BETA-RAY DISTRIBUTION OF ENERGY 65 



Beta Rays 



The beta rays from the disintegration products of radium consist of 

 streams of negative electrons possessing a wide range of velocities, the 

 swiftest having a velocity nearly equal to that of light. RaC is out- 

 standing in this respect, as seen in Table II— 3. 



One naturally is curious about the origin of such very high-speed 

 electrons. It has been determined experimentally that, on the average, 

 for every pair of disintegrating RaB and RaC atoms, 2.3 electrons are 

 emitted, about 1.25 coming from RaB and 1.05 from RaC. These 

 electrons may be either nuclear electrons or planetary electrons. The 

 accompanying gamma rays have their origin in the nucleus. The 

 number of electrons in excess of the one coming from the nucleus are 

 planetary electrons, emitted as the result of the absorption of the gamma 

 rays as they pass through the planetary electrons, and are referred to 

 as photoelectrons. 



RaE emits only extremely weak gamma radiation. No photoelectrons 

 are mixed in with the disintegration electrons which come from the 

 nucleus. Emeleus [1924] actually found that, in the disintegration of 

 each RaE atom, only one electron was emitted. 



RaD emits statistically about 1.5 electrons at each atomic disinte- 

 gration. It was found that two of every three electrons are slow dis- 

 integration electrons, and the third is emitted with high speed because 

 of the internal conversion of the gamma radiation into electron emis- 

 sions. 



Beta-Ray Distribution of Energy 



The distribution of energy among the emitted beta rays is determined 

 by their electron spectra, obtained by means of their relative deflections 

 by a magnetic field. As shown in Fig. II— 5, the strong magnetic field 

 bends the negative electrons into arcs of circles. Their radii of curva- 

 ture are proportional to their momentum (mv) of emission. Chadwick 

 as early as 1914 showed that the magnetic field acted as a lens to focus 

 all those electrons leaving the source with the same velocity. Advan- 

 tage is taken of this fact in the design of a device for measuring the 

 velocity distribution of a source of beta-ray electrons. The source, 

 in the form of some radioactive material deposited on a fine wire, is 

 fixed at S, Fig. II-5, perpendicular to the plane of the diagram. A wide 

 slit is placed at A. Beyond, in the same plane with the slit is placed a 

 photographic plate. The whole is contained in a light-tight evacuated 

 box and placed between the poles of an electromagnet, with its field 

 perpendicular to the plane of the paper. The magnetic field intensity 



