182 Prof. A. Stanley Mackenzie on Secondary Radiation 



11 mm. thick is put before the ionization-chamber the leak 

 is still 1*92, or nearly 2 per cent, greater than without any 

 plate. In other words, you can increase the leak in a vessel 

 in certain circumstances by merely making the walls thicker. 

 I£ the rays here approaching the cylinder are of an absorbable 

 type, we could explain this phenomenon by saying that when 

 they meet the plate they start up both a radiation of similar 

 type as well as a very penetrating type, the sum o£ the two 

 being a maximum for a lead plate 1 mm. thick, more only 

 acting as an absorber. If they are of the y type, we could 

 say that they themselves pass easily through the thickest 

 screen used, and at the same time start up a secondary 

 radiation in the cylinder of ionizing activity more than enough 

 to make up for the absorption of the original rays. We 

 shall have evidence later of the existence of both these 

 phenomena, but the latter explanation is probably the main 

 one. In order to put this peculiar behaviour to a further 

 test, a plate of 6 mm. thickness was put at A instead of the 

 cone and plate used before ; this must absorb all but a few 

 of the very fastest /3 rays and let through a quantity of 

 7 radiation cut out before. The results were similar to those 

 just described, but the changes were not so great. The leak 

 with no screen at S was 1'92 ; with S^mm. thick the leak 

 was not measurably different from that with no screen ; with 

 S '225 mm. the leak increased to 1*96 ; and later decreased 

 again. The proportion of less penetrable secondary air-rays 

 due to the impact of the less penetrable primary rays has 

 begun to mask the effect due to the rays coming from the 

 impact of the very penetrable ones ; and there must be a 

 certain thickness of plate A for which the phenomenon in 

 question would just disappear. It is not present when the 

 whole beam is used. Further discussion of this behaviour is 

 reserved until experiments on " transmitted " rays are 

 described. Finally, with regard to the data of columns 2 

 and 3, we may say that the 7 rays passing through air 

 produce no great amount of radiation except the peculiar 

 kind just described, and that practically all the absorbable 

 radiation produced is by the ft rays, and that it cannot 

 penetrate more than 1 mm. of lead ; the leak shown in 

 column 2 for thicknesses greater than 1 mm. is that due to 

 the 7 rays referred to, since the numbers are the same as the 

 corresponding ones in column 3 within the limits of accuracy 

 of the experiments. 



Before giving the results obtained from investigating the 

 secondary rays reflected from varying thicknesses of lead 

 bombarded by the primary rays, the proportion of the 



