BASIC DIFFICULTIES IN TRACER METHODOLOGY 63 



row, spleen, and liver, has been utilized for the therapeutic delivery of 

 radiation to these sites. However, interpretations of physiological and 

 metabolic behavior ma}^ be made quite difficult l)y the use of these radio- 

 tracers in an ill-defined physical state. 



Parent-Daughter Relationships. Special attention must be paid to 

 radioisotopes that decay to ,yield other radioactive spetiies with different 

 chemical or physical properties. Examples are 2{)-yr Sr'-"', which decays 

 to 60-hr Y-'", and 65-day Zr'^ which decays to 35-day Nh^K There are 

 two general types of parent-daughter relationships: (a) Shorter-lived par- 

 ent— li the parent is shorter-lived than the daughter, then no eciuilibrium 

 is reached between the parent and daughter. Starting with the parent 

 activity initially free of the daughter, as the parent decays, the amount of 

 daughter will increase and eventually decay with the half-life of the 

 daughter. (6) Longer-lived parent — If the parent is longer-lived than the 

 daughter, it becomes possible to use the measured amount of daughter 

 present in a sample as an indication of the amount of parent present. 

 The important point is that, starting with the parent activity, an equi- 

 librium will be reached after a time equal to about 6 to 10 daughter half- 

 lives. At equilibrium the daughter will be decaying at the same rate at 

 which it is formed, and therefore the amount present will be proportional 

 to the amount of parent present. Also the apparent half-life of the 

 daughter will be the same as that of the parent. An illustration of how 

 this behavior is employed in biological studies is presented on page 65. 

 When the parent is being constantly replenished, as in pile bombardment, 

 or is very much longer-lived than the daughter, the term secular equilib- 

 rium is employed. In this case the parent does not decay measurably 

 during many daughter half-lives. When the parent is relatively fittle 

 longer-lived than the daughter, the term transient equilibrium is used. 

 Clearly, secular equilibrium is the limiting case of transient equilibrium, 

 and whether or not the equations for secular equilibrium can be applied 

 depends upon the accuracy desired and the particular experiment. The 

 text of Cook and Duncan (13) may be consulted for a mathematical treat- 

 ment of the growth and decay curves of parent-daughter couples. 



In many ca.ses the radiation characteristics of the short-lived daughter 

 make radioassay of the daughter much easier than measurement of the 

 parent. Also, when chemical separations of the daughter are convenient, 

 it may lead to analytical methods that have the following advantages: 

 (a) The radioactivity present in the chemically separated fraction rep- 

 resents only that from the parent activity, and therefore the results are 

 unaffected by any radiocontaminants in the original sample which arc 

 not carried through with the daughter fraction, {b) The .separated 

 daughter activity is usually present in a smaller amount of extraneous 

 mass and can therefore be measured with less self-absorption losses than 



