io6 



Supplement to ''Nature,'' August 25, 1923 



For this reason the vcloiity of the u-particle cannot 

 be followed with certainty below 0-38 Vq. We must 

 bear in mind that even at the lowest velocity at 

 which it is possible to detect the a-particle by the 

 scintillation or photographic method, it is still moving 

 at a high speed compared with the positively charged 

 particles generated in an ordinary discharge tube. 



It is clear that ultimately the a-particle must be 

 slowed down to such an extent that it captures electrons 

 and becomes a neutral atom, but until recently no 

 evidence of this process of capture of electrons had 

 been obtained. G. H. Henderson (Proc, Roy. Soc. 

 A, 102, p. 496, 1922) has recently added much to 

 our knowledge of this subject by examining the 

 deflexion of a-rays in a magnetic field in a very good 

 vacuum. For the success of these experiments it is 

 essential that the apparatus in which the deflexion is 

 observed should be exhausted to a very low pressure, 

 corresponding to that required for a good X-ray tube. 

 The reason of this will be seen later. When a narrow 

 pencil of a-rays was deflected in a magnetic field two 

 bands were observed on the photographic plate, one 

 the main band, due to ordinary a-particles carrying 

 two positive charges, and another midway band 

 which he supposed to consist of particles which had 

 captured one electron, i.e. to singly charged helium 

 atoms. At low velocities he also obtained evidence 

 of the existence of neutral a-particles resulting from 

 the capture of two electrons by the helium nucleus. 

 In these experiments Henderson employed Schumann 

 plates, where the film is so thin that low velocity 

 particles produce as much or more photographic effect 

 than the swifter particles. 



I have repeated these experiments, by the scin- 

 tillation method, and confirmed the deduction of 

 Henderson. By observing the deflexion of the mid- 

 way band in an electric as well as in a magnetic field 

 I find there is no doubt the particles composing the 

 midway band consist of particles of mass 4 and 

 charge i, i.e. to singly charged helium atoms which 

 have the same speed as the doubly charged particles 

 comprising the main band. 



Some recent experiments have been made by me to 

 throw light on the conditions under which the flying 

 tt-particles may gain or lose an electron. The general 

 arrangement of the experiment is shown in Fig. i. 

 A fine platinum wire coated with radium B + C, by 

 exposure to the emanation (radon) serves as a nearly 

 homogeneous source of a-rays, since the a-particles are 

 emitted only from the atoms of radium C, which are 

 too few in number to form a film on the platinum of 

 even one molecule thick. The a-rays from this source 

 pass through a narrow slit about 0-3 mm. wide and fall 

 on a screen of zinc sulphide. The distribution of 



a-parti<lcs on the .screen is determined by the scii 

 tiliation mcthfjd in :i dark room, using a microsco] 

 outside th( scl containing the source and 



screen is cornpii itiy exiiaustcd by means of a Gaedc 

 and mercury diffusion pump, and if necessary the 

 residual pressure can l>e measured by a Macleod gauge. 

 The lx)x is placed between the plane pole pieces of 

 large electromagnet so that the i)cn(il of a-rays is bt: 

 in the direction shown in the figure. Usually tl 

 distance between the source and screen was 16 en; 

 with the slit midway. The whole path of the rays w. 

 exposed to a nearly uniform magnetic field and tl 

 deflexion of the pencil of rays was proportional to tli. 

 strength of the magnetic field. Under normal ex- 

 perimental conditions the jjencil of a-rays from tl 



Fig. I. 



bare radium C wire was bent a distance on the screen 

 of about 15 mm. from the zero position without field. 

 The field of view of the microscope was sufficient to 

 take in the depth of the whole pencil of a-rays without 

 the field. 



Special precautions were taken to prevent con- 

 tamination of the screen by the escape of active 

 matter from the wire in a low vacuum. It must be 

 borne in mind that the type of wire source employed 

 always introduces some heterogeneity in the beam of 

 a-rays even from the uncovered source. This is diip 

 to the escape from the back of the wire of a-particli 

 which are reduced in velocity in passing through the 

 material. This effect is clearly manifest when the 

 pencil of a-rays is deflected by a magnetic field ; for 

 in addition to the main band of a-rays there is ahva} 

 a distribution of particles extending beyond the mai: 

 beam. The intensity of this heterogeneous beam at 

 any point is generally less than one per cent, of tli- 

 main beam and does not seriously interfere with tin 

 accuracy of the deductions discussed in this lecture. 



In Figs. 2 and 3 are given illustrations of the di? 

 tribution of singly and doubly charged a-partick 



\ 



