Scintillation Method Research Articles

Long-range Particles from Radium-active Deposit

WITH reference to the communication of G. Kirsch and H. Pettersson in the issue of NATURE of September 15, p. 394, on the “Sources of long-range H-particles,” the results of an examination by the scintillation method of the particles emitted by radium-active deposit, in which we have been engaged for the past six months, are of interest.

On the analysis of α-ray photographs

The beautiful method devised by Mr. C. T. R. Wilson of making visible the tracks of ionising particles, by the condensation of water drops on the ions left in their trail, has been used in this investigation to study the passage of α -particles through gases. As is well known the tracks of these particles appear in these cloud photographs to be very nearly straight except for occasional sudden bends due evidently to collision with the nuclei of the atoms of gas. These bends occur very much more frequently towards the end of the range where the velocity of the particles is low. The theory of the occurrence of such close collisions between the α -particles and the nuclei, based upon the assumption of a force varying inversely as the square of their distance apart, has been given by Rutherford and Darwin and verified by Geiger and Marsden, Chadwick and others. These investigations confirmed in general the hypothesis but indicated a departure from the law of force when the two particles approach very close. Now it is also certain that at sufficiently great distances from the nucleus, the effective force on the particle must be different from that given by the inverse square law owing to the partial or total screening of the nucleus by the electrons. An investigation of the field at such greater distances can be done most sensitively by using particles of such a low velocity that they will not approach closer to the nucleus than the distance considered. As no particles have been detected with certainty with velocities less than about 0.4 v 0 , where v 0 is the initial velocity of the α -particles from radium c , by either the photographic or scintillation methods, it is necessary to make use of the Wilson expansion system.

On the Automatic Registration ofα-Particles,β-particles andγ-Ray and X-Ray Pulses

Synopsis. A device is described which makes it possible for $\ensuremath{\alpha}$-particles, $\ensuremath{\beta}$-particles and $\ensuremath{\gamma}$-ray and X-rays pulses to make records automatically on chronograph paper, which records can be conveniently and accurately studied at leisure. Historically, the method developed from Rutherford's method of counting $\ensuremath{\alpha}$-particles and the last previously described development was by the writer in which he used a telephone receiver whose diaphragm was actuated by the current to a needle electrode, after amplification by means of a 3-electrode vacuum tube. In the present method the amplified current actuates a sensitive relay which in turn operates the chronograph pen by means of a local circuit. The accuracy of the records was checked up for the $\ensuremath{\alpha}$-particles by the scintillation method of counting and for the $\ensuremath{\beta}$-particles by determining the charge carried by the $\ensuremath{\beta}$-particles. Records of $\ensuremath{\gamma}$-rays and X-rays are similar to those of the $\ensuremath{\alpha}$- and $\ensuremath{\beta}$-particles and some arguments are given supporting the idea that the records represent individual pulses or short trains of pulses. A record of $\ensuremath{\beta}$-particles taken over a 7-hour period was studied and conclusion deduced that the variations in the number of $\ensuremath{\beta}$-particles emitted in a given time interval follow the probability law.

On the scattering of the α-particles by matter

In the course of the experiments undertaken by Professor Rutherford and myself to determine accurately the number of α-particles expelled from 1 gramme of radium, our attention was directed to a notable scattering of the α-particles in passing through matter. The effect of scattering is well known in the case of β -particles. A narrow pencil of β -rays emerges after passing through a metal plate as an ill-defined beam. A similar effect, but to a much smaller extent, was known to exist also for the α-particles. Professor Rutherford* showed that the image of a narrow slit produced by the α-rays on a photographic plate broadens out when the slit was covered with a thin sheet of mica, while a well-defined image was obtained in vacuum with the uncovered slit. The question of the actual existence of the scattering effect of the α-particles has been discussed further by Kucera and Masek,† by W. H. Bragg,‡ L. Meitner,§ and E. Meyer.|| Some experiments have been made, using the scintillation method to determine the magnitude of the scattering of the α-particles in passing through matter. The apparatus used is shown in fig. 1. The main part consists of a glass tube nearly 2 metres in length and of about 4 cm. diameter. The α-particles from a strong but small source placed at R passed through a narrow slit S and produced an image of this slit on a phosphorescent screen Z, which was cemented to the end of the glass tube. The breadth of the slit was 0.9 mm., and the breadth of the geometrical image on the screen was about 2 mm., depending upon the dimensions and the distance of the source. The numbers of scintillations at different points of the screen were counted directly by means of a suitable microscope M, of 50 times magnifi­cation. The area of the screen which could be seen through the microscope was about 1 mm. 2 The number of scintillations counted varied between two or three a minute and about 80 per minute. As regards the microscope and the most convenient method to count thescintillations, the hints given by E. Regener* in his recent paper proved very useful. The microscope was mounted on a slide PP so that the scintillations produced at varying distances from the centre of the beam could be observed. The actual position of the microscope was read on a millimetre scale fixed to the slide.

The Origin of Radium

IN an earlier communication to NATURE (September 26, p. 544) I mentioned some experimental proof which had been obtained of the existence in uranium minerals of a new radio-active element differing from those which have previously been identified. More conclusive evidence of the individuality of this new substance has now been obtained through the examination of the properties of its radiation. The rays which it a emits are much more readily absorbed by aluminium than the a rays from polonium, with which it has been directly compared. Their apparent range in air determined by the scintillation method is less than 3 centimetres, arid a more accurate determination is somewhat difficult, since it has not yet been found possible to obtain the new substance entirely free from thorium. Certain operations are now being carried out with considerable quantities of a uranium mineral containing no thorium which it is hoped will result in the separation of a highly active preparation of the new body free from other radio-active substances. The short range of the particles is, however, sufficiently characteristic to serve as a definite means of identification.