⚛️ nuclear experiments

Measurement of the $^{39}$ Ar specific activity in atmospheric argon with DArT at the Canfranc Underground Laboratory

Using the DArT detector at the Canfranc Underground Laboratory, researchers measured the specific activity of $^{39}$ Ar in atmospheric argon to be $0.955 \pm 0.008~\text{Bq}/\text{kg}$ , establishing the most precise determination to date and validating the detector for future dark matter experiments.

Original authors: P. Agnes, H. O. Back, S. Bharat, W. M. Bonivento, M. Boulay, M. Caboni, N. Canci, N. Cargioli, P. Castello, S. Cebrián, L. Consiglio, R. Crampton, D. Diaz Mairena, D. Franco, I. Fuente-Ortega, F. Gabr

Published 2026-10-01

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CC BY 4.0

Original authors: P. Agnes, H. O. Back, S. Bharat, W. M. Bonivento, M. Boulay, M. Caboni, N. Canci, N. Cargioli, P. Castello, S. Cebrián, L. Consiglio, R. Crampton, D. Diaz Mairena, D. Franco, I. Fuente-Ortega, F. Gabriele, D. Gahan, C. Galbiati, P. Garcia Abia, T. Hessel, S. Horikawa, A. Ianni, M. Kuźniak, L. Luzzi, M. Martínez, P. Organtini, A. Ortiz de Solórzano, R. Pavarani, P. Pegoraro, V. Pesudo Fortes, M. Razeti, O. U. Rehman, L. Romero, D. Sablone, E. Sánchez García, R. Santorelli, C. Seoane, J. Sosiak, S. Sulis, C. Tierney, S. Tullio, G. Vera Díaz

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

Deep beneath the Earth's surface, in a quiet laboratory carved into the Pyrenees mountains of Spain, scientists are listening for the faintest whispers of the universe. They are searching for dark matter, the invisible substance that makes up most of the mass in the cosmos but refuses to interact with light. To catch a glimpse of it, researchers use tanks filled with liquid argon, a noble gas that glows briefly when struck by a particle. This glow acts as a signal, a flash of light that tells a detector a collision has occurred. However, there is a problem with the argon we breathe. The argon found in our atmosphere contains a tiny amount of a radioactive isotope called argon-39. This isotope acts like a constant, low-level static noise, emitting electrons that can mimic the very signals scientists are trying to find. To hear the faint whisper of dark matter, they need to know exactly how loud this static is, and they need to find a source of argon that is completely free of it.

A team of researchers recently conducted a precise measurement to determine exactly how much of this radioactive noise exists in atmospheric argon. They used a small, highly sensitive detector called DArT, which sits inside a cryostat filled with liquid nitrogen to keep the argon cold and liquid. The experiment was designed to be as clean as possible, using a unique reference point: a sample of "underground argon." This special gas was extracted from deep wells in Colorado, where it has been isolated from the atmosphere for millions of years, meaning it has almost no radioactive argon-39 left in it. By comparing the noise level in the atmospheric argon against the near-silent underground argon, the team could measure the specific activity of the radioactive isotope with unprecedented accuracy.

The researchers filled their detector first with the atmospheric argon and then with the underground sample. Inside the detector, the liquid argon acts as both the source of the radiation and the medium that captures the light. When an argon-39 atom decays, it releases an electron that creates a flash of light. The detector, equipped with ultra-sensitive light sensors, counts these flashes. Because the underground argon has such a low level of radioactivity, it served as a perfect background reference. The team did not need to rely on complex computer simulations to guess what the background noise should look like; they could simply subtract the signal from the underground sample from the signal of the atmospheric sample. This direct comparison removed many of the uncertainties that usually plague such measurements.

The study yielded a very precise number for the specific activity of argon-39 in the atmosphere: 0.955 decays per second for every kilogram of argon. This result is consistent with previous measurements but is significantly more precise, reducing the uncertainty by a factor of three compared to the best previous data. The researchers confirmed this finding using two different methods: a straightforward counting technique and a more complex statistical analysis of the energy spectrum. Both approaches led to the same conclusion, reinforcing the reliability of the result. They also carefully checked for other sources of interference, such as nitrogen leaks or other radioactive contaminants, and found that their detector remained stable and pure throughout the long data-taking periods.

This measurement is a critical piece of the puzzle for future experiments that aim to detect dark matter. Projects like DarkSide-20k and LEGEND-1000 plan to use hundreds of tons of liquid argon to hunt for dark matter particles. For these massive detectors to work, they must use argon that has been purified to remove the radioactive argon-39. Knowing the exact starting amount of radioactivity in the raw atmospheric gas allows scientists to calculate how much purification is needed and to predict the background noise levels in their final experiments with confidence. The success of the DArT detector in this test also validates the technology that will be used in the larger underground facilities, proving that the sensors and the cryogenic systems can operate reliably in the harsh conditions required for these deep-space investigations.

The findings also have implications beyond particle physics. Argon-39 is used to date groundwater and ice, helping scientists understand climate history and water movement over timescales of decades to a thousand years. The uncertainty in the atmospheric activity level directly affects the accuracy of these dates. By pinning down this value with such precision, the researchers have provided a more reliable reference for dating young water samples. The work demonstrates that by using a simple, direct comparison between two gas samples—one noisy and one quiet—scientists can strip away the guesswork and reveal the true nature of the materials that make up our world and the universe beyond.

Technical Summary: Measurement of the $^{39}$ Ar Specific Activity in Atmospheric Argon with DArT

Problem Statement
Liquid argon (LAr) is a premier target for direct dark matter searches due to its scalability and pulse-shape discrimination capabilities. However, atmospheric argon (AAr) contains the cosmogenic isotope $^{39}$ Ar, a $\beta$ -emitter with an activity of approximately 1 Bq/kg. This intrinsic background limits the sensitivity of large-scale experiments. While previous measurements of the specific activity ( $a_{AAr}$ ) exist, future experiments involving hundreds or thousands of tonnes of LAr require a precise determination of this value, as small uncertainties in specific activity translate into large uncertainties in total radioactive decay rates. Furthermore, $^{39}$ Ar is used for dating groundwater and ice, where reference uncertainties propagate directly into calculated ages.

Methodology
The authors utilized the DArT detector, a compact, single-phase liquid-argon instrument located at the Canfranc Underground Laboratory (LSC) in Spain. DArT features a cylindrical ultra-pure Oxygen-Free High-Conductivity (OFHC) copper vessel containing an active volume of $\sim$ 169 mm height and $\sim$ 85.5 mm diameter, lined with wavelength-shifting TPB and read out by two cryogenic silicon photomultipliers (SiPMs).

The measurement strategy relied on a differential approach to minimize dependence on background simulations:

Reference Sample: The detector was first filled with a sample of underground argon (UAr) from the DarkSide-50 experiment. This sample has a known, negligible $^{39}$ Ar activity ( $0.71 \pm 0.11$ mBq/kg) and serves as a low-background reference.
Target Sample: The detector was subsequently filled with atmospheric argon (AAr).
Calibration and Monitoring: The detector response was calibrated using a $^{137}$ Cs source to establish the energy scale and light yield ( $\sim$ 0.39 PE/keV). Purity was monitored by measuring the scintillation triplet time constant ( $\tau_{slow}$ ) to ensure nitrogen contamination remained below the ppm level.
Analysis Techniques: Two independent methods were employed to extract the specific activity:
- Cut-and-Count: A direct subtraction of the UAr event rate (scaled by live-time) from the AAr event rate within a defined energy window ( $<250$ PE), corrected for selection efficiency derived from simulation.
- Binned Maximum-Likelihood Fit: An extended fit of the $Q_{tot}$ (total charge) spectrum in the range 5–300 PE, treating the UAr spectrum as a background component and the $^{39}$ Ar signal as a simulated template. Systematic uncertainties regarding spectrum shape were addressed using phenomenological corrections to the charge response.

Key Contributions

Robust Background Subtraction: By using a physically measured UAr sample as a reference rather than relying solely on background simulations, the measurement reduces systematic uncertainties associated with modeling.
Detector Validation: The paper validates the DArT detector design, including the cryogenic SiPM readout and mechanical structure, establishing it as a key component for the DArTInArDM project.
Comprehensive Systematic Evaluation: The study provides a detailed breakdown of systematic uncertainties, including SiPM gain calibration, light collection asymmetry, electronic noise, and potential radon contamination.

Results
The study reports the specific activity of $^{39}$ Ar in atmospheric argon using both analysis methods, which yielded consistent results:

Cut-and-Count Method: $a_{AAr} = 0.955 \pm 0.003 \text{ (stat)} \pm 0.007 \text{ (syst)} \text{ Bq/kg}$ .
Likelihood Fit Method: $a_{AAr} = 0.954 \pm 0.015 \text{ Bq/kg}$ (total uncertainty).

The authors select the cut-and-count result as the final value due to its smaller dependence on the theoretical shape of the $^{39}$ Ar $\beta$ -spectrum. The final result is:
$a_{AAr} = 0.955 \pm 0.008 \text{ Bq/kg}$

This value is consistent with previous measurements from experiments such as DEAP-3600, WARP, and ArDM.

Significance
The paper claims this result provides the most precise determination of the $^{39}$ Ar specific activity in atmospheric argon to date, with a total uncertainty approximately three times smaller than the previous most precise measurement (DEAP-3600). This precision is critical for:

Dark Matter Experiments: Informing the design of trigger systems, background models, and sensitivity projections for upcoming large-scale LAr experiments like DarkSide-20k and LEGEND-1000.
Argon Purification: Validating the DArT detector's capability to assay the $^{39}$ Ar activity of argon extracted from deep underground wells (e.g., in Colorado) for use in future experiments.
Geochronology: Providing a refined reference value for dating groundwater and ice using $^{39}$ Ar.

The authors emphasize that the result validates the DArT detector as a reliable tool for future radioactivity assaying in the context of the DArTInArDM project.

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