ATOMIC ENERGY — JOHNS 187 



run several months, after which the uranium rods are removed and 

 the pkitonium created in them is separated out. It is like hunting 

 a needle in a haystack, for 50 kilograms of uranium may have 10 

 grams of plutonium in it. 



FISSION PRODUCTS 



I have already remarked that the kinds of atoms now discovered 

 number upwards of 600, and of these a very large number, well over 

 200, have arisen as fission products by neutron bombardment of 

 uranium. These fission products range all the way from zinc to sama- 

 rium. Strangely enough they break into two groups. The light group 

 consists of those from zinc with mass number about 72 to palladium 

 with mass number about 108, and shows the greatest concentration 

 around krypton with mass number 94. The heavy group goes from 

 palladium to samarium with mass number 150 and shows the greatest 

 concentration around barium with mass number 140. Actually about 

 6 percent is krypton and 6 percent barium, with 97 percent of all fission 

 products grouped closely around these. The minimum is around tin 

 with mass number 117 and the yield there is about 0.01 percent of the 

 whole. 



For the lighter elements in the periodic table the number of neu- 

 trons in the nucleus is about equal to the number of protons. For in- 

 stance carbon has 6 of each. But as we ascend in the table the propor- 

 tion of neutrons to protons gradually increases until in uranium 238 

 there are 146 neutrons to 92 protons. So the fission products from 

 uranium, which are near the middle of the table, are overloaded with 

 neutrons and are likely to be unstable and give off beta-rays in succes- 

 sion imtil a stable product is formed. There are 64 such mass chains of 

 transformations now known, 31 in the light group and 33 in the heavy, 

 and they involve about 164 known active products, and about 64 stable 

 ones. McMaster University, under Dr. Thode's direction, is given 

 credit for the discovery of eight of the stable products, and one active 

 one, namely, Kr^^ with a half-life of 10 years. 



The problems presented to the chemist by these fission products 

 were appalling. He had to separate the minute quantities of these 

 bewildering products from the original uranium and the neptunium 

 and plutonium created there as well. He had to determine what ele- 

 ments were there and what isotope. If the product was radioactive, 

 what was its half -life ; was it formed directly or was it part of a chain ; 

 and if of a chain, how was it related to other fission products? What 

 energy had the ^- and y-rays emitted? These and many other ques- 

 tions confronted the chemist, and he had to work with exceedingly 

 minute quantities and he had to work fast. Any product with half- 

 life of less than 2 seconds cannot yet be identified. One instrument. 



