184 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 7 



and "res.n.act." (resonance neutron activation) refers to the identification of 

 a nuclear isomer by observing both isomers upon irradiation with filtered 

 neutrons. With the mass spectrographic assignment of mass numbers there 

 are some instances in which the mass number is known with greater certainty 

 than the element. Such cases are assigned the appropriate code letter such 

 as "E" followed by "m.s." 



The per cent abundance of the stable isotopes is listed in column three. 



The fourth column lists the type of radiation, with the following meaning 

 for the symbols: 



jS - = negative beta particles (negatrons) 

 /3 + = positive beta particles (positrons) 

 7 = gamma rays 

 a = alpha particles 

 n = neutrons 



er = internal-conversion electrons 



K = K-electron capture (or in more general terms, orbital electron 

 capture) 



I.T. = isomeric transition (transition from upper to lower isomeric state) 

 In the cases where it is certain that no gamma rays are emitted, this fact is 

 expressed explicitly in column seven by the term "No 7." Annihilation 

 gamma rays and x-rays are not listed. It may be assumed that x-rays have 

 been observed or actually identified in almost all cases of orbital electron 

 capture listed. 



The half-life is given in the fifth column. In most cases the determination 

 is direct, either by measuring the decay rate, by weighing a long-lived isotope 

 of known purity, or by comparing the activity with that of a genetically 

 related isotope of known half-life. A number of half-lives are known only 

 from the yield of activity resulting from a nuclear reaction of known or 

 estimated cross section. Half-lives estimated in this manner are indicated 

 by the term "yield." Usually for the cases where more than one value for 

 the half-life has been reported, an attempt has been made to list the best 

 value (an experimental value thought to be taken under the most favorable 

 conditions) rather than a mean value; more than one value is listed where a 

 choice does not seem obvious. Among the natural radioactivities an average 

 value is often used which was taken from an international committee sum- 

 mary report. 



In the columns headed " Energy of radiation," the energy value is followed 

 by a description of the method used for the energy determination. The beta- 

 particle energies correspond to the observed upper limits of the spectra; in 

 those cases where only the Konopinski-Uhlenbeck extrapolated value has 

 been reported, this is listed, followed by the designation "K.U." For alpha 

 particles reported only by a range the "mean range in air" vs. energy relation- 



