276 BELL SYSTEM TECHNICAL JOURNAL 



speed of the particle. So, the ratio of the kicks caused respectively 

 by a fragment and an a-particle is the ratio of their initial energies, 

 provided the chamber is so "deep" that they both run their courses 

 completely to the end in the gas thereof. If on the other hand the 

 chamber is so shallow or "thin" that fragment and a-particle shoot 

 across it and only a small part of the total course of each is comprised 

 within it, then the ratio of the kicks may be the ratio of the densities- 

 of-ionization along the two tracks. Both the initial energy and the 

 density-of-ionization are known for the a-particles, permitting the 

 calibration. Also the constant value of the energy-expended-per-ion- 

 pair is known (it is about 30 ev.) so that if the experimenter can meas- 

 ure the actual amount of charge set free in his ionization-chamber he 

 need not bother with the a-particles.^ In Fig. 1, by the way, the alpha- 

 particle tracks are quite lost in the black band of the "background." 



The second grand experiment, then, consisted in showing that when 

 the neutrons were falling upon the uranium, there instantly appeared 

 among the smallish kicks due to the a-particles others which were 

 much greater — ten- and twenty-fold greater. This was done in four 

 places "^ at least in America in the closing days of January 1939; in 

 Copenhagen, however, a fortnight earlier. 



The greatness of the kicks when the ionization-chamber is deep 

 signifies the greatness of the initial energies of the fragments: I shall 

 presently quote the latest data of these. But when the chamber is 

 thin, the kicks due to the fragments again stand out very much over 

 those due to the a-particles; and this signifies that the ionization- 

 density along the fragment-tracks is great. (Take note, by the way, 

 that one and the same chamber may be thin or thick, according as the 

 density of the gas within is low or high — a very convenient fact.) The 

 fragments, then, not only have remarkably great energy to start with, 

 but also spend it at a remarkable rate in ionization along their courses. 

 The course or "range" of a fragment must therefore be much shorter 

 than would be that of an a-particle of the same energy. This is a 

 verifiable fact, the ranges being easily measured by this method. We 

 have just seen how Joliot was able to estimate them earlier, wiping 

 out by this observation the possibility that each of the great kicks 

 may be due to many a-particles starting off together. From the ion- 



^ All this is contingent upon the ions being completely gathered in by the collector 

 of the ionization-chamber before any serious fraction of them is annulled by re- 

 combination, or (failing that) upon the loss by recombination being the same in 

 proportion for a-particles and for fragments. Owing to the (unprecedently) great 

 density of ion-pairs along the tracks of the fragments, this is by no means sure. 



^ New York (Columbia), Baltimore (Johns Hopkins), Washington (Carnegie 

 Institution), Berkeley (University of California). In these cases the suggestion 

 originated with Fermi. 



