Speaker
Description
Miniature in-core neutron detectors are of interest for a variety of experiments in research reactors, for instance for highly local flux measurements, multi-physics high-resolution measurements, or even in-core noise experiments. The typical challenge associated with in-core detectors is the limited space available within the core between fuel elements. To address the geometrical challenges, miniature scintillation detectors (in the range of mm³ or below) provide cost-effective local detection means, as demonstrated recently at EPFL in the thermal energy range (MiMi detectors and SAFFRON array). As radiation in the core can perturb or damage commonly used light collection systems such PMTs, and especially SiPMs, one common solution is to fiber-couple the scintillator, thereby enabling out-of-core signal analysis, and further increasing space-saving within the core.
To detect fast neutrons, plastic scintillators capable of pulse shape discrimination can be employed. Thanks to the difference in pulse decay shape by electron-induced scintillation vs proton-recoil-induced scintillation, individual pulses can be classified and discriminated into gamma ray or neutron-induced pulses. The relatively compact dimensions of the detectors, while keeping reasonable sensitivity, might also lead to the reduction of gamma-induced signals, thereby enhancing the specificity of neutron signals. The general application case of such small fast neutron scintillators is to observe local fast flux changes in the reactor core, at reduced cost allowing for scalability, which is a hitherto relatively poorly explored domain of reactor measurements.
In this contribution, we present the design and first testing of a fiber-coupled plastic scintillator for fast neutron detection. We outline the design, electronics, and challenges encountered. Fast neutron scintillators, due to the inherently smaller interaction cross-section, could provide experimental results at powers up to 100kW, which is relevant for the instrumentation developments and experiments in the framework of the Horizon Europe project EVEREST dedicated to multi-physics validation. Our detector comprises a cube of EJ-276D plastic scintillator coupled to an optical fiber (Super ESKA with 2 mm core and 3 mm total diameter). Two plastic cube scintillators are tested: a 10x10x10 mm cube and a 5x5x5 mm cube. An aluminum holder is used to ensure the centering of the fiber on the scintillator. The fiber is then coupled to a Hamamatsu H3178-51 Photo-Multiplier Tube (PMT). To optically couple the surfaces we use the EJ 550 optical grease. For signal acquisition we use a 500MS/s CAEN DS 5730S digitizer using the DPP-PSD firmware.
The first tests were conducted with a PuBe neutron source in the water of the CARROUSEL facility at EPFL to demonstrate the feasibility of a pulse shape discrimination with the 10x10x10 mm and 5x5x5 mm cubes. To solve light losses and pile-up issues, we investigated the effect of fiber diameter and fiber length to determine practical limits. We also investigated different pulse shape discrimination parameters and post-processing strategies (e.g. pulse integration times) to recover the neutron-induced signals. Future developments include the testing of other organic scintillator materials, such as organic glass scintillator and stilbene, sub mm3 detector volumes, and wavelength shifting fibers. In-core tests are carried out in the EPFL reactor CROCUS to assess the detectors’ performance. Further experiments at the Slovenian JSI TRIGA and the Hungarian BRR reactors are planned within EVEREST.
