Performance Characterization of a Plastic-Scintillator Sensor for Fast-Neutron, Thermal-Neutron, and Gamma-Ray Discrimination

This paper demonstrates that a compact plastic-scintillator sensor coupled with a thermal-neutron screen and a single photomultiplier tube can effectively discriminate between fast neutrons, thermal neutrons, and gamma rays in mixed radiation fields, with specific configurations achieving high separation figures of merit.

The Gist

Imagine you are standing in a crowded room where three very different types of people are shouting at once:

1. Gamma Rays: Like a swarm of fast, invisible bees buzzing everywhere.
2. Fast Neutrons: Like heavy, bouncy rubber balls zooming through the air.
3. Thermal Neutrons: Like slow-moving, sleepy turtles wandering around.

In nuclear physics, knowing who is shouting what is crucial for safety and experiments. The problem is, they all look the same to a standard detector. This paper describes how the authors built a special "listening device" (a sensor) that can tell these three groups apart, even when they are all mixed together.

The Detective's Toolkit: Two Different Sensors

The researchers built two versions of this sensor using a "sandwich" technique. Think of it like a two-layer cake where each layer reacts differently to the shouting.

The Ingredients:

• The Cake (Plastic Scintillator): This is the main body of the sensor. When a particle hits it, it flashes light. They used two types of "cake":
  • EJ200: A fast-reacting cake that flashes instantly but doesn't tell you what hit it, only that something hit it.
  • EJ276: A smarter cake that changes its "flashing style" depending on whether a bee (gamma) or a rubber ball (fast neutron) hit it.
• The Frosting (Thermal Neutron Screen): This is a thin, special layer (EJ426) placed on the other side. It's designed to catch the slow turtles (thermal neutrons). When a turtle gets caught, it produces a very slow, lingering flash of light, unlike the quick flash of the cake.
• The Ears (Photomultiplier Tube): A single device that listens to the light flashes from both layers.

How It Works: The "Flash Speed" Trick

The secret sauce is Pulse Shape Discrimination (PSD). Instead of just counting how bright the flash is, the sensor measures how long the flash lasts.

• Gamma Rays (Bees): Make a very quick, sharp flash.
• Fast Neutrons (Rubber Balls): Make a slightly longer flash (in the EJ276 sensor).
• Thermal Neutrons (Turtles): Get caught in the frosting layer and make a very slow, long-lasting flash.

By looking at the "shape" of the light signal, the sensor can sort the crowd.

The Results: What the Sensors Found

The team tested their sensors using a radioactive source that mimics a mixed crowd of bees, balls, and turtles. They also added layers of plastic (HDPE) to slow down the rubber balls, turning them into turtles, to see how the sensor handled the change.

1. The Simple Sensor (EJ200 + Frosting)

• Performance: This version was excellent at separating the Turtles (Thermal Neutrons) from the Bees (Gamma Rays).
• The Score: They gave it a "separation score" (called Figure of Merit) of over 5. In this world, a score above 1 is good; 5 is fantastic. It clearly saw the slow turtles and ignored the buzzing bees.
• Limitation: It couldn't tell the difference between the bees and the rubber balls (fast neutrons).

2. The Smart Sensor (EJ276 + Frosting)

• Performance: This version was a three-way champion. It successfully identified three distinct groups:
  • The Bees (Gamma Rays).
  • The Rubber Balls (Fast Neutrons).
  • The Turtles (Thermal Neutrons).
• The Catch: While it could separate the Turtles from the others perfectly, telling the Bees apart from the Rubber Balls was tricky when the balls were moving slowly (low energy). However, once the rubber balls were moving fast enough (equivalent to energy above 1 MeV), the sensor could clearly tell them apart from the bees.

The "Moderator" Effect

The researchers wrapped the sensors in different thicknesses of plastic foam (HDPE).

• Thin Foam: The rubber balls (fast neutrons) mostly stayed fast.
• Thick Foam: The foam slowed the rubber balls down, turning them into turtles.
• Result: As the foam got thicker, the sensor saw fewer rubber balls and more turtles, proving the sensor could track how the crowd changed as the environment changed.

The Bottom Line

The paper concludes that these "sandwich" sensors are a promising, compact way to handle mixed radiation.

• If you just need to find Thermal Neutrons amidst a sea of Gamma rays, the Simple Sensor (EJ200) is your best bet.
• If you need to sort through Fast Neutrons, Thermal Neutrons, and Gamma Rays all at once (and the neutrons are energetic enough), the Smart Sensor (EJ276) is the tool to use.

The authors emphasize that while the sensors work well, they haven't yet calculated exactly how many particles they catch (efficiency) and that the "Fast Neutron vs. Gamma" separation isn't perfect for very low-energy particles. But for a compact, single-device solution, it's a significant step forward.

Technical Summary

1. Problem Statement

In mixed radiation fields (containing gamma rays, fast neutrons, and thermal neutrons), effective discrimination between these particle types is critical for radiation monitoring, reactor measurements, and background suppression in nuclear experiments. While hybrid scintillation detectors offer a path toward compact sensors, existing solutions often struggle to simultaneously distinguish all three particle types with high efficiency in a single-readout architecture. The challenge lies in developing a compact, robust sensor capable of separating thermal neutrons from gamma rays and, if possible, distinguishing fast neutrons from gamma rays without requiring complex multi-detector setups.

2. Methodology

The authors investigated two compact detector assemblies based on a "phoswich-like" configuration, where different scintillators are optically coupled to a single photomultiplier tube (PMT) and distinguished via Pulse Shape Discrimination (PSD).

• Detector Configurations:

  1. EJ200 + EJ426: A standard plastic scintillator (EJ200) coupled to a thermal-neutron-sensitive screen (EJ426, containing $^6$LiF/ZnS(Ag)). EJ200 lacks intrinsic fast-neutron/gamma PSD but provides a fast response.
  2. EJ276 + EJ426: A pulse-shape-discriminating plastic scintillator (EJ276) coupled to the same EJ426 screen. EJ276 offers intrinsic fast-neutron/gamma discrimination.
  • Geometry: The EJ426 screen was sandwiched between PMMA plates and optically coupled to a 6 cm plastic scintillator cube, which was then coupled to an XP3232 PMT.
  • Readout: Signals were acquired using a LeCroy HDO4104A digital oscilloscope (1 GHz bandwidth, 10 GS/s).

• Experimental Setup:

  • Calibration: Gamma-ray energy calibration was performed using $^{137}$Cs, $^{22}$Na, and $^{60}$Co sources. Due to the low density of plastic scintillators, calibration relied on Compton-edge analysis rather than full-energy peaks.
  • PSD Measurement: An Am–Be neutron source was used to generate mixed fields. High-Density Polyethylene (HDPE) moderators of varying thicknesses (1 cm, 2 cm, 3 cm) were placed between the source and detector to modulate the neutron energy spectrum (thermalizing fast neutrons).
  • Analysis: The PSD parameter was defined as $PSD = 1 - (Q_{short} / Q_{long})$, where $Q_{short}$ and $Q_{long}$ are charge integrals over 75 ns and 800 ns gates, respectively. Separation performance was quantified using the Figure of Merit (FOM).

3. Key Contributions

• Dual-Configuration Comparison: The study systematically compares a standard plastic scintillator (EJ200) against a PSD-capable plastic (EJ276) when coupled with a thermal-neutron screen, demonstrating how material choice dictates the discrimination capabilities of the hybrid system.
• Three-Component Discrimination: The EJ276+EJ426 configuration successfully resolves three distinct signal populations (gamma rays, fast neutrons, and thermal neutrons) in a single detector readout, a significant advancement for compact mixed-field sensors.
• Energy-Dependent Characterization: The authors established a linear gamma-equivalent energy calibration and demonstrated that the fast-neutron/gamma discrimination capability of the EJ276+EJ426 assembly is strongly energy-dependent, becoming effective only above specific energy thresholds.

4. Key Results

A. Energy Calibration

Both assemblies exhibited a linear response between the photoelectron (PE) yield and gamma-equivalent energy.

• EJ200+EJ426: Light yield $\approx 716.5$ PE/MeV.
• EJ276+EJ426: Light yield $\approx 781.9$ PE/MeV.

B. EJ200 + EJ426 Performance (Thermal Neutron vs. Gamma)

• Discrimination: This configuration effectively separated thermal-neutron capture events (high PSD) from gamma-ray events (low PSD).
• FOM: The Figure of Merit for thermal-neutron/gamma separation was consistently high, exceeding 5.0 (specifically $5.23 \pm 0.06$ for 1 cm HDPE and $5.22 \pm 0.05$ for 3 cm HDPE).
• Conclusion: This setup is ideal for thermal-neutron tagging in gamma-dominated backgrounds.

C. EJ276 + EJ426 Performance (Fast Neutron, Thermal Neutron, and Gamma)

• Discrimination: This configuration resolved three distinct populations:
  1. Low PSD: Gamma rays.
  2. Intermediate PSD: Fast neutrons (recoil protons).
  3. High PSD: Thermal neutrons (capture in EJ426).
• FOM Values:
  • Thermal Neutron / Gamma: Excellent separation ($FOM > 5$).
  • Thermal Neutron / Fast Neutron: Strong separation ($FOM \approx 3.7 - 4.5$).
  • Fast Neutron / Gamma: Limited separation in the low-energy region ($FOM < 1.0$).
• Energy Dependence: The fast-neutron/gamma separation improves significantly with energy. The FOM exceeds the effective separation criterion of 1.27 (equivalent to $3\sigma$) only above approximately 1 MeV gamma-equivalent energy.
• Moderator Effect: Increasing HDPE thickness increased the fraction of thermal neutron events (from 0.46% to 0.57%) while decreasing the fast neutron fraction, confirming the system's sensitivity to neutron moderation.

5. Significance

This work demonstrates a practical, compact approach for mixed-field radiation detection using a single PMT readout.

• Versatility: The study proves that detector architecture can be tuned for specific needs:
  • Use EJ200+EJ426 for robust thermal-neutron detection in high gamma backgrounds.
  • Use EJ276+EJ426 for comprehensive discrimination of all three particle types, provided the analysis focuses on energies above ~1 MeV.
• Compactness: The "phoswich" design eliminates the need for multiple detectors or complex coincidence logic, making it suitable for portable monitoring and space-constrained environments.
• Future Outlook: While the system shows promise, the authors note that absolute detection efficiencies and low-energy fast-neutron/gamma discrimination need further optimization through geometry adjustments and advanced modeling.

In summary, the paper validates that coupling a thermal-neutron screen with either a standard or PSD-capable plastic scintillator creates a powerful, compact sensor capable of distinguishing thermal neutrons from gammas, with the EJ276 variant offering the added, energy-dependent capability to separate fast neutrons.