138 ANNUAL REPORT SMITHSONIAN INSTITUTION, 194 5 



to the world for nothing by the Curies (6), was now selling at a very 

 high price. 



But away from the distractions of the world of industry the first 

 successful "artificial" disintegration of ordinary chemical elements 

 was made by Rutherford in 1919 at Cambridge by passing alpha par- 

 ticles through nitrogen which produced scintillations on a screen when 

 the absorbing matter present was greater than that which corresponded 

 to the ordinary range of alpha particles (4). Rutherford concluded 

 that the scintillations were due to particles ejected with great speed 

 from the nitrogen nucleus by impact of an alpha particle, and sug- 

 gested that these particles were hydrogen nuclei or protons. Evidence 

 of the capture of an alpha particle by the nucleus of the nitrogen atom 

 was obtained by Blackett (7) a few years later by photographing al- 

 pha-ray tracks in a Wilson expansion chamber. In 1934, Joliot and 

 Curie (daughter of Madame Curie) gave the first chemical proof of 

 artificial transmutation and of the capture of the alpha particle (8). 



The alpha particle, with its two positive charges, has twice the 

 potential barrier to surmount in order to penetrate an atomic nucleus 

 as a single charged particle, and consequently search was made for a 

 means of accelerating hydrogen nuclei or protons carrying a single 

 positive charge. Most investigators used high voltages for the pur- 

 pose, but among those interested in the subject was E. O. Lawrence, 

 born 2 years after the discovery of radium, who saw a paper by the 

 German physicist, Wideroe, describing a method for energizing ionized 

 particles by hollow cylindrical electrodes in a tandem pair, each elec- 

 trode being connected to the leg of an oscillating high-frequency cir- 

 cuit. Lawrence conceived the idea of raising the efficiency of the 

 system, without increasing the length of the apparatus to unmanage- 

 able proportions, by giving the particles a circular motion by employ- 

 ing a powerful magnetic field within which a hollow electrode was 

 fixed. 8 A design for "a magnetic resonance accelerator for ions" was 

 made, and a model of the apparatus tried out. It worked, and was 

 given the nickname of "cyclotron" (9). With a linear accelerator and 

 an oscillating current of 40 kv., 30 tandem electrodes would be required 

 to achieve an ion energy of 1.2 mev. ; 150 revolutions in the cyclotron 

 with an oscillating current of only 4 kv., however, produces ions of 

 equal energy (10). The result has been a succession of cyclotrons (see 

 table 1) built at the University of California, Berkeley, by a team 

 which were said "not to know the meaning of a regulated existence . . . 

 as day and night the work went on" (10). 



8 A single hollow D-electrode was used In the early apparatus ; later a pair of D-shaped 

 electrodes, now a characteristic feature of cyclotron design, were used. 



