Development of a Modular Current-Mode NaI(Tl) Detector Array for Parity Odd (n,{\gamma}) Cross Section Measurements

This paper presents the design, construction, and testing of a modular 24-detector NaI(Tl) array with custom pulse and current-mode electronics, developed by the NOPTREX Collaboration to measure parity-odd asymmetries in neutron-nucleus interactions, as demonstrated by the successful observation of the 0.7 eV parity-violating resonance in 139La at LANSCE.

The Gist

The Big Picture: Hunting for a "Ghost" in the Machine

Imagine you are trying to listen to a very quiet whisper (a fundamental force of nature called the Weak Interaction) in the middle of a roaring stadium (the Strong Interaction). In the world of atoms, the "whisper" is so faint that it's usually drowned out by the "roar."

However, scientists have found that at certain specific moments—when a neutron hits a nucleus just right—the whisper becomes loud enough to hear. This happens during a "resonance," like pushing a child on a swing at exactly the right moment to make them go higher.

The NOPTREX collaboration (a team of scientists from the US and Japan) built a special machine to listen for these whispers. Their goal? To find violations of Parity.

What is Parity?
Think of parity as a mirror. If you look in a mirror, your left hand becomes your right hand. Most laws of physics work the same way in a mirror. But the "Weak Interaction" is a rebel; it doesn't care about mirrors. It prefers one "handedness" over the other. The scientists want to prove this by watching how neutrons behave when they hit a target.

The Problem: Too Many Neutrons, Too Fast

To hear this whisper, you need a massive crowd of neutrons hitting a target. But here's the catch: when you have that many neutrons hitting at once, the detectors get overwhelmed.

• Pulse Mode (The Old Way): Imagine trying to count individual raindrops hitting a roof. If it's a light drizzle, you can count "one, two, three." But if it's a hurricane, you can't count individual drops; you just hear a continuous roar.
• Current Mode (The New Way): Because the neutron "storm" is so intense, the scientists couldn't count individual drops. Instead, they built a detector that measures the total weight of the rain (the electrical current) rather than counting drops. This allows them to handle the "hurricane" of neutrons without breaking.

The Solution: A Modular "Egg Carton" of Detectors

The team built a giant, modular array of 24 detectors made of NaI(Tl) (Sodium Iodide crystals).

The Analogy:
Imagine a giant, hollow egg carton made of 24 cups.

• The Center: In the very middle of the carton, they place the "target" (a block of Lanthanum-139 atoms).
• The Cups: The 24 cups surround the target. When a neutron hits the target, it gets absorbed and immediately spits out a burst of energy (gamma rays), like a firework exploding.
• The Job: The 24 cups act as a net to catch every piece of that firework. Because they are arranged in a ring, they can catch the explosion from almost every angle (covering about 11 "steradians," which is a fancy way of saying "a huge chunk of the sky").

How They Built It (The "Swiss Army Knife" of Detectors)

The paper details how they built this machine from scratch, which is impressive because they had to solve several tricky problems:

1. The Magnetic Shield (The Invisible Bubble):
   The experiment uses magnets to spin the neutrons. But magnets can mess up the electronics inside the detectors.

   • The Fix: They wrapped each detector in a special "mu-metal" shield (a nickel-iron alloy). Think of it like a Faraday cage for magnets. It creates a bubble where the magnetic field is zero, protecting the sensitive electronics inside while letting the light from the explosion pass through.

2. The Light Guide (The Fiber Optic Tube):
   The detectors use a device called a Photomultiplier Tube (PMT) to see the light. But the PMT hates magnets.

   • The Fix: They used a clear plastic "light guide" to connect the crystal to the PMT. It's like a fiber optic cable. They found the perfect length (0.75 inches) so the PMT was far enough away from the magnets to be safe, but close enough to catch all the light.

3. The "Blind" Switch (The Gain Blank):
   When the neutron beam starts, there is a blinding flash of gamma rays (like a camera flash going off right in your face). This could burn out the detectors.

   • The Fix: They built a switch that instantly "blinds" the detectors for a tiny fraction of a second during that initial flash, then turns them back on to catch the real signal.

The Test Runs: From Japan to New Mexico

Before using the machine for the big experiment, they tested it in two places:

1. J-PARC (Japan): They tested two detectors in a neutron beam. They successfully saw the "signature" of the Lanthanum atoms, proving the machine could "hear" the neutrons.
2. LANSCE (New Mexico, USA): This was the main event. They used the full array of 24 detectors.
   • The Result: They successfully measured the "parity violation" in Lanthanum. They saw that when the neutrons were spinning one way, the reaction was slightly different than when they spun the other way.
   • The Proof: The data showed a clear "wiggle" (asymmetry) at the exact moment the neutrons hit the 0.7 eV resonance. It was like seeing a specific note in a song that only plays when the singer is facing left, but not when facing right.

Why Does This Matter?

This isn't just about counting neutrons.

• The "Whisper" is Key: By measuring these tiny differences, scientists learn more about the Weak Force, which helps explain why the universe is made of matter and not just antimatter.
• Future Tech: The design is "modular." This means they can take the array apart and use it for other experiments, like searching for other types of symmetry violations or even testing new theories about how neutrons behave with "twisting" wave packets (a very advanced physics concept).

Summary

The NOPTREX team built a 24-detector "net" that can survive a neutron hurricane by measuring current instead of counting. They wrapped it in magnetic armor and gave it a quick blindfold to survive the initial flash. They tested it in Japan and New Mexico, and it worked perfectly, proving they can now hunt for the universe's deepest secrets: why the laws of physics aren't perfectly symmetrical.

Technical Summary

1. Problem Statement

The primary scientific goal is to measure Parity Violation (PV) in neutron-nucleus interactions, specifically the weak interaction between hadrons. While the weak interaction is approximately seven orders of magnitude smaller than the strong interaction, PV effects are dramatically enhanced near p-wave neutron-nucleus resonances due to interference between s-wave and p-wave amplitudes.

Two main challenges exist in measuring these effects:

• Target Limitations: Traditional transmission asymmetry measurements require thick targets (kilograms of material), limiting studies to abundant isotopes.
• High Event Rates: Direct measurement of helicity-dependent neutron capture cross-sections ($\sigma_{(n,\gamma)}$) requires high statistical precision. This necessitates high neutron fluxes, which often result in event rates too high for standard "pulse counting" mode due to detector deadtime and pulse pileup.
• Background Rejection: In high-rate environments, individual event energy information is often lost when switching to "current mode" (integrating the signal). This makes standard background subtraction difficult. However, since parity violation is a spin-dependent effect, background processes (which are spin-independent) cancel out in the asymmetry measurement, provided the apparatus is stable.

2. Methodology and Design

The NOPTREX collaboration developed a modular array of 24 NaI(Tl) detectors designed to operate in both pulse and current modes to address these challenges.

A. Detector Array Design

• Geometry: The array consists of two square rings, each containing 12 NaI(Tl) crystals ($3'' \times 3'' \times 5''$), arranged to cover approximately 11 steradians of solid angle around the target.
• Materials:
  • Scintillators: 24 refurbished NaI(Tl) crystals (38 photons/keV light yield, ~230 ns decay time) chosen for high efficiency in the 6–8 MeV gamma cascade range typical of neutron capture.
  • Photomultipliers (PMTs): Hamamatsu R550 PMTs coupled to the crystals via custom lightguides.
  • Shielding: The housing uses mu-metal (nickel-iron alloy) to shield PMTs from external magnetic fields (critical for preserving electron trajectories) and lead/borated polyethylene to block external neutrons and gammas.
• Optical Coupling: A custom lightguide design (0.75" length) was optimized to position the PMT photocathode deep within the magnetic shield while keeping the anode accessible, minimizing magnetic field sensitivity.

B. Electronics and Data Acquisition

• Dual-Mode Operation: Custom electronics boards allow operation in:
  • Pulse Mode: For energy resolution and calibration.
  • Current Mode: For high-flux measurements where individual pulses cannot be resolved. The signal is integrated into a continuous current proportional to the instantaneous gamma flux.
• Gain Control: A unique feature allows the electronic gain to be "blanked" (disabled) during the intense initial gamma flash from the spallation source to prevent PMT saturation.
• DAQ System: Utilized CAEN digitizers (DT5560SE at JPARC, DT5740 at LANSCE) with custom FPGA firmware. The system performs real-time decimation (downsampling) to manage data rates, sampling at ~1.95 MS/s to capture the time-of-flight (TOF) spectrum over 5.12 ms per neutron pulse.

C. Experimental Setup

• Beamlines: Testing occurred at JPARC (Japan) and the final experiment at LANSCE (USA).
• Target: A polarized neutron beam (spin flipper upstream) interacts with a target (e.g., $^{139}\text{La}$).
• Spin Transport: A 48-inch aluminum tube wrapped in copper coils generates a magnetic field to maintain neutron polarization from the flipper through the detector array to the target.
• Neutron Shielding: The target region is surrounded by $^6\text{Li}$-rich material to absorb scattered neutrons, preventing activation of the NaI detectors.

3. Key Contributions

1. Custom Modular Array: Successful construction of a 24-detector NaI(Tl) array with integrated magnetic shielding and custom current-mode electronics.
2. Dual-Mode Capability: Demonstration of a system capable of switching between high-resolution pulse counting and high-rate current integration, essential for PV experiments requiring high flux.
3. Optimized Magnetic Shielding: Detailed characterization of PMT lightguide lengths and mu-metal placement to ensure detector stability in the presence of magnetic fields required for neutron spin manipulation.
4. Background Suppression Strategy: Validation that current-mode detection, combined with spin-flip asymmetry measurements, effectively isolates parity-violating signals from background noise without needing individual event energy reconstruction.

4. Results

• Detector Characterization:
  • Energy resolution at 662 keV ($^{137}\text{Cs}$) averaged 18 ± 3% across all 24 detectors.
  • MCNP simulations confirmed that adding corner crystals increased the intrinsic efficiency ($\epsilon_{int}$) by ~12% at 2 MeV, reaching ~75% total array efficiency.
• JPARC Test Run:
  • Successfully observed the known s-wave and p-wave resonances of $^{139}\text{La}$ in current mode.
  • The 0.7 eV p-wave resonance was clearly identified at the expected time-of-flight (1.14 ms), validating the timing and integration capabilities.
• LANSCE Parity Violation Experiment:
  • Collected ~75 hours of data with a polarized beam on a $^{139}\text{La}$ target.
  • Observation: A clear parity-odd asymmetry was observed at the 0.7 eV p-wave resonance (1.96 ms in the spectrum), while no asymmetry was seen at the s-wave resonance.
  • This confirmed the array's ability to detect PV effects and validated the absence of significant systematic errors.

5. Significance

• Enabling Rare Isotope Studies: By using the current-mode approach with thin, isotopically enriched targets, this array enables PV studies on rare isotopes where large target masses (required for transmission methods) are unavailable.
• Future Physics: The modular design allows for future searches for Time-Reversal (T) violation and CP symmetry violation.
• New Kinematic Regimes: The array is well-suited for future experiments involving "twisting" neutron wave packets (orbital angular momentum), which could overcome traditional angular momentum barriers in neutron-nucleus scattering and potentially enhance resonance amplitudes.
• Technological Advancement: The successful revival and integration of refurbished NaI crystals with custom electronics demonstrates a cost-effective path to building large-scale, high-efficiency gamma detector arrays for nuclear physics.

In conclusion, the NOPTREX collaboration has successfully developed and validated a robust, modular detector array capable of measuring parity violation in neutron capture reactions, opening new avenues for studying fundamental symmetries in nuclear physics.