RECOIL EFFECTS FOLLOWING NEUTRON CAPTURE 45 



Neutron Interactions with Lithium and Boron 



Among the light elements, lithium and boron are unique in that they 

 can capture slow neutrons and give off alpha particles. The cross section 

 for this reaction is abnormally large, being at ordinary thermal energy 

 about 900 barns for Li® (70 barns for the natural isotopic mixture of Li® 

 and Li^) and 3800 barns for B^° (710 barns for the natural mixture). 

 The Li® alpha particle is ejected with an energy of 4.6 mev, and the 

 B^^ alpha particle with an energy of about 2.5 mev. The high, slow 

 neutron cross section, coupled with the short path length and high 

 ionization density of the alpha particles, makes these reactions attractive 

 for the production of local dense ionization. 



Kruger (3) and Zahl and his collaborators (4) have attempted to make 

 use of these reactions in selective tumor irradiation. Kruger found that 

 the high ionization density worked well with tumors soaked in boric 

 acid in vitro, but neither group was able to devise a method by which 

 the boron or the lithium could be selectively absorbed by the living 

 tumor. 



Recoil Effects Following Neutron Capture 



The 2.2-mev gamma ray given off when a proton captures a slow 

 neutron causes the deuteron to recoil with considerable energy. This 

 energy is at a maximum when a single quantum is emitted after neutron 

 capture, the usual case for light nuclei. Under these conditions the 

 recoil energy {Er in mev) from a single gamma-ray emission is given by 

 the expression Er = 536 X 10~® E^/M, where E is the gamma-ray 

 energy in mev, and M the mass of the recoiling atom in atomic mass 

 units. The deuteron will recoil with an energy of 1300 ev. This tre- 

 mendous energy can be compared with the ionization potential for 

 hydrogen of 13.6 ev. In slow neutron capture by C^^, the product nucleus 

 has a recoil energy of 945 ev from the 4.1-mev gamma ray, to be com- 

 pared with the carbon-hydrogen bond strength of 3.8 ev. The bond 

 strength of the C — O bond is 3.0 ev; the C — C bond 2.6 ev; and the 

 C— N bond 2.1 e v. 



Szilard and Chalmers (5) in 1934 made use of these high recoil energies 

 to obtain radioactive halogen atoms of high specific activity. When a 

 pure liquid hydrocarbon halide is exposed to slow neutron bombard- 

 ment, the halogen bond is ruptured by recoil, and the halogen can then 

 be extracted by water. However, it has been observed that even though 

 more than enough energy is available to break the bonds, some of the 

 halogen is retained by the parent liquid (6). The fraction retained in 

 the organic layer, called the retention, varies from 31 per cent in irradi- 



