544 Sir E. Rutherford on Collision of 



§ 6. Scintillations due to source and absorbing screens. 



When the containing vessel was exhausted of air, scintil- 

 lations were always observed on the screen proportional in 

 number to the activity of the source. The number fell off 

 rapidly between 7 and 12 cm. air absorption and then more 

 slowly, but a few could be observed nearly to 28 cm. The 

 variation of number with amount of absorption in terms of 

 cms. of air is shown in fig. 2. This refers to a heated brass 

 source, 3*3 cm. from the screen, with a heated silver plate 

 of stopping power 6 cm. of air just before the screen. 



9 13 17 21 25 



/Panye in cms. of a/'r 



These scintillations appear to be due mainly to H atoms 

 excited partl) T in the source and partly in the absorbing- 

 screens. Thin foils of aluminium, for example, placed close 

 to the source increase the number of scintillations. This is 

 due to the occlusion of hydrogen, which can be removed by 

 heating the aluminium in an exhausted furnace just below 

 the melting-point. Similar effects were observed with silver 

 b>ut not with gold. In practice, all screens to be used 

 in the path of the a rays were heated to drive off occluded 

 gases as far as possible. This is very necessary when small 

 numbers of scintillations have to be counted. Usually a 

 silver plate was used to absorb the a. rays. Gold was found 

 to be very free from hydrogen, but it could not be used in 

 place of silver close to the screen on account of the marked 

 luminosity set up on the screen well beyond the range of the 

 a particles. This peculiarity of gold had been previously noted 

 by Marsden, but I was surprised to observe the magnitude of 

 the effect with strong- sources of radiation. A fuller account 



