The Measurement of Radiation 189 



Neutrons 



The consideration of the measurement of neutron radiation 

 follows on naturally from that of X- and gamma radiation, for 

 neutron radiation also produces ionization by an indirect means, 

 namely, through the agency of secondary particles. 



A neutron is a material particle of mass approximately unity 

 on the atomic scale, that is, its mass is very similar to the mass 

 of the nucleus of the hydrogen atom, the proton. But, whereas 

 the proton has a positive unit elementary charge, the neutron 

 has no charge at all, and so, unlike radiations consisting of 

 charged particles, it is unable to drag electrons out of the atoms 

 near which it passes. Thus it loses practically no energy by 

 ionization, and will penetrate very much greater thicknesses of 

 matter than, say, a proton of similar energy. 



The interaction of the neutron is almost entirely with the 

 nuclei of the atoms, and the commonest process is a simple 

 collision which deflects the neutron with a reduced energy, and 

 causes the nucleus to recoil with the balance of the original 

 energy. The average energy transfer in a collision is greatest 

 when the neutron and the nucleus have equal masses, and be- 

 comes progressively less as the mass of the recoiling nucleus 

 increases. The energy transfer is greatest in hydrogen, when 

 the neutron energy is on the average reduced to about 37% at 

 each collision. 



In addition to these scattering collisions, a neutron may be 

 captured by a nucleus and provoke nuclear disintegrations of 

 various kinds, sometimes resulting in the production of "artificial 

 radioactivity." The relative probability of such processes is 

 generally small, however, until the neutron has been made very 

 slow by repeated collisions. In the case of biological material, 

 these nuclear disintegrations may usually be ignored in consider- 

 ing the energy communicated to the medium by a beam of 

 neutrons. It may be mentioned in passing, that the induced 

 radioactivity produced in suitable substances is of help in making 



