130 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 5 



approximated by a Maxwell distribution for a temperature T because of the 

 large number of levels affected at high energies, the cross section for emission 

 of two neutrons is given by the expression 



ff(7>.,2n) — 7riv-£ 



(, + «-&) .*?*] 



1-11 + ^-^— " e l cm 2 



where T = temperature correlated to observed energy distribution, 



T = 2(5E n /A)K mev 



£ = sticking probability for neutrons of high energy (> 1.0 mev) 

 At high energies E n — E b » T, and the (n, 2n) process becomes more prob- 

 able than (n, n). The process involving the emission of more than two neu- 

 trons becomes increasingly more important at very high energies, i.e., when 

 Ei » E h . 



d. Char ged-par tide Emission (n, p), (n, d), (n, a), . . . . Reactions 

 leading to the emission of charged particles are probable only when the 

 incident neutron energy is sufficiently high so that the available energy is 

 great enough to allow a charged particle to pass over the electrostatic poten- 

 tial barrier. Assuming the average cross section for high energies to be of the 

 form 



a q = TR^ cm 2 



the charged particle width is the product of the emission width without the 

 barrier r' 9 and the penetrability factor P = e~ f . Because of the large num- 

 bers of levels excited by neutrons with energies high enough to result in 

 charged-particle emission, the energy distribution of the emitted particles is 

 continuous and probably roughly Maxwellian. 



Ordinarily, the cross section for charged-particle emission is small because 

 of the strong competition from the more favorable (n, n) process or at very 

 high energies, from (n, 2n), (n, 3n), etc. 



e. Fission (see Fission, Chap. 6). 



/. Other Processes. At energies of the order of 100 mev and greater, 

 neutron bombardment frequently leads to the emission of great numbers of 

 particles and probably complete disintegration of certain nuclei. Such 

 processes are still under investigation at the present time (184-in. synchro- 

 cyclotron at the Radiation Laboratory of the University of California). 



5.6. Neutron Diffusion. For practical purposes neutrons are obtained 

 either by the reactions (a, n) and (7, n) from mixtures of a radioactive isotope 

 such as radium, and a "target" material such as beryllium, or from appro- 

 priate targets bombarded with heavy particles in high-energy accelerators. 

 However, all such sources provide only fast neutrons, and when slow neutrons 

 are required it is necessary to slow down the fast neutrons with appropriate 



