Physicists Measure Ionization Potential of Astatine, Rarest Naturally Occurring Element on Earth

In ground-breaking experiments conducted at the radioactive isotope facility ISOLDE at CERN, a multinational research team has investigated the atomic structure of astatine (At), the rarest atom on our planet.

Resonance ionization laser ion source of ISOLDE (CERN)

Astatine was discovered by D. Corson and co-workers in 1940 by bombarding a bismuth target with alpha particles. The most stable isotope of this element has a half-life time of only 8.1 hours. In 1964, McLaughlin studied a 70 ng sample of artificially produced radioactive isotopes of astatine and was first to observe two spectral lines in the UV region. Apart from this, no other data on astatine’s atomic spectrum was known before the ISOLDE experiments.

Astatine is also of significant interest as its decay properties make it an ideal short-range radiation source for targeted alpha therapy in cancer treatment.

Writing in the journal Nature Communications, the ISOLDE group has measured for the first time the ionization potential of the astatine atom. This represents the essential quantity defining chemical and physical properties of this exclusively radioactive element. The results fill a long-standing gap in Mendeleev’s periodic table, since astatine was the last element present in nature for which this fundamental property was unknown. As binding energy of the outermost valence electron, the atomic ionization energy is highly relevant for the chemical reactivity of an element and, indirectly, the stability of its chemical bonds in compounds.

“Astatine is of particular interest because its isotopes are interesting candidates for the creation of radiopharmaceuticals for cancer treatment by targeted alpha therapy. The experimental value for astatine serves also for benchmarking the theories used to predict the atomic and chemical properties of super-heavy elements, in particular the recently discovered element 117, which is a homologue of astatine,” explained Prof Andrei Andreyev of the University of York.

According to the ISOLDE physicists, the ionization potential of the astatine atom is 9.31751(8) eV.

At ISOLDE, short-lived isotopes created in nuclear reactions induced by a high energy proton beam release from target material and can immediately interact with laser beams inside the hot cavity of laser ion source.

Once the wavelengths of lasers are tuned in resonance with selected atomic transitions the atoms are step-wise excited and ionized due to absorption of several photons with total energy exceeding the ionization threshold.

Among these, is a study of short-lived nuclides by in-source resonance ionization spectroscopy using a highly sensitive detection of nuclear decay. The first laser-ionized ions of astatine were observed and identified by its characteristic alpha-decay in these experiments. Also the ionization threshold of astatine was found by scanning the wavelength of ionizing UV laser.

A second phase of the study of the atomic spectrum of astatine took place at a radioactive isotope facility in Canada, where new optical transitions in the infrared region of spectrum were found. With the newly found transitions a highly efficient three-step ionization scheme of astatine was defined and used at ISOLDE for further study of astatine spectrum. The team probed the interesting region around the ionization threshold and found a series of highly excited resonances – known as Rydberg states. From this spectrum the first ionization potential of astatine was extracted with high accuracy.

“The in-source laser spectroscopy today is a most sensitive method to study atomic properties of exotic short-lived isotopes. For artificially produced elements, like super-heavy ones, this could be a real way to probe their spectra,” said Dr Valentine Fedosseev from CERN.

“This development allows several new phenomena to be investigated, such as the size (radii) of astatine nuclei, along with a very exotic type of nuclear fission. Our collaboration has recently initiated a series of experiments to reach these goals,” Prof Andreyev added.

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Bibliographic information: S. Rothe et al. 2013. Measurement of the first ionization potential of astatine by laser ionization spectroscopy. Nature Communications 4, article number: 1835; doi: 10.1038/ncomms2819