552 



H. GRAY 



specimen must be surrounded on all sides by material having the same 

 hydrogen content as itself. 



TABLE VI 

 Relative Dose in Equivalent Roentgens of Various Materials Exposed to the Same 

 Beam of Neutrons Generated by 8 m.e.v. Deuterons Bombarding Beryllium (91 ) 



Energy 



absorbed per g. 



material 



exposed to 



8 m.e.v. 



Material Composition neutrons" 



Tissue (Aebersold and Anslow) Co.sHgOs.sNo.u 



Tissue (Gray and Read) C0.8H8O4 



Paraffin CH2 



Amber CsHgOo.s 



Bakelite CjHsOo 



Celluloid Ca.sHgOa.sNn.T 



Air O0.4N1.6 



Aerion (Zimmer) C10.9H8O2.7 



" Arbitrary units. 



1.00 



0.94 



1.34 



1.03 



0.70 



0.64 



0.195 



0.53 



Since the neutron output of a source is usually quoted in the physi- 

 cal literature in terms either of the total number of neutrons emitted 

 in all directions or of the neutron flux at a given distance from the 

 source, it may be convenient to note the follo^ving relation between 

 the energy absorbed per unit mass of a medium, E^, and the number, 

 N, of neutrons incident per unit area, namely: 



Er„, = NE-^rii (Ti 



2Ai 



(1 + A^y 



(20) 



where E is the energy of individual neutrons, fit is the number of atoms 

 of type i per unit mass of the medium, o-j is the atomic scattering 

 cross section of atoms of type i for neutrons of energy E, and Ai is 

 the atomic weight of atoms of type i. The sign S indicates that the 

 contributions of all types of atom are to be added together. The 

 formula implicitly assumes that the scattering of neutrons by all 

 types of atom is isotropic, which is not strictly true. It is, however, 

 true of hydrogen, which contributes about 92.5% of the energy ab- 

 sorption in tissue. Indeed a good approximation to the energy ab- 

 sorption for neutrons of any given energy may be obtained by multi- 



