32 LIFE: ITS NATURE AND ORIGIN 



watts. The graphite matrix is needed to slow down high-speed 

 neutrons sufficiently to obtain the desired fission effect. The 

 intense radiation fields of the pile changed the electric resistance, 

 the heat conductivity, and the elasticity of the graphite. 



To maintain the pile in operation, conditions had to be chosen so 

 as to maintain a "chain reaction" by continuous formation of sufficient 

 neutrons, allowing always for the waste due to non-effective neutron 

 collisions, absorption of neutrons by impurities, and escape of neutrons 

 from the pile. Since the object of the pile is to produce plutonium 

 by chain reaction, it is desirable to absorb all excess neutrons in U 238 , 

 leaving just enough neutrons available to maintain the chain by their 

 action on U 235 , aided by high-energy fission of U 238 and thermal fission 

 of Pu 239 . While collisions between neutrons and U 238 occur with 

 neutrons of all energies, they are most probable with those whose 

 energies lie in the "resonance" region, located somewhat above ther- 

 mal energies. So great is the tendency of the dominant isotope U 238 

 to absorb neutrons compared to their tendency to cause fission of the 

 140 times less numerous U 235 atoms, that this fission had to be favored 

 by use of materials of high purity, by a suitable lattice, use of a 

 moderator, etc. 



After sufficient plutonium had been produced in the pile, the 

 slugs of uranium were withdrawn and dissolved, and the plu- 

 tonium was separated by co-precipitation with some other ele- 

 ment, an expedient common in radioactive chemistry. The chem- 

 istry of plutonium had been worked out previously, on an ultra- 

 microchemical scale. 



Because of the extremely dangerous radiations emitted through- 

 out the several processes, they must all be operated by remote 

 control, the personnel being shielded by heavy concrete and metal 

 walls. Smyth states that plutonium is one of the most dangerous 

 substances known if it gets into the body; and the fission products, 

 which include some thirty elements, are also very active and 

 troublesome. These major fragments of the fission of uranium 

 are released in considerable quantity when the slugs of uranium 

 are dissolved, and high stacks had to be built to carry off the 

 deadly gases without endangering the surrounding countryside. 



Most of the technical difficulties involved in assembling the 

 "atomic bomb" arose from the fact that the time interval between 

 the beginning and the end of a chain reaction is extremely short. 

 The efficiency of the bomb depends on the ratio of (a) the speed 

 with which neutrons emitted by the first fissions can get to other 



