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.