284 BELL SYSTEM TECHNICAL JOURNAL 



are in effect assuming that all the nuclei in the layer are equally liable 

 to being fissured by these. To remain faithful to the well-grounded 

 assumption that only the nuclei U^*^ are liable thus, we must multiply 

 by 140, since only one nucleus in one hundred and forty is of this 

 isotope. ^^ The resulting value is large-sized for the nuclear scale, 

 though not unprecedented: there are elements which absorb thermal 

 neutrons so voraciously (without however suffering fission) that the 

 cross-section for absorption is found to be hundreds of times more 

 extensive. 



Now in conclusion we turn to the particles other than nuclei, which 

 go forth into space when or after the fission occurs. These comprise 

 photons, electrons, and newborn or "secondary" neutrons; and the 

 last are by far the most sensational. 



Of the electrons, almost all has been said that should find place in 

 this account. I recall that by virtue of the second argument from the 

 masses (page 279) the nuclei of the fission -products should go from 

 instability over to stability by emitting electrons which are negative. 

 Observation shows that the emitted electrons are negative indeed (and 

 yet there must be many among the products for which the sign has not 

 been ascertained). Unstable nuclei emitting positive electrons are not 

 at all unknown; indeed they are formed in many transmutations; their 

 absence from among the fission-products is therefore significant. 

 Many of the electrons coming forth are of "secondary origin," i.e. 

 released by photons from the electron-families of the atoms. When 

 classified with the many radioactive bodies formed by other modes of 

 transmutation, some of the fission-products are found to be identical 

 with some of those others, and the rest are in no wise peculiar. 



Of the photons, some are X-ray photons engendered as I have re- 

 cently described (page 282). Others are of the gamma-ray type, i.e., 

 they spring from unstable nuclei among the fission-products. Their 

 existence not being in the least surprising, they have in the main been 

 left for future study. 



Coming now to the secondary neutrons, I will begin by dividing 

 them into the "delayed" and the "instantaneous." The former come 

 forth and are detected during an appreciable time— a few seconds up 

 to a few minutes — after the fissions cease. Here then are radioactive 

 bodies, of which the radioactivity consists in the emission of neutrons ! 

 Nothing of the sort had ever been known, and the discovery (made at 

 the Carnegie Institution of Washington) created a sensation. In num- 

 ber they are much fewer than the "instantaneous" neutrons, define 



i«The figure is from Nier, Phys. Rev. 55, 150 (1939), who gives 139 ± 1 as tiie 

 abundance-ratio of U^^"* and U^^*. 



