from a Plate exposed to Rays from Radium. 



183 



secondary beam from lead due to the /3 and to the 7 rays 

 and the penetrating powers in each case may be referred to 

 and compared with the corresponding results for air just 

 described. These results are given in the last three columns 

 of Table II. for a reflector of thickness 1*8 mm., which was 

 found to be more than sufficient to produce the maximum 

 effect. In the first place, the j3 rays produce a greater 

 proportion of penetrating radiation from lead than from air ; 

 for whereas in the case of air only one-fifth of the stream of rays 

 gets through ^ mm. of lead, now one-half of it does so. As 

 before, the leak does not fall to the possible minimum of 1*89 

 for an absorbing screen of 11 mm., but after the thickness of 

 S reaches beyond 1 mm. the leak remains in the neighbour- 

 hood of 1*95 for the 7 rays, and "06 for the /5 rays. Another 

 obvious result is that the 7 rays now produce considerable 

 absorbable radiation, sufficient to mask entirely the peculiar 

 effect noticed in the case of air. The final leak after 11 mm. 

 of lead is interposed is 1"91, and is diminishing very slowly 

 with increasing thickness of lead, and is nearly 3 per cent, 

 greater than that we know to be a possible minimum. The 

 last column of the table shows that the secondary rays set 

 up by (3 rays are still effective in the case of the greatest 

 thickness of screen S used, that is, are able to produce by 

 striking lead rays of the highly penetrating kind. 



Secondary Radiation from the Front Surface of a Lead Plate. 

 " Reflected Rays."' 



The plate was put in position B, of the diagram,, and various 

 thicknesses were used in order to find when the intensity of 

 the reflected beam reached a maximum. The position of the 

 front surface of each plate was always the same. Observations 

 were made with no absorbing plate at A, and with 15"4 mm. 

 of lead at A, and their differences taken, as before. Table III. 

 contains the results. 



Table III. 



! 



Thickness of reflector ~ 9 ~ 

 in mm. 



■040 -060 1-080 



•095 



•190 



•225 



•45 



1-00 ■ 1-80 



3-60 



5-40 



7-40 



No plate 

 at A. 



-2 



CO 





 



P+7 62-1 

 rays. 



79-4 87-7 91-7 



95-2 



101-0 



104-2 





104-2 104-2 



104-2 





... 



15-4 mm. 

 lead at A. 



y 2-12 

 rays. 



2-30 2 39 2-49 



2-58 





2-73 



2-77 



2-79 2-82 



2-83 



2-84 ! 2-84 



Differ- 

 ence. 



""is 



13 60-0 

 rays. . 



774 85-3 89-2 



92-G 





101-5 



101*4 101-4 



101-4 







