Targeted alpha therapy is a method used to treat cancer tissue at close proximity using radioactive alpha radiation. Astatine has already been used in studies e.g. of ovarian cancer treatment at the Sahlgrenska University Hospital in Gothenburg.  Astatine is purely radioactive, with its longest living isotope having a lifetime of 8.1 hours. According to estimations, there are less than one gram of astatine in the Earth’s crust, making it the rarest naturally occurring element. Hence, in order to study its properties, it must be produced artificially. However, the short lifetime and the limited production rate hinder the experimental work significantly. Therefore, it is of great importance to give insight in astatine’s chemical behavior and to benchmark theoretical chemistry models, in order to facilitate the development of a radiopharmaceutical using astatine. The electron affinity (EA), the energy gained when binding an electron to a neutral atom, and the ionization potential (IP) are fundamental properties that need to be measured precisely. In the recently released paper a group of scientists have now determined the EA of astatine at CERN’s  nuclear physics facility ISOLDE.

“It’s a complicated experiment with a lot of components that all have to work at the same time. This is a large collaboration with around 40 scientists from almost 20 different universities and research groups”, says Moa Kristiansson, a researcher from Stockholm University who took part in the experiment. Moa also works on EA measurements and other types of experiments involving negative ions at the DESIREE facility here at Fysikum as part of her PhD thesis work. The negative ion research performed at DESIREE is very similar to that in the recent publication and a close collaboration with many of the people involved have already lead to several other discoveries in the field of advanced negative ion spectroscopy. 

Schematic diagram of the experimental setup GANDALPH. A beam of negative ions enter the vacuum chamber to the left ans is overlapped with a laser beam. The neutrals created in photodetachment continue to the right and hit the detector, and the negative ion beam is bent to hit the ion detector.


At ISOLDE a 1.4 GeV proton beam from the Proton Synchrotron Booster was used to bombard a thorium target, producing astatine among other elements through spallation reactions. The neutral atoms were subsequently negatively ionized and accelerated to form a 20 keV ion beam which was then mass selected. The beam of negative 211At ions was guided into the GANDALPH (Gothenburg ANion Detector for Affinity measurements by Laser PHotodetachment) setup (see figure), where it was overlapped with a frequency tune-able laser beam. Here, the neutral atoms yielding from the photodetachment process were recorded as a function of photon energy and from this photodetachment spectrum the EA of astatine was determined to be 2.415 78(7) eV.

Photodetachment cross section as a function of photon energy. The threshold onset corresponds to the EA of 211 At.


“From this result and the previously determined IP we were finally able to derive astatine’s fundamental chemical properties such as its electronegativity based on high precision measurements, which could facilitate astatine’s use as an agent for targeted alpha therapy”, says David Leimbach, lead author of the study.

In addition to the experimental results, state-of-the-art relativistic quantum mechanical calculations were also performed to theoretically predict the EA. Calculations of electronic properties of heavy elements such as astatine requires the highest possible electron-electron correlation effects to be included. The results from these complex calculations are in excellent agreement with the experimental findings.

“When you are the first to observe something, it is always helpful to have the theory to support your work, especially when previous calculations have given varied results”, says Moa.

“With the present result of the determination of the electron affinity and the previous measurement of the ionization potential (IP), we conclude a 10 years experimental campaign at ISOLDE to determine the fundamental properties of astatine” says Sebastian Rothe, spokesperson of the astatine experiment at CERN and lead author of the previous IP study [2].

The GANDALPH setup at ISOLDE will further be used to study other rare isotopes of atomic anions using a similar technique. The most imminent measurement planned is the mass shift in the EA of chlorine. Mass shift measurements of the EA of oxygen are also planned to take place at the DESIREE facility at Fysikum. The facility at DESIREE also allows for additional properties of negative ions to be studied. In fact, while GANDALPH allows for a single pass setup, DESIREE consists of two rings where negative ions can be stored, allowing interactions with the ions not only as a function of photon energy but also as a function of time. Several negative ion species and the lifetimes of their excited states have already been studied in detail. A recent project nearing its completion is a precision measurement of the EA of oxygen where the most accurate EA measurement to this day is performed. 


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