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Unlocking Auger electron therapy via heavy-ion measurements

This paper presents a novel vacuum-based experimental approach using 197Au measurements that successfully validates the BrIccEmis atomic model, offering a critical pathway to resolving Auger-electron multiplicity challenges and enabling accurate dosimetry for targeted cancer therapy.

Original authors: Jacob Heery, Jack Henderson, Tibor Kibédi, David Jenkins, Claus Müller-Gatermann, Darek Seweryniak, Marco Siciliano, Michael Carpenter, Cameron Cassells, Sean Collins, Daniel Doherty, Aysegul Ertoprak

Published 2026-10-15

📖 6 min read🧠 Deep dive

CC BY 4.0

Original authors: Jacob Heery, Jack Henderson, Tibor Kibédi, David Jenkins, Claus Müller-Gatermann, Darek Seweryniak, Marco Siciliano, Michael Carpenter, Cameron Cassells, Sean Collins, Daniel Doherty, Aysegul Ertoprak, John Fuller, Heshani Jayatissa, Vasil Karayonchev, Amel Korichi, Filip Kondev, Torben Lauritsen, Gavin Lotay, Bondili Sreenivasa Nara Singh, Edward O'Sullivan, Walter Reviol, Nirupama Sensharma, Robert Shearman, Ragandeep Singh Sidhu

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

In the fight against cancer, doctors often use radiation to destroy malignant cells, but the challenge lies in delivering that energy precisely. Traditional radiation beams can be like a broad-spectrum flashlight, illuminating the tumor but also casting light on the healthy tissue nearby, causing collateral damage. A more refined approach, known as Auger electron therapy, aims to act like a microscopic scalpel. This method relies on a specific type of atomic event where an unstable atom releases a burst of very low-energy electrons. Because these electrons carry so little energy, they travel only a tiny distance—roughly the width of a single strand of DNA—before stopping. If a radioactive atom can be placed directly inside a cancer cell's nucleus, these electrons would deposit all their destructive energy into the cell's genetic code, killing the cell while leaving its neighbors unharmed. However, for this therapy to work safely and effectively, scientists must know exactly how many of these tiny electrons are released during each atomic decay and how much energy they carry. Without this precise count, doctors cannot calculate the correct dose, making the treatment too risky to use widely.

For decades, determining the exact number of these electrons has been a stubborn problem. The process is chaotic; when an atom relaxes after a nuclear event, it can emit a cascade of electrons in a random pattern, making it difficult to count them all, especially the very low-energy ones that are easily lost or absorbed before they can be measured. A new study by a team of physicists from institutions including the University of Surrey, Argonne National Laboratory, and the Australian National University has introduced a novel way to solve this puzzle. Instead of trying to catch the electrons directly, the researchers measured the change in the electrical charge of the atoms themselves. They reasoned that every time an atom ejects an electron, it loses a negative charge and becomes more positively charged. By counting how much more positive the atom becomes after a specific event, they could deduce exactly how many electrons were emitted.

To test this idea, the team conducted an experiment at the ATLAS facility at Argonne National Laboratory in Illinois. They fired a beam of heavy argon ions at a thin sheet of gold. The collision knocked gold atoms loose, sending them flying forward at high speeds. As these gold atoms traveled, they passed through a very thin carbon foil. This foil acted as a reset button, stripping away any extra electrons the gold atoms had gained or lost during the collision and setting them to a standard, known charge state. The researchers then waited for a specific moment: they looked for gold atoms that had an excited nucleus, which would eventually calm down by releasing energy. If the nucleus released this energy by firing out an electron (a process called internal conversion), the atom would lose that electron and become more positive. Crucially, because the atom had already passed through the reset foil, any increase in its positive charge could only be caused by the electron emission happening after the reset.

The team focused on a specific energy level in the gold atom that lasts for about 1.9 billionths of a second before decaying. This delay was long enough that the atom would travel past the reset foil before the decay occurred. By using a sophisticated array of detectors to identify the specific gamma rays emitted during this decay, the researchers could separate the atoms that had undergone this specific event from those that had not. They then measured the electrical charge of these atoms. They found that the atoms which had undergone the decay were indeed more positive than the others. By comparing the charge of the "decayed" atoms to the "non-decayed" ones, they calculated the average number of electrons released in the cascade.

The results showed that for a single decay event in a neutral gold atom, the process releases an average of roughly 7.7 Auger electrons. This number is significant because it confirms the theoretical models scientists have been using to design these therapies. The researchers compared their experimental findings with a state-of-the-art computer model called BrIccEmis, which simulates these atomic processes. The model's predictions matched the experimental data almost perfectly, giving scientists high confidence that the model is accurate. This is a vital step forward because it validates the tool used to predict how much radiation a patient would receive. The study also demonstrated that the method works regardless of the initial speed or charge of the atoms, suggesting it can be applied to many different elements.

While the experiment was performed in a vacuum, the researchers noted that the method provides a clean, controlled environment to test the fundamental physics without interference from surrounding matter. In a real medical setting, where these atoms would be inside a patient's body, the surrounding water and tissue might allow the atom to grab extra electrons from its environment, potentially increasing the number of Auger electrons released even further. The new technique allows scientists to measure the baseline number of electrons in a vacuum and then use the computer models to adjust for the conditions inside the human body. This ability to benchmark the models against real data means that doctors can eventually calculate radiation doses with much greater precision.

The study does not claim that Auger electron therapy is ready for immediate clinical use, nor does it suggest that all the hurdles have been cleared. Rather, it offers a reliable path to resolving one of the biggest uncertainties in the field: the exact count of electrons emitted. By proving that the charge-reset method works and that current models are accurate, the team has provided a crucial piece of the puzzle. This work paves the way for the development of safer, more effective cancer treatments that can target tumors with surgical precision, sparing healthy tissue from the harsh effects of radiation. The ability to measure these tiny, fleeting events with such clarity brings the promise of this targeted therapy one step closer to reality.

Technical Summary: Unlocking Auger Electron Therapy via Heavy-Ion Measurements

Problem Statement
Auger Electron Therapy (AET) represents a promising modality for targeted radiotherapy, utilizing low-energy electrons emitted during atomic relaxation to deposit energy within the nanometer scale of a cell's DNA, thereby minimizing damage to surrounding healthy tissue. Despite its theoretical advantages over beta or alpha emitters, the clinical deployment of AET is hindered by a critical barrier: the inability to comprehensively determine the dose deposited and its spatial disposition. This uncertainty stems largely from the difficulty in accurately measuring the number (multiplicity) and energy of electrons emitted during a single decay process. Conventional electron-spectroscopy techniques fail to detect low-energy electrons (<10 keV) effectively, and the stochastic nature of the Meitner-Auger effect, which competes with X-ray emission, complicates simple counting. Consequently, the average number of Auger electrons emitted per decay ( $N_{AE}$ ) for medically relevant isotopes remains poorly constrained, preventing the development of accurate dosimetry models required for regulatory approval and patient safety.

Methodology
To address these measurement limitations, the authors present a novel experimental approach utilizing heavy-ion beam physics to measure Auger electron multiplicity in a vacuum environment. The study focuses on $^{197}\text{Au}$ , the daughter nucleus of the medically relevant isotope $^{197}\text{Hg}$ , which is a candidate for AET.

The core of the methodology is the charge-reset method, adapted from nuclear lifetime measurements. The experimental setup at the ATLAS facility (Argonne National Laboratory) involved:

Beam and Target: A $^{40}\text{Ar}$ heavy-ion beam was accelerated to two different energies (150 MeV and 101 MeV) and impinged upon a self-supporting $^{197}\text{Au}$ target. This Coulomb interaction excited the $^{197}\text{Au}$ nuclei, causing them to recoil forward.
Charge Reset: A thin carbon foil (61 $\mu\text{g/cm}^2$ ) was placed 5 mm downstream from the target. As recoiling ions passed through this foil, their charge states were "reset" to an equilibrium value determined by their velocity, effectively erasing their prior electronic history.
Selective Decay: The experiment targeted the 77.3 keV excited state of $^{197}\text{Au}$ , which has a half-life of 1.91 ns. Due to the flight time (~0.5–0.7 ns) to the foil, a significant fraction of these ions decay after passing through the charge-reset foil.
Detection and Analysis:
- Internal Conversion (IC): The 77.3 keV state decays via internal conversion with an 81% probability, ejecting a conversion electron and triggering an Auger cascade.
- Charge State Measurement: Ions that decayed after the foil via IC experienced an increase in charge state ( $\Delta Q = +1$ for the conversion electron + $+N_{AE}$ for the Auger cascade). These were separated from ions that decayed via $\gamma$ -ray emission (no charge change) or decayed before the foil (charge reset).
- Instrumentation: The Fragment Mass Analyzer (FMA) measured the mass-to-charge ( $M/Q$ ) ratio of the recoils, while the GRETINA $\gamma$ -ray array identified the specific nuclear transitions (e.g., the 191.4 keV $\gamma$ -ray feeding the 77 keV state) to correlate charge states with specific decay events.

Key Contributions

Novel Measurement Technique: The paper introduces a method to determine Auger electron multiplicity by measuring the shift in ionic charge state distributions in a vacuum, bypassing the detection limits of conventional electron spectroscopy for low-energy electrons.
Benchmarking Theoretical Models: The study provides high-precision experimental data to benchmark the BrIccEmis code, a state-of-the-art atomic model used to predict Auger emission spectra.
Vacuum Environment Validation: By performing measurements in a vacuum, the study isolates the atomic cascade process from environmental interactions (such as electron capture from a medium), providing a "clean" test of the fundamental atomic physics models used in dosimetry.

Results
The experiment successfully isolated the charge-state distribution associated with the decay of the 77 keV state in $^{197}\text{Au}$ .

Charge Increase: For an initial low-charge distribution ( $\tilde{Q}_L \approx 22.2e$ ), the average charge increase was measured as $\Delta Q = 3.8(6)e$ . For a higher initial charge distribution ( $\tilde{Q}_L \approx 25.8e$ ), the increase was $\Delta Q = 3.3(7)e$ .
Auger Multiplicity: By accounting for the internal conversion electron ( $\Delta Q = +1$ ) and extrapolating the results to a neutral atom using the BrIccEmis model, the authors determined an average Auger emission multiplicity of 7.7 electrons per single atomic cascade for neutral $^{197}\text{Au}$ .
Model Agreement: The BrIccEmis calculations, which required over 14 million atomic configurations to simulate the highly charged ion systems, reproduced the experimental charge-state distributions and the scale of the Auger multiplicity with high fidelity.

Significance and Claims
The authors assert that this work provides a crucial step toward resolving the "Auger-electron multiplicity challenge," a primary obstacle in the clinical implementation of AET.

Dosimetry Accuracy: By validating the BrIccEmis model against experimental data derived from a vacuum environment, the method offers a pathway to generate accurate dosimetry data for AET candidates.
Scalability: The technique is described as broadly applicable at heavy-ion facilities worldwide, requiring only standard equipment (recoil separators and HPGe detector arrays). The authors suggest that by varying beam masses and energies, the method can be extended to measure lower initial charge states, further reducing systematic uncertainties in extrapolating to neutral atoms.
Future Outlook: The paper posits that this approach has the potential to completely resolve the question of Auger multiplicity within the next decade, thereby eliminating a major concern in the deployment of Auger electron therapy. The authors emphasize that while the current study focuses on $^{197}\text{Au}$ , the method is applicable to other elements, allowing researchers to select the most favorable isotopes for therapy based on robust, experimentally verified atomic data.

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