NUCLEAR FISSION 269 



present resources of energy we could not tamper with any of these 

 nuclei, had we not at our disposal those chargeless particles the 

 "neutrons" with which to assail them. 



One might of course foretell that mighty powers of transmutation 

 would be possessed by a particle which is not repelled as it approaches 

 a nucleus. Actually the transmuting powers of the neutron are greater 

 than, I should think, anyone can have expected; nor can many people, 

 if any, have foreseen that the slow neutron — the neutron having 

 no more speed and kinetic energy than a molecule of air at room- 

 temperature — would prove to be more potent than the fast one. Yet 

 so it is. When the other agents of transmutation were first applied — 

 alpha-particle, proton, deuteron, photon, fast neutron — it took years 

 to get proof of the transmutation of even a few elements; but when the 

 slow neutron was first applied, Fermi and his half-a-dozen colleagues 

 at Rome managed to do something to almost every element in a very 

 few months! Let me recall that neutrons mostly are what we call 

 fast — i.e., they have energies of millions of electron-volts — when they 

 start their careers. Slow neutrons are initially-fast ones which have 

 been sent through layers of paraffin or water, and have lost nearly the 

 whole of their initial energy by making elastic impacts with hydrogen 

 nuclei. We shall later have to distinguish between the fast neutrons 

 and the slow as agents of nuclear fission. 



Now to supply a fitting historical background to the discovery of 

 fission, I must draw attention to a theorem which until the end of 1938 

 was believed to govern the whole of transmutation, and which still 

 governs nearly the whole of the field. It is this: with the exceptions 

 presently to be related, no transmutation ever produces a change in 

 atomic number greater than 2 or a change in mass-number greater than 4. 

 I am going to illustrate this theorem by writing in symbolic form three 

 of the reactions of transmutation produced by neutrons and recognized 

 before the end of 1938. Here I use E as the general symbol for ele- 

 ment; Z and A as the general symbols for atomic number and mass- 

 number; and a, p, d, n, and for alpha-particle, proton, deuteron, 

 neutron and photon ; and I recall that the mass-numbers of these five 

 particles are 4, 1, 2, 1,0 respectively. 



zE^ (n, a) z-2E^-'; zE^ {n, p) z-iE^; zE^ {n, <p) zE-'+K 



The first of these (for example) is to be read : a neutron enters a nucleus 

 (Z, A) and an alpha-particle comes out, leaving behind a nucleus 

 {Z — 2, A — 3). There was a similar (not identical) rule setting a limit 

 on the changes of atomic number and mass-number suffered by radio- 

 active bodies. Every radioactive nucleus emits either a positive 



