RECOIL EFFECTS FOLLOWING NEUTRON CAPTURE 47 



Other competing reactions are possible, including the formation of a 

 chlorine hydrocarbon. As Miller and Dodson point out, the system may 

 also be treated solely as a set of competing chemical reactions, as is 

 usual for thermal kinetic systems. Williams and Hamill (8), who have 

 used such a treatment in study of the pressure dependence of neutron 

 capture by bromine in mixtures of HBr, C2H4, and C2H5Br, were able 

 to account qualitatively for their effects by a kinetic scheme. The "hot" 

 atom reactions may differ widely from those that take place at thermal 

 energies, as was shown by Miller and Dodson, who were unable to 

 detect any change in yields by a 25° decrease in temperature. 



The experiments of Siie and Kayas (9) cast doubt on the theoretical 

 assumption that the initial act, in all cases, is that of bond rupture. 

 They have shown that, in a series of cobalt complex compounds, the 

 retention of free cobalt atoms increases markedly as the central cobalt 

 atom is protected by a more complex external structure. The retention 

 of free radioactive cobalt by a solution of Co(NH3)6(N03)3 is 14 per cent, 

 compared with a retention of 90 per cent by a solution of Co(NH2CH2- 

 CH2NHCH2CH2NH2)2(N03)3. Control experiments showed that the 

 retention of the latter salt was not increased when it was irradiated with 

 an equal amount of cobalt nitrate, thus indicating that the additional 

 "hot" Co atoms from the cobalt nitrate could not enter the complex. 

 Within the limits of the control, it appears that in large molecules the 

 struck atom often never severs (in any permanent sense) its chemical 

 bonds. 



The process of "hot" atom exchange can also be used in chemical 

 synthesis, as illustrated by the production of radioactive CS2 from C2CI6 

 [see Edwards, Nesbett, and Solomon (10)]. In a pile, S^^ can be prepared 

 with a much higher yield by an (n, p) reaction on chlorine than by an 

 (n, 7) reaction on sulfur, the second important example of this (n, p) 

 process in light elements. Preliminary cyclotron experiments showed 

 that "hot" S^^ atoms from C2CI6 would exchange with CS2 dissolved in 

 the hexacliloroethane. A solution of 1 gram of C2CI6 in 1 ml CS2 was 

 then bombarded in the Oak Ridge pile for 1 month. At the end of this 

 time 12 per cent of the radioactive sulfur was found in the CS2, yielding 

 a product with a specific activity of 1 millicurie per gram. 



Ball, Rodkey, Cooper, Davison, and Solomon (11) have investigated 

 the possibility of utilizing inverse Szilard-Chalmers processes for the 

 production of radioactive compounds. The crystalline amino acid 

 cystine was chosen for preliminary studies. Table 2 shows its compo- 

 sition, and the relative neutron capture cross sections of the sulfur and 

 other atoms. Of the total sulfur capture cross section of 1.6 barns, only 

 0.011 barn is effective in the production of S^^; in view of the small total 



