Performance of large-scale 6Li-doped pulse-shape discriminating plastic scintillators

This paper reports the successful kilogram-scale production and characterization of EJ-299-50, a $^6$Li-doped plastic scintillator with pulse-shape discrimination capabilities that exhibits optical properties comparable to liquid scintillators, high neutron capture efficiency, and long-term stability.

The Gist

Imagine you are trying to listen to a specific whisper in a very noisy, crowded room. That is essentially what scientists do when they try to detect tiny particles like neutrons and anti-neutrinos. For decades, they have used "liquid" detectors—basically giant, sensitive buckets of glowing fluid—to do this. These liquids are great at telling the difference between different types of particles, but they are messy, flammable, and hard to move around.

This paper introduces a new, solid alternative: a special kind of plastic that acts like a high-tech, glowing sponge. Specifically, the researchers are testing a new material called EJ-299-50.

Here is a breakdown of what they did and what they found, using simple analogies:

1. The "Magic" Ingredient: Lithium-6

Think of this plastic as a sponge soaked in a special ingredient called Lithium-6.

• The Problem: Regular plastic can glow when hit by particles, but it's hard to tell which particle hit it.
• The Solution: The Lithium-6 acts like a specialized magnet. When a slow-moving (thermal) neutron hits it, the Lithium-6 "catches" it and throws a very specific, bright flare. This allows the detector to say, "Aha! That was a neutron, not a gamma ray!"
• The Challenge: Putting Lithium-6 into plastic is like trying to dissolve sugar into oil; they usually don't mix well. The team had to invent a new recipe to get the Lithium to dissolve evenly in the plastic without ruining its ability to glow.

2. Making Big Bars (The "Giant Popsicles")

The researchers didn't just make small test tubes; they cast 44 giant bars of this plastic.

• The Size: Each bar is about 20 inches long and 2 inches wide (roughly the size of a large ruler).
• The Goal: They needed to prove that this material works just as well in these huge bars as it does in tiny samples. If you make a giant bar, the light has to travel a long way to get to the sensors. If the plastic is "cloudy," the light gets lost, and the signal fades.

3. Testing the "Flashlight" (Light Output and Clarity)

To test the bars, they shined a controlled beam of gamma rays (a type of light) at different spots along the length of the bars.

• The Result: They found that the plastic is very clear. The light travels through the long bars almost as well as it does through the best liquid detectors.
• The "Wrapped" vs. "Bare" Test:
  • Bare: Measuring the plastic with nothing around it (like a naked stick).
  • Wrapped: Wrapping the plastic in a special shiny foil (like wrapping a gift in mirror paper) to bounce light back to the sensors.
  • Finding: When wrapped, the plastic glows about twice as bright as standard plastic bars. This means it's very efficient at capturing the light it produces.

4. The "Noise Cancellation" (Pulse-Shape Discrimination)

This is the most important trick. Imagine two people shouting in the room: one shouts in a short, sharp burst (a gamma ray), and the other shouts in a long, drawn-out groan (a neutron).

• The Tech: This plastic is smart enough to listen to the shape of the shout. It can tell the difference between the "sharp burst" and the "long groan."
• The Score: The researchers gave the plastic a score (called the Figure of Merit) to see how well it separates these two sounds. While it's slightly harder to separate them in a giant solid bar than in a tiny liquid drop, the plastic still did a very good job, successfully distinguishing neutrons from background noise.

5. The "Neutron Trap" Efficiency

They tested how good the Lithium-6 was at catching neutrons.

• The Result: About 85% of the neutrons that entered the plastic were successfully caught by the Lithium-6 and identified. This is a very high success rate, meaning the detector is very sensitive.

6. The "Aging" Test (Will it rot?)

Plastic can sometimes get brittle or cloudy over time, especially if the chemicals inside start to "sweat" or leak out.

• The Test: They left samples of the plastic in the air for months, and even heated some up to 60°C (140°F) to simulate harsh conditions.
• The Finding: The plastic held up remarkably well.
  • There was a tiny issue where a chemical (PPO) sometimes "sweated" out and stuck to the wrapping, but wiping it off with alcohol fixed it immediately.
  • The light output and the ability to distinguish neutrons did not degrade significantly over the test period (about 19 weeks).

Summary

The paper concludes that this new EJ-299-50 plastic is a "goldilocks" material:

1. It is solid (easy to move and safe, unlike flammable liquids).
2. It is clear and bright (works well in large sizes).
3. It is smart (can tell neutrons apart from other particles).
4. It is durable (doesn't fall apart over time).

The researchers successfully proved that you can make large, solid blocks of this material that perform nearly as well as the traditional liquid detectors, opening the door for easier-to-deploy neutron and anti-neutrino detectors.

Technical Summary

Technical Summary: Performance of Large-Scale 6Li-Doped Pulse-Shape Discriminating Plastic Scintillators

Problem Statement
Liquid scintillators have long been the standard for reactor-antineutrino detection and neutron spectroscopy due to their ability to facilitate Inverse Beta Decay (IBD) and provide Pulse-Shape Discrimination (PSD) between electronic and nuclear recoils. However, their liquid state introduces significant challenges regarding safety, handling, transportation, and the maintenance of specific operating environments. While plastic scintillators offer a solid-state alternative with improved mobility and safety, developing large-scale plastics that simultaneously possess high-performance PSD capabilities and thermal-neutron capture agents (such as $^6$Li) has been a significant materials science challenge. The primary difficulties involve balancing the solubility of $^6$Li-bearing organic salts in monomers like styrene, controlling curing conditions, and maintaining satisfactory light output and optical properties at the kilogram scale.

Methodology
The authors characterized 44 kilogram-scale bars of a new plastic scintillator, commercially identified as EJ-299-50, produced by Eljen Technology. The bars measured 5.5 cm × 5.5 cm × 50 cm. The study employed a multi-faceted experimental approach:

• Optical Characterization: Light output and effective attenuation lengths were measured using a collimated $^{22}$Na gamma-ray source moved along the longitudinal axis of the bars. Measurements were taken in both "bare" configurations (to ensure consistent bar-to-bar comparison) and "wrapped" configurations (using 3M™ DF2000MA reflectors and optical grease to simulate optimal detector performance).
• PSD and Neutron Identification: The material's ability to distinguish between gamma-rays and neutrons was tested using $^{137}$Cs and $^{252}$Cf sources. The Pulse-Shape Discrimination (PSD) parameter was calculated based on the ratio of tail charge to total charge in the waveform.
• Neutron Capture Analysis: Thermal-neutron capture efficiency was evaluated by analyzing prompt-delayed coincidences from $^{252}$Cf sources. The study distinguished between captures on $^6$Li (producing an $\alpha$ and triton) and $^1$H (producing a 2.2 MeV gamma). These results were compared against Geant4 simulations to estimate absolute capture efficiencies.
• Stability Testing: Long-term stability was assessed through two methods: monitoring the attenuation length of a specific bar over two months of repeated measurements, and conducting aging tests on two formulation variations ("Original" and "Current") exposed to air and heat treatment (60°C) for up to 19 weeks.

Key Results

• Optical Performance: The EJ-299-50 material demonstrated effective attenuation lengths ($\lambda$) ranging from approximately 83 cm to 110 cm, with two distinct subgroups observed likely due to production batch variations. The effective light output in a bare configuration was measured at $141 \pm 12$ PE/MeV, increasing to $421 \pm 32$ PE/MeV in a wrapped configuration. This performance is comparable to $^6$Li-doped liquid scintillators.
• PSD Capability: The material exhibited good separation between gamma-ray and fast-neutron events. In small cylindrical samples, the Figure of Merit (FOM) between bands was 2.11. In the larger 50 cm bars, the FOM was lower ($1.27 \pm 0.11$ for fast neutrons vs. gammas) due to reduced light collection efficiency in larger volumes, but still sufficient for discrimination.
• Neutron Capture: The scintillator showed an effective capture efficiency on $^6$Li of approximately 85% in simulation, with a mean capture time of $\sim 15$ $\mu$s. The capture fraction of $^6$Li relative to $^1$H was found to be $\sim 16.8\%$ in data, consistent with simulations. The material successfully identified the "capture island" at $\sim 0.4$ MeVee.
• Stability: Aging tests revealed that while the "Original" formulation showed a decrease in light output after heat treatment, the "Current" formulation maintained stable performance. No intrinsic degradation in optical properties or PSD capabilities was observed over several months of observation. A minor issue with PPO leaching (outgassing) was noted when bars were wrapped for extended periods, but this did not permanently degrade performance and could be mitigated by cleaning.

Significance and Claims
The paper claims that EJ-299-50 represents a significant advancement in solid-state neutron detection technology. By successfully producing kilogram-scale, $^6$Li-doped plastic scintillators with PSD capabilities, the material offers a viable alternative to liquid scintillators for applications requiring near-field, readily deployable systems.

Specifically, the authors highlight that EJ-299-50 is a promising candidate for:

• Reactor Monitoring: Future safeguards and reactor monitoring experiments utilizing antineutrino detection.
• Neutron Spectroscopy: Applications requiring the distinction between fast neutrons, thermal neutrons, and gamma rays.
• Multiplicity Counting: Neutron multiplicity counting applications.

The study concludes that the material's optical properties are comparable to liquid counterparts while offering the geometric versatility and increased mobility inherent to solid-state detectors. The demonstrated long-term stability of the formulation variations further supports the viability of this material for long-term deployment in nuclear safeguards and monitoring.