Technetium-99

⁹⁹TcWhen nuclear physics hits the headlines, it's usually in unpleasant contexts, such as leakage of radioactive substances from reactors, the unsolved problem of disposal of nuclear waste, or the proliferation of nuclear weapons.

So, I was surprised by I story I had heard on the radio a few weeks ago, and read about again a while later: It was about the temporary shortage in the supply of a radioactive isotope heavily used in medicine, Technetium-99.

Technetium-99, a nucleus made up of 43 protons and 56 neutrons, has an excited state which decays with a half-life of about 6 hours. Similar to an excited atom emitting a photon of visible light when converting back to the ground state, the decay of the exited nucleus comes along with the emission of a γ photon. This γ photon has an energy of 140 keV – about 100.000 times the energy of a photon of red light. Here is schematic representation of the energy levels and the transitions involved:



To create nuclei of Technetium-99 in the excited state – also called Technetium-99m, where "m" stands for "metastable" –, one resorts to another isotope, Molybdenum-99. This isotope undergoes a β decay with a half-life of 66 hours, thereby ending up as Technetium-99 in the excited state. Like all isotopes of Technetium, Technetium-99 isn't stable either and finally ends up, following another β decay, as Ruthenium-99.

Technetium-99 thyroid uptake scans. Scan (A) shows the normal, healthy result. (from Petros Perros: Thyrotoxicosis and Pregnancy, PLOS Medicine 2(12): e370)

Now, what is this good for? As it came out from investigations at the Brookhaven National Laboratory done in the 1960s, the decay reaction of Technetium can be adapted as a very elegant and practical tool for medical diagnosis. To this end, "freshly produced" Tc-99m is extracted chemically from a probe of Mo-99, bound to suitable large molecules, and administered intravenously to the blood circuit. Then, the 140 keV γ photons emitted at the decay of Tc-99m map from inside the body the distribution of blood. They trace regions of lacking blood supply, for example after a stroke, or highlight spots with enhanced metabolisms, which could be tumours. Energy of the γ photon and lifetime of Tc-99m are just so that such an exploration does not produce to high a radiation exposure, and can be done in a very reasonable time.

To use this technique, Molybdenum-99 is needed, and this is where the current shortage comes from: Molybdenum-99 is created in nuclear reactions, either by bombarding more common isotopes with neutrons, or by fissioning of U-235 in highly enriched uranium targets. This second source, which is the most important one, is of course highly linked to weapon-grade stuff, so there are only a handful of civilian reactors in the world that produce Molybdenum-99. In Europe, the main source is a reactor in Petten in the Netherlands, which is currently shut down for maintenance and inspections. It seems, however, that the consequential shortage of Molybdenum-99 and Technetium-99m for medical purposes is not critical.

While trying to get some background on this news story, I realised that the decay scheme of Molybdenum-99 and Technetium-99m involves a few interesting questions:

Why does the decay of Mo-99 not end up in the ground state of Tc-99? Why is the lifetime of the γ decay of Tc-99m so long? Usually, γ transitions happen within fractions of a second. And finally, what a strange element is technetium in the first place, as without stable isotopes, it marks a gap in the middle of the periodic table? And as it comes out, the answers to these questions touch upon a few concepts very central to nuclear physics.

But this will be the stuff of another post.