CONTEMPORARY ADVANCES IN PHYSICS 393 



random distribution of the lengths of these delays, it is customary 

 and proper to refer to these phenomena as "induced radioactivity." 



Examples of induced radioactivity have already been provoked with 

 all of the four known agents of transmutation: alpha-particles acting 

 on B, Na, Mg, Al and P, — protons acting on boron and carbon — 

 deutons acting on boron and carbon and a number of othens — neutrons 

 acting on a large variety of elements. The half-periods reported when 

 neutrons are the agents have ranged from a few seconds to a couple 

 of days, while in all other cases they are of the order of a few minutes. 



The nature of the ejected particles resulting from the ultimate 

 disruption is of course of the greatest importance. The Joliots found 

 them to be positive electrons or orestons ^ in their pioneering experi- 

 ments, and this was confirmed by Ellis and Henderson at the Cavendish 

 Laboratory; the tests have been made by applying magnetic fields to 

 tracks made visible in the Wilson chamber or to beams of particles on 

 their way to photographic or other detectors, and are doubtless to be 

 regarded as conclusive, though no details have yet been published. 

 Induced radioactivity provoked by a-par tides, in the few cases so far 

 known, thus results in the emission of orestons.^ This seems also to 

 be the rule when it is provoked by deutons or protons, as is shown by 

 splendid Wilson-chamber photographs (Figs. 2, 4) obtained by 

 Anderson when samples of various elements (boron in the form of 

 B2O3, carbon, aluminium, beryllium) were first bombarded for several 

 minutes and then put right into the chamber itself. The tracks of the 

 particles springing from the samples have the specific aspect of electron- 

 tracks,^ and in the imposed magnetic field of 800 gauss they have a 

 curvature of which the sense proves the particles to be positive. On 

 the other hand it is stated by Fermi that the radioactivity induced 

 by impacts of neutrons involves the emission of negative electrons, 

 though in his very brief reports there is no intimation as to how this 

 is shown. 



For each individual case it is important to inquire whether the 

 half-period is independent of such circumstances as the kinetic energy 

 Ka of the impinging particles. If so, it is sufficient to postulate a 

 single kind of unstable nucleus resulting from the collisions; otherwise, 

 not. This has been investigated in the cases of radioactivity induced 

 by alpha-particle impact; the Joliots reduced Kq from 5.3 to 1 MEV, 

 without observing any change in the half-perod. 



^ As an occasional alternative to "positive electron" I adopt Dingle's beautiful 

 word "oreston" (Orestes, in Greek mythology, was the brother of Electra). 



* Excepting that the Joliots have lately reported that magnesium emits electrons 

 of both signs, which they attribute to different isotopes. 



^ "The Nucleus, First Part," p. 303. 



