PLUTONIUM — SEABORG 213 



case of the pitchblende a transuranium fraction which would contain 

 neptunium and plutonium was carefully isolated from the completely 

 dissolved carnotite ore, about 5 kilograms being used in this case. 

 Again an alpha-radioactivity was found, the concentration of the 

 corresponding Pu-^^ being comparable to that found in the pitchblende. 

 It was originally intended to extend the search for transuranium 

 elements to a number of other ores, but the exigencies of the investi- 

 gations in connection with the Plutonium Project made it impossible to 

 carry out this program. Such a program did not seem justified in 

 view of the small amounts which had been found in the pitchblende 

 and carnotite. The results of the investigation tempt one toward the 

 conclusion that transuranium elements do not exist in practical amounts 

 on the face of the earth. There will, of course, be some neptunium, in 

 the form of the isotope Np^^^, present in pitchblende, carnotite and 

 other uranium-bearing ores formed as the result of reaction 3 above, 

 but it seems very likely that the concentration of this is even smaller 

 than the concentration of the plutonium. The amounts of americiura 

 and curium on the basis of present indications would appear to be even 

 smaller. However, there is just an outside possibility that there might 

 exist some transuranium isotope or isotopes, perhaps, whose radiation 

 characteristics have not yet been characterized, formed by a mechanism 

 as yet not conceived. Thus, although it appears that transuranium 

 elements do not exist on the earth in any ores in concentrations larger 

 than some one part per 10^*, it might be a little premature to make this 

 statement too definite and further searches for such elements might be 

 worth while. 



AMERICIUM 



Americium, the element with atomic number 95, was the fourth 

 transuranium element to be discovered, its first identification taking 

 place late in 1944 and early in 1945. The first isotope of this element 

 was identified in the experiments of G. T. Seaborg, R. A. James, and 

 L. O. Morgan at the Metallurgical Laboratory of the University of 

 Chicago. 



The bombardment of U"* with very high-energy (40 to 44 Mev.) 

 helium ions in the cyclotron leads to the formation of the isotope of 

 americium with mass 241 — that is, Am^''\ The Am-" is the daughter 

 of a relatively long-lived ^-emitting Pu-" which is formed in the 

 primary reaction of U^^® and helium ions. The reactions therefore 

 are as follows : 



U^^'^-f 2He^ > Pu^-'i + n (7) 



^- 



Pu2" > Am^"! (8) 



long 



The isotope Am^" emits a-particles with a half -life of 500 years. 



