MEASUREMENT OF IONIZING RADIATIONS 161 



ing his experiments under conditions allowing the accurate measurement 

 or calculation of energy absorbed in his material. 



Neutrons. The interaction of neutrons with matter leads in general to 

 a variety of nuclear reactions which result in the release of protons, a rays, 

 and 7 rays and in the creation of radioactive nuclei. The type and prob- 

 ability of occurrence of these reactions are critically dependent on the 

 type of nucleus and on the energy of the neutron involved. They are not 

 expressible in simple terms of such general properties of matter as density, 

 mass, atomic number, or neutron energy as is the case for electromagnetic 

 radiation, but are instead available as empirical graphical relations of 

 total probability of reaction per nucleus per neutron per square centimeter 

 as a function of neutron energy (Adair, 1950). This probability has been 

 called "total cross section." Often, when alternative types of reaction 

 occur, the probability of occurrence for each is not known sufficiently well 

 to permit calculations of satisfactory accuracy. This deficiency in basic 

 knowledge, acute in earlier days, led to the provisional adoption of an 

 empirical unit of dose, the n, which served the essential purpose of provid- 

 ing reproducibiUty of irradiation, but precluded quantitative comparison 

 of the effects produced by neutrons to those produced by equal doses of 

 other types of radiation (Aebersold and Anslow, 1946). 



Because of the complicated nature of neutron reactions it is more con- 

 venient to express the dose in terms of neutron flux instead of energy flux. 

 Therefore, the dose D should be expressed, in general, as 



u — Zjj -iji^ e^ e„ th 



in order to account for different types of reactions {if) that take place 

 between nuclei of type i and neutrons of energy Ej. At points where the 

 energy imparted to the material is identical to the energy converted, 

 namely, where e *j = 1, the dose may be expressed as 



where 



D = ^i^^J: (11) 



'■c 



in which n^ = neutron flux in neutrons per square centimeter character- 

 ized by energy Ej 

 7il = number of nuclei of type i per gram of material 

 ffii = cross section of the reaction in square centimeters per 

 nucleus, namely, the probability that the reaction ij 

 occurs when a flux of one neutron of type j per square 

 centimeter impinges on one atom of type i 

 E^' = the average kinetic energy imparted to the ionizing 

 particle or particles which are released by the reaction ij 



