546 L. H. GRAY 



stopping power ratio if the dose is expressed in energy units per min- 

 ute. 



If the source is quite clean and not covered by any kind of film 

 so that the energy of the a particles is accurately known, it may be 

 easier to count the number of a particles emitted within a known 

 small solid angle and apply the general equation given on p. 525: 



dose rate at 1 cm. = 1.10/ A r.e.p./min. (14) 



= 0.99/ A energy units/min. 



for a point-source of 1 millicurie strength, i.e., a source emitting 3.7 

 X lO'^a particles isotropically, where A is the ion density of the par- 

 ticles in water and/ as above is 1.09. 



Nonionizing Primary Radiation. Fast Neutrons. The diffi- 

 cult problem that arises by virtue of the fact that the absorption of 

 neutron energy per unit mass of air bears little relation to the ab- 

 sorption in tissue was met in America by recording always the reading 

 of a particular commercial X-ray ionization chamber, the Victoreen 

 100 r. chamber, when exposed to the neutron beam, but expressing 

 the fact that the ionization was produced by neutrons and not by X 

 rays by substituting the symbol n for the symbol r. This was recog- 

 nized as a temporary expedient that might at least enable particular 

 conditions of exposure to be reproduced pending further research on 

 neutron dosimetr3^ Owing to the fact that the Bakelite walls of the 

 Victoreen ionization chamber contained hydrogen the measured 

 ionization is in part due to recoil protons and bears a closer relation 

 to the absorption of neutron energy in tissue than would the ioniza- 

 tion produced in free air. As a result of a very careful subsequent 

 study of the problem, Aebersold and Anslow {91) concluded that 

 under most of the conditions that obtained in the previous biological 

 investigations carried out in the United States with fast neutrons, 

 the energy dissipation in tissue could be reckoned to have been just 

 about double that employed by exposure to 1 r. of X radiation. Thus 

 it was concluded that for fast neutrons : 



1 n = 2 to 2.5 r.e.p. = 168 to 210 ergs/g. tissue 



The problem of the measurement of fast neutron dose is that of 

 measuring as accurately as possible and in absolute units, with the 

 aid of ionization chambers, the amount of energy lost by ionizing par- 

 ticles per unit mass of tissue when that tissue is exposed to a beam of 



