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The Radioactive Background of the JUNO Calibration System

This paper presents a comprehensive study of the radioactive background induced by the JUNO calibration system, demonstrating through material screening, Monte Carlo simulations, and in-situ data validation that its contribution is well-understood, consistent with predictions, and negligible (less than 76 mHz) compared to the design requirement of 200 mHz.

Original authors: Rui Li, Youhui Yun, Zhangming Chen, Junting Huang, Jiaqi Hui, Haojing Lai, Jianglai Liu, Yankai Liu, Yue Meng, Akira Takenaka, Ziqian Xiang, Feiyang Zhang, Ping Zhang, Qingmin Zhang, Tao Zhang, Yuanyu

Published 2026-08-20

📖 4 min read🧠 Deep dive

CC BY 4.0

Original authors: Rui Li, Youhui Yun, Zhangming Chen, Junting Huang, Jiaqi Hui, Haojing Lai, Jianglai Liu, Yankai Liu, Yue Meng, Akira Takenaka, Ziqian Xiang, Feiyang Zhang, Ping Zhang, Qingmin Zhang, Tao Zhang, Yuanyuan Zhang

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, shielded by hundreds of meters of mountain rock, scientists are listening for the faintest whispers from the heart of the sun and distant nuclear power plants. They are hunting for neutrinos, ghostly particles that pass through almost everything without leaving a trace. To catch them, researchers built a massive detector filled with 20,000 tons of a special, ultra-clean liquid that glows when a neutrino hits it. The goal is to determine the ordering of the neutrino masses and to precisely measure their oscillation parameters. However, this listening device is incredibly sensitive. Just as a whisper can be drowned out by the hum of a refrigerator, the tiny signals from neutrinos can be masked by the natural radioactivity found in the very materials used to build the detector. Even the tools scientists use to check if the detector is working correctly can introduce their own background noise, potentially confusing the precious data.

The Jiangmen Underground Neutrino Observatory, known as JUNO, faces a strict challenge: the total background noise from all sources must stay below a specific limit to ensure the neutrino signals remain clear. A significant part of this challenge involves the calibration system, a complex set of cables, sensors, and robotic units that move radioactive sources into the detector to test its accuracy. Because these components sit close to the liquid that catches the neutrinos, their own natural radioactivity could theoretically create false signals. The researchers behind this study set out to prove that the calibration system would not interfere with the experiment. They began by treating every single piece of the calibration machinery like a suspect in a crime scene investigation, but instead of looking for a criminal, they were looking for the faintest traces of radioactive elements like uranium, thorium, and potassium. They procured multiple batches of materials, such as stainless steel cables and plastic anchors, and measured them with extreme precision using high-purity germanium detectors located deep underground to block out cosmic rays. They found that some batches were far cleaner than others, and they selected only the purest materials for the final construction, discarding those that were even slightly too "dirty."

Once the cleanest materials were chosen, the team built a detailed digital model of the entire calibration system to simulate how its radioactivity would behave inside the detector. They calculated how many false signals these components would generate, taking into account the specific shapes of the cables and the position of the sensors. The simulation predicted that the total noise added by the entire calibration system would be less than 76 millihertz, a rate well below the allowed limit of 200 millihertz. To be absolutely certain, the scientists then turned to real data collected by the detector itself. They developed a clever method to separate the signals coming from the calibration cables and sensors from the general background noise of the liquid. By mapping the exact path of the cables and the specific locations of the sensors, they could isolate the events caused by these tools. The results from the real detector matched the predictions from the computer simulations perfectly. The actual noise generated by the cables, the ultrasonic receivers, and the guide tube sensors was consistent with what was expected, and in some cases, even lower.

The final conclusion is a reassuring one for the future of the experiment. The calibration system, which is essential for keeping the detector accurate, does not add enough noise to spoil the neutrino measurements. The extra background it introduces is so small that it has a negligible effect on the search for neutrino mass ordering. This work confirms that the careful selection of ultra-pure materials and the rigorous testing of every component were successful. It demonstrates that the JUNO team has mastered the art of building a low-background environment where the faintest cosmic signals can be heard clearly, free from the interference of the very tools used to listen for them.

Technical Summary: The Radioactive Background of the JUNO Calibration System

Problem Statement
The Jiangmen Underground Neutrino Observatory (JUNO) aims to determine the neutrino mass ordering and precisely measure oscillation parameters using a 20-kton ultra-pure liquid scintillator (LS) detector. A critical requirement for the reactor antineutrino analysis is maintaining the total single-event background rate from radioactivity below 10 Hz within the fiducial volume (radius < 17.2 m) in the energy range of 0.7–12 MeV. The calibration system, essential for correcting energy non-linearity and detector non-uniformity, introduces components (cables, sensors, anchors) that are either permanently installed within or in close proximity to the LS. These components pose a significant risk of contributing to the background budget. The design requirement stipulates that the calibration system's contribution must not exceed 2% of the total singles background (approximately 200 mHz).

Methodology
To address this challenge, the authors employed a multi-faceted approach combining rigorous material screening, Monte Carlo simulations, and in-situ data analysis:

Material Screening and Selection:
- Techniques: Components were assayed using High-Purity Germanium (HPGe) detectors at the China JinPing Underground Laboratory (CJPL) and Neutron Activation Analysis (NAA) at INFN Milano Bicocca. HPGe measured gamma rays from daughter nuclides to derive chain activities under secular equilibrium assumptions, while NAA directly measured parent U and Th nuclei to provide mass fractions insensitive to radon escape.
- Selection Strategy: Multiple batches of materials (e.g., stainless steel for Cable Loop System (CLS) cables, custom ultrasonic receivers) were tested. The batch with the lowest radioactivity (or best sensitivity upper limits) was selected. For instance, CLS cable batch #3 was chosen based on superior upper limits for $^{238}$ U, $^{232}$ Th, and $^{40}$ K.
- Components: Special attention was given to permanently installed elements: CLS cables and anchors, Ultrasonic Positioning System (USS) receivers, and Guide Tube Calibration System (GTCS) proximity sensors.
Monte Carlo Simulation:
- A Geant4-based simulation within the SNiPER framework modeled the detector geometry, including the specific spatial distribution of calibration components (e.g., catenary shapes for CLS cables, localized points for USS/GTCS).
- Radioactive decays ( $^{238}$ U, $^{232}$ Th, $^{40}$ K, $^{60}$ Co, $^{137}$ Cs) were generated uniformly within components. Early and late sub-chains of U/Th were treated independently to account for potential radon migration.
- Detector response (energy and position resolution) was modeled conservatively to bracket worst-case performance near the detector periphery.
- Survival probabilities ( $P$ ) were calculated after applying energy ( $E > 0.7$ MeV) and fiducial volume ( $R < 17.2$ m) cuts. The expected background rate was estimated using $R = A \times P$ , where $A$ is the activity.
In-Situ Data Analysis:
- Using the first 59 days of physics data (August 2025), the authors isolated calibration-related backgrounds using dedicated spatial selection methods.
- CLS Cables: Events were fitted to a catenary function describing the cable trajectory. A cylindrical signal region (1 m radius) around the fitted curve was compared against reference planes to subtract bulk LS background.
- USS and GTCS: Cylindrical selection regions (0.5 m radius) were defined around sensor positions. A data-driven subtraction method using a reference location (free of calibration components) isolated the excess events.
- Spectral Fitting: The residual spectra were fitted with templates of simulated energy spectra for specific isotopes to extract activity rates.

Key Contributions and Results

Material Characterization: The study successfully characterized the radioactivity of all calibration system materials. The selected batches showed significantly reduced radioactivity compared to typical commercial materials.
Background Estimation:
- The total estimated radioactivity contribution from the calibration system is less than 76 mHz (specifically < 75.95 mHz), which is well within the design requirement of 200 mHz (2% of the total budget).
- The breakdown shows CLS cables contribute < 38.35 mHz, GTCS sensors < 30.18 mHz, and USS receivers/cables < 7.35 mHz.
Accidental Background Impact: The increase in the accidental coincidence background (IBD mimicry) due to the calibration system is estimated at approximately $1 \times 10^{-3}$ events per day. This is negligible compared to the expected IBD signal rate of ~48 events per day.
Data-Simulation Agreement:
- CLS Cables: The data-driven fit yielded a total rate of 7.29 mHz, consistent with the conservative HPGe+MC prediction of < 25.19 mHz.
- USS Receivers: The extracted rate was $(3.24 \pm 0.10)$ mHz, consistent with the prediction of < 4.01 mHz.
- GTCS Sensors: The extracted rate was $(3.93 \pm 0.19)$ mHz, consistent with the prediction of < 30.18 mHz (noting that HPGe results were upper limits, making the prediction conservative).
- The in-situ results confirm that the background contributions are well understood and agree with predictions within uncertainties.

Significance
The paper demonstrates that the low-background design and material control strategies for the JUNO calibration system are effective. The calibration-induced background is proven to be negligible for reactor antineutrino measurements under standard IBD selection criteria, requiring no dedicated calibration-specific fiducial volume cuts. The methodology presented—combining rigorous material assay, detailed simulation, and spatially resolved in-situ validation—provides a validated reference for background assessment and material optimization in future low-background neutrino experiments.

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