280 BELL SYSTEM TECHNICAL JOURNAL 



physics: the fact that when mass-number is plotted against atomic 

 number, the points representing the stable nuclei cluster about a con- 

 cave-upward curve. Moreover, whichever pair of fragments we as- 

 sume, there will be a superfluity of rest-mass which will manifest 

 itself in a high kinetic energy of the two fragments. This derives 

 from another fundamental fact : when the percentage of excess of mass- 

 number over true mass is plotted against the mass-number (or for 

 that matter the atomic number) the points representing the nuclei 

 cluster along an upward-trending curve. 



And so, the kinetic energy of the fragments and the facts of the 

 emerging negative electrons tell us neither which is the initial fragment- 

 pair, nor even whether the initial pair is in all cases the same! Can 

 these questions be answered out of the study of radioactive substances? 

 Some of these we can indeed exclude by observing that they grow out 

 of others; but as to these others, we shall never be able to exclude the 

 possibility that they grow out of still others so short-lived, as to be 

 quite unidentifiable. The half-period of a radioactive substance must 

 be appreciable, if the substance is to be detected and its chemical 

 character recognized; and "appreciable" thus far has signified, among 

 the products of fission, "several seconds or more." It is true that 

 certain tours de force, whereby much shorter half-periods have been 

 measured among the natural radioactive bodies, have not yet been 

 applied to the fission-products (so far as publications tell) ; they might 

 prove workable. ^° 



Thus it may be necessary for the nonce to lay aside the problem of 

 deciding which is the true initial fragment-pair (or pairs) and be con- 

 tented with identifying as many as possible among the radioactive 

 substances and tracing their interrelations. Of these — hereafter to be 

 called "the fissi on -products " — there is indeed a multitude. Among 

 them, chemical elements have been recognized as follows: 34Se, ssBr, 

 seKr, svRb, ssSr, 39Y, 4oZr, 4iCb, 42M0, 52Te, 53I, 54Xe, ssCs, seBa, and 

 57La. Yet by counting these one does not count all of the distinguish- 

 able products; experimenters say that they can tell apart three isotopes 

 of barium, three of strontium, four of iodine and no fewer than seven of 

 tellurium! Some of these agree in their half-periods with radioactive 

 isotopes of those same elements already formed by the older ways of 

 transformation, and frequently we can thus identify their mass-num- 

 bers with a fair degree of certainty. (Ba^^^, which I introduced into 

 the hypothetical reaction of page 272, is such a one; but we shall see 



1" I refer particularly to the use of a rapidly-turning wheel to carry a target swiftly 

 from a place where it is under bombardment (or receiving a deposit of radioactive 

 nuclei) to another place where it is opposite a detector; and the measurement of the 

 radioactivity of a beam of fast-flying nuclei at various points along the beam (the 

 method applied by Jacobsen to RaC). 



