CONTEMPORARY ADVANCES IN PHYSICS 



395 



One next inquires whether all of the orestons resulting from a given 

 type of impact spring off with the same energy. Experience with 

 natural radioactivity shows that while alpha -particles are emitted 

 either with a single definite energy or with one of several definite 

 discrete energies characteristic of the particular process, negative 

 electrons (beta-particles) are always emitted with a very wide and 



F"ig. 4 — Induced radioactivity resulting from bombardment of boron oxide by 

 0.9-MEV deutons: tracks of positive electrons, some springing from gas adjoining 

 the target, as though a radioactive gas had diffused out of the boron oxide block. 

 (Anderson.) 



continuous distribution-in-energy. Short as is the time which has 

 elapsed since January last, and weak as are the beams of positive 

 electrons resulting from induced radioactivity, it is already assured 

 that in several cases at least it is the latter rule which is followed and 

 not the former. The best distribution-curves are those derived at 

 Pasadena from a statistical study of oreston-tracks made visible in a 

 Wilson chamber and curved by an imposed magnetic field; they refer 

 to radioactivity provoked by 0.9-MEV protons falling on carbon, and 

 by 0.9-MEV deutons falling on Be, B, C and Al. I reproduce one of 

 these curves as Fig. 3 (another curve obtained with 0.7-MEV protons 



