235U and slow neutrons."/>
Figure 3.7 Chain reaction involving 235U and slow neutrons.
Fission
In this process, the desired radionuclide is one of the fission fragments of a heavy element (Z > 92), either the fuel atom itself or the atoms of a target placed inside the reactor. The by‐product is chemically separated from the other fission fragments. The fission reaction is denoted as
For example, the formation of iodine‐131 and molybdenum‐99 are written as
Neutron capture
In neutron capture the target atom captures a neutron. The new atom is radioactive and emits gamma photons or charged particles to produce the daughter nuclide (Figure 3.8). A gamma photon is emitted following capture of a thermal (slow) neutron. This reaction is written as
For example:
When the target atom captures a fast neutron a proton can be emitted. This capture reaction is sometimes referred to as transmutation and is symbolized as
For example:
A list of common medical nuclides and their methods of production, modes of decay, and decay products is provided in Appendix A.
Figure 3.8 Neutron capture by target nuclide placed in a reactor. Thermal (slow) neutron capture causes gamma emission (top), fast neutron capture results in proton emission (bottom).
Radionuclide production
There is often more than one way to make a radionuclide. For example, 111In is most commonly made in a cyclotron using the 112Cd(p,2n)111In reaction but it can also be made from a cadmium‐111 target using the 111Cd(p,n)111In reaction or from a silver‐109 target in the reaction 109Ag(α,2n)111In. The tradeoffs involve the relative efficiencies of production for various cyclotron energies and the issues involved in separating and purifying the product. In general, bombardment of a target by charged particles in a cyclotron increases the charge to mass ratio and so nuclei in the product are likely to be charge‐rich (neutron‐poor) and thus decay via beta‐plus decay or electron capture. This is the reason that most PET radionuclides, such as 18F, are produced using a cyclotron. Conversely, bombarding a target with neutrons in a reactor tends to increase the mass of the target nuclei rendering them neutron‐rich and thus likely to decay by beta‐minus decay.
Questions
1 Which of the following statements are true about medical radionuclide generators:The parent nuclide always has a shorter half‐life than the daughter nuclide.If the T1/2 of the parent nuclide is 50 times greater than the T1/2 of the daughter nuclide the equilibrium portion of the activity curve is basically flat and is categorized as “secular” equilibrium.The parent nuclide is less tightly bound to the column than the daughter nuclide.All of the above.None of the above.
2 True or False: During an equilibrium state within a 99Mo–99mTc generator the total activity of 99mTc is always less than the total activity of 99Mo because 14% of 99Mo decays directly to 99Tc, bypassing the metastable state.
3 Which of the following is an example of a generator that reaches secular equilibrium?99Mo–99mTc.82Sr–82Rb.
4 Associate each of the following terms (a) through (k) with the most appropriate of the three methods of nuclide production listed here (a term can apply to more than one method):Reactor.Cyclotron.Generator.Moderator.Chain reaction.Thermal neutron.Dee.Control rod.Target.Cow.Pig.Column.Elution.By‐product.
5 Associate each of the following nuclear reactions (a) through (d) with the most appropriate of the three listed production methods.Cyclotron.Fission reaction (reactor).Neutron capture (reactor).111Cd(p,n)111In.68Zn(p,2n)67Ga.235U(n,f)99Mo.98Mo (n,γ)99Mo.
6 18F is produced:In a generator.In a cyclotron.From by‐product material of nuclear fission.By neutron bombardment in a nuclear reactor.
7 99Mo is produced (select all that apply):In a generator.In a cyclotron.From by‐product material of nuclear fission.By neutron bombardment in a nuclear reactor.
8 99mTc is produced:In a generator.In a cyclotron.From by‐product material of nuclear fission.By neutron bombardment in a nuclear reactor.
9 The half‐life of 99mTc is 6 hours and the half‐life of 99Mo is 66 hours. At 8 AM Monday morning, the Tc/Mo generator is eluted of all 99mTc and the yield is 86 mCi. Approximately how much total 99mTc will we be able to elute from the generator on Friday at 8 AM.Essentially zero since more than 10 half‐lives have elapsed.31 mCi.86 mCi.54 mCi.
Answers
1 (e) None of the above: (a) the parent half‐life is always longer than the daughter half‐life; (b) if the half‐life of the parent is between 10 and 100 times greater than the half‐life of the daughter, the activity curve is downward sloping and the equilibrium is termed “transient”; and (c) the daughter nuclide is less tightly bound, thereby it can be removed or eluted for use.
2 True.
3 (b) The half‐life of the parent nuclide, 82Sr, is more than 100 times that of the daughter nuclide, 82Rb.
4 (a),
