An international team of physicists at the radioactive-beam facility ISOLDE at CERN have for the first time measured the ionization potential of the rare radioactive element astatine*.
The value for astatine, published in the journal Nature Communications, could help chemists to develop applications for the element in radiotherapy, and will serve as a benchmark for theories that predict the structure of super-heavy elements.
The ionization potential of an element is the energy needed to remove one electron from the atom, thereby turning it into an ion. This measurement is related to the chemical reactivity of an element and, indirectly, to the stability of its chemical bonds in compounds.
Astatine occurs naturally in only trace amounts on Earth** but physicists at ISOLDE can make artificial isotopes of astatine by proton-induced reactions and use wavelength-tuneable lasers to study their atomic structure through a technique known as in-source laser resonance ionization spectroscopy.
High-energy proton beams from CERN's Proton Synchrotron Booster are fired at uranium targets. The collisions produce a shower of chemical elements, which diffuse inside a metal cavity at 2000°C. Shining laser beams of chosen wavelengths into this cavity results in selective ionization of some of the neutral atoms inside. An electric field extracts the positively charged ions, which are sent through tuneable magnets, configured to allow transmission of only a chosen mass. The result is a pure ion beam of one isotope that is sent to a detector. By applying this technique whilst carefully scanning the laser wavelengths, ISOLDE physicists measured the ionization potential of astatine to be 9.31751 electronvolts.
The measurement fills a long-standing gap in the periodic table; astatine is the last element present in nature for which this fundamental property remained unknown. The element is of particular interest because isotopes of astatine are candidates for the creation of radiopharmaceuticals for cancer treatment by targeted alpha therapy.
"None of the many short-lived isotopes used in medicine exist in nature; they have to be artificially produced by nuclear reactions," says Bruce Marsh of the resonance ionization laser ion source (RILIS) at ISOLDE. "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 the recently discovered element 117, an astatine homologue.
“In-source laser spectroscopy today is a most sensitive method to study atomic properties of exotic short-lived isotopes," says RILIS team leader Valentin Fedosseev. "It is well suited to explore the spectra of artificially produced elements, like the super-heavy ones. The success in this study of astatine has added confidence for similar projects started recently at GANIL, France and at JINR, Russia.
CERN, the European Organization for Nuclear Research, is the world's leading laboratory for particle physics. It has its headquarters in Geneva. At present, its member states are Austria, Belgium, Bulgaria, the Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Italy, the Netherlands, Norway, Poland, Portugal, Slovakia, Spain, Sweden, Switzerland and the United Kingdom. Romania is a candidate for accession. Israel and Serbia are associate members in the pre-stage to membership. India, Japan, the Russian Federation, the United States of America, Turkey, the European Commission and UNESCO have observer status.
*In 1940, D Corson and co-workers discovered the element astatine by bombarding a bismuth target with alpha particles. The most stable astatine isotope has a half-life of only 8.1 hours. In 1964 McLaughlin studied a 70-nanogram sample of artificially produced radioactive isotopes of astatine and was first to observe two spectral lines in the UV region. No other data on the atomic spectrum of astatine were known prior to the ISOLDE study described in this press release.
**A 1953 estimate by Isaac Asimov put the worldwide total of astatine in nature at 0.07 grams.