Rays of Radioactive Substances, 325 



§ 9. Comparison of the Amount of Ionization Produced 

 by Deviable and Non-deviable Rays. 



In 1899 one of us* showed that uranium oxide emits two 

 types of radiation which were for convenience termed the 

 a and /3 radiations. The /3 radiation was far more pene- 

 trating in character than the a-rays. By the electrical 

 method we have found that the /3 radiation is made up almost 

 entirely of deviable rays of a penetrating character. 



For brevity and convenience we will call the non-de viable 

 rays of all radioactive substances a-rays and the deviable rays 

 fi-rays. 



The magnetic-deviation apparatus of fig. 1 is not suitable 

 for a direct comparison of the ionizing action of two types of 

 rays. Since, however, the a-rays are in all cases easily ab- 

 sorbed, and we have experimentally found that the greater 

 proportion of the penetrating rays of uranium, thorium, and 

 radium are magnetically deviable, a simple indirect method 

 can be employed. 



The ionization current between two large parallel metal 

 plates was observed with a large P.D. between the plates, 



(1) with the radioactive substances bare, 



(2) with a layer of metal over the substances sufficiently 

 thick to absorb all the non-deviable rays. 



In (1) we have the effect of the a- and /3-rays, and in 

 (2) that of the /3-rays alone. 



Since the amount of the a-rays emitted reaches a practical 

 maximum for a very thin layer of radioactive substance a 

 comparison of the ionizing effects of the two kinds is best 

 made with a very thin layer. 



In the apparatus employed about ^ gr. of the finely 

 powdered radioactive substance was uniformly spread over an 

 area of about 80 sq. cms. The distance between the testing- 

 plates was 5" 7 cms. and the P.D. between the plates 300 volts. 



The results of previous experiments have shown that the 

 a-radiations of uranium, thorium, and radium are almost 

 completely absorbed in passing through a distance of 5 cms. 

 of air, so that the current with the bare substance was a 

 measure of the total number of ions produced by a-rays, 

 together with a small fraction only of the ions produced by 

 /3-rays. 



A layer of aluminium "009 cm. thick was found sufficient 

 to completely absorb the a- radiation. The following table 



- E. Rutherford, Phil. Mag. Jan. 1899. 



