Characterization of GS20 and CLYC Detectors for Neutron Resonance Transmission Analysis in High Radiation Environments

This paper demonstrates that while GS20 is a proven portable neutron detector, CLYC detectors are superior for neutron resonance transmission analysis in high gamma-radiation environments typical of thorium-based safeguards due to their strong pulse-shape discrimination capabilities, which yield significantly more precise measurements despite longer decay times.

The Gist

Imagine you are trying to identify a specific type of coin hidden inside a thick, heavy box. You can't open the box, but you have a special flashlight that shoots out tiny, invisible "neutron bullets." When these bullets hit the coin, they get absorbed in a very specific way, creating a unique "shadow" or pattern that tells you exactly what the coin is. This is the basic idea behind Neutron Resonance Transmission Analysis (NRTA), a high-tech method used to inspect nuclear materials without breaking the seal.

However, there's a catch. In the world of advanced nuclear reactors (specifically those using Thorium), the "box" isn't just heavy; it's also glowing with an intense, blinding light called gamma radiation. This radiation is so strong that it acts like a blizzard of snow, making it hard to see the tiny shadows cast by your neutron bullets.

This paper is a race between two different "cameras" (detectors) trying to take a clear picture of the coin in the middle of this blizzard.

The Two Contenders

The researchers tested two types of detectors to see which one could handle the job best:

1. The Speedster: GS20 (The "Fast but Blurry" Camera)

• What it is: A glass detector that is very fast at reacting to neutrons.
• The Good: It's quick and efficient at catching neutrons.
• The Bad: It's easily confused. When the intense gamma radiation (the blizzard) hits it, the detector gets overwhelmed. It can't easily tell the difference between a neutron bullet and a gamma ray. It's like trying to hear a whisper in a rock concert; the background noise drowns out the signal.
• The Result: When the "blizzard" got heavy, this detector's measurements became shaky and less accurate.

2. The Smart Filter: CLYC (The "Slow but Sharp" Camera)

• What it is: A crystal detector that is a bit slower to react but has a superpower called Pulse Shape Discrimination (PSD).
• The Good: This is the magic trick. Even though it's slower, it can look at the shape of the signal it receives. It knows: "Oh, this squiggly line is a gamma ray, I'll ignore it. This sharp spike is a neutron, I'll count it." It's like having a bouncer at a club who can instantly tell the difference between a VIP guest (neutron) and a rowdy fan (gamma ray) and only let the VIP in.
• The Bad: It's physically slower (takes longer to reset after a hit) and has a tiny flaw: the crystal itself contains a tiny bit of a different element (Cesium) that creates its own tiny "shadows" which could theoretically confuse the reading.
• The Result: Despite being slower and having that tiny flaw, it was incredibly good at ignoring the blizzard. It kept taking perfect, sharp pictures even when the radiation was at its worst.

The Experiment: The "Fake" Blizzards

The researchers set up a 2-meter long track (a "flight path") and fired neutrons at a piece of Tungsten metal (the "coin").

• Round 1 (Clean Air): They measured the metal with no extra radiation. Both cameras did a decent job.
• Round 2 (The Blizzard): They added a powerful radioactive source (simulating the dangerous environment of Thorium fuel) to blast the detectors with gamma rays.

The Outcome:

• GS20 (The Speedster): As the gamma radiation increased, its measurements got messy. The "error bars" (the margin of doubt) grew huge. It was struggling to see the wood for the trees.
• CLYC (The Smart Filter): It didn't even flinch. Its measurements remained just as precise as they were in the clean air. It successfully filtered out the noise and gave the exact same answer as before.

The Big Takeaway

The paper concludes that for inspecting advanced nuclear fuel (like Thorium-based reactors), speed isn't everything.

While the fast detector (GS20) is great for quiet environments, the "smart" detector (CLYC) is the winner for high-radiation environments. Even though it's slower and has a tiny internal quirk, its ability to ignore the noise makes it far more reliable.

The Analogy:
Imagine you are trying to count how many people are walking through a doorway.

• GS20 is a person who counts everyone who walks through. If a storm starts throwing leaves at the door, they start counting leaves as people, messing up the count.
• CLYC is a person who looks at how the person walks. They know a leaf flutters differently than a human walks. Even in the storm, they only count the humans.

Why does this matter?
This technology is crucial for international safety inspectors. They need to verify that countries aren't secretly building weapons. With new Thorium reactors, the fuel is so radioactive that old inspection tools fail. This paper proves that using "smart" detectors like CLYC allows inspectors to see clearly through the radiation, ensuring nuclear safety even in the most dangerous environments.

Technical Summary

1. Problem Statement

The development of advanced nuclear reactors utilizing the thorium fuel cycle presents significant challenges for international nuclear safeguards. Unlike conventional uranium-fueled systems, thorium cycles produce $^{232}$U as a byproduct. The decay chain of $^{232}$U includes $^{208}$Tl, a potent gamma emitter (~2.6 MeV), creating an intense radiological environment.

This high gamma background complicates existing Non-Destructive Assay (NDA) techniques:

• Passive Gamma Spectroscopy: Saturated by the high background.
• Active Neutron Interrogation: Traditional tools struggle to distinguish between $^{233}$U (the fissile product of the thorium cycle) and $^{235}$U in mixed samples.

Neutron Resonance Transmission Analysis (NRTA) is a promising alternative due to its isotopic specificity in the epithermal energy range (1 eV–100 eV) and robustness against non-resonant shielding. However, deploying NRTA in thorium safeguards requires detectors that can maintain timing performance and quantitative accuracy amidst intense gamma fields. The paper addresses the critical need to identify which detector technology best balances neutron detection efficiency, timing resolution, and gamma discrimination in these harsh conditions.

2. Methodology

The authors conducted a comparative characterization of two candidate scintillator detectors using a portable NRTA setup at the MIT Vault Laboratory.

• Detectors Compared:

  1. GS20 ($^6$Li:Ce Glass): A standard detector for portable NRTA known for fast decay times (~70 ns) and high efficiency but limited neutron-gamma discrimination (relying on Pulse Height Analysis, PHA).
  2. CLYC (Cs$_2$LiYCl$_6$:Ce): A crystal offering superior Pulse Shape Discrimination (PSD) and energy resolution but with a longer decay time (~50 ns for neutrons/gammas, though the paper notes microsecond-scale concerns for pileup) and intrinsic resonances from $^{133}$Cs.

• Experimental Setup:

  • Source: D-T neutron generator (14.1 MeV) moderated to the epithermal range.
  • Geometry: 2-meter flight path.
  • Target: A 1.50 mm thick natural tungsten (W) sheet.
  • High-Radiation Emulation: To simulate the intense gamma environment of a $^{233}$U target, a $^{232}$Th source was placed near the detector. This source provided a passive gamma count rate comparable to previous $^{233}$U measurements, with energies overlapping the neutron detection regions.

• Data Analysis:

  • Transmission spectra were analyzed using NeuFIT, an open-source code that incorporates setup-specific Time-of-Flight (TOF) response functions derived from Geant4 simulations.
  • The study compared "clean" conditions (no external gamma source) against "high gamma" conditions (simulated by concatenating passive gamma runs with target-in runs).
  • Pileup Check: A specific test was conducted with the CLYC detector flush against the $^{232}$Th source to verify that the slower decay time did not cause unacceptable signal pileup.

3. Key Contributions

• Direct Comparative Characterization: This is the first study to directly compare GS20 and CLYC specifically for portable NRTA applications under simulated high-radiation safeguards conditions.
• Validation of CLYC in High Gamma Fields: The paper demonstrates that despite CLYC's slower decay time and intrinsic $^{133}$Cs resonances (which overlap with some actinide resonances), it outperforms GS20 in high-background scenarios.
• Quantitative Uncertainty Analysis: The study provides empirical data showing that CLYC maintains consistent quantitative accuracy (areal density extraction) even when gamma backgrounds are increased, whereas GS20 suffers significant degradation in precision.
• Feasibility of PSD in NRTA: It validates that Pulse Shape Discrimination is a viable and superior method for filtering gamma backgrounds in NRTA compared to the Pulse Height Analysis used by GS20.

4. Results

• Accuracy: Both detectors successfully recovered the tungsten target thickness (1.50 mm) with good accuracy in all conditions.
• Precision and Uncertainty:
  • GS20: In the presence of the $^{232}$Th source, the uncertainty in the predicted thickness increased significantly (errors were ~50% larger than in clean conditions). The uncertainty grew from $\pm 0.15$ mm to $\pm 0.19$ mm.
  • CLYC: The uncertainty remained constant ($\pm 0.13$ mm) regardless of the gamma background. The predicted thickness (1.522 mm) did not change between clean and high-radiation conditions.
• Pileup Performance: The CLYC detector showed negligible pileup even when placed directly against the intense gamma source, confirming that its microsecond-scale decay time is acceptable for the count rates encountered in this specific NRTA configuration.
• Resonance Interference: While CLYC contains $^{133}$Cs with resonances at 5.9 eV and 22.5 eV (overlapping with $^{238}$U and $^{232}$Th), the study found that the 22.5 eV resonance is not saturating. Therefore, target resonances can still be detected and quantified, provided the internal resonances do not completely obscure the target absorption lines.

5. Significance

The findings have profound implications for the future of nuclear safeguards, particularly for the thorium fuel cycle:

1. Superior Detector Choice: For scenarios requiring the identification and quantification of $^{233}$U in the presence of intense gamma backgrounds (from $^{232}$U daughters), CLYC-like detectors with strong PSD capabilities are the more reliable choice over traditional GS20 glass.
2. Overcoming "Self-Protection": The high gamma activity of recycled thorium fuel, often cited as a "self-protection" feature that hinders inspection, can be effectively mitigated using CLYC detectors, allowing for precise NRTA measurements without heavy shielding or large standoff distances that degrade statistics.
3. Future Development: The paper suggests that while CLYC is a strong candidate, future work should explore LiI:Ce scintillators, which may offer a combination of high neutron capture cross-sections and PSD capabilities, potentially offering the best of both worlds.

In conclusion, the paper establishes that the trade-off of a slower decay time in CLYC is outweighed by its superior gamma rejection capabilities, making it a critical enabler for next-generation nuclear safeguards in high-radiation environments.