Decay scheme
Introduction
The technetium radionuclide generator appears to produce a simple product. Elution of the generator provides a sterile saline solution that we describe as 99mTc. We may also be aware that the parent 99Mo produces this daughter following a beta decay process. Although these statements and conceptions are generally true the decay processes are in fact complex and a variety of intermediates and products are produced that require a bit more explanation and understanding.
Decay formulae
The parent radionuclide 99Mo can be prepared by either neutron activation or by induced nuclear fission. Both these processes tend to produce radionuclides that are unstable by virtue of having a “neutron excess” or so called “neutron rich” species. Neutron rich species generally decay by beta decay. In this process a neutron is converted into a proton in the nucleus thereby helping to alleviate the instability caused by the extra neutrons. A full explanation of the beta decay process will be provided elsewhere (Link to NUC-PHYS). 99Mo decays eventually to 99Tc which is also a radioactive species with a long half-life (2.14 x 10 5 years). Materials with half-lives this long are barely radioactive and can be treated as stable. Much of what we know about technetium physical and chemical properties has been discovered from using small quantities of this isotope since there are no stable isotopes of this element.
The decay of radioactive species can be depicted in a formula style which usually gives sufficient information to understand the general processes at work. For the processes described above we would have:
1. 99Mo → 99Tc o- γ δ (66 hour half-life, multiple beta and gamma energies). This direct to ground state decay occurs about 12.5% of the time. The symbol u indicates an emission known as a neutrino which although not detected carries away part of the decay energy.
2. 99Mo → 99mTc o- γ δ (66 hour half-life, metastable intermediate, multiple beta and gamma energies). This decay to the metastable intermediate occurs about 87.5% of the time.
3. 99mTc → 99Tc γ (Isomeric transition, 6 hour half-life, gamma energy is 0.1405 MeV).
4. 99Tc → 99Ru o- δ (Essentially pure o- decay, 2.14 x 10 5 year half-life.)
A number of web links can be found by searching under “isotopes” or "nuclear data" and in general they present detailed tabulated data for the known nuclides. For example:
http://icg.harvard.edu/~chem15/reference/all_isotopes_CRC.pdf
http://ie.lbl.gov/education/parent/Mo_iso.htm
http://atom.kaeri.re.kr/
A second method of presenting radionuclide data uses a diagrammatic process. So called "Energy Level Diagrams" are constructed using an energy scale with annotated and tabular data. The best compilation is found in a reference work entitled Table of Isotopes edited by Richard B. Firestone, Lawrence Berkeley Laboratory, and Virginia S. Shirley, Lawrence Berkeley Laboratory. Published by John Wiley & Sons, Inc. ISBN 0471-14918-7 2 volume print set / 3,168 pages / cloth Packaged with Interactive CD-ROM. http://wiley.com/legacy/products/subject/physics/toi/
A simplified energy level diagram is presented in Figure 1. The y-axis is an energy scale and the horizontal lines represent energy levels for the parent and the daughter. There are three main beta emissions shown but there are 9 other minor beta decays that are not illustrated. Each of these beta emissions results in a daughter 99Tc in an excited nuclear state. These excited daughters emit gamma rays and progress to the ground state of the daughter. In this particular situation one of the excited states has some stability and the main beta decay leads directly to this "metastable" state. In addition the gamma emissions of the other minor beta decay processes may also lead to this metastable state. A percentage of the decays (about 12.5%) bypass the metastable state and produce the ground state of the daughter rapidly (nanoseconds) and directly. Those transitions which leave the daughter in the metastable state (87.5%) produce 99mTc which then decays with the characteristic half-life of 6 hours to the ground state of the daughter with the emission of a 0.1405 MeV gamma ray. This process is called isomeric transition. The daughter for this decay is 99Tc which is also radioactive but has a very long half-life (2.14 x 10 5 years). 99Tc decays by beta emission to ruthenium-99 which is stable.
To further complicate the situation not every decay of 99mTc results in the emission of a gamma ray. About 11% of the decays result in the absorption of the gamma ray by inner shell electrons and resultant emission of these electrons with kinetic energy. These "conversion electrons" account for much of the radiation dose that is received by patients following injection of technetium radiopharmaceuticals.
