170 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1941 



basis of Einstein's mass-energy relation that each splitting or fission, 

 as the process is called, of a uranium atom into two approximately 

 equal parts releases an energy of about 200 million electron-volts, 

 which is millions of times more heat per atom than is given off when 

 ordinary fuel is burned. Thus, calculations show that 100 pounds 

 of uranium would yield a billion kilowatt hours, which at 1 cent 

 per kilowatt-hour would be 10 million dollars' worth of electrical 

 energy. 



For some time these considerations were largely academic because 

 no way was known for producing fission of the heavy elements. But 

 interest in the matter has now become extremely lively as a result 

 of the discovery that fission of uranium is actually brought about by 

 bombarding it with neutrons. 



The phenomenon has, during the past 2 years, received intensive 

 study in laboratories all over the world and several salient facts have 

 emerged. First, the rare U^^° isotope undergoes fission after absorp- 

 tion of a slow neutron. Second, the energy released in the fission 

 process has been measured ; and, as expected, it is found that, when 

 a neutron having an energy less than an electron-volt enters the U^^'' 

 nucleus, about 200 million electron-volts of energy is released. Third, 

 it is found also that the fission process is so violent that usually the 

 U^^^ nucleus does not break up into two parts only, but more often 

 several neutrons are given off in addition to the two large fragments. 



That neutrons are generated in the fission process is of the greatest 

 interest because it opens up the possibility of a chain reaction, a 

 series of nuclear reactions wherein the neutrons liberated in one fission 

 process go to produce additional fissions in other atoms which in 

 turn give rise to more neutrons which produce further fissions and 

 so on. It is this possibility of a chain reaction that has excited the 

 interest in uranium as a practical source of atomic energy. 



Without going into further detail, it is perhaps sufficient to say 

 that there is some evidence now that, if IJ-^^ could be separated in 

 quantity from the natural mixture of the isotopes, a chain reaction 

 could, indeed, be produced. But herein lies the catch, for there is 

 no practical large-scale way in sight of separating the isotopes of 

 the heavy elements, and certainly it is doubtful if a way will be 

 found. 



But I should not want to indicate that the uranium matter is a 

 disappointment, that after all we shall never find a way to bring 

 about fission of the heavy elements for useful purposes. Quite the 

 contrary ! 



The present situation is not unlike the circumstances 50 years ago 

 surrounding the then great question of whether man would ever be 

 able to fly. In those days the fundamental laws of classical mecha- 



