278 ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 3 



Written as a chemical equation, the process is 



Be{9) +He{J^) >C{12) +n{l) (1) 



Similarly boron behaves like beryllium in giving off neutrons accord- 

 ing to 



Bill) +He{4) >N{U) +n{l) (2) 



Here the alpha particle is, of course, a helium nucleus of mass 4, 

 and the products of transmutation are carbon, nitrogen, and 

 neutrons. 



One striking feature of this transmutation is that the products 

 are heavier atoms than the original atoms. This is a process of atom 

 building and not atom disintegration as in the previously known 

 cases of transmutation, radioactivity, and Rutherford's artificially 

 produced nuclear disintegration. It is highly important to know 

 that atoms may be built up as well as broken down. 



The third and last success of the modern alchemists, to date, was 

 the transmutation of lithium when bombarded by swift protons by 

 Cockroft and Walton about 6 months ago. Here the reaction is 



Lii7)+H{l)>£ He{4) (3) 



This is peculiarly interesting for several reasons. In the first 

 place it is the first instance of transmutation produced by a particle 

 whose speed had been produced by laboratory methods. In the 

 previous cases the bombarding projectiles were alpha particles whose 

 speeds were fixed beyond man's control by the inherent nature of the 

 radioactive process — except that man could slow them down as 

 desired by interposing absorbing screens in their path. In the pres- 

 ent case, however, protons produced by ionization of hydrogen and 

 speeded up by applied voltages up to 600,000 volts were used as the 

 bombarding agents. 



In the second place, such a source of bombarding particles may 

 be made ever so much more powerful than the previous sources of 

 alpha particles, for currents of microamperes or even milliamperes 

 of protons may be used instead of the tiny natural currents of alpha 

 particles which, from the high-speed sources like polonium, come 

 out at the rates of only a few thousand or hundred thousand particles 

 per second. Thus we may hope to carry on these transmutation 

 processes on a chemical rather than an atomic scale. 



In the third place, the proton has only half the charge of an alpha 

 particle and therefore suffers only half the repulsive force as it 

 approaches an atomic nucleus. For this reason we can hope to shoot 

 protons much farther into nuclei than alpha particles can penetrate. 

 Protons thus have in a certain measure the advantage of neutrons, 



