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BELL SYSTEM TECHNICAL JOURNAL 



be analyzed; four lines only are visible upon the reproduction, but 

 some plates after long exposure have shown as many as six.* The 

 fine-structure of AcC consists of a pair of lines, which were detected 

 as peaks in the distribution-in-range curve obtained at the Cavendish 

 with a differential ionization-chamber. Instead of showing this curve 

 I have chosen the corresponding curve for RaC, albeit it shows only a 

 single hump (Fig. 8).^^ The unsymmetrical shape of this hump, how- 



Fig. 10 — Fine-structure of alpha-ray spectrum of ThC (not completely brought out 

 in picture) obtained with Bellevue magnet. (S. Rosenblum.) 



ThC 



ThC 



I I 

 ThC RaA 



Fig. 11 — Alpha-ray spectra of several elements (those of Po and AcC shifted 

 slightly to the right with respect to the rest). (S. Rosenblum, Origine des rayons 

 gamma, Hermann & Cie.). 



ever, implies that it really consists of a pair of overlapping peaks; 

 and so it does; for when the Cavendish magnet was applied to an 

 a-ray beam from this element, the curve of detector-reading vs. field- 

 strength displayed two equal peaks quite sharply separate (Fig. 9). 



According to Rosenblum's latest census (February 2, 1934) there 

 are now eight known examples of fine-structure: from the radium 

 series, Ra (two groups), RaC (2); from the thorium series, RdTh (2), 

 ThC (6) ; from the actinium series, RdAc (no fewer than eleven groups, 

 the richest case of all!), AcX (3), An (3), AcC (2). According to 

 Lewis and Wynn-Williams, there are (or were, in the spring of 1932) 



* I am indebted to Dr. Rosenblum for a print from which Fig. 10 was made. 



'^ This curve was the first to disclose the a-rays of RaC, previously known only 

 by inference (though it was very compelling inference). Being of somewhat lesser 

 range than the much more numerous (to be precise, 3000 times as numerous) a- 

 particles emanating from the RaC atoms with which RaC is always inevitably 

 mingled, they were completely hidden from observation by any method known 

 before the use of the differential chamber and the powerful magnet. 



