[eve] 



[PRIMARY AND SECONDARY GAMMA RAYS 



43 



hick iron screens do not harden the rays, but lead screens do so 

 rapidly. 



When the U-shaped curves shown in the preceding figures have 

 the left branch high compared with the right, it seems to indicate 

 hard rays; when low, soft rays. In order to put this to the test I 

 examined the ;- rays from uranium X, and also the secondary y 

 rays from iron produced by radium C. Both types of radiation 

 are known to be softer than the primary j rays of radium. 



Secondary y Rays. 



A strong'and satisfactory source of secondary y rays was obtained 

 by the arrangement shown in fig. 4. Fourteen miUigrams of radium 

 bromide, placed in a hollow cylinder H, sent a cone of rays through 

 the platform P to the secondary radiator V. The secondary ;- rays 

 were measured by the electroscope E, after passing through the 

 plates A and B. Both electroscope and plates were well screened 

 from the radium by much lead. A correction was made for natural 

 leak and for all rays entering the electroscope otherwise than through 



Fig. 4. 



A and B, and the amount of this deduction could be well estimated 

 by placing many thick plates between V and E. Three different 

 platforms were tried, of lead, iron and wood respectively, each about 

 6 mm. thick. The results obtained differed somewhat, but as the 

 differences were too small to measure correctly the mean values 

 were taken. When a large lead cylinder served as radiator at V, 

 the secondary y rays were feeble, because they were to a large extent 

 absorbed in the lead itself. Four bricks placed on end, close together, 

 proved a good radiator. A tinned iron pail, 17 cm. high, 15 cm. 

 in^diameter, was also used and could be filled with iron filings, water, 

 sand or coal. Such different radiators did not give well marked 



