Thermal/epithermal neutron detection via LiO$_{2}$-doped fiber bundles: A computational study through GEANT4 simulations

Technical Summary: Thermal/Epithermal Neutron Detection via LiO2-Doped Fiber Bundles

Problem Statement
The detection of thermal and epithermal neutrons is critical for nuclear applications such as space dosimetry, activation analysis, and reactor instrumentation. However, neutrons are uncharged and require a conversion process involving interaction with a nucleus to generate detectable secondary particles. While lithium is a preferred material due to its high neutron capture cross-section and the production of high linear energy transfer (LET) secondary particles (alpha-triton pairs), integrating lithium into flexible, high-surface-area detector geometries remains a challenge. This study addresses the feasibility of utilizing lithium-doped plastic scintillating fibers to detect incident neutrons by capturing the secondary charged particles generated during neutron capture.

Methodology
The authors employed GEANT4 simulations to model a fiber bundle system based on Kuraray Y11-200(M) fibers. The core methodology involved the following steps:

• Material Modification: The polystyrene core of the fibers was doped with 10 wt.% LiO₂. The fibers were modeled as double-cladded cylinders with a core radius of 0.5 mm, surrounded by PMMA and fluorinated PMMA claddings.
• Simulation Physics: The simulation utilized the FTFP_BERT_HP physics list for hadronic interactions and EmStandardPhysics_option4 for optical photon generation. The neutron capture reaction $^6\text{Li} + n \rightarrow ^4\text{He} (2.05 \text{ MeV}) + ^3\text{H} (2.73 \text{ MeV})$ was tracked, with the resulting alpha-triton pairs depositing their energy within the fiber core.
• Optical Readout: A pixelated photo-sensor grid (1.36 × 1.36 mm² pixels) was placed at both ends of the fiber bundle to collect scintillation photons. The scintillation yield was set to 10 optical photons per keV, with a time constant of 10 ns.
• Experimental Configurations:
  1. Thermal Neutron Optimization: A monoenergetic beam of 0.025 eV thermal neutrons was used to determine the optimal number of fiber layers required for efficient detection.
  2. Epithermal Neutron Discrimination: Two configurations were tested to distinguish epithermal neutrons (1 eV) from thermal neutrons:
     • A double fiber bundle system separated by a Boron Carbide (B₄C) absorber layer.
     • A single fiber bundle system with a B₄C absorber layer positioned above the fibers.
       The thickness of the B₄C absorber was varied (0.5 mm to 2.0 mm) to optimize the ratio of detected epithermal to thermal neutrons.

Key Results

• Optimal Fiber Array: For thermal neutrons (0.025 eV), the number of detected alpha-triton pairs showed diminishing returns beyond 20 fiber layers. A 20-layer bundle (with 34 fibers per layer) was determined to be sufficient, yielding a minimum detection efficiency of approximately 30%.
• Epithermal Detection: In the same 20-layer configuration without an absorber, approximately 15% of incident 1 eV epithermal neutrons generated detectable alpha-triton pairs.
• Discrimination via Absorbers:
  • Double Bundle System: A 0.5 mm B₄C absorber layer effectively reduced the thermal neutron detection fraction to the target ~5% while allowing significant epithermal detection (1637 epithermal vs. 85 thermal neutrons detected in the bottom bundle).
  • Single Bundle System: This configuration proved more effective. A 0.5 mm B₄C absorber placed above the fiber bundle resulted in 6971 detected epithermal neutrons and 450 thermal neutrons, maintaining the desired ~5% thermal fraction while offering higher overall epithermal detection efficiency compared to the double-bundle setup.
• Signal Characteristics: The simulation demonstrated that the short range of alpha-triton pairs ensures energy deposition occurs almost entirely within the fiber core. Consequently, the emitted optical photons are trapped within the specific fiber of origin, allowing a pixelated readout to correlate specific pixel hits with individual neutron capture events.

Significance and Claims
The study concludes that LiO₂-doped plastic scintillating fiber bundles are a viable instrument for detecting thermal and epithermal neutrons. The authors claim that the proposed system offers a flexible detector design capable of distinguishing between thermal and epithermal neutrons through the strategic use of B₄C absorber layers. Specifically, the single fiber bundle configuration combined with a 0.5 mm B₄C absorber is identified as the optimal solution for maximizing epithermal neutron detection while minimizing thermal neutron interference. The paper asserts that these findings support the utility of such fiber bundles in diverse applications requiring thermal/epithermal neutron detection and discrimination, without proposing specific future experimental implementations beyond the scope of the current computational study.