NUCLEAR FISSION 



275 



target. When in defiance of this the radioactivity crossed over to the 

 collector, the trans-uranic elements were doomed. 



In these experiments, then, the fragments of the initial explosion 

 are found en masse together with their descendants upon the distant 

 plate. In those of the other grand type, they are detected each by 

 itself en route. Being charged particles of great momentum, they 

 cleave through any gas in nearly linear paths, along which quantities 

 of ions stay behind.^ The Wilson chamber may be used to make these 

 visible, and has indeed already been so used (by Joliot, and by Corson 

 and Thornton) ; but another device gave the first and as yet most 

 instructive results. This is the ionization-chamber equipped with 

 linear amplifier and oscillograph. In the first, the ions due to the pas- 

 sage of a single particle are drawn to a collector and their charges 

 united; in the second, the united charge is multiplied by a large fixed 

 factor; in the third, the multiplied charge produces a sharp sidewise 

 motion of the oscillograph-beam and the spot which this last produces. 

 On the photographic plate the moving spot produces a line, the length 

 of which is measured. Instances of these lines appear in Fig. 1. 



Fig. 1 — Records of fission-fragments obtained with ionization chamber, linear ampli- 

 fier, and oscillograph. The short lines due to alpha-particles are lost in the hazy 

 dark band beneath. (Courtesy of J. R. Dunning) 



Uranium is a spontaneous emitter of alpha-rays (this is how the 

 radioactivity of U^'^ and U^'* is manifest) and so the apparatus will 

 show "kicks" even when neutrons are absent. This is an advantage 

 really, since when the neutrons are admitted and the kicks due to 

 the fragments appear they are so much the larger that there is no 

 danger of confusing them with alpha-particle kicks, while these last 

 may be pressed into service for calibrating the device. The calibration 

 reposes upon a theorem of very great value in physics: viz., the (aver- 

 age) amount of energy expended by a fast charged particle in producing 

 an ion-pair is fixed and constant, whatever the charge and mass and 



^ This is correct whether they travel as isolated nuclei, or are attended by some 

 though not the full quota of orbital electrons which would environ them were they 

 already the nuclei of completed atoms. Capture of electrons along the course is al- 

 most certain (it has been proved to occur with alpha-particles). 



