XV. ELECTRONS, NEUTRONS, AND ALPHA PARTICLES 551 



The expression for the neutron dose, in equivalent roentgens, de- 

 Hvered to the material of the walls of the chamber or to any material 

 having the same atomic composition as the walls, when the ionization 

 produced in the air in the chamber at 0°C. and 760 mm. of mercury- 

 is Jj, electrostatic units per cubic centimeter, is given by : 



dose = Pm X ^l I" Jv r.e.p. (19) 



61.D 



where pm and (IFp)air are mean values for the particular energy distri- 

 bution of recoil protons generated by the neutrons. 



TABLE V 

 Computed Variation of TFair with Particle Energy for Protons and a Particles 



a particle energy, m.e.v. 12345678 

 Proton energy, m.e.v.. 0.25 0.5 0.75 1.0 1.25 1.5 1.75 2.0 



^^'air for an a particle 

 of given instantane- 

 ous energy 36.9 35.8 34.8 34.5 34.3 34.2 34.1 34.0 



Wair relative to that for 



an 8 m.e.v. a particle. 1.085 1.053 1.024 1.015 1.009 1.006 1.003 (1.000) 



Wa.\T for an a particle of 



given initial energy . . 38.6 37.3 36.5 36.0 35.7 35,4 35.2 35.0 



PTair relative to that for 



an 8 m.e.v. a particle. 1.103 1.066 1.043 1.028 1.020 1.011 1.006 (1.000) 



Table V shows the manner in which W probably varies with en- 

 ergy for a particles and protons, as computed by Gray {23) from a 

 survey of experimental data. The mean initial energy of the protons 

 may be taken to be half that of the neutrons under consideration. 



In attempting to match the atomic composition of the chamber 

 wall with that of the tissue being irradiated the important element to 

 consider is the hydrogen, since this usually gives rise to rather over 

 90% of total energy absorption. It will generally be impossible to 

 obtain an exact match but, when interpolating between two sub- 

 stances, one more rich and one less rich in hydrogen than tissue, ad- 

 vantage may be taken of the fact that the energy absorption increases 

 linearly with hydrogen content. Aebersold's observations showed 

 that neutron energy absorption in amber is very close to that in tis- 

 sue. The figures given in Table VI are taken from Aebersold and An- 

 slow's paper {91). It is clear that the wall thickness of the ionization 

 chamber must be at least equal to the range of the most energetic 

 recoil protons, and similarly that in the biological irradiations the 



