Alpha Particles from Light Atoms. 535 



It follows that the expected abnormalities on the basis of 

 Prof. Rutherford's results might be detected in possibly the 

 case of aluminium, but not with certainty in carbon because 

 of the small number of scintillations. 



2. The ranges of the recoiling alpha particles from light 

 atoms are on the whole very short. The ranges for particles 

 deflected through 170° for An, Al, and C are as follows. 

 These are calculated from Darwin's theory. 



Au = 6'2 cm. air equivalent. 

 Al = 2-9 cm. „ 



C = 0-89 cm. 



The shortness of the recoil range below Al complicates the 

 problem as follows: — . 



(a) It prohibits use of diaphragms with absorbing power 

 more than 3 mm. of air which are needed to prevent con- 

 tamination. 



(b) It reduces the thickness of the deflecting layer by 

 reducing the thickness of the layer through which the 

 deflected alpha particles may escape. 



(c) It reduces the intensity of the scintillations, thus 

 rendering counting very difficult. 



3. With powerful sources of radium active deposit the 

 work is complicated by the intense gamma radiations. These 

 cause a luminosity of the ZnS screen such that the counting 

 of weak scintillations is impossible. It is necessary with 

 such sources to conform to the following rules : — 



(a) The distance from source to ZnS screen should be not 

 less than 3 cm. 



(b) The source and screen should be separated by at least 

 1*5 cm. of lead faced by carbon or paper on the side towards 

 the screen. 



(c) The ZnS screen should be as thin as possible without 

 having too many holes in it. 



4. Work with carbon was prevented and with aluminium 

 hampered by a phenomenon termed contamination. On 

 account of the short range of recoil particles the box used 

 for counting had to be evacuated. The low pressures 

 permit a sort of diffusion of particles of active deposit from 

 the source all over the box. The resulting scintillations on 

 the ZnS screen due to their number and brilliance render 

 counting impossible. Once contamination had set in due to 

 any cause whatever, work had to be abandoned for 24 hours 

 to permit decay of the contamination. Attempts were made 

 to reduce this effect by using a current of H 2 gasat 500 mm. 



2 P 2 



