IONIZATION AND BIOLOGICAL EFFECTS 115 



that this can be strictly true only for one definite set of conditions. In 

 particular when the quality of radiation is varied, the balance of loss and 

 gain of ionization by the presence of the wall may be upset considerably. 

 By a suitable adjustment of materials, wall thickness, and volume, closed 

 ionization chambers have been constructed which are remarkably free 

 from wall effect in the range of wave-lengths commonly employed in 

 practice. The error introduced by the wall effect on the long-wave side 

 may be ascertained by comparison with a standard ionization chamber 

 (cf. L. S. Taylor, Paper II). On the short-wave side, approaching the 

 gamma-ray region, this comparison cannot be made, for reasons which 

 cannot be given in detail here.^^ 



KNOWLEDGE OF IONIZATION PRODUCED IN IRRADIATED ORGANISM 



ALL IMPORTANT 



The point which is of chief interest to the biologist is this : Placing a 

 living organism at a certain point m a beam of radiation (X-rays or 

 gamma rays), he wants to know the number of ions'" liberated per second 

 per cubic centimeter at every point of the material. Knowing the dura- 

 tion of the irradiation (exposure time), he can calculate the total number 

 of ions produced per cubic centimeter at any point of the material during 

 the treatment. Then he has something which he can reasonably call 

 the "cause" and can relate it to the observed "effect." Now granting 

 that this is the desideratum, it follows that the biologist is not particu- 

 larly interested in the ionization produced in the atmosphere at a point 

 where he expects to place his material, unless he can deduce therefrom 

 the ionization which is subsequently produced in the biological object. 

 It has been shown how placing an ionization chamber with solid walls in 

 a beam of radiation in general disturbs the ionization in the air at that 

 point. Since it is assumed that the chamber is made of organic materials, 

 whose atoms react individually with the radiation in the same way as the 

 atoms of air (or living tissue), the disturbance is due to the greater con- 

 centration of atoms in the chamber materials. Or, to be more specific, 

 it is due to the introduction of solid materials. Obviously the living 



*^ The results of the author's work on this subject were presented recently at the 

 Fourth International Congress of Radiology, held in Zurich. They will be published 

 in the near future in "Acta Radiologica" (Stockholm). 



2" The biologist should note that the number of ions of both polarities is the con- 

 trolling factor in his case. Physicists are primarily interested in ion pairs, and in 

 general when they speak of a "certain number of ions" produced in an ionization 

 chamber, for instance, they mean actually the number of ion pairs. This is rather 

 confusing at times, and the biologist who makes use of such data given in textbooks 

 on physics should ascertain whether the numbers refer to single ions or ion pairs. 

 In this paper both terms have been used, but — it is hoped — without introducing 

 any uncertainty. 



