Reactor-based Search for Axion-Like Particles using CsI(Tl) Detector

This paper presents a reactor-based search for axion-like particles using a low-background $\sim100~\mathrm{kg}$ CsI(Tl) detector, demonstrating the potential to probe previously unexplored parameter spaces for ALP masses between 1 keV and 10 MeV, including the cosmological triangle.

The Gist

Imagine the universe is a giant, dark ocean. For decades, scientists have been trying to find a specific, elusive fish in this ocean called Dark Matter. They've been using big nets (experiments) to catch heavy, slow-moving fish called WIMPs, but so far, the nets have come up empty.

Now, a team of scientists at Texas A&M University is trying a different approach. Instead of looking for heavy fish, they are hunting for something much lighter and faster: a ghostly particle called an Axion (or an Axion-Like Particle, ALP). These particles are like invisible whispers that might explain why the universe exists the way it does and solve a major puzzle in physics called the "Strong CP problem."

Here is how they went about hunting these ghosts, explained simply:

1. The Hunting Ground: A Nuclear Reactor

Instead of waiting for these particles to drift in from deep space, the scientists decided to make them. They set up their experiment next to a nuclear reactor (a machine that splits atoms to create energy).

Think of the reactor as a giant, chaotic lightbulb. Inside, atoms are splitting apart, creating a massive storm of invisible light particles (photons). The scientists believe that when these photons crash into the reactor's metal walls, they might accidentally "bump" into the fabric of space and turn into Axions. It's like shaking a snow globe so hard that a few snowflakes turn into tiny, invisible fairies.

2. The Trap: A Giant Crystal Box

To catch these invisible fairies, the team built a detector. Imagine a 100-kilogram (220-pound) block of glowing ice made of a special crystal called CsI(Tl).

• How it works: If an Axion flies into this crystal, it might turn back into a flash of light (a photon) or a tiny electron. The crystal is like a glow-in-the-dark trampoline; when the invisible particle hits it, the trampoline lights up.
• The Eyes: Attached to the crystals are super-sensitive cameras (called Photomultiplier Tubes) that can see even a single photon of light, like a night-vision camera that can spot a firefly in a pitch-black room.

3. The Shield: A Fortress Against Noise

The problem with hunting ghosts is that the world is full of "noise." Radioactive rocks, cosmic rays from space, and even the reactor itself are constantly bombarding the detector with real light and particles. It's like trying to hear a whisper in a rock concert.

To solve this, the scientists built a fortress:

• Passive Shielding: They wrapped the crystal box in thick layers of lead and copper (like a heavy, lead-lined safe). This blocks out most of the outside noise.
• Active Shielding (The Bouncer): This is the clever part. They arranged the crystals in a grid (5 by 5). The inner 9 crystals are the "VIP area" where they look for the Axion. The outer 16 crystals act as bouncers.
  • If a particle hits the outer bouncers at the same time as the inner VIPs, the bouncers shout, "Fake! That's just background noise!" and the computer ignores the event.
  • If a particle hits only the inner VIPs, the computer says, "Okay, this might be a real Axion!" and records it.

4. The Air Purge: Blowing Away the Smoke

They also noticed a specific type of "smoke" (a radioactive gas called Argon-41) created by the reactor that was messing up their readings. To fix this, they installed an air-purge system, essentially a giant fan blowing fresh, clean air over the detector to blow the radioactive smoke away, much like opening a window to clear out cigarette smoke.

5. The Results: A Quiet Room

After running the experiment for a while, they achieved something amazing: they created a super-quiet room.

• They managed to lower the background noise to a level where they could hear the faintest whispers (signals) in the MeV energy range.
• They didn't find the Axions yet (the fish haven't been caught), but they successfully proved that their "net" is sensitive enough to catch them if they are there.

Why This Matters

This experiment is like opening a new door in the Dark Matter house.

• Previous experiments looked for heavy Axions or very light ones.
• This experiment is looking in the middle range (between 1 keV and 10 MeV), a region that was previously a "no-man's land" because it was too hard to detect.
• They found that their setup is sensitive enough to probe a specific area called the "Cosmological Triangle," which is a theoretical sweet spot where Axions might actually be the Dark Matter that holds the universe together.

In summary: The team built a super-sensitive, crystal-based "ear" next to a nuclear reactor, wrapped it in a lead fortress with bouncer-crystals, and successfully silenced the noise. While they haven't caught the Axion yet, they have shown that their method works and can now listen for these particles in a part of the universe that no one has been able to hear before.

Technical Summary

1. Problem Statement

Despite extensive searches for Weakly Interacting Massive Particles (WIMPs), conclusive dark matter signals remain elusive. This has shifted focus toward alternative candidates like Axions and Axion-Like Particles (ALPs), which also solve the Standard Model's strong CP problem. While various experiments (helioscopes, haloscopes, beam-dumps) cover wide parameter spaces, a significant gap remains in the MeV mass range ($1 \text{ keV} < m_a < 10 \text{ MeV}$). Existing constraints in this region are sparse, particularly regarding the coupling between ALPs and photons ($g_{a\gamma\gamma}$) and electrons ($g_{ae}$). The challenge lies in detecting these rare interactions amidst high background radiation from nuclear reactors and the environment.

2. Methodology

Experimental Setup

• Location: The experiment was conducted at the Texas A&M University Nuclear Science Center (NSC) using a 1 MW TRIGA research reactor.
• Source: The reactor core produces an intense flux of photons ($\sim 9 \times 10^{11} \text{ photons/cm}^2/\text{s}$) via fission. These photons generate ALPs through Primakoff conversion ($\gamma + A \to a + A$), Compton-like scattering, and nuclear de-excitation.
• Detector: A scalable $\sim 100 \text{ kg}$ CsI(Tl) scintillation detector was employed.
  • Configuration: Initially a $3 \times 3$ prototype (9 crystals, $\sim 3.5 \text{ kg}$ fiducial mass), upgraded to a $5 \times 5$ array (25 crystals, $\sim 31.5 \text{ kg}$ fiducial mass).
  • Readout: Each crystal is coupled to a Photomultiplier Tube (PMT). The system uses a CAEN V1740D digitizer for real-time pulse processing.
  • Performance: The detector achieves a low energy threshold of $\sim 60 \text{ keV}$ and linear energy response up to $1.3 \text{ MeV}$.

Background Mitigation

To achieve the required sensitivity, a multi-layered background suppression strategy was implemented:

1. Passive Shielding: The detector was encased in 4 inches of lead (selected for low intrinsic radioactivity) and, in early runs, copper and water bricks.
2. Active Veto (Anti-coincidence): The outer layer of crystals in the array acts as a veto. Events depositing energy in the central "fiducial" crystals and the outer "veto" crystals within a $10 \mu\text{s}$ window are rejected. This isolates single-scatter events (expected for ALPs) from background multi-scatter events.
3. Air Purge: An air-purge system was installed to remove airborne radioactive isotopes, specifically $^{41}\text{Ar}$ (produced by neutron activation), which emits a characteristic $1.29 \text{ MeV}$ gamma ray.

Data Acquisition

• Duration: Data was collected during 2021–2022, yielding 2 days of Reactor-ON data and 7 days of Reactor-OFF data.
• Analysis: The Reactor-OFF spectrum serves as the background baseline. The signal is defined as the excess in the Reactor-ON spectrum after applying active veto cuts.

3. Key Contributions

• Novel Detection Strategy: Successfully demonstrated a reactor-based search for MeV-scale ALPs using a large-mass CsI(Tl) detector, a configuration distinct from traditional beam-dump or haloscope experiments.
• Background Reduction: Achieved a sub-100 DRU (Differential Rate Unit) background level in the MeV energy range through the combination of passive shielding and the single-scatter veto technique.
• Scalability: Validated a modular $5 \times 5$ detector architecture that allows for mass scaling (up to $\sim 100 \text{ kg}$ in this phase, with potential for $\sim 1000 \text{ kg}$ in future iterations) while maintaining background rejection efficiency.
• Air Purge Implementation: Demonstrated the efficacy of active air circulation in suppressing specific reactor-induced background lines ($^{41}\text{Ar}$).

4. Results

• Background Levels: The experiment achieved background levels of $\sim 0.1\text{--}10 \text{ DRU}$ in the $2\text{--}10 \text{ MeV}$ energy range after applying the anti-coincidence cut.
• Signal Observation: A clear separation between Reactor-ON and Reactor-OFF spectra was observed, with the ON spectrum showing the expected excess consistent with the reactor photon flux.
• Exclusion Limits: Using a single-energy-bin analysis with a $90\%$ Confidence Level (CL), the experiment established new exclusion limits on ALP couplings:
  • Photon Coupling ($g_{a\gamma\gamma}$): Sensitivity reaches $g_{a\gamma\gamma} \gtrsim 10^{-6}$ for ALP masses between $1 \text{ keV}$ and $10 \text{ MeV}$.
  • Electron Coupling ($g_{ae}$): Sensitivity covers the range $10^{-8} < g_{ae} < 10^{-4}$.
• Parameter Space Coverage: The results probe the "cosmological triangle" region for MeV-scale ALPs, a parameter space previously unexplored by direct detection experiments.

5. Significance

• Complementarity: This work complements existing constraints from astrophysical observations (e.g., SN1987A, Horizontal Branch stars) and other laboratory experiments (CAST, beam-dumps), filling a critical gap in the MeV mass range.
• Feasibility for Future Searches: The study proves that reactor-based searches with large-mass scintillators are a viable and powerful method for probing ALP parameter space.
• Future Potential: The authors project that by moving the detector closer to the core ($\sim 2 \text{ m}$) and scaling the mass to $\sim 1000 \text{ kg}$ with a 3-year data-taking period, the experiment could reach the cosmological triangle boundary, potentially discovering ALPs that constitute dark matter or solve the strong CP problem.

In summary, this paper presents a successful proof-of-concept for a reactor-based ALP search, demonstrating the ability to suppress backgrounds to levels that allow sensitivity to previously inaccessible regions of the ALP parameter space.