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Neutrons and Their Special Effects; 

 Recoil Effects 



A. K. SOLOMON 



Biophysical Laboratory 



Harvard Medical School 



Cambridge, Massachusetts 



The neutron is a unique tool for the production of ionization, being 

 set apart from the more familar alpha, beta, gamma, and x-radiation 

 both in its history and in its properties. Its existence was predicted by 

 Rutherford in his famous Bakerian lecture in 1920; indeed, he was so 

 convinced of its existence that he spent some time in a fruitless search 

 for it in electric discharges. It was not until 12 years later that Chadwick, 

 hot on the trail of an aberrant gamma ray, discovered the neutron. It 

 is a radioactive particle, of charge zero, and mass a little greater than the 

 proton. It decays, with a half life of 10-20 min, to a proton and an 

 electron with a maximum energy of 760 kev. 



The interaction of the neutron depends very much on its kinetic 

 energy; indeed, as shown by Fermi in 1934, slow neutrons are in general 

 much more effective than fast ones in the production of artificial radio- 

 activity. For our purposes fast neutrons may be classed as those with 

 energy greater than tens of thousands of electron volts. The remainder, 

 the slow neutrons, include the special category of thermal neutrons 

 which have only the energy available at room temperature, approxi- 

 mately 0.025 ev. The average thermal-neutron velocity is 2200 meters 

 per second. 



Neutrons are formed in nuclear reactions with energies of the order of 

 millions of volts; to produce slow neutrons it is necessary to strip the 

 fast ones of most of their energy. This may be done by passing fast 

 neutrons through a scatterer, called a moderator, as in a nuclear pile. 

 However, since the neutron has no charge, it can neither give up its 

 energy by direct reaction with the extranuclear electrons nor ionize or 

 excite other atoms directly. It must give up its energy through the 

 agency of another nucleon, either by collision or by reaction, and thus 

 relies on secondary effects to produce its ionization. 



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