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Thick-target Yield of $^{65}$ Cu( $\alpha,n)^{68}$ Ga Near Threshold and Implications for Nuclear Medicine, Deep Underground Detector Backgrounds, and Nucleosynthesis

This study reports new thick-target yield measurements for the $^{65}$ Cu( $\alpha,n)^{68}$ Ga reaction near threshold, which align with previous single-foil data and current IAEA estimates at 14 MeV but reveal significantly lower yields at 8 MeV, while confirming the accuracy of existing cross-section estimates used for detector backgrounds and nucleosynthesis calculations.

Original authors: Zach Meisel, Gula Hamad, Kristyn Brandenburg, Carl R. Brune, Don E. Carter, Joseph Derkin, Yenuel Jones-Alberty, Tom N. Massey, Doug Soltesz, Shiv K. Subedi, Alexander V. Voinov

Published 2026-10-02

📖 4 min read🧠 Deep dive

CC BY 4.0

Original authors: Zach Meisel, Gula Hamad, Kristyn Brandenburg, Carl R. Brune, Don E. Carter, Joseph Derkin, Yenuel Jones-Alberty, Tom N. Massey, Doug Soltesz, Shiv K. Subedi, Alexander V. Voinov

Original paper licensed under CC BY 4.0 (http://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 quiet corners of the universe, deep beneath the earth's surface, scientists listen for whispers from the cosmos. They search for neutrinos, ghostly particles that pass through everything, and dark matter, an invisible substance that holds galaxies together. To hear these faint signals, detectors must be shielded from the constant chatter of cosmic rays and natural radioactivity. However, even in the deepest mines, a subtle background noise persists. This noise often comes from neutrons, tiny particles that can mimic the signals scientists are hunting. One source of these unwanted neutrons is the interaction between alpha particles—helium nuclei emitted by natural radioactive elements in the ground—and the copper used to build the detectors themselves. Understanding exactly how often this happens is crucial for distinguishing a true cosmic signal from a local disturbance.

At the same time, this same interaction holds the key to creating a vital medical tool. Hospitals rely on a radioactive form of gallium, known as gallium-68, to create images of the body's internal workings, particularly for detecting cancer. This isotope is often made by firing alpha particles at a target of copper. The challenge lies in finding the perfect energy level: too low, and the reaction won't happen; too high, and it creates unwanted radioactive impurities that make the medical product unsafe. For years, scientists have debated how much gallium-68 can be produced at these specific energy levels, with different studies offering wildly different estimates. This uncertainty has made it difficult to plan efficient medical production or to accurately calculate the background noise in deep underground experiments.

A team of researchers set out to settle this debate by measuring the reaction directly, rather than relying on indirect methods that had led to confusion. They worked at a particle accelerator, a machine that speeds up helium ions to high velocities. Instead of using the complex, multi-layered targets that had been common in previous studies, they used a single, thick block of natural copper. They fired a beam of helium ions at this block, varying the energy of the beam from just above the point where the reaction begins up to a higher, more powerful level. As the helium ions struck the copper, they triggered a nuclear reaction that released neutrons. The researchers counted these neutrons with a specialized detector designed to capture them from all directions, allowing them to calculate exactly how many gallium atoms were being created for every unit of beam energy used.

The results provided a clear picture that contradicted several long-standing estimates. The team found that the amount of gallium-68 produced at lower energies was significantly less than what some previous models had predicted. Specifically, at an energy of eight million electron volts, a level where the reaction could produce pure medical isotopes without contamination, their measurements showed a yield nearly four times smaller than the current standard estimate used by international health organizations. This suggests that producing this medical isotope using natural copper at this specific energy is more difficult and requires more powerful equipment than previously thought, though the researchers confirmed it remains technically possible with high-intensity beams.

However, the story is not one of failure but of clarification. When the researchers looked at the higher energy range, where the reaction is most efficient for medical production, their new data aligned perfectly with the current international standard. This agreement gives scientists confidence in the numbers used for making medical isotopes at those higher energies. Furthermore, the team compared their findings to the calculations used to predict the background noise in deep underground detectors. They found that the models used by physicists searching for dark matter and neutrinos were already accurate. The cross-section, a measure of how likely the reaction is to occur, used in those deep-earth calculations matches their new measurements, meaning those sensitive experiments do not need to be recalibrated based on this new data.

The study also reached out to the field of astrophysics, where scientists try to understand how heavy elements are forged in the violent environments of exploding stars. The rate at which copper turns into gallium in these cosmic furnaces is a key ingredient in computer models of the universe. The researchers found that their new measurements support the reaction rates currently used in these astrophysical models, particularly those that rely on sophisticated computer simulations of nuclear forces. This confirms that our understanding of how elements are created in the stars is on solid ground. By measuring the reaction directly and carefully, the team has removed a layer of uncertainty, ensuring that doctors, deep-earth explorers, and cosmic theorists all have the same reliable numbers to work with.

Technical Summary: Thick-target Yield of 65Cu(𝛼, 𝑛)68Ga Near Threshold and Implications for Nuclear Medicine, Deep Underground Detector Backgrounds, and Nucleosynthesis

Problem Statement
The nuclear reaction $^{65}\text{Cu}(\alpha, n)^{68}\text{Ga}$ is critical for three distinct fields: the production of the medical isotope $^{68}\text{Ga}$ , the estimation of neutron backgrounds in deep underground neutrino and dark matter detectors, and astrophysical nucleosynthesis calculations. However, existing data on the reaction's thick-target yield and cross-section near the threshold energy ( $E_\alpha \approx 6.14$ MeV) are inconsistent. Previous measurements, predominantly utilizing the stacked-foil activation method, suffer from compounded uncertainties in beam energy, energy loss, and straggling, leading to significant discrepancies in derived yields. Some evaluations suggest yields up to an order of magnitude higher than others, creating uncertainty regarding the feasibility of radioimpurity-free $^{68}\text{Ga}$ production using natural copper targets and the accuracy of background models for underground detectors.

Methodology
To resolve these discrepancies, the authors conducted direct thick-target yield measurements at the Edwards Accelerator Laboratory using the $^{3}\text{He}/\text{BF}_3$ Giant Barrel (HeBGB) neutron detector.

Experimental Setup: Helium ions ( $E_\alpha = 6.533 - 7.996$ MeV) were accelerated and directed onto a natural abundance copper target. The setup utilized a 4.5 MV T-type tandem Pelletron with a beam energy uncertainty of 0.1%.
Neutron Detection: Neutrons were detected via $^{3}\text{He}$ and $\text{BF}_3$ proportional counters embedded in a polyethylene moderator with nearly $4\pi$ solid-angle coverage. The detection efficiency was determined to be $\eta = 7.5 \pm 1.2\%$ .
Background Subtraction: A significant challenge was distinguishing the reaction signal from background neutrons generated by carbon contamination on the target. The authors measured yields using a $^{13}\text{C}$ target to characterize this background, scaling the results to subtract the carbon-induced neutron yield from the copper target data.
Analysis: Measured yields were compared against prior single-foil and stacked-foil data. Furthermore, Hauser-Feshbach calculations were performed using the Talys 2.0 code with various $\alpha$ -optical model potentials (OMP) to extrapolate the cross-section behavior up to $E_\alpha = 14$ MeV and to calculate astrophysical reaction rates.

Key Results

Measured Yields: The study reports new thick-target yields for $^{65}\text{Cu}(\alpha, n)^{68}\text{Ga}$ in the range $E_\alpha = 6.5 - 8$ MeV. These results align with prior single-foil irradiation data (Roughton et al. and Stelson & McGowan) but are substantially lower (by a factor of 3–6) than estimates derived from stacked-foil activation data and current IAEA evaluations in this energy regime.
Extrapolated Yield at 14 MeV: By anchoring Hauser-Feshbach calculations (specifically using $\alpha$ -OMP models 1–6, with model 3 as the central estimate) to the new low-energy data, the authors estimate the thick-target yield for $E_\alpha = 14$ MeV on 100%-enriched $^{65}\text{Cu}$ to be $161^{+5}_{-24}$ MBq/ $\mu$ Ah (for He $^{++}$ ). This value agrees well with the current IAEA estimate of 156 MBq/ $\mu$ Ah.
Yield at 8 MeV (Natural Copper): For $E_\alpha = 8$ MeV using a natural copper target (to avoid radioimpurities from $^{63}\text{Cu}$ ), the authors estimate a yield of $0.12 \pm 0.02$ MBq/ $\mu$ Ah. This is approximately four times lower than the IAEA estimate for these conditions.
Cross-Section and Backgrounds: The measured data support the cross-section estimates currently employed for neutrino and dark matter detector backgrounds (specifically those by Kudryavtsev et al.), confirming that existing background models are accurate.
Astrophysical Rates: The recommended reaction rate, derived from the Talys calculations anchored to the new data, agrees with rates calculated using the state-of-the-art Atomki-V2 $\alpha$ -optical potential within a factor of two over the 3–5 GK temperature range relevant for $\nu$ -driven wind nucleosynthesis.

Significance and Claims
The paper claims to resolve the long-standing discrepancy in $^{65}\text{Cu}(\alpha, n)^{68}\text{Ga}$ yield data near the threshold by providing direct neutron-counting measurements that validate single-foil results over stacked-foil extrapolations.

For Nuclear Medicine: The authors demonstrate that while the yield at $E_\alpha = 8$ MeV on natural copper is lower than previously estimated, it is technically feasible to produce radioimpurity-free $^{68}\text{Ga}$ for medical use. They calculate that a 5 mA beam current could produce approximately 210 MBq of $^{68}\text{Ga}$ , a quantity sufficient for medical imaging, provided standard target cooling and chemical separation protocols are employed.
For Detector Backgrounds: The work validates the cross-section estimates used in current models for deep underground detectors, suggesting that the neutron background contributions from copper components are accurately accounted for.
For Nucleosynthesis: The study confirms that the astrophysical reaction rates derived from modern $\alpha$ -optical potentials (such as Atomki-V2) are consistent with experimental constraints, providing a reliable basis for nucleosynthesis calculations involving copper isotopes.

The authors conclude that the uncertainty in the $^{65}\text{Cu}(\alpha, n)$ cross-section and reaction rate is approximately 25%, a level they deem sufficient for current applications in detector background modeling and astrophysical calculations.

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