Researchers at CERN’s ISOLDE facility have successfully deployed a novel electrostatic trap that efficiently recycles rare, negatively charged ions, enabling unprecedentedly precise measurements of the fundamental properties of the heaviest and most elusive elements. This breakthrough technique significantly boosts the efficiency of studying atoms that can only be produced in minuscule quantities and decay within minutes or seconds, opening a new window into the nature of matter at the extreme edge of the periodic table.
The new device, known as GANDALPH, has already achieved a landmark result by performing the first-ever successful measurement of the electron affinity of astatine, the rarest naturally occurring element on Earth. Electron affinity, the energy released when an atom gains an electron, is a critical parameter that governs chemical bonding and reactivity. By accurately measuring this value for astatine, the team has not only clarified the element’s chemical identity but has also validated a powerful method for probing the superheavy elements that lie beyond.
A Novel Instrument for Scarce Isotopes
Studying the universe’s heaviest elements presents a profound challenge: they are extraordinarily unstable, often vanishing moments after their creation in particle accelerators. This leaves scientists with vanishingly small samples to investigate. At CERN’s Isotope Separator On-Line facility, or ISOLDE, scientists create these exotic atoms by firing high-energy protons at specialized targets. For the recent experiment, a thorium target was bombarded to produce various elements, including isotopes of astatine.
From this atomic spray, the desired atoms must be separated and analyzed with extreme speed and sensitivity. The GANDALPH apparatus, which stands for Gothenburg ANion Detector for Affinity measurements by Laser PHotodetachment, was developed specifically for this purpose. Unlike conventional magnetic traps that can interfere with delicate measurements, GANDALPH is an electrostatic ion beam trap. It captures negatively charged atoms, or anions, using only electric fields, creating a cleaner environment for analysis. Inside the trap, a precisely tuned laser is fired at the captured anions to knock off their extra electron. By systematically adjusting the laser’s frequency, scientists can pinpoint the exact energy threshold required for this detachment, a value that directly corresponds to the atom’s electron affinity.
The Power of Anion Recycling
The key innovation of the GANDALPH system is its ability to recycle ions. Given the incredibly low production rates for elements like astatine—sometimes just a few atoms per second—every single anion is precious. In a conventional single-pass experiment, an anion that does not interact with the laser beam would be lost forever. Such inefficiency would make measurements of the rarest isotopes nearly impossible.
The electrostatic trap design overcomes this limitation by forcing the anion beam to travel in a closed loop. Anions that pass through the laser interaction zone without being neutralized simply continue along the circuit and are guided back for another attempt. This recycling process is repeated thousands of times per second, vastly increasing the probability that each anion will eventually be struck by a laser photon. This dramatic boost in efficiency makes it feasible to conduct high-precision experiments with samples consisting of only a handful of atoms, turning what was once a prohibitive obstacle into a manageable challenge. This capability is crucial for extending these techniques to the even more ephemeral superheavy elements.
First Subject: Earth’s Rarest Element
To commission their new instrument, the research team focused on astatine, an element so scarce that the entire Earth’s crust is estimated to contain a mere 70 milligrams at any given time. Its extreme rarity and high radioactivity have left its fundamental chemical properties largely to theoretical predictions. Yet astatine holds significant promise for medicine, particularly as an agent for targeted alpha therapy, a cutting-edge cancer treatment.
A Landmark Measurement
After producing astatine-211 isotopes at ISOLDE and converting them into negative ions, the team fed them into GANDALPH. The successful experiment yielded the first reliable measurement of astatine’s electron affinity: 2.416 electronvolts. This result confirms that astatine behaves as the heaviest member of the halogen family, which also includes chlorine and iodine, but it also provides a critical benchmark for quantum theories that model complex atoms. “This is a breakthrough,” said Dag Hanstorp, a professor of atomic physics at the University of Gothenburg who was involved in the work. “Though the amounts we have obtained are very small, we can now fundamentally clarify and describe how the astatine atom works.”
From Medical Advances to Superheavy Elements
The precise measurement of astatine’s electron affinity has implications that span from practical medical applications to the most fundamental questions of chemistry and physics. Understanding how readily astatine forms negative ions is essential for developing radiopharmaceuticals, as it affects how the element will interact with both cancerous and healthy tissues in the body.
Charting the Periodic Table’s Edge
Beyond its medical relevance, the achievement serves as a crucial stepping stone toward understanding the superheavy elements. For atoms with extremely large nuclei, such as Tennessine (element 117), the immense positive charge of the protons forces the innermost electrons to orbit at near the speed of light. This introduces strong relativistic effects that can alter the expected electron shell structure and dramatically shift chemical properties, potentially blurring the neat columns of the periodic table. Theoretical models that predict these properties have been difficult to test. The new value for astatine provides a firm anchor for these theories, increasing confidence in their predictions for its even heavier cousins. As Julia Karls, the doctoral student who built the detector, explained, “With these results we are paving the way for measuring elements that are heavier than astatine.”
A Five-Year Effort Culminates in Success
The successful measurement was the result of a five-year effort to design, build, and commission the highly specialized GANDALPH detector. The collaboration first tested the system on more stable and abundant ions of iodine to prove its functionality before moving on to the formidable challenge of radioactive astatine. “We have been working towards this for five years and have finally managed to produce a result!” Karls stated. The team’s success not only solves a long-standing question about astatine but also establishes a robust and efficient new tool in the experimental physicist’s arsenal. The future of the GANDALPH experiment involves applying this powerful technique to a host of other rare isotopes produced at ISOLDE, promising to shed further light on the forces that govern the structure of matter and the ultimate limits of the periodic table.
