46 NEUTRONS AND THEIR SPECIAL EFFECTS 



ated C2H4Br2 to 75 per cent in C2H5Br. By far the largest part of the 

 radioactivity retained is found in the initial parent molecule. To 

 account for these high retentions, it is necessary to assume that the 

 "hot" recoiling atom re-enters the parent molecule by exchange. When 

 the target halide is diluted by substances whose atomic weight is far 

 different from that of the halide, the retention decreases, and, as might 

 be expected, approaches zero when the target is in the gas phase. 



Libby and Miller et al. (6) have developed approximate theories to 

 account for some of the simple recoil processes. Their theory is based 

 on two assumptions: first, that every neutron captured leads to bond 

 rupture; and, second, that the collisions of the "hot" recoiling atoms are 

 non-ionizing and elastic. Slow neutron bombardment of a pure liquid, 

 say C2H5Br, produces as primary product radioactive bromine because 

 the thermal-neutron capture cross sections are high, 1.11 barns for the 

 production of 34-hour Br^\ with equally high ones for the production of 

 other radioactive bromine isotopes. The "hot" radioactive bromine will 

 then lose its recoil energy by elastic collisions. To give up a large part 

 of its recoil energy in a single impact, it is necessary for the bromine to 

 collide with another bromine atom, for the C and H atoms are far too 

 light to carry away enough energy. If the bromine is to re-enter the 

 parent C2H5Br, it must satisfy two conditions: first, the "hot" bromine 

 ion must have enough energy, v, so that it can knock off a cold bromine 

 from the molecule, leaving a free radical; and, second, the "hot" ion 

 must be left with so little energy, e, that it cannot escape from the 

 "cage" of surrounding molecules. 



The caged ion will then give up the rest of its excess energy by knock- 

 ing against the walls of the cage until it can easily combine with the 

 free radical. Ions left with energy less than v are removed from further 

 consideration because they cannot again form a free radical. The 

 theoretical expression for the retention R is then given by R = e/v, and 

 € is found to lie, for organic halides, in the range of 0.8-1.9 ev (bond 

 strength 2.6 ev). 



Later experiments by Miller and Dodson (7) show that the theory 

 has to be modified in important respects when more than one reacting 

 substance competes for the "hot" atom. Experiments with mixtures of 

 ecu and SiCU give good agreement with theory for the variation of 

 retention with composition. However, as soon as a hydrocarbon re- 

 places SiCU as the diluent, sharp disagreements with theory arise. In 

 order to reconcile experiment with theory, it is necessary to postulate 

 that every "hot" recoil atom first undergoes reaction with the hydro- 

 carbon to form an unstable intermediate, which may subsequently de- 

 compose to free the radioactive chlorine atom for entrance into CCU- 



