910 Mr F. P. Slater on Excitation of y Radiation 



difference between the two curves, the one for lead and the 

 one for paper, for the same absorbing thickness, can probably 

 only arise from radiation set up by the impact of the 

 a particles on the lead, though a more detailed discussion 

 will be given later in this paper. 



Thus by determining the initial difference in ordinate 

 between the paper curve and the curve with any other 

 substance, for different thicknesses of absorbing material — 

 lead in this case — an idea of the quality of the radiation can 

 be obtained. The theoretical rise cannot be taken as the 

 standard from which this initial ionization is measured, 

 as it disregards contamination, which is always a variable 

 quantity. It is reasonably safe to use the amount of initial 

 activity obtained when paper is the target bombarded by the 

 a particles as the standard from which the extra initial 

 activity obtained when tin and lead are the targets can be 

 measured. This initial standard activity was obtained by 

 taking the mean of a number of results, and it incorporates 

 the average initial activity due to contamination, thereby 

 reducing the error which the latter would otherwise intro- 

 duce. The actual amount of initial contamination activity, 

 if present at all, would probably vary from experiment to 

 experiment, and since it cannot be determined definitely for 

 each investigation the method of using the paper standard is 

 the only satisfactory system, and this procedure was adopted 

 throughout these investigations. Paper contains atoms of 

 low atomic number only, and from the results of these 

 experiments its use seems fully justified. 



Since the amount of the difference of initial activity 

 between the radiation from paper and any other element can 

 arise only from the bombardment of the emanation a particles, 

 its magnitude in divisions per minute per milligram of ema- 

 nation used was calculated for each particular absorption 

 thickness of lead used. From these amounts the absorption 

 coefficients of the excited tin and lead radiations are deduced 

 in the usual way. 



The metals tried were lead, tin, and aluminium, which 

 give a wide range of atomic number. The amount of radia- 

 tion from lead and tin was sufficiently large to measure with 

 some degree of accuracy, but aluminium gave quantities too 

 small to determine the quality of the radiation with any 

 certainty. 



Results.— Lead. The absorption curve in lead for the lead 

 radiation is shown by curve LL, fig. 3, where the logarithms 

 of the ionization are plotted against the thickness of the 



