THE SMALLEST PARTICLES OF MATTER 21 



of the Curie-Joliot discovery, Lawrence bombarded sodium with 

 2 mev (million electron-volt) deuterons, from his cyclotron, and 

 found that it became radioactive. The radio-sodium formed has 

 a half-life of only 15 hours; it changes into a stable isotope of 

 magnesium, giving off an electron and a gamma ray. 



When a high-speed deuteron from the cyclotron crashes into a beryl- 

 lium nucleus, an unstable, radioactive isotope of boron is formed, 

 which becomes a stable isotope of boron by ejecting a neutron of 

 21 mev energy. Unstable nuclei are also capable of undergoing 

 multiple and branch disintegrations. For example, when boron is 

 bombarded by protons it forms an unstable carbon nucleus, 6 C 12 , which 

 breaks down in steps into three helium nuclei; and on bombarding 

 lithium with deuterons, the unstable 3 Li 6 formed may break down 

 either into 3 Li t + 1 H x , or into 2 He 4 + 2 He 4 . 



Professor Glenn T. Seaborg (California) has described 10 the trans- 

 uranium elements thus far produced, as well as the astonishing ultra- 

 microchemical methods used in following the work, including the use 

 of quartz fiber microbalances with a sensitivity of 0.02 ^g. The new 

 elements are neptunium (Np 239 , half-life 2.3 days), plutonium (Pu 239 , 

 half-life 24,000 years), americum (Am 95 , half-life 500 years) and curium 

 Cm 96 , half-life of isotope 240, one month; of isotope 242, five months). 

 Both neptunium and plutonium have other isotopes than the main 

 ones above indicated. On Aug. 18, 1942 the first pure chemical com- 

 pound of plutonium was produced, and Seaborg states: "This memor- 

 able day will go down in scientific history to mark the first sight of a 

 synthetic element and the first isolation of a weighable amount of an 

 artificially produced isotope of any element." 



Enrico Fermi and Nuclear Fission 



Shortly after Chadwick had discovered the neutron, Professor 

 Enrico Fermi (Nobel prize, 1938) made a number of new radio- 

 active isotopes by exposing various elements to this uncharged 

 missile. Usually the nucleus would capture the neutron, and 

 often the atom would return to a stable state by emitting a beta 

 ray, thereby yielding an element having an atomic number one 

 unit higher in the Periodic Table than the parent atom. Fermi 

 wanted to see what would happen in the case of uranium, the last 

 element in the atomic table with an atomic number of 92. After 

 prolonged exposure to neutron bombardment, the activity showed 

 the existence of particles with jour half-lives, and indications of 

 others; but natural uranium has only three isotopes. This led to 

 the notion that one of the supernumerary activities might be due 



