CHAPTER 5 

 NEUTRONS 



5.1. General Properties 



Atomic weight [1]: = 1.008937 + 0.0000075 



Magnetic moment [2]: = —1.91307 + 0.0006 nuclear magnetons 



Half-life: - — ' 20 min 



Statistics: Fermi-Dirac 



The absence of charge makes the interaction of neutrons with matter 

 strikingly different from that of charged particles. Interaction with electrons 

 is entirely negligible, and interaction with coulomb fields of nuclei does not 

 occur; hence, neutrons do not lose energy by ionization or radiation. Con- 

 sequently there does not exist a range-energy relation such as that which 

 describes the behavior of charged particles traversing absorbing media. But 

 by virtue of their electrical neutrality, neutrons of all energies, down to those 

 with nearly zero kinetic energy, have free access to nuclei, with which they 

 readily combine to form unstable compound nuclei. The effects of neutrons 

 on matter, therefore, must be described in terms of the nuclear reactions they 

 produce. More specifically, the essential properties that must be known are 

 the probabilities or cross sections for the various nuclear interactions that 

 may be induced as a function of neutron energy and the kind of nuclei that 

 constitute the matter. 



The very marked dependence of neutron processes on energy has led to the 

 universal use of the terms fast, slow, and thermal to indicate broad and rather 

 indefinite ranges of neutron kinetic energy. Fast neutrons are those with 

 energies greater than tens of kev; slow neutrons are those with energies less 

 than this. Included in the latter group are thermal neutrons that possess 

 energies of the order of kT, ~ 0.02 ev, where k is Boltzman's constant and T 

 is the absolute temperature. 



Neutrons from nearly all sources are fast since they are emitted in nuclear 

 reactions with kinetic energies of the order of 1 mev. Consequently slow 

 neutrons can be obtained only by slowing down fast neutrons through elastic 

 collisions, usually with light nuclei. This may be accomplished with exten- 

 sive volumes of substances such as graphite or water surrounding the fast 

 neutron source. After losing their kinetic energy the neutrons tend to 

 diffuse through matter in much the same way as gases. Under such condi- 



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