186 ANNUAL REPORT SMITHSONIAN INSTITUTION, 194 7 



and about 3 feet of concrete, and yet the only way to stop them is to put 

 something in their way. A tennis ball rebounds from the hard court 

 with its velocity only slightly lessened, but if it collides head on with 

 another tennis ball, its energy is passed on to the other and it stops 

 dead. At each collision with a ball heavier than itself part of its 

 energy would be given up. So moderators must have nuclei with 

 masses comparable to the mass of the neutron, if the neutron is to be 

 slowed up at each collision. Also to be useful it must itself not absorb 

 neutrons. The substances with both these characteristics turned out 

 to be carbon and heavy water, and both had to be of high purity. In 

 the first pile at Chicago carbon was used, while at Chalk River heavy 

 water is the moderator. This heavy water was made at Trail, British 

 Columbia, tested at McMaster University, and used at Chalk River. 



MAN-MADE ELEMENTS 



When a slow neutron is absorbed by a nucleus of U^^, a nucleus of an 

 unstable isotope, U^^®, is formed. Its half -life is only 23 minutes and 

 from its disintegration a new element, neptunium, with atomic number 

 93 and mass number 239 is formed. This new element also is unstable 

 with a half-life of 2,3 days and disintegrates to form a second new 

 element, plutonium, with atomic nimaber 94 and mass number 239. 

 The reactions are 



92U^« + on^ > 92U^^+T 



slow 



23 minutes 



93N/3« , 94Pu^«+_ieO-f7 



2.3 days 



This isotope 94Pu^^^ of plutonium does slowly decay by the emission 

 of an a-particle to 92^^^^, but its half -life is 24,000 years, so that it is 

 really very stable. Recently the discovery of two more man-made 

 elements has been announced. Since the astronomers in their search 

 of planets beyond Pluto have not kept pace with the discovery of 

 elements beyond plutonium, it is proposed to call these two elements 

 americium and curium respectively, in honor of the Americas and of 

 Pierre and Marie Curie. 



Plutonium has qualities which make it so tremendously impor- 

 tant that it is largely replacing U^^^ in both military and peacetime 

 applications. Though usually stable, it is fissionable by slow 

 neutrons just as U^^^. It is potentially more abundant since it is 

 created from the much more plentiful U^^^. It is a different element 

 from uranium and hence can be separated from it by chemical 

 means. So the atomic-energy plant using uranium is allowed to 



