48 NEUTRONS AND THEIR SPECIAL EFFECTS 



capture cross section of carbon (0.0049 barn), a much smaller cross 

 section was expected for the formation of C^^. 



* Results corrected to insulin molecular weight of 36,000. 



Bombardment of the first few samples was disappointing because the 

 cystine was returned so charred by the heat in the pile that it could not 

 be used. Finally, a cool enough place was found, and the returned sample 

 was subjected to an exhausting variety of chemical tests which proved 

 that all the radioactivity was present as S^^ and that it was contained in 

 a molecule which could be chemically characterized as cystine. This 

 proves that the cross section for (n, 7) capture by carbon leading to 

 formation of C^* has a very low value, probably less than 0.0004 barn. 



Although exact figures of neutron flux are unavailable, approximate 

 calculations show that about 6.8 per cent of the S^^ radioactivity was 

 retained in the cystine. The final product had a specific activity of 0.7 

 microcurie per millimole of cystine. 



This result was so promising that it was decided to bombard ciystalline 

 insulin, a relatively heat-stable protein containing 12 per cent cystine. 

 The cystine plays an important role in the structure of insulin, for its 

 sulfur atoms serve as the cross links which hold the protein chains 

 together. It is to be expected that insulin, as shown in Table 2, would 

 be considerably damaged by gamma recoil effects, for the preponderance 

 of hydrogen and carbon would cause the disruption of a great many 

 chemical bonds. No appreciable yield of C^^ was anticipated. Insulin 



