Rarest natural element study could enable new radiotherapy

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Nuclear physicists at the ISOLDE radioactive-beam facility at CERN have for the first time measured key properties of astatine, the rarest element on Earth.

This new study, with input from the universities of York and Manchester, is said to fill a long-standing gap in the periodic table; astatine, atomic number 85, is the last element present in nature for which this fundamental property remained unknown.

The element is of particular interest because isotopes of astatine could be used to create radiopharmaceuticals for cancer treatment by targeted alpha therapy.

This research, co-funded by the Science and Technology Facilities Council (STFC) and published in Nature Communications, could help chemists to develop applications for astatine in radiotherapy, as well as developing theories that predict the structure of super-heavy elements.

By looking at the ionization potential of astatine - namely, the energy needed to remove one electron from the atom, and thereby turning it into a positive ion - the scientists have been able to understand more about the chemical reactivity of astatine and the stability of its chemical bonds in compounds.

Astatine is a naturally occurring trace element and less than 28 grams exist on Earth at any time. Physicists at ISOLDE can make artificial isotopes of astatine by bombarding uranium targets with high-energy protons.

By shining a series of precisely wavelength-tuned lasers at the astatine atoms, the team that operates the resonance ionization laser ion source (RILIS) at ISOLDE measured the ionization potential of astatine to be 9.31751 electron volts.

‘None of the many short-lived isotopes used in medicine exist in nature; they have to be artificially produced by nuclear reactions,’ said Dr Bruce Marsh from CERN and Manchester University. ‘The possible medical isotopes of astatine are not so different in this respect. What is different about astatine is that its scarcity in nature makes it difficult to study by experiment, which is why this measurement of one of the fundamental properties is a significant achievement.’

‘The experimental value for astatine also serves for benchmarking theories that predict the atomic and chemical properties of super-heavy elements, in particular a recently discovered element 117, which shares very similar characteristics to astatine,’ added York University’s Prof Andrei Andreyev in a statement.