Secondary 7 Radiation. 431 



§n. 



To investigate the secondary incidence 7 rays the ioni- 

 zation-chamber (lig. 1), was inverted, and the large plates 

 which served as radiators were laid directly npon the ioniza- 

 tion-chamber as at p, p. This enabled, the secondary inci- 

 dence rays from the radiator to enter the ionization-chamber, 

 not only by the sides i, i bnt also through the top of the 

 chamber, and should more than compensate for any decrease 

 in effect due to the primary 7 rays falling upon a larger 

 area than they did in the case where the emergence radiation 

 was measured. 



The secondary incidence radiation increased in amount as 

 the thickness of the radiator was increased. In fig. 6 is 

 shown the relation between the mass of the radiator and the 

 amount of the secondary incidence 7 radiation for C, Al, 

 Zn, and Pb. 



C and Al gave maximum values of nearly the same 

 amount, viz., 300 ; Zn not quite so much, viz., 270 ; while 

 Pb gave only 100. Comparing these with the corrected 

 values lor the emergence radiation from Pb and Zn respect- 

 ively, which were b'80 and 1-400 approximately, it is seen 

 that the lack of symmetry between the effects on the two 

 sides of a plate is very marked — the effect on the emergence 

 side being from about 4*5 to 6'5 times that on the incidence 

 side. The quantities on the incidence side are consequently 

 so small that they become difficult to measure with any great 

 accuracy, especially when an attempt is made to determine 

 the quality of the radiation. 



To measure the quality of the incidence radiation the top 

 of the ionization-chamber was covered with a lead sheet 

 about 1 cm. thick, from the centre of which was cut a cir- 

 cular hole 17*5 cm. diameter. One of the larger domes was 

 then inverted and placed so as to intercept the radiation from 

 the radiator, which was placed in the large circular hole just 

 mentioned. A thickness of radiator sufficient to give the 

 maximum effect was used in all cases. There appeared to 

 be very little difference in the quality of the radiation 

 returned from C, Al, and Zn. A Pb dome 4 mm. thick re- 

 duced the effect to approximately 20 per cent., indicating a 

 radiation for which the value of X is 4, if we can suppose it 

 homogeneous. 



In the case of Zn and Al it will be seen from Table I. that 

 a 4 mm. Pb dome reduces the effect of the emergence radia- 

 tion to about 45 per cent. There is thus a considerable 

 difference in the quality of the emergence and the incidence 

 7 radiations from these substances. 



2 G 2 



