Technetium is the first element (by atomic number) in the periodic table to have no stable isotopes. Despite this, it has found a variety of uses, in both medicine and with proposed uses in preventing corrosion and as a catalyst.
Technetium was first discovered by Emilio Segrè and Carlos Perrier in 1937, by analysing radioactive molybdenum foil previously part of a cyclotron (a type of particle accelerator which accelerates charged particles using a magnetic field). Several people had previously claimed to have discovered element 43 but had merely not recognised elements that were already known (with the exception of Masataka Ogawa, who, after finding the then-unknown element rhenium, mistook it for technetium, as they belong to the same group). In addition, a group of scientists observed emission spectra in 1925 which seemed to correspond with element 43, which they duly named masurium, after Masuria, a region in Eastern Prussia. Unfortunately, these results were not repeatable, and as such, the experiment was ignored.
Like manganese, technetium is commonly found in the oxidation state 7 as part of pertechnetate, which assumes a tetrahedral structure. The most commonly used isotope of technetium is technetium-99m (metastable technetium-99), which has a half-life of six hours and is used as a radioactive tracer in medical imaging. Technetium-99 is the only isotope of technetium that can be found in minute traces on Earth, as it can be created by the spontaneous nuclear fission of uranium, and has a half-life of roughly 211,000 years. As a result, technetium is found in pitchblende at a concentration of roughly 0.2ng/kg. Technetium-98 has a much longer half-life of 4.2 million years but does not occur naturally on Earth. Nevertheless, it has been observed in red giant stars, such as K Geminorum. The discovery of technetium-98 in the stellar spectra of these stars provided proof that nuclear fusion was occurring in these stars, as the half-life of technetium-98 is much too short for it to have been present before the birth of the star.
Technetium-99m is used as a radioactive tracer in approximately 85% of circumstances in which a radioactive tracer is used in nuclear medicine. As a result, it has roughly 20 million uses annually. Technetium-99m only emits gamma radiation, as it releases energy to decay into the more stable technetium-99. While it cannot be transported long distances, as most of the sample would decay before it reached its destination, it can be produced as a decay product of molybdenum-99, which has a half-life of 66 hours. As a result, the molybdenum-99 is transported to hospitals on a weekly basis in shielded cylinders, which generate technetium-99m through decay.
Technetium-99m is then injected into the patient. The syringe used is radiation-shielded, to protect the person giving the dose, as they may administer hundreds of these procedures a year, which could exceed their dose limit. A gamma camera is then used to detect the gamma radiation given off. As gamma radiation rarely interacts with the atoms it passes through, it has a very high penetrative power, and low ionising power, making it ideal for radioactive tracing, as it harms the patient less, and is more easily detectable. The gamma camera contains sodium iodide crystals, with photomultipliers behind them. It is also shielded from background radiation by lead, for more accurate data. When a gamma ray hits part of the crystal, an electron is briefly promoted up energy levels, before returning to its original state and emitting energy in the form of a photon. The photomultiplier then registers this as a gamma ray, allowing the rate of gamma radiation being emitted to be calculated.
Unfortunately, a shortage of technetium-99m now exists, due to complications surrounding two nuclear reactors which together are responsible for two-thirds of the world's molybdenum-99 production, the only reliable way of obtaining technetium-99m in hospitals. One of them, the National Research Universal reactor, was shut down in 2007 for maintenance, again in 2009 due to a heavy water leak, and was finally decommissioned in 2018. The condition was particularly problematic in 2010, as the Petten nuclear reactor, in the Netherlands, was also shut down, leading to worldwide shortages.
Technetium, like any other very radioactive material, can pose a serious threat to health. Once ingested, it concentrates in the thyroid gland and gastrointestinal tract. Luckily, the body tries to excrete technetium once it is ingested. The only isotope of technetium which most people are likely to come in contact with is technetium-99. The most common sources of technetium for ordinary people are through contaminated water, and through ingesting plants that have been contaminated. Unfortunately, certain aquatic plants accumulate technetium-99 over time, which can be a problem near nuclear power stations, the main source of technetium. The way in which technetium accumulates in plants and animals was studied using technetium-95m, which has a much longer half-life than technetium-99m and also emits gamma radiation, allowing its movement to be tracked over a much longer period of time. Since the introduction of nuclear power, several tonnes of technetium have found their way into the environment, however, this should not pose a threat, as this technetium is distributed across the world, and the risk of being injured by it is so low as to be non-existent. Technetium is not currently a health problem.
However, the radioactivity of technetium has limited its use in other areas. Potassium pertechnetate has been found to be an excellent corrosion inhibitor for steel, with a concentration of 55ppm in steel enough to prevent corrosion at temperatures of 250°C, at roughly one-tenth of the concentration that potassium chromate needs to achieve the same results. Unfortunately, due to concerns over technetium's radioactivity, currently, technetium is not used as a corrosion inhibitor. Technetium also has similar catalytic properties to rhenium, as they are in the same group, and even in some cases outperforms rhenium, but is not used as a catalyst due to its radioactivity.
Overall, technetium's current uses are entirely dictated by its extremely short half-life and radiation. For the foreseeable future, technetium will continue to be used in medicine. It may also acquire an industrial role, such as corrosion proofing parts in nuclear waste pools, where its radioactivity is not a problem, but mostly it will be unable to be used, due to the inherent cancer risk of using it near people.
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