212 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1947 



time, undeveloped and questionable nuclear chain-reaction for produc- 

 tion purposes. 



The ore pitchblende was chosen for this first search because it was felt 

 that a source rich in radioactive material would offer the most hope 

 and also because pitchblende is known to contain a large number, some 

 40, of different elements. About 0.5 kilogram of pitchblende ore was 

 completely dissolved and subjected to an exhaustive chemical process 

 designed to separate and isolate the elements neptmiium and plu- 

 tonium. A small quantity of a-radioactivity was found in this trans- 

 uranium fraction and these investigators attributed this to the plu- 

 tonium isotope, Pu'^^. The amount of plutonium in the pitchblende 

 corresponded to about one part in 10^*, an amount which could not 

 possibly have been f oimd had the chemical properties not been known. 

 Thus, although the experiment proved that this could not be a prac- 

 tical source for the production of plutonium, it also gave good evidence 

 that this element does exist in measurable quantities on the face of 

 the earth. 



The relatively short half -life of Pu^^^ compared to the age of the earth 

 makes it necessary that it be continuously formed in order that it be 

 present on the earth in any detectable amount. There is a mechanism 

 for its continuous formation which can both qualitatively and quanti- 

 tatively account for its presence in pitchblende. Uranium undergoes 

 spontaneous fission according to the following reaction : 



^238 ^ fission products+ neutrons + energy (6) 



238 



and the rate corresponds to a "half -life" for this process of some 10 

 years. If all the neutrons from this process are reabsorbed by U 

 to form Pu^^^ according to reaction 2 above, the amount of Pu'^^ in the 

 pitchblende in equilibrium with its parent U^^^ would be (assuming 

 two neutrons per spontaneous fission) 2X24,000/10^^= approximately 

 500 parts in 10". This corresponds to some 100 or 200 parts of Pu^^^ 

 per 10" parts of pitchblende. Thus only about 1 percent of the spon- 

 taneous fission neutrons need be absorbed by TJ-^, the remainder either 

 escaping or being reabsorbed by the many neutron-absorbing impuri- 

 ties in the pitchblende, in order to account for the Pu^^^ present. 



There are, of course, other sources of neutrons which may be of com- 

 parable importance in the formation of this Pu-^^. For example, 

 uranium, and especially its decay products, emits a-particles which 

 can give rise to neutrons according to the well-known (a, n) reaction 

 as a result of their reaction with light nuclei — for example, lithium, 

 boron, beryllium, fluorine, oxygen, etc. — in the pitchblende. 



A further search for the presence of transuranium elements in nature 

 was made during the summer of 1942. Another radioactive ore — 

 namely, carnotite — was chosen this time, the investigation being car- 

 ried out by C. S, Garner, N. A. Bonner, and G. T. Seaborg. As in the 



