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PENETRATING POWER OF THE GAMMA RAYS 



183 



absorption of the 7 rays in lead was then observed in the usual way. 

 The initial reading was 15 divisions per minute. The results are given 

 in the table below. 



The third column gives the average mass absorption co-efficient 

 of the rays issuing from the different absorbing screens. For example 



= 3-67 for the 1st screen. 



Testing the rays by aluminum 12 mms. thick, we find that for 

 the same rays p/pAl = 0-135. The radiation coming through 0-76 

 mm. of lead is reduced from 2-48 to 1-21 divisions per minute by a 

 screen 0-76 mm. thick, giving p/pPb = 0-82. The same rays tested 

 by the aluminium screen gave p/pAl = 0- 100. 



For the rays of the radium C, Ai/pAl = 0-0424, and p/pPb = 0044, 

 after the rays have passed through a centimetre of lead. 



The atomic number for lead = 82 and consequently the H series 

 for lead should have about the same penetrating power as the 7 rays 

 of Ra C. The table shows that the 7 rays excited in the lead are 

 decidedly less penetrating than would be the rays of the H series. . 



Even the most penetrating rays only correspond in penetrating 

 power to the 7 rays of actinium, although the readings are so 

 small that it is difficult to measure them accurately. On the other 

 hand, there is a possibility that part of the more penetrating radiation 

 is due to a very slight amount of radium, from which the radium D 

 had been separated. However, for the rays of actinium p/pAl = 0-080 

 app. and this agrees, approximately, with the values obtained in the 

 last column. 



It is therefore concluded that although the /3 rays of radium E 

 have more than sufficient speed, they do not excite the characteistic 

 radiation of series H in lead, and the experiment makes it doubtful 

 whether the H series can be excited in this way. 



