554 L. H. GRAY 



where Wn is the number of nitrogen atoms per gram of tissue, a^ is 

 the atomic cross section for disintegration, and £'n is the energy re- 

 lease per disintegration. Assuming 2.5% by weight of nitrogen in 

 tissue, (7N = 1-7 X 10-24 cm.^ and E^ = 0.62 m.e.v. (94), the 

 dose is related to the neutron flux by: 



dose = 2.18 X lO-^i N r.e.p. (23) 



The most direct method of measuring the dose due to slow neutrons 

 is to measure the ionization produced in air in a chamber large com- 

 pared with the range of the disintegration protons — i.e., large com- 

 pared with 1 cm. The ionization observed will be a measure of the 

 energy loss in a medium in which the percentage of nitrogen by 

 weight is the same as in air, namely, 75.5%. The dose received by 

 tissue is therefore related to the ionization per unit volume of air at 

 0°C. and 760 mm. of mercury by: 



dose = 1.11 J, X (p/0.755) r.e.p. (24) 



= l.OJ,, X (p/0.755) energy units 



where p is the proportion by weight of nitrogen in tissue. The factor 

 1.11 is the ratio of the energy expended per ion pair formed in air by 

 disintegration protons relative to fast electrons. When the dose is 

 expressed in energy units, this factor is exactly balanced by the ratio of 

 the electron densities of tissue and air. 



Mixed Radiations. The measurement of the dose in equivalent 

 roentgens received by tissue exposed to mixed 7-ray, fast neutron, 

 and slow neutron beams presents no new problem in principle. It 

 requires only that a chamber be constructed in which the elements 

 vital to the correct recording of neutron dose, namely, hydrogen and 

 nitrogen, should be present in the chamber wall and in the gas in cor- 

 rect proportions. If this is accomplished, the chamber (with appro- 

 priate allowance for W, the average energy expended in the produc- 

 tion of a pair of ions in the gas mixture) will correctly record the dose 

 due to all three types of radiation together in equivalent roentgens, 

 except for the energy dissipated by epithermal neutrons, which has 

 no proportional ionization associated with it. ^his omission is not 

 likely to be serious in practice. 



It unfortunately happens, however, that, in the practical problem 

 of monitoring research establishments to ascertain that the permis- 

 sible dose levels are not exceeded, the total effect as regards injury 

 to health of the ionization due to fast and slow neutrons is weighted 



