[ 467 ] 



XXXV. 



ON" THE DISTRIBUTION OF ACTIVITY IN EADIUM THEEAPY 

 UNDER DIFFERENT CONDITIONS OF SCREENING. 



By H. H. POOLE, M.A., Sc.D., 

 Chief Scientific Officer, Royal Dublin Society. 



[Read January 24. Published April 13, 1922.] 



Introduction. 



A GREAT deal of work has been done by various experimenters on the passage 

 of a, |3, and y rays from radioactive substances through various kinds of 

 matter, and on the secondary radiations set up.' Some of the results so 

 obtained are rather inaccessible to medical workers in this country, and, 

 moreover, their application to specific problems is generally rendered difficult 

 by the complexity of the radiations, and by the necessity of combining the 

 effect of absorption with the geometrical effect of distance. 



An attempt has accordingly been made to reduce the user's problem 

 to its simplest form by carrying out tests on the screening effects of various 

 materials for the combined j3 and y radiation emitted by an emanation 

 tube at a fixed distance, and combining the results so obtained with the 

 geometrical law of distance, so as to calculate the variation of activity 

 with distance from the source in some typical cases.^ 



It seems probable that we may take the ionisation produced in the 

 tissues as a measure of the physiological action. This ionisation may be 

 due to : — (1) a rays, (2) primary j3 rays, (3) secondary /3 rays produced by 



'For a review of recent work on the absorption of y rays see " Radium," October, 

 1921. 



= The validity of this method has been questioned by Kroenig and Freidrich 

 [" Strahlentherapie," xi, p. 20, 1920]. Their direct measurements of y radiation at a 

 depth of 10 cms. in water gave I'esults about twice as great as they obtained by 

 calculation. They attributed the discrepancy to scattered radiation. On the other 

 hand, Sievert ["Acta Radiologica," i, p. 89, 1921] obtained results at smaller depths 

 according excellently with theory. It would seem unlikely that, under the conditions 

 prevalent in radium therapy, scattered radiation could cause a large error for distances 

 up to 10 cms. 



SOIENT. PROC. E.D.S., VOL XVI, NO. XSXV. 3 E 



