Principles of radionuclide generators
(Parent and daughter relationships between radionuclides and how they allow the dispensing of radionuclides. )
Radionuclide generators are made possible by the occurrence of radioactive decays where the daughter is also radioactive. Commonly a radioactive decay proceeds from a radioactive parent to a daughter that is stable. For example when 32P decays by beta emission the daughter is 32S, which is not radioactive. In some decays however the daughter is itself radioactive and will undergo a decay process. There are many examples of this type of situation and in some of these useful generators can be constructed. In general the generators will allow isolation and utilization of the radioactive daughter.
There are a number of requirements in order to have a useful generator system:
A radionuclide generator (these are sometimes referred to as "cows") will contain a long-lived radionuclide (also called "parent"), which then decays into a short-lived radionuclide (also known as "daughter") of interest. There are a number of useful isotopes that can be obtained from generator systems that have applications in medical diagnosis (imaging) and therapy as well as applications in radionuclide tracer work. There are generally two types of parent-daughter generator systems. The first one is "transient equilibrium generator" where the parent radionuclide half-life is somewhat greater than the daughter’s. For example, the basic concept of the 99Mo/99mTc generator relies on the availability of a relatively long-lived parent radionuclide that decays to a relatively short-lived daughter radionuclide that has useful physical and decay properties. (See Figure 1) In a generator the parent is adsorbed strongly on a suitable material; whereas the daughter will have different physical and chemical properties and can be eluted from the parent-daughter mixture. Although the 99Mo → 99mTc radionuclide generator is the most common and best-known radionuclide generator, there are a variety of other examples of generators that fit this description (Other generators).
The daughter radionuclide is a different element than the parent and will therefore often be in a quite different chemical form than the parent. With this difference in chemical characteristics between parent and daughter radionuclides, the latter can usually be separated by an elution method (this process is commonly called “milking” the generator or "cow"). The concept of "transient equilibrium" is described in Equilibrium concepts and equations (This site is not yet available) but briefly we find that the daughter radionuclide after some time has passed will grow to a maximum and then appear to have a half-life that parallels the parent. Once the activity of the daughter is eluted there is a growth of the daughter until it again reaches a maximum and is again in equilibrium with the parent. This elution and regrowth can be continued as long as there are useful amounts of the parent radionuclide available. Elution may be performed before equilibrium is reached, and the amount of daughter activity eluted will depend on the time elapsed since the last elution.
Another type of generator is called the "secular equilibrium generator"; where the half-life of the parent is much longer than the half-live of the daughter. The parent will not decay noticeably during many daughter half-lives. This situation is called "secular equilibrium" (See Figure 2). This example of a secular equilibrium generator has the parent / daughter system 81Rb → 81mKr (the rubidium / krypton generator) where the parent 81Rb has a T 1/2 = 4.58 hours and the daughter 81mKr has a T1/2 = 13 seconds. Like the transient equilibrium generator, the rate of daughter production initially is greater than its rate of decay and the daughter activity will continue to increase until it reaches a state where the rate of production equals the rate of decay. At this point the daughter appears to decay with the parent half-life. There are a wide variety of generator systems that have been developed or proposed as shown in GEN-11 but very few have been widely available due to the availability of the parent radionuclides and the technical complexity of most separation techniques.