Beyond the Alchemists’ Wildest Dreams

by Nicholas Mee on August 15, 2020

Technetium cannot be dug out of the ground and extracted from its ore like other metals. It is akin to the gold of the alchemists, forged through transmutation of the elements. But technetium is far more precious than gold, and the way it is created is beyond the alchemists’ wildest dreams. Indeed, technetium was the first artificial element ever made, as described in a previous post: Scrap Metal from the Proton Merry-Go-Round. Today technetium is used in hospitals all over the world, saving numerous lives every year. 

Technetium was first isolated by Emilio Segrè and Carlo Perrier in 1937. The following year Segrè went to the Rad Labs at the University of Berkeley, California to further investigate his new element. Segrè met a young chemist there named Glenn Seaborg and together they worked to isolate and understand the various isotopes of technetium. All these isotopes are radioactive, the longest lived—technetium-97 and technetium-98—have half-lives of just 4.2 million years, which is why barely a trace of naturally occurring technetium exists on Earth. (Minuscule quantities have been detected among the decay products from the spontaneous fission of uranium and thorium in ores of these elements.)

Emilio Segrè (furthest left) and Glenn Seaborg in front of the cameras in 1957, being filmed for Wide, Wide World.

The most important discovery that Segrè and Seaborg made about technetium was the existence of a metastable excited state of the technetium-99 nucleus that decays by emitting a gamma ray with a half-life of six hours. It is known as technetium-99m, where the ‘m’ means metastable.

Nuclear Medicine

Technetium-99m SPECT images.

The full medical potential of technetium-99m only became apparent through further research carried out during the 1950s and 1960s. Technetium-99m has a combination of features that make it ideal for medical imaging.

Firstly, the gamma ray emitted when technetium-99m decays has a relatively low energy (140 keV), in the same range as more conventional medical X-rays. And the half-life of six hours gives sufficient time for medical procedures, but all the technetium-99m decays within a couple of days, so the patient does not emit gamma radiation for too long after leaving hospital. The resulting technetium-99 undergoes beta decay and emits high-energy electrons. But, with a half-life of 211,000 years, the decay is rather slow and not too much radiation is emitted before it flushes through the patient’s system, which again takes a couple of days. Furthermore, the ruthenium-99 derived from the decay of technetium-99 is chemically inert and not radioactive. So, all in all, the patient is not subject to too much damaging radiation other than the gamma rays that are required for the imaging.

X-ray SPECTs

Technetium-99m is used to X-ray a patient’s organs from the inside—a technique known as SPECT (Single Photon Emission Computer Tomography). It is used for around twenty million medical imaging procedures each year.

Technetium-99m can be used to image a whole range of organs including the skeleton, blood, brain, heart, kidneys, thyroid, lungs and liver. Technetium atoms are chemically combined with a molecule known as a ligand, which transports the technetium to the appropriate part of the body. Each ligand is designed to be selectively absorbed by a particular organ, for instance technetium-99m-sestamibi is used to image the heart. A few of these molecules are shown below.

Compounds in which a technetium atom (Tc) binds to a ligand chosen for its ability to transport technetium to the appropriate part of the body.

 

Milking a Moly Cow

You might wonder how hospitals obtain an element that doesn’t exist naturally on Earth. Well, they produce technetium-99m by milking a Moly Cow. It is all rather ingenious.

The process usually begins with a nuclear reactor. An enriched uranium sample is slid into the core of the reactor where it undergoes neutron bombardment causing the uranium to fission. The irradiated sample is then removed and fission products such as molybdenum-99 are chemically extracted. Molybdenum-99 decays into technetium-99m with a half-life of sixty-six hours, so it must be delivered to hospitals with some expediency and added to a device known as a Moly Cow where the technetium-99m decay product is extracted. A schematic diagram is shown below.

The Moly Cow. Credit: SHINE medical.

The Moly Cow contains aluminium oxide powder (alumina) with molybdenum-99 added in the form of the molybdate ion (MoO42–), which binds to the surface of the powder. But when a molybdenum-99 nucleus transforms by beta decay into a technetium-99m nucleus the molybdate ion (MoO42–) becomes a pertechnate ion (TcO4). Beta decay has transformed a neutron in the molybdenum nucleus into a proton to form a technetium nucleus. The nucleus now has an extra positive charge and so the ion’s charge decreases from minus two to minus one. This means that the pertechnate ion does not stick to the aluminium oxide powder as strongly as the molybdate ion. It can be washed off with a saline solution, and this is what happens within the Moly Cow. The sodium pertechnate is the precious cow’s milk.

Incidentally, technetium is chemically similar to manganese as Mendeleyev had predicted. So sodium pertechnate is chemically analogous to potassium permanganate, the well known household antiseptic used for treating various skin ailments and fungal infections.

All this alchemical magic produces a potion that is more valuable than gold and has saved many more lives than the philosopher’s stone or the elixir of life.



Further Information

There is more about the manufacture of technetium-99m and other medical radio-isotopes here:  SHINE medical

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