chadwick and Dr C. D. Ellis for many valuable suggestions and discussions.
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.
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