NTTCLEAR FISSION — DARKOW 157 



between two and three electron-volts for each pair of reacting mole- 

 cules; and in the notorious explosives of industry and war, such as 

 TNT and nitroglycerine, not even so much as that. 



Now I have said that fission occurs when a slow neutron impinges 

 on a uranium nucleus, and that an enormous amount of energy is 

 released, and that the resulting new-born elements are radioactive; 

 but I have not yet said what these new-bom elements are. This is 

 the second of the astonishing features of fission. All other transmu- 

 tations have resulted in changing the target element to some other 

 not more than two steps away from it in the periodic table of the 

 elements. In this periodic table, uranium stands at the ninety-second 

 and last place; but the no fewer than 16 different elements thus far 

 identified among the "fission-products" (as they are called) stand 

 in places ranging from the thirty-fifth to the fifty-seventh I What 

 happens in fission is therefore something never before observed — the 

 division of a massive nucleus into two nearly equal fragments. In 

 ordinary transmutations of heavy nuclei, a particle small in both 

 charge and mass pops into a nucleus, and another particle small in 

 charge and mass pops out. In this kind of transmutation a particle 

 of small mass and no charge at all wanders into a uranium nucleus, 

 and the nucleus promptly bursts apart into two pieces not exactly 

 alike indeed but not very different from one another. Fission in 

 biology is the division of a cell into two which are very much alike 

 in size, and this is the source of the name. 



As for the fact that fission results in so many different types of 

 nucleus instead of just two, that probably has a double meaning. 

 Many of the radioactive bodies which are observed during and 

 after fission are clearly not the original fragments of the explosions, 

 for after the neutron influx is suspended they increase for a while 

 in amount instead of diminishing. It is clear that these are descend- 

 ants of the original fission fragments, and the question as to which 

 are really the original ones is at present a very live one. Theory 

 suggests that the initial fragment-pair need not always be the same. 

 One nucleus, on being entered by a neutron, may burst into barium, 

 and krypton, another into xenon and strontium, another perhaps into 

 caesium and rubidium. (Note that the members of these element- 

 pairs are so chosen that their atomic numbers add up to 92, which 

 is a way of saying that the entire positive charge of the uranium 

 nucleus must be found upon the two initial fragments immediately 

 after the explosion.) Wliatever the initial fragment-pair may be, 

 one at least of its members must be the parent of a long chain of 

 radioactive bodies, and probably both are. This sufficiently accounts 

 for the fact that the fission process produces radioactive elements 

 in a profusion and variety beyond any other which is known. 



