675 



TABLE 733.— THE ORIGINAL NAMES OF CERTAIN RADIOACTIVE MATERIALS* 



* At times the prefix cca was used to designate the element following certain elements either in the periodic 

 table or in radioactive series. t At one time all these materials were called Emanation, i.e., RaEm, AcEm, 



ThEm. 



TABLE 734.— THE FOUR RADIOACTIVE FAMILIES 



The radioactive isotopes of the heavy materials arrange themselves into four families, 

 or series, that are known either by the parent of the family or by the member of the series 

 with the longest life. Before the various isotopes had been established some of the differ- 

 ent members of the families had special names. (See Table 733.) These families or series 

 are also designated by the numerical relation of the particular isotopes of the family in- 

 volved and the number 4. Thus the four families or series are: (1) Thorium, or 4» 

 series; (2) Neptunium,* or 4>i -f- 1 ; (3) Uranium, or 4« -f- 2 ; (4) Actinium, or 4n -f- 3. 



Generally, tables of these families show the type of radiation emitted, the energy of the 

 radiation, the end product, and two or three factors that describe the time characteristics 

 of the disintegrations; i.e., T the half-life (that is, the time it takes for one-half of the 

 given material to disintegrate, which can be accurately measured T a , the average life, and 

 X, the decay constant. From the law of disintegration which radioactive materials have 



been found to follow, the three constants are shown to be related as follows : X = '„ 

 1 l 



and T a =— • 



A 



There are a number of isomers 232 in the series as shown in Table 742, as for instance, 

 see Uranium Xi, Radium C, Actinium, etc. As a result of recent work on the artificial 

 production of radioactive isotopes many more isomers could be given. Also, the first 

 member of some of the series might be different. Thus, the 4n + 3 series (the Actinium 

 group) might start in this manner : 



* Almost all the isotopes of this family are artificial products and are not now found in the earth. 

 232 Serge, Emilio, and Helmholtz, A. C, Rev. Mod. Phys., vol. 21, p. 271, 1949. 



(continued) 



SMITHSONIAN PHYSICAL TABLES 



