Primary and Secondary y Bays. 



929 



II. Secondary y Rays. 

 (1) The Distribution of Secondary y Radiation. 



Apparatus. — The electroscope was of lead 3 mm. thick and 

 7 cm. cube. In the first experiments, it was supported on a 

 wooden arm which could be revolved so that the electroscope 

 moved round the arc of a circle of radius 25 cm. The centre 

 of this circle was approximately the centre of the radiator. 

 The radium, about 300 mgrs.* of RaBr 2 , was contained in a 

 platinum vessel, and the electroscope was screened from the 

 direct radiation by a mass of lead. The arrangement was 

 similar to that shown in fig. 1. The radiator consisted of iron 

 plates ll'l cm. square. The electroscope was turned into 

 the different positions, and the readings taken with and 



div./inin. 



5 cms. 



//cms. | 



Tt//CK/V£SS 



Relation between amount of secondary radiation and thickness of 

 iron radiator. 



without the radiator, Seven positions of the electroscope 

 are taken; the first one measures the direct radiation and the 

 last one the secondary radiation at right angles to this. The 

 other positions are intermediate. 



In fig. 3 curves are plotted showing the effect of varying 



* This was kindly lent for the purpose by Professor Kutherford. It was 

 sealed up in order to determine the rate of production of helium from it. 



