One of the first versions of the periodic table that the Russian chemist Dmitry Mendeleev presented more than a century and a half ago contained four striking open holes. Three were quickly filled with scandium, gallium and germanium. But the fourth hole remained empty for more than six decades. The hole at element 43 was a thorn in the side of chemists.
Over the years, several chemists have claimed to have discovered the missing element. But none of these discoveries lasted. In 1925 it finally seemed to happen. The German chemists Walter Noddack, Otto Berg and Ida Tacke thought they had found traces of two new elements in a mineral: element 75, which they called rhenium (after the Rhine) and element 43, which they named masurium (after an area in Prussia). The existence of rhenium was confirmed. Not that of masurium. Place 43 remained empty.
It was not until 1937 that the Italian chemists Carlo Perrier and Emilio Segrè found the missing element. They did not do this by sifting through natural minerals, like other element hunters. They took a different approach.
The first cyclotron
In 1936, Segrè was visiting the American physicist Ernest Lawrence, who a few years earlier had built the first cyclotron, a particle accelerator with which he produced radioactive elements. There, Segrè learned that during this process, strips of metal containing molybdenum – element 42 – were bombarded with radioactive radiation. Segrè asked Lawrence to send him pieces of this irradiated cyclotron scrap so that he could analyze it with Perrier. They soon discovered element 43 in it, which was created by the irradiation of molybdenum.
Segrè and Perrier gave the element the name technetium, after the Greek word technetos, which means ‘artificial’. Technetium is the first artificial, man-made element. And it is the only element discovered in Italy.
Yet technetium does occur in nature. Tiny amounts of it have been found in the mineral uraninite. And it has been observed in certain stars. The fact that natural technetium is scarce is because there are no stable isotopes – the longest-lived variants have a half-life of 4 million years, a fraction of the age of the Earth. Artificial technetium can be found in radioactive waste from nuclear power plants.
Worldwide, 100,000 people undergo medical examinations with technetium-99m every day. It is the most commonly used medical isotope. Technetium-99m is attached to a tracer. This is a substance that is absorbed by tumors or other tissues. Because technetium-99m emits easily measurable gamma radiation, tumors can be imaged, for example. Because half of the technetium-99m has decayed after six hours and does not accumulate in the body, the radiation dose patients receive is very low.
Technetium-99m is created by the radioactive decay of molybdenum-99, which is mainly made in nuclear reactors. Work is also underway on production with cyclotrons, the instrument that led to the discovery of technetium 86 years ago.