XV. ELECTRONS, NEUTRONS, AND ALPHA PARTICLES 553 



plying the hydrogen term in the energy absorption by 100/92.5 = 

 1.08. Thus, very simply, we have the approximate relation : 



Em = l.OSNEnnau X 0.5 = 0.54:NEniiaii (21) 



If we wish to convert to dose in equivalent roentgens, the expression 

 (21) must be divided by 84 if energies are expressed in ergs or 

 5.23 X 10^^ if measured in electron volts. To convert energy units, 

 divide by 93 or 5.81 X 10^^, respectivel}'. 



Table VII gives the dose in equivalent roentgens due to a flux 

 of N neutrons per square centimeter of the stated energy, calculated 

 from published experimental values of an for neutrons (23).. The 

 dose per unit neutron flux evidently varies relatively slowly with neu- 

 tron energy. 



TABLE VII 



Approximate Dose Received by Tissue Exposed to Neutron Flux of 10^ Neutrons 



per Square Centimeter as a Function of Neutron Energy 



Neutron energy, m.e.v 0.9 2.-14 2.88 4 25 



Dose, r.e.p 2.03 3.39 4.15 4.23 5.95 



Slow and Thermal Neutrons. So little is known about the ioniza- 

 tion and excitation caused by neutrons having less than about 0.2 

 m.e.v. energy, which we have classified as slow neutrons, that their 

 measurement must be regarded as a research in itself. It is fairly 

 clear, however, from a consideration of alternative modes of absorp- 

 tion of thermal neutrons in tissue that when neutrons of energy less 

 than about 0.05 m.e.v. fall on tissue the energy associated with the 

 disintegration of the nitrogen content of the tissue, which is produced 

 by some 8% of these neutrons, will contribute at least as much energy 

 as is contributed in all other forms in the course of the gradual slowing 

 down of the neutrons. Thus, in the case of very slow neutrons and 

 thermal neutrons, we may concentrate attention on the disintegration 

 of the nitrogen in the tissue element under consideration and the 

 irradiation of this element by the 2.2 m.e.v. 7 rays which come from 

 the absorption of thermal neutrons elsewhere. 



The relation between energy absorption per unit mass of tissue, 

 Em, and thermal neutron flux, A'', is given by an equation analogous 

 to equation (21), namely. 



Err, = Nn^a^E^ (22) 



