20 LIFE: ITS NATURE AND ORIGIN 



1932 by Sir James Chadwick (Nobel prize, 1935). This opened 

 the way for understanding how the heavy proton and the agile 

 electron can be associated with still other particles in atomic 

 nuclei. In 1932 Carl David Anderson (Nobel prize, 1936) dis- 

 covered the positron, or "positive electron." We have as yet no 

 complete explanation of how, under sufficient impact or excita- 

 tion, electrons (beta rays) and positrons may emerge from atomic 

 nuclei. Rutherford suggested that within the confines of the 

 nucleus where particles are at close grips and subject to enormous 

 forces, protons may change to neutrons, and vice versa. P. A. M. 

 Dirac (Nobel prize, 1933), predicted that if a high frequency 

 gamma ray (a high-energy photon) were to pass near enough to 

 an atomic nucleus, the powerful nuclear field would "annihilate" 

 the gamma ray, and "create" a pair of particles, an electron and a 

 positron. This phenomenon, known to physicists as pair produc- 

 tion, was soon observed by P. M. S. Blackett and others. 9 



Joliot and Induced Radioactivity 



In 1934, Jean Frederick Joliot and his wife Irene Curie-Joliot 

 (Nobel prize, 1945) while bombarding aluminum with alpha par- 

 ticles from polonium, found that even after the polonium emitter 

 had been removed, the aluminum still continued to emit radia- 

 tion — that is, it had become radioactive. What happened may be 

 understood from the following: 



jHe<+uAl" , 16 p30 +onl 



\ 



M Si 3 °+_ie 



The entering alpha particle had changed the aluminum into an 

 unstable isotope of phosphorus with the ejection of a high-speed 

 neutron; and the phosphorus nucleus (half-life 2.55 min.) gradu- 

 ally changed into a stable isotope of silicon. On dissolving the 

 radiated aluminum in HC1, adding some ordinary inactive phos- 

 phorus-containing salt, and then carrying out an ordinary analyt- 

 ical group separation, the radioactivity followed the phosphorus. 



Ernest O. Lawrence and the Cyclotron 



In 1932, Professor Ernest O. Lawrence of Univ. of California 

 (Nobel prize, 1940) announced the cyclotron. Hundreds of syn- 

 thetic isotopes have been recognized, and these have been mainly 

 produced by use of the cyclotron. Following the announcement 



