5 .6 



Table 625 

 RADIOACTIVITY 

 INTRODUCTION. THE URANIUM FAMILY 



(References: Kovarik, McKeehan, Nat. Res. Council. Bull. 51 (reprint 1929); Andrade, Structure of the atom, 

 3rd ed., 1926; Rutherford, Radiations from radioactive substances, 1930; Kohlrausch, Radioactivitat, 1929; 

 Radioactive constants of 1930, Report International Radium-Standards Commission, Rev. Mod. Phys., 3, 

 427, 1931.) 



Certain elements (about 40) of high atomic weight (also slightly K and Rb) are unstable 

 in that they spontaneously change to elements of lower atomic weight with the production 

 of heat and the emission of a, p, or y rays. Radioactivity is an additive property of the atom, 

 dependent only on the particular element and not on the chemical compound into which 

 this element enters nor on the physical conditions controlling ordinary reactions — tem- 

 perature, whether solid, gaseous or liquid, etc. The lives of these elements vary from io 10 

 yrs. to io -11 sec. (See Table 625.) 



TABLE 625.— The Uranium Family, T, X, t 



At. Wt. = atomic weight; P. 

 years; d = days; h = hours; r 

 life; X = decay constant. 



No. = proton number; At. No. = atomic number; yr = 

 1 = minutes; s = seconds; T = half-period; t = average 



* Earlier values still in use. 



Notes on Decay Constants: For Ui the calculation is based on Z = no. of a particles from 

 1 g Ra per sec. = 3.70 X io 10 ; Ra/U = 3.40 X icr 7 ; Avogadro's No. = 6.064 X io 23 ; no 

 account is taken of the branching Ac series. The values given are for T and t, upper, for X 

 lower limits. 



For UXi, the lowest value T = 23.8 is mentioned as well as the one preferred by the 

 Commission. 



UII. The adoption of 3 X io 5 yr. is recommended. 



ThC. Mme. Curie has recently calculated X = about io'sec 1 . (Geiger-Nuttal Law). 

 In view of the uncertainty of the values, T< io -6 sec. has been proposed. 



AcC". 150 curves give T = 4.71 min., 9, T = 4.76 min. Both values are given. 



Smithsonian Tables 



