520 Table 629 



RADIOACTIVITY UNITS 



(Taken from 1930 Report International Radium Standards Commission, Rev. Mod. Phys., 



3, 427, 1931- ) 



Radium- content is expressed in g or mg of radium, regardless of its chemical combina- 

 tion. It is always desirable to know the total weight and nature of the compound. 



Radon (radium emanation). — A curie: Quantity of Rn in equilibrium with 1 g Ra. 

 One curie Rn has a vol. 0.66 mm 3 at o° C 760 mm. One curie (Rn without decay products) 

 can with complete utilization of the a-particles keep by its ionization of air a saturation 

 current of 2.75 X io 6 e.s.u. (0.92 milliampere). 



Sub-units: Millicurie, microcurie, milli-microcurie (io~ 9 ). 



Eman. = io" ia curie per 1 (io~ 13 curie/cm 3 ) — concentration unit used for Rn content of 

 the atmosphere. 



Mache Unit (M. E.) : Concentration unit referred to the Rn content of 1 1, = quantity 

 Rn/1 which without decay products and with complete utilization of the a particles can 

 maintain by its ionization of air a saturation current of io" 3 e.s.u. One M. E. corresponds 

 to 3.64 X io" 10 curie/liter = 3.64 eman. 



It is recommended that the curie be extended to include the equilibrium of any decay 

 product of radium, specifying the element as 1 curie Rn. The unit quantity of any radio- 

 active element may be expressed in terms of the mass equivalent to 1 g Ra with respect to 

 the effect of the rays as to the number of atoms decaying per sec, e.g., 1 mg Ra equivalent 

 is that amount of an element whose number of atoms decaying per sec. equals that of 

 1 mg Ra (3.y X io 7 atoms/sec). 



Polonium. — " 1 curie Po " = amount equiv. to 1 g Ra emitting 3.7 X io 10 a particles 

 per sec. = quantity in radioactive equilibrium with 1 g Ra = 2.24 X io -4 g Po. 



That quantity of Po whose a radiation directed to one side only is fully utilized to 

 ionize air and which can support a current of 1 e.s.u. corresponds to 1.68 X io" 10 g Po or 

 0.75 X io" 6 curie Po. 1 curie Po would, in the utilization of its rays in all directions, sup- 

 port a saturation current in air of 2.66 X io~" e.s.u., 1 microcurie (one-sided radiation) 

 = 1.33 e.s.u. 



Mesothorium. — "1 mg MsTh" usually signifies that 7-ray equivalent of 1 mg Ra-RaC, 

 compared after absorption by 5 mm of lead. (This definition is inexact and open to 

 criticism.) 



All determinations of content of Ra, Ru, Ms, Th, Po, etc., must be dated. (Condensed 

 from Report of the International Radium Standards Commission, Rev. Mod. Phys., 1931.) 



Smithsonian Tables 



