The electron affinity of astatine

Abstract

One of the most important properties influencing the chemical behavior of an element is the electron affinity (EA). Among the remaining elements with unknown EA is astatine, where one of its isotopes, ²¹¹At, is remarkably well suited for targeted radionuclide therapy of cancer. With the At^- anion being involved in many aspects of current astatine labeling protocols, the knowledge of the electron affinity of this element is of prime importance. Here we report the measured value of the EA of astatine to be 2.41578(7) eV. This result is compared to state-of-the-art relativistic quantum mechanical calculations that incorporate both the Breit and the quantum electrodynamics (QED) corrections and the electron-electron correlation effects on the highest level that can be currently achieved for many-electron systems. The developed technique of laser-photodetachment spectroscopy of radioisotopes opens the path for future EA measurements of other radioelements such as polonium, and eventually super-heavy elements.

Fig. 1. Electron affinities across the periodic table.

The height corresponds to the measured value of the electron affinity of the corresponding element^,,. Astatine is highlighted in red. Blue indicates elements that are experimentally determined to have a positive EA, i.e., to form stable negative ions. Elements that are predicted to form stable negative ions but have not yet been experimentally investigated are indicated in green, while those in light gray are predicted to not form a stable negative ion, i.e., have a negative EA. Finally, elements that neither have been experimentally observed nor investigated theoretically, are indicated with dark gray.

Fig. 2. Schematic diagram of the experimental setup.

From left to right: a beam of negative astatine ions (blue circles) is guided into GANDALPH^,, where the ion beam is overlapped with a frequency tuneable laser beam (red line) in the interaction region in either co- or counter-propagating geometry. By absorbing a photon (Inset 1), an electron can gain enough energy to be ejected from the ion, thereby creating a neutral atom (green circles, Inset 2). After the interaction region, the charged particles are deflected into an ion detector, while neutralized atoms continue moving straight to the graphene-coated glass plate downstream and create secondary electrons (white circles), which are detected by a channel electron multiplier.

Fig. 3. Threshold scan of the photodetachment of astatine.

The neutralization cross section is measured as a function of the photon energy. The data points are the experimental measurements with one standard deviation represented by error bars, and the solid line is a fit of Eq. (1). The onset corresponds to the EA of ²¹¹At. The inset shows the region around threshold, where the different onsets in the fit function represent the detachment to the hyperfine levels of the groundstate of the neutral atom.

Fig. 4. Production of a negative astatine ion beam.

Astatine atoms (green circles) are created in a spallation reaction of thorium (white circles) with 1.4 GeV protons (red circles). Subsequently, the atoms are negatively ionized and extracted as a mono-energetic beam (blue circles) with an energy of 20 keV. The ²¹¹At isotopes are then mass separated with an electromagnetic mass separator and directed to the GANDALPH spectrometer.