180 Prof. A. Stanley Mackenzie on Secondary Radiation 



rays are the most effective in producing these air-rays, there 

 is an appreciable amount of radiation set up by the 7 rays, 

 or, at least, set up after a layer of lead is interposed sufficient 

 to cut out all the /3 rays ; equal to several per cent, of the 

 maximum. It will, however, be shown later that there is a 

 large radiation sent out from the side of the lead plate which 

 is not bombarded, what we may call for convenience 

 "transmitted" rays; and although the apparatus is so 

 arranged that these transmitted rays cannot directly enter 

 the ionization-chamber, yet they may produce air-rays to an 

 unknown extent, and it is impossible now to estimate how 

 much of the leak which we observe when a plate say of 

 6 mm. is at A is due to the 7 rays which strike the air after 

 getting through the plate, and how much is due to the rays 

 which are set up in the lead and then strike the air. If 

 these numbers are plotted, it will be seen that the law of 

 absorption over the whole range is very far from being an 

 exponential one, being too steep at first and then too flat. 

 But before trying to find an equation to the curve, it would 

 be necessary to subtract from the numbers given the leak 

 which is always present due to spontaneous ionization and 

 the direct 7 rays and their secondary effects. This I have 

 not found a way of estimating with any accuracy. The leak 

 when the radium is out of range and out of the room (a room 

 never contaminated by having had open radium in it) is 

 small) being about '21 on the scale adopted. 



In order to test the type of these rays as to penetrability, 

 the leak was observed with lead screens S of various 

 thicknesses over the aluminium end of the cylinder. The 

 results are found in the second column of Table II. It will 

 be seen that these air-rays are mainly of the easily absorbable 

 kind, and that about two-thirds of the whole is absorbed by 

 lead-foil '02 mm. in thickness, and five-sixths by £ mm. of 

 lead. One of the most interesting results is that the remaining 

 one-sixth is of a seemingly very penetrating type, like 7 rays, 

 the leak through 11 mm. of lead being only about 3 per 

 cent, less than through ^ mm. The question as to whether 

 these are 7 rays or not will be considered later. By referring 

 to Table I. it will be seen that as a rough statement we may 

 say that the proportion of the beam of secondary rays of any 

 given penetrability is about the same in order of magnitude 

 as the effectiveness of primary rays of that penetrabilit}^ in 

 producing secondary air-rays. These results are in the main 

 in agreement with those found by others. 



