Particles of long range emitted by the active deposits of radium, thorium and actinium

Abstract

For a considerable time after the discovery of radio-activity, it was thought that all the α-rays emitted by a given substance possessed a definite velocity and hence a definite range. It was shown, however, by Hahn, in 1906, that the active deposit of thorium emitted two distinct sets of α-rays with ranges 4·8 and 8·6 cm. respectively. Although these were at first thought to arise from successive products, it was subsequently established that they were the result of two different methods of the disintegration of the parent atom, thorium C. Marsden and Barratt were able to show that when thorium C breaks up, 65 per cent. of the atoms contribute α-rays of the longer range and 35 per cent. of the shorter. In 1911, Fajans, in investigating the recoil atoms from radium active deposit, discovered a new product radium C'' which had a half-value period of 1.38 minutes. The fraction of the radium C atoms, which disintegrated with the formation of this product, was found to be only 3 in 10,000. In 1914, Marsden and Perkins discovered that actinium C emitted a small number of particles whose range was 6·4 cm. This was followed in 1916 by Rutherford and Wood's discovery of a few particles of range 11·3 cm. from thorium active deposit, and finally in 1919 Rutherford established the presence of particles or range 9·0 cm. from radium active deposit. It will be noticed that, in all these cases except the first cited, the large majority of the atoms break up in what might be termed the "normal" manner, while comparatively few produce α-rays of range different from the normal range. The above results suggested that perhaps the various active deposits emitted, in even smaller numbers, other particles of definite ranges, and a thorough search was undertaken by means of the scintillation method for particles of long range, advantage being taken of the great improvements which have recently been achieved in the methods of counting α-particles. These methods involve the use of microscopes specially designed to increase the brightness of the scintillations observed and in particular to increase the area of the zine sulphide screen, and consequently the number of scintillations, under observation. The microscope used in the present research was of the type described by Rutherford and Chadwick, with a holoscopic objective of numerical aperture 04∙5 and 16 mm. focal length, together with a specially constructed eye-piece consisting of two large plano-convex lenses and a smaller double convex eye-lens. The field of view was about 20 Sq. mm. in area, whilst the fields formerly employed rarely exceeded 3 sq. mm. We have thus been able without greatly increasing the intensity of the radioactive sources to detect the presence of several groups of particles from the active deposits of radium and thorium which had previously not been observed.