Design and testing of a fiber-coupled fast neutron scintillation detector for low-power research reactors

¹ école Polytechnique Fédérale de Lausanne (EPFL), Switzerland
² Paul Scherrer Institut (PSI), Switzerland

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Published online: 6 November 2025

Abstract

We present a new fiber-coupled organic scintillator fast neutron detector. Plastic scintillators are used in many applications to detect fast neutrons and gamma rays. The interest in small, fiber-coupled scintillation detectors is in their ability to perform high-resolution neutron flux measurements to ultimately provide multi-physics flux data in research nuclear reactors. Such measurements are planned in the framework of the EURATOM EVEREST project, specifically in the JSI-TRIGA and BME reactors to produce data for code validation.

A 5x5x5 mm EJ-276D scintillator was tested with and without optical fiber coupling to a PMT with Pu-Be and ¹³⁷Cs sources. The Pulse Shape Discrimination histograms with coupling were still showing a region of interest for neutron detection. We produced a prototype reactor sensor with a surface of 1 mm² (V=0.3 mm³) for high resolution experiments. The 0.3 mm³ scintillators were coupled to a 20 m optical fiber for a set of experiments with a strong Co⁶⁰ (≈250 GBq) source and in the zero-power reactor CROCUS. The neutron detection capability was assessed in CROCUS for different power levels. The fiber-coupled small-sized scintillators have shown a linear response up to 25 W (total flux around 3.8x10⁸ cm⁻².s⁻¹). Ongoing developments include testing alternative scintillation materials such as stilbene and organic glass. Future work will focus on experiments in the JSI-TRIGA reactor at higher neutron fluxes and smaller detector volumes.