522 



Table 633 

 RADIOACTIVITY 



o Particles: Range, Velocity, Ionization 



RaC (84 Rail) taken as standard: Vstd= 1.922 X io 9 cm/sec. 



(Table taken from Report International Radium Standards Commission, 1930, 

 Rev. Mod. Phys., 1931.) 



As an a particle passes through matter, energy is dissipated, principally in ionization; 

 its velocity diminishes. Ultimately the a particle can not be detected. Rutherford was 

 able to detect by their scintillations, a particles of velociy 0.15 Vs and Blackett in his 

 study of cloud-tracks of RaC a rays found tracks corresponding to a velocity greater 

 than 0.04 Vs. When the kinetic energy becomes equal to that of an electron fallen a p.d. 

 of 13.5 volts, then an a particle should not produce even a single pair of ions. 



When a particles encounter successive thin layers nearly the same number emerge as 

 enter up to a certain thickness. Beyond, the number transmitted decreases rapidly with 

 small increments in the thickness. If the ionization in a thin layer of gas at increasing 

 distance is plotted against distance, the decrease from max. ionization is rapid, the ioniza- 

 tion descending nearly to zero along a steeply sloping line, becoming asymptotic to the 

 distanceaxis. Marsden-Perkins have defined the range as the abscissa of the point where 

 the straight line portion of the curve, produced, intersects the distance axis. 



For the discussion of ranges see especially Rosenblum, C. R., 190, 1124, 1930, and 

 Rutherford, Chadwick, Ellis, " Radiations from Radioactive Substances," 1930, pages 82 

 et seq. and 86. For two decimal places the relation v 3 = aR gives sufficient accuracy for 

 normal ranges. The basic value for ion production of a particles is that for RaC ; 

 K = 2.2X io 5 . For the velocity of a particles from ThC, Rutherford, Chadwick, Ellis 

 chose 1. 701, while Mines. Curie and Joliet-Curie propose 1.698 X io 9 cm/sec. 



Ranges/Velocities and Ion Productions 



In the Table for R, v, k (range, velocity, ion production) the directly observed values 

 are denoted by +. The calculation of the other values for v and k was made by using 

 the basic values denoted ++. 



Ranges at o° C and 760 mm Hg in Air (R ) ; at 15 C (Ri-,). Velocity (v) and Ion 



Production (k) 



Smithsonian Tables 



