THE SMALLEST PARTICLES OF MATTER 31 



of all explosive weapons. We read the Smyth Report with 

 mingled feelings — pride in the splendid scientific achievements 

 revealed; joy for the speedy and enforced surrender of those who 

 had set out to enslave the world, and for the saving of millions 

 of lives on both sides — both soldiers and civilians — who would 

 otherwise have perished had the war been continued; sorrow for 

 the much lesser number who died in the nuclear explosions, and 

 for the fact that these epoch-making scientific discoveries were 

 prostituted to military uses, instead of being devoted to the 

 benefit of humantiy. 



It was already known that the isotope U 235 would undergo nuclear 

 fission by slow or thermal neutrons; but natural uranium contains 

 only about 0.71 per cent of this isotope (AcU 235 ), 0.006 per cent of 

 isotope 92 UII 234 , but 99.28 per cent of isotope 92 UI 238 . So besides 

 rinding out how to separate the isotope 235 for immediate use, a 

 major problem was to find a way to utilize the major portion of the 

 natural metal, bv converting: the U 238 into something: fissionable. 

 Thomas Graham had developed the mathematical expression or "law" 

 regarding gaseous diffusion, and though many other methods were 

 investigated, the separation was successfully made by differential dif- 

 fusion of uranium hexafluoride, UF 6 . Since the rate of diffusion of 

 gases through chemically indifferent septa is inversely proportional to 

 the square roots of their molecular weights, the difference in the rate 

 of diffusion between U 235 F 6 and U 238 F 6 , whose molecular weights are 

 349 and 352 respectively, is very slight indeed. Immense technical 

 difficulties had to be overcome in handling so corrosive and dangerous 

 a substance during the 4000 diffusion cycles necessary. 27 



Meanwhile, it had been found that by radiating 92 U 238 with neu- 

 trons, it could be transformed into 92 U 239 , by taking the neutron into 

 its nucleus and emitting /3-radiation. This new 239 isotope of ura- 

 nium has a half-life of only 23 minutes; it emits an electron and 

 becomes an entirely new element, neptunium, 93 Np 239 , which has a 

 half-life of 2.3 days. Neptunium, in turn emits an electron and 

 becomes another new element, plutonium 94 Pu 239 , which is relatively 

 stable, having a half-life of 24,000 years, and emits alpha rays (helium 

 nuclei). 



Plutonium was "manufactured" in a "pile," consisting of a 

 matrix of highly purified graphite with tubular passages into 

 which were inserted aluminum "cans" containing uranium rods, 

 protected from the cooling water needed to carry off the intense 

 heat generated; for the production of 1 gram of plutonium per 

 day involves liberation of energy at the rate of 500 to 1,500 kilo- 



