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How come technetium is a synthetic material right in the centre of all those naturally-occurring substances in the periodic table? Could there be a deposit of naturally-occurring technetium out there somewhere?

If the early periodic table had been a party, technetium would have been a particularly late arrival.

However its tardy entrance hasn't prevented it from becoming an important element in nuclear medicine.

Russian chemist Dmitri Mendeleev predicted technetium's existence when he published the first periodic table of the then 63 known elements in 1869, but it was nearly another 70 years before it was discovered.

Mendeleev's periodic table was unique at that time because he arranged the elements by their atomic number, but also by their properties: with metals in the left hand side of the table, transition elements in the middle and nonmetals on the right.

This meant when he predicted that technetium was missing, he was also able to predict its properties, says Dr Ron Weiner, head of ANSTO's Radiopharmaceutical Research Institute.

It wasn't until 1937 that two Italian scientists, Emilio Segrè and Carlo Perrier, were able to isolate and finally prove the existence of the elusive element 43 — a reference to technetium's 43 protons.

The reason technetium has proved such an unwilling guest is that it is not found in nature. Indeed its very name is derived from the Greek word technetos meaning artificial.

Technetium can only be produced artificially because most forms or isotopes of it (atoms of the same chemical element with different numbers of neutrons) have an excess of neutrons, making it very unstable.

"If an atom has an equal number of neutrons and protons then it is generally stable, if it has too few or too many neutrons then that particular element is unstable," Weiner explains.

Unstable elements undergo radioactive decay into stable elements. Technetium is the lightest radioactive element on the periodic table and its isotopes decay into a variety of other elements including stable ruthenium.

So while it's highly unlikely we'll ever find a mother lode of technetium on Earth, it has been detected in the spectra of some stars, confirming the theory that elements are produced by nuclear reactions within stars.

"The idea of stars of course is they're fusion devices," says Weiner. "They start with helium and hydrogen and lithium and then build up, and that's basically one of the signatures that suggest how old a star is."

Technetium isotopes are also found occasionally in naturally-occurring 'reactors', says Weiner. These reactors are uranium deposits which naturally underwent sustained nuclear reactions in Gabon, western Africa about 1.9 billion years ago.

Technetium isotopes like the excited isotope technetium-99m are produced today because of their importance in nuclear medicine.

"About 80 per cent of the nuclear medicine procedures in the world use this isotope, and that's about 10 million procedures a year," says Weiner.

The big advantage of technetium-99m (half-life six hours) is that it is produced by decay from the much longer lived isotope molybdenum-99 (half-life 67 hours). This means molybdenum-99 can be stored in the hospital and the technetium-99m isolated for patient use when necessary.

ANSTO's OPAL reactor produces molybdenum-99 using uranium-235, and ANSTO is hoping to increase its production of this isotope with the world facing a potential shortfall in supplies as older reactors are temporarily closed for maintenance.

Technetium-99m is a good isotope to use as a radioactive tracer because it doesn't harm the body too much, and the single gamma ray it emits enables doctors to get a very good picture of where the isotope is in the body, Weiner says.

However the downside of the procedure is it produces waste in the form of technetium-99 (the ground or unexcited state of technetium-99m) which has to be immobilised to prevent harm to the environment.

Dr Ron Weiner was interviewed by Suzannah Lyons.