244 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 5 



therefore, is an example of a discovery in pure science which ahnost 

 immediately found a practical application in furnishing to the 

 nuclear physicist a very efficient projectile just at a time when such 

 ammunition was to become exceedingly valuable. 



ARTIFICIAL ATOMIC TRANSMUTATIONS 



Fundamentally, the method of effecting atomic transmutations is 

 to allow one nucleus traveling at a high velocity to strike another 

 nucleus and then to watch for evidence of a transmutation. Ruther- 

 ford, in his first work in 1919, used the alpha particles emitted by 

 radium. The alpha particles, or nuclei of helium atoms, were al- 

 lowed to strike nitrogen nuclei; and the ejection of protons, or 

 hydrogen nuclei, was observed. In the following years the work m 

 this field progressed only slowly, chiefly because of the limitations 

 imposed upon the method by the need of relying upon natural radio- 

 active bodies to supply the projectiles. The scarcity of radioactive 

 materials and the minute size of the nucleus target, which permitted 

 only one in more than a million of the alpha particles to make a hit 

 sufficiently direct to produce a transmutation, made the number of 

 transmutations which could be effected by this method very small 

 indeed. 



But in April 1932 the experiments of J. D. Cockcroft and E. T. S. 

 Walton in the Cavendish Laboratory, England, gave a tremendous 

 impetus to this fascinating and important phase of physics. In 

 place of Nature's own source of high-si^eed nuclei they substituted 

 a large glass tube in connection with a high-voltage electrical circuit 

 and observed for the first time atomic disintegrations produced by 

 ammunition artificially flung. The electrical method of producing 

 high-velocity streams of particles has the advantage that one is not 

 limited only to alpha particles such as radium supplies but may also 

 use various other nuclei. Cockcroft and Walton first used protons. 

 The newly discovered deuton has proved to be a projectile par 

 excellence and is now a favorite among the atom smashers. 



The speed given to the projectile depends upon the voltage applied 

 to the tube. The first work was done using 700,000 volts; and, 

 although it is not necessary to use voltage of such high values, the 

 effectiveness for transmutation increases rapidly with voltage — the 

 higher the voltage the more transmutations produced. The voltages 

 employed, and the currents, which determine, respectively, the speed 

 and the number of projectiles fired per second, are limited only by 

 the technical difficulties involved. Already these have been over- 

 come to such an extent that in this country, C. C. Lauritsen and 

 R. H. Crane at the California Institute of Technology are using 

 daily a tube operating at close to 1,000,000 volts and employing 



