Particle background characterization and prediction for the NUCLEUS reactor CE$ν$NS experiment

This paper presents a comprehensive prediction of particle-induced backgrounds for the NUCLEUS experiment at the Chooz nuclear power plant, demonstrating through environmental measurements and Geant4 simulations that the setup achieves sufficient background rejection to enable the detection of Coherent Elastic Neutrino-Nucleus Scattering with a signal-to-background ratio greater than one in the sub-keV energy range.

The Gist

Imagine you are trying to hear a single, tiny whisper in a room that is currently being blasted by a rock concert, a thunderstorm, and a construction crew all at once. That is essentially the challenge the NUCLEUS experiment is facing.

Here is a simple breakdown of what this paper is about, using everyday analogies.

The Goal: Catching the "Ghost" Whisper

The scientists want to detect a phenomenon called Coherent Elastic Neutrino-Nucleus Scattering (CEνNS).

• The Neutrino: Think of a neutrino as a ghost. It's a tiny particle that zips through everything without stopping. It rarely bumps into anything.
• The Interaction: When a neutrino does hit an atom in their detector, it gives the atom a tiny "nudge."
• The Problem: This nudge is incredibly weak. It's like a mosquito landing on a bowling ball. The ball (the atom) barely moves. The energy of this movement is so small it's measured in electron-volts (sub-keV), which is a billion times smaller than the energy of a visible light photon.

To hear this "whisper," the detector needs to be incredibly sensitive and, most importantly, very quiet.

The Location: A Noisy Basement

The experiment is located at the Chooz nuclear power plant in France.

• The Good News: They are right next to the reactor, so there are plenty of neutrinos (the "whisperers").
• The Bad News: They are in a basement that is very close to the surface (only about 3 meters of water equivalent of rock/earth above them).
• The Noise: Because they are so close to the surface, the "room" is filled with cosmic rays (particles from space), natural radioactivity from the concrete walls, and radiation from the nuclear reactor itself. It's like trying to hear that mosquito whisper while standing next to a jet engine.

The Solution: The "Fortress" Strategy

The paper details how the team built a "fortress" to block out the noise so they can hear the whisper. They used a multi-layered approach, like an onion or a set of Russian nesting dolls.

1. The Outer Walls (Passive Shielding)

They built a box around their detector using special materials:

• Lead (Pb): Like a heavy blanket to block gamma rays (high-energy light).
• Boron-loaded Plastic: Like a sponge that soaks up neutrons (the most dangerous noise for this experiment).
• The Catch: If you use too much heavy lead, the cosmic rays hitting the lead actually create more noise (secondary particles). So, they had to find the perfect thickness—just enough to block the outside noise, but not so much that it creates new noise inside.

2. The Active Guards (Veto Detectors)

This is the clever part. Instead of just blocking noise, they put "guards" around the detector that scream if they see anything suspicious.

• The Muon Veto: A plastic shield that detects cosmic rays (muons) hitting the outside. If it sees a hit, it tells the main detector, "Ignore everything happening right now!"
• The Cryogenic Outer Veto (COV): This is the star of the show. It's a ring of super-cooled Germanium crystals surrounding the main detector. It acts like a motion sensor. If a particle hits the guard before hitting the main detector, the guard triggers an alarm, and the main detector ignores that event.
• The Inner Veto: A tiny sensor right next to the detector to catch any "surface noise" (dust or particles touching the detector from the outside).

The Simulation: The "Digital Twin"

Before building the real thing, the scientists ran massive computer simulations (using a tool called Geant4).

• Imagine they built a perfect virtual replica of their basement, their shielding, and their detectors inside a computer.
• They then "shot" billions of virtual particles (neutrons, muons, gamma rays) at this virtual setup to see what would get through.
• They tested different wall thicknesses and guard placements to find the perfect recipe.

The Results: Can They Hear the Whisper?

After running the simulations and measuring the actual noise in the basement, here is what they found:

1. The Noise is Loud: Without their fortress, the background noise would be millions of times louder than the neutrino signal.
2. The Fortress Works: Their combination of heavy shielding and "screaming guards" reduces the noise by a factor of 100 to 1,000 (two orders of magnitude).
3. The Final Tally: Even with the best shielding, the biggest remaining noise comes from cosmic-ray neutrons sneaking through the thin roof of the basement.
4. The Verdict: Despite the noise, the signal-to-noise ratio is predicted to be greater than 1. This means they expect to hear the "whisper" (the neutrino signal) at least as clearly as they hear the background noise.

The "Low Energy Excess" Mystery

The paper also mentions a weird glitch called the "Low Energy Excess" (LEE).

• In many ultra-sensitive detectors, scientists see a sudden spike of tiny energy events at the very bottom of their range.
• It's like hearing a static hiss that doesn't seem to come from any known source.
• The paper admits this is a mystery. It might be a flaw in the detector design itself, not actual particles. Solving this is the next big challenge.

Summary

The NUCLEUS team has designed a highly sophisticated, multi-layered "noise-canceling headphone" system for a nuclear reactor basement. Their computer models predict that this system is good enough to finally hear the faint "nudge" of neutrinos hitting atoms, opening a new window into the universe's most elusive particles.

In short: They built a super-quiet room in a noisy basement, used smart sensors to ignore the noise, and calculated that they will finally be able to hear the ghostly whisper of neutrinos.

Technical Summary

1. Problem Statement

The NUCLEUS experiment aims to detect Coherent Elastic Neutrino-Nucleus Scattering (CEνNS) using reactor antineutrinos at the Chooz nuclear power plant in France. This detection presents significant challenges:

• Low Energy Signature: Reactor antineutrinos (max ~10 MeV) interacting with heavy nuclei produce nuclear recoils with energies typically below 1 keV, often in the 10–100 eV range.
• Shallow Overburden: The experimental site, the Very Near Site (VNS), is located in a basement less than 100 meters from the reactor core. It offers a very modest overburden of only ~3 meters water equivalent (m w.e.), providing minimal shielding against cosmic rays.
• Background Dominance: At such shallow depths, cosmic-ray-induced muons and neutrons, along with environmental gamma rays and intrinsic material radioactivity, create a background rate that can easily overwhelm the faint CEνNS signal.
• Mitigation Constraints: The experiment must fit within a small footprint room, limiting the size of passive shielding, while requiring ultra-low energy thresholds (<100 eV) where "Low Energy Excess" (LEE) backgrounds and material contamination are critical concerns.

2. Methodology

The authors employed a comprehensive strategy combining experimental characterization and Monte Carlo simulation to predict and mitigate backgrounds.

A. Experimental Characterization (Input Data)

The team conducted extensive measurements at the VNS to characterize the background environment:

• Cosmic Muons: Measured using a "cosmic wheel" apparatus (rotating plastic scintillators) to determine the muon flux and angular distribution. The measured attenuation factor was 1.41 ± 0.02, confirming the ~2.9 m w.e. overburden.
• Fast Neutrons: Characterized using Bonner spheres (polyethylene spheres with a LiI(Eu) thermal neutron detector) to measure the flux reduction of neutrons >10 MeV.
• Gamma-Ray Ambiance: Measured using a high-purity Germanium (HPGe) detector to identify natural radioactivity ($^{40}$K, $^{232}$Th, $^{238}$U) in the concrete walls and airborne radon levels.
• Material Screening: Components of the detector and shielding (lead, polyethylene, copper, B4C, etc.) were screened at the Stella facility (LNGS) using low-background HPGe detectors to quantify intrinsic radioactivity (primordial and cosmogenic).

B. Simulation Framework

• Tool: Extensive simulations were performed using Geant4 (v10.7.3).
• Geometry: A high-fidelity model of the VNS building, the shielding layers, and the cryogenic detectors was implemented.
• Physics Lists: Used the "Shielding" reference list for neutron transport and the "Livermore" constructor for electromagnetic interactions (crucial for sub-keV accuracy).
• Validation: The simulation was validated against the experimental measurements (muon attenuation, neutron flux reduction, and gamma spectra) to ensure accurate prediction of residual rates.

C. Shielding Strategy

The experiment utilizes a multi-layered approach:

1. Passive Shielding:
   • Outer: 5 cm plastic scintillator (Muon Veto), 5 cm low-radioactivity Lead (Pb), and 20 cm boron-loaded HDPE.
   • Inner (Cryostat): Additional Pb and HDPE layers, plus a 4 cm Boron Carbide (B4C) layer to capture thermal neutrons.
2. Active Veto Systems:
   • Muon Veto (MV): Outer plastic scintillator.
   • Cryogenic Outer Veto (COV): A hermetic array of HPGe crystals (2.5 cm thick) operating at mK temperatures surrounding the target detectors. This is critical for rejecting gamma rays and low-energy nuclear recoils.
   • Inner Veto (IV): Instrumented holder for the target detectors to reject surface events.

3. Key Contributions

• First Comprehensive Background Model for VNS: This paper provides the first detailed prediction of particle backgrounds for a CEνNS experiment at a shallow reactor site with such low overburden.
• Validation of Geant4 for Sub-keV Physics: The work validates the simulation framework's ability to model particle transport and energy deposition down to the 10–100 eV regime, a critical requirement for CEνNS detection.
• Optimization of the COV: The study demonstrates the pivotal role of the cryogenic HPGe veto (COV) in suppressing gamma and neutron backgrounds, allowing for a compact shielding design that fits the VNS constraints.
• Quantification of the "Low Energy Excess" (LEE) Context: While the paper focuses on particle backgrounds, it contextualizes the prediction against the known LEE issue, highlighting that the predicted particle background is manageable, leaving LEE as the primary remaining challenge.

4. Key Results

• Background Rejection Power: The combined shielding and veto system is predicted to achieve a total rejection power of >2 orders of magnitude (factor of 100–1000) for particle-induced backgrounds.
• Residual Background Rate: In the CEνNS signal region of interest (10–100 eV), the total predicted particle background rate in the CaWO$_4$ target detectors is ~250 d$^{-1}$ kg$^{-1}$ keV$^{-1}$.
• Dominant Background Source: The residual background is strongly dominated by cosmic-ray-induced fast neutrons (specifically those >10 MeV that penetrate the building and shielding).
  • Breakdown in 10–100 eV (CaWO$_4$):
    • Atmospheric Neutrons: ~131 counts/day/kg/keV
    • Material Radioactivity: ~7.1 counts/day/kg/keV
    • Environmental Gammas: ~6 counts/day/kg/keV
    • Atmospheric Muons: <14 counts/day/kg/keV
• Signal-to-Background Ratio (S/B): The predicted S/B ratio in the 10–100 eV region is $\gtrsim$ 1 (specifically ~0.9 to 1.5 depending on systematic uncertainties).
  • Expected CEνNS signal: ~218 d$^{-1}$ kg$^{-1}$ keV$^{-1}$.
  • This meets the experimental requirement for a viable detection.
• Sensitivity to Thresholds: The S/B ratio is highly sensitive to the COV energy threshold. A threshold of 1 keV$_{ee}$ is required to maintain S/B $\ge$ 1. Raising the threshold to 10 keV$_{ee}$ significantly degrades performance, though it remains acceptable.

5. Significance

• Feasibility Confirmation: The results confirm that detecting CEνNS from a reactor is feasible even at a very shallow site (3 m w.e.) provided a sophisticated combination of passive shielding and active cryogenic vetoes is used.
• Design Validation: The study validates the NUCLEUS design, proving that the compact shielding and the HPGe COV are sufficient to suppress the overwhelming cosmic-ray background to a level where the CEνNS signal can be extracted.
• Future Roadmap: The paper identifies the absolute normalization of the neutron flux at the VNS and the mitigation of the Low Energy Excess (LEE) as the remaining critical challenges. It sets the stage for the upcoming technical run at Chooz to refine the background model and validate the detector's performance in the sub-keV regime.
• Broader Impact: The methodology and simulation framework developed here serve as a blueprint for future low-threshold neutrino experiments operating in non-underground environments.