632 BELL SYSTEM TECHNICAL JOURNAL 



the power of forming ions or evoking scintillations, however fast they 

 move; but this would be too pessimistic; there is every reason to sup- 

 pose that they are charged corpuscles, therefore possessed of the same 

 powers as alpha-rays.) 



In all likelihood, many atom-kernels are disrupted and their fate 

 goes unperceived, because the "products of disintegration" move too 

 slowly; but sometimes these are fast enough to be detected in any of 

 the three aforesaid ways, as we shall see. There is, however, yet an- 

 other peril. Consider the alpha-particles which pass close to nuclei 

 without disrupting them. They are deflected, but the nuclei them- 

 selves suffer a reaction which sets them into motion. If these belong 

 to elements of atomic weight greater than 30, or let us say 40 to err 

 on the safe side, their masses are so much larger than those of the alpha- 

 corpuscle that the speed they acquire is negligible. But if they belong 

 to one or another of the half-dozen lightest elements, they may acquire 

 a speed so great that of themselves they can make ions in a gas or 

 scintillations on a screen. If an alpha-particle, being itself a helium 

 nucleus, flies straight against the kernel of a helium atom but does not 

 fracture it, then obviously the struck nucleus must take up the entire 

 speed of the striking corpuscle. If it is a carbon or an oxygen nucleus 

 which is thus squarely struck, without being broken, its final speed 

 must be one half or four tenths that of the alpha-particle. And if it is 

 a hydrogen nucleus or proton which is the victim of a square and central 

 impact, it must go off with no less than sixteen-tenth s of the speed of the 

 impinger. Incidentally, the latter is slowed down to compensate for 

 the kinetic energy acquired by the kernel which it strikes. 



The dangerous consequence is, that in a stratum of matter of low 

 atomic weight which is bombarded by alpha-rays, there must be intact 

 but rapidly-moving kernels which may be confused, which indeed one 

 can hardly help confusing, with the expected products of disintegration. 

 Moreover, even in a stratum of an element of higher weight, a metal 

 film or a tube of gas, there may be hydrogen enough to provide so many 

 low-mass targets for the alpha-rays, that the region is filled with fast- 

 flying protons which are not tokens of disruption. In every case where 

 corpuscles are observed which are thought to be parts of fractured 

 nuclei, it must be proved that they are not of this kind, nor yet are 

 scattered alpha-particles. 



Now as an index of the initial speed of an alpha-particle, people 

 generally take its "range." The trail of water-droplets which the 

 particle leaves along its path through suddenly-cooled moist air comes 

 to a sudden end (Figs. 10, 11); the length of the trail, measured to its 

 end from the point where the particle entered the air, is its range in the 



