Performance and pulse shape discrimination of glass scintillator SG101 for neutron detection

This paper demonstrates that the glass scintillator SG101 is a promising candidate for high-efficiency thermal neutron detection and precise event tagging, exhibiting superior pulse shape discrimination performance and significant physical correlations when coupled with organic scintillators like EJ200 and EJ276.

The Gist

Imagine you are trying to listen to a specific conversation in a very noisy, crowded room. The room is filled with two types of people: Gamma Rays (who are loud, fast-talking chatterboxes) and Neutrons (who are quieter, slower, and move differently). Your goal is to find the "Thermal Neutrons" (the slow, heavy ones) without getting confused by the fast ones or the chatter.

This paper is about testing a new, high-tech "ear" (a detector) called SG101 to see if it can hear these specific neutrons better than the old, standard "ear" (called EJ426).

Here is the breakdown of their experiment and findings, using simple analogies:

1. The New Ear vs. The Old Ear

• The Old Ear (EJ426): This is like a thick, fuzzy blanket. It's good at catching neutrons, but it's heavy, opaque (you can't see through it), and when it catches a particle, it takes a long time to "settle down" and tell you what happened. It's like a slow, muffled response.
• The New Ear (SG101): This is a thin, clear sheet of glass. It's transparent and very fast. When it catches a neutron, it flashes quickly and clearly.
• The Result: The researchers found that the new glass ear (SG101) is 6 to 8 times better at catching neutrons than the old blanket. It's also much sharper, giving a clearer picture of the energy of the particle it caught.

2. The "Party" Setup (The Hybrid System)

The glass alone is great at catching slow neutrons, but it can't tell the difference between a fast neutron and a gamma ray (the loud chatter). To fix this, the researchers built a "two-person team":

• The Glass (SG101): Sits on one side, catching the slow, heavy thermal neutrons.
• The Plastic (EJ200 or EJ276): Sits next to it, catching fast neutrons and gamma rays.

Think of this like a security checkpoint with two guards:

• Guard A (Plastic): Stops the fast runners and the loud talkers.
• Guard B (Glass): Only stops the slow, heavy walkers.

3. The "Shape-Shifter" Test (Pulse Shape Discrimination)

How do they tell the particles apart? They look at the shape of the signal (the "pulse") when a particle hits the detector.

• The Analogy: Imagine dropping a marble and a feather into a pool of water.
  • The Marble (Neutron) makes a big splash that ripples for a long time.
  • The Feather (Gamma Ray) makes a tiny splash that stops almost instantly.
• The researchers used a special math trick to measure the "ripple time."
  • Team 1 (Glass + Fast Plastic): They could perfectly separate the "slow walkers" (thermal neutrons) from the "loud talkers" (gamma rays). The separation was so clear it was like distinguishing between a whisper and a shout.
  • Team 2 (Glass + Smart Plastic): This team was even better. They could separate three groups: Gamma rays, Fast Neutrons, and Thermal Neutrons. It's like being able to tell the difference between a sprinter, a jogger, and a slow walker all at once.

4. The "Buddy System" (Coincidence Analysis)

The most exciting part is how they checked if the particles were actually related.

• The Scenario: Sometimes, a fast neutron hits the plastic, slows down, and then gets caught by the glass as a thermal neutron. They are "buddies" from the same family.
• The Test: The researchers watched for a "Fast Neutron" hitting the plastic, followed immediately (within a blink of an eye, or 100 microseconds) by a "Thermal Neutron" hitting the glass.
• The Finding: They saw way more of these "buddy pairs" than they would expect by pure chance. It's like walking into a room and seeing 100 pairs of twins standing together, when you'd only expect 10 by random luck. This proves the detector is working correctly and can track the life-cycle of a neutron.

5. Why Does This Matter?

This new glass detector (SG101) is a game-changer for a few reasons:

• It's Clear: Because it's transparent glass, you can see through it, which helps in building complex detectors.
• It's Fast: It doesn't get "confused" or "tired" like the old materials.
• It's a Team Player: When paired with plastic, it can sort out different types of radiation with incredible precision.

The Big Picture:
This technology is like upgrading from a muddy, old pair of binoculars to a high-definition, laser-focused camera. It will help scientists in nuclear security (finding hidden radioactive materials), fusion research (clean energy), and even neutrino detection (studying the most elusive particles in the universe) by filtering out the "noise" and letting them see the "signal" clearly.

Technical Summary

1. Problem Statement

Thermal neutron detection is critical for applications in nuclear security, fusion research, and neutrino physics. Conventional thermal neutron detectors often rely on LiF/ZnS:Ag composite scintillators (e.g., EJ426). While effective, these materials are opaque, have relatively slow decay times, and often suffer from broad signal distributions that limit energy resolution and pulse shape discrimination (PSD) capabilities. There is a need for a transparent, high-efficiency thermal neutron scintillator that can be effectively coupled with organic scintillators to enable multi-particle discrimination (separating gamma rays, fast neutrons, and thermal neutrons) and precise event tagging.

2. Methodology

The study employed a systematic experimental approach to characterize a novel $^6$Li-doped transparent glass scintillator (SG101) and compare it against the standard EJ426 scintillator.

• Materials:
  • SG101: A transparent glass disc (5 cm diameter, 1.07 mm thick) doped with 7.5 wt% $^6$Li and Ce$^{3+}$ ions. It relies on the $^6$Li(n, $\alpha$)t reaction.
  • EJ426: A conventional opaque LiF/ZnS:Ag sheet (42 cm square, 0.32 mm thick).
  • Organic Scintillators: EJ200 (no PSD capability) and EJ276 (excellent PSD capability) were coupled with SG101 to form hybrid detector systems.
• Experimental Setup:
  • Detectors were coupled to an XP3232 Photomultiplier Tube (PMT) using optical silicone grease.
  • Shielding included lead bricks (for $\gamma$-ray background) and high-density polyethylene (HDPE) bricks (for ambient neutrons).
  • Irradiation was performed using an Am-Be neutron source.
  • Data acquisition utilized a Teledyne LeCroy HDO4104A oscilloscope (for waveform analysis) and a CAEN DT5751 digitizer (for coincidence analysis).
• Analysis Techniques:
  • Pulse Shape Discrimination (PSD): Used a short-long gate integration method ($PSD = 1 - Q_{short}/Q_{long}$) to distinguish particle types based on decay time differences.
  • Energy Linearity: Calibrated using standard $\gamma$-ray sources ($^{22}$Na, $^{137}$Cs, $^{60}$Co) by fitting Compton edges to determine photoelectron yield per MeV.
  • Coincidence Analysis: Analyzed time-correlated events between fast neutrons (detected in EJ276) and thermal neutrons (detected in SG101) within a 100 $\mu$s window to distinguish physical correlations from accidental background.

3. Key Contributions

• Comprehensive Characterization of SG101: Provided the first detailed performance benchmark of the SG101 glass scintillator against the industry-standard EJ426, highlighting its superior optical transparency and signal stability.
• Hybrid Detector Architecture: Demonstrated the efficacy of coupling a thin, transparent thermal neutron glass (SG101) with thick organic plastic scintillators (EJ200/EJ276) to create a system capable of simultaneous thermal neutron detection and fast neutron/$\gamma$ discrimination.
• Multi-Particle Discrimination: Successfully resolved three distinct particle populations (gamma rays, fast neutrons, and thermal neutrons) in a single detector system using the SG101-EJ276 configuration.
• Coincidence Verification: Validated the physical correlation of fast-thermal neutron events and triple-coincidence events ($\gamma$-fast-thermal), proving the system's utility for background suppression in complex radiation fields.

4. Key Results

A. Comparison: SG101 vs. EJ426

• Detection Efficiency: Under identical Am-Be irradiation conditions, SG101 achieved 6 to 8 times higher event counts than EJ426. This is attributed to SG101's higher optical transparency, which allows more efficient photon collection despite its thinner geometry.
• Signal Stability: SG101 exhibited a much narrower distribution of integrated signal amplitudes (Standard Deviation: 0.81 V·ns) compared to EJ426 (Standard Deviation: 5.96 V·ns), indicating superior energy resolution and signal reproducibility.
• Waveform Characteristics: SG101 produces a narrow scintillation pulse (53 ns decay) due to Ce$^{3+}$ emission, whereas EJ426 has a broad pulse (200 ns decay) with significant tailing.

B. Energy Linearity

• Both hybrid systems (SG101+EJ200 and SG101+EJ276) demonstrated excellent linear energy response in the 0.3–1.3 MeV range.
• Photoelectron Yields:
  • SG101+EJ200: ~495.7 p.e./MeV.
  • SG101+EJ276: ~673.1 p.e./MeV (higher yield due to better light collection).

C. Pulse Shape Discrimination (PSD) Performance

• SG101 + EJ200: Achieved a Figure-of-Merit (FOM) of 3.81 for separating thermal neutrons from gamma rays. The separation exceeded 9$\sigma$.
• SG101 + EJ276: Successfully resolved three distinct populations:
  1. Thermal Neutrons vs. Gamma Rays: FOM = 3.46 (8.1$\sigma$ separation).
  2. Thermal Neutrons vs. Fast Neutrons: FOM = 2.21 (5.2$\sigma$ separation).
  3. Fast Neutrons vs. Gamma Rays: Inherited EJ276's high discrimination capability (FOM > 1.27).

D. Coincidence Analysis

• Fast-Thermal Correlation: In a 10-minute run, the system observed 2,008 correlated fast-thermal events, significantly exceeding the expected accidental background of ~908 events.
• Triple Coincidence: The system detected 108 triple-coincidence events ($\gamma$-fast-thermal), far exceeding the accidental expectation of ~25 events.
• Time Constant: The time difference distribution between correlated events yielded a characteristic time constant of $\tau \approx 11.3$ $\mu$s, which is advantageous for inverse beta decay (IBD) event selection in neutrino detectors.

5. Significance

This study establishes SG101 as a superior candidate for next-generation thermal neutron detectors. Its unique combination of high optical transparency and high thermal neutron sensitivity allows for:

1. Compact Design: The thin glass geometry minimizes gamma-ray sensitivity while maintaining high neutron efficiency.
2. Advanced Background Rejection: When coupled with PSD-capable organic scintillators, the system can simultaneously tag thermal neutrons and reject gamma/fast neutron backgrounds with high statistical confidence (up to 9$\sigma$).
3. Neutrino Detection Applications: The ability to perform time-correlated event tagging (fast-thermal coincidence) makes this system highly promising for reactor antineutrino monitoring and other neutrino experiments where background suppression is critical.

The results suggest that SG101-based hybrid detectors offer a practical pathway for developing compact, high-performance neutron detection systems for nuclear security and fundamental physics research.