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This article presents the development, full-characterization, and in-core testing of a miniature neutron detector for the investigation of highly localized in-core neutronics effects in the zero-power reactor CROCUS, operated at the Ecole Polytechnique federale de Lausanne (EPFL), Switzerland. A ZnS: 6LiF screen, mixing inorganic scintillator and neutron converter, with a surface of 1 mm2 and a thickness of 0.2 mm, was coupled with a silicon photomultiplier (SiPM) via a 10-m optical fiber. Analog readout electronics were adopted in this prototype version to process the SiPM output signal. A first campaign was carried out to assess the detection system capabilities in terms of neutron detection and counting in mixed neutron-gamma radiation fields. The miniature detector was thoroughly characterized with a Pu-Be source installed in the CARROUSEL facility and was subsequently tested inside a control rod guide tube of the CROCUS reactor in different reactor conditions, i.e., at shutdown, startup, and critical. The detector shows a linear response with a reactor power increase up to 6.5 W (i.e., around $10 {^{\mathrm{ 8}}}\,\,\text {cm}^{-2}\cdot \text {s}^{-1} $ total neutron flux) and excellent neutron counting capabilities, when compared to other localized detection systems available in CROCUS, such as the miniature fission chambers and single-crystal vapor deposited (sCVD) diamond detectors. In addition, the exposure of the miniature detector to a 60Co source with an activity of ~250 GBq, combined with the computation of the gamma flux in CROCUS with the Serpent 2 Monte Carlo code, confirmed the gamma insensitivity of the system in a mixed neutron-gamma field.

It is generally accepted that the count rate capability of ZnS:6LiF neutron detectors is strongly limited due to the long afterglow of the scintillator. However, realized as an array of individual sensitive elements (pixels) with independent signal readout and processing, such detectors can be operated with blocking times down to 1 $\mu \text{s}$ and promising counting rates up to 100 kHz/pixel. In this paper, we investigate how the long emission of the ZnS scintillator influences the detector performance apart from requiring a finite blocking time to suppress the probability of multiple counts. We also study the potential of the ZnO:6LiF scintillator with lower afterglow as a possible replacement of ZnS:6LiF.

An advanced neutron detection system for highly localized measurements in nuclear reactor cores was developed and tested in the Laboratory for Reactor Physics and System Behaviour (LRS) at the École polytechnique fédérale de Lausanne (EPFL), Switzerland, in close collaboration with the Detector group of the Laboratory for Particle Physics (LTP) at the Paul Scherrer Institute (PSI), Switzerland. The miniature-size detector is based on the coupling of a ZnS:6LiF scintillator/converter screen of 1 mm2 and 0.2 mm thickness with a 10-m optical fiber, the latter being connected to a silicon photomultiplier (SiPM). In this development version, the output signal is processed via analog read-out electronics. The present work documents the characterization of a detection system prototype in the mixedradiation fields o f t he C ARROUSEL f acility a nd i ts t esting in the CROCUS zero-power reactor operated at LRS. The fibercoupled scintillator shows a linear response with the reactor power increase up to 6.5 W (i.e. around 108 cm-2s-1 total neutron flux), with a s ubsequent l oss o f l inearity d ue t o e lectronic dead time of the analog system. Nevertheless, the detector shows excellent neutron counting capabilities whether compared to other localized detection systems available at LRS, e.g. miniature fission chambers and an sCVD diamond detector.

The efficiency of detecting signals from neutron absorption events (trigger efficiency) in detectors using ZnS scintillators loaded with neutron absorbers (6Li, 10B) is prone to be limited to values well below 100%: the non-transparency of the scintillator results in a wide dynamic range of the detected signals with some fraction always falling under the detection threshold set to satisfy the requirements on the gamma sensitivity, multi-count ratio, and the background count rate of the detector. The question of the limited trigger efficiency of such detectors is very seldom discussed in publications, yet it is very important as the product of the trigger efficiency and the probability of neutron absorption in the detector defines its overall efficiency. Available data indicate that the trigger efficiency of PMT-based detectors used in neutron diffraction experiments can be as low as $\sim 50$ %. In this work we evaluate the trigger efficiency of a SiPM-based ZnS:6LiF scintillation neutron detector developed to replace a 3He-detector of the POLDI time-of-flight diffractometer at the Paul Scherrer Institut in Switzerland. Requiring the gamma sensitivity with a 60Co source $\le 10^{-7}$ , the multi-count ratio $\le 10^{-3}$ with the detector dead time set to $10~\mu \text {s}$ , and the background count rate $\le 10^{-3}$ Hz we obtain the maximum possible trigger efficiency of $\sim 85$ % with the main restriction coming from the gamma sensitivity condition. Considering the neutron absorption probability of $\sim 80$ % at 1°A, the detector efficiency at this wavelength amounts to $\sim 70$ %, which is comparable to that of the current 3He-detector of the POLDI instrument.

In this paper we present the development of a one-dimensional multi-channel thermal neutron detection system for the application in neutron scattering instrumentation, e.g. strain-scanning diffractometers. The detection system is based on ZnS(Ag):6LiF neutron scintillator with embedded WLS fibers which are read out with a SiPM. A dedicated signal processing system allows us to suppress the SiPM dark counts and to extract the signals from the neutron absorption events. For a single-channel detection unit which represents the elementary building block of this detection system we achieved a neutron detection efficiency of ~65% at 1.2 A, a background count rate 10 − 3 Hz and a gamma-sensitivity 10 − 6 (measured with a 60Co source), while the dead time is ~20 μs and the multi-count ratio is 1 % . This performance was achieved even for SiPM dark count rates of up to ~2 MHz.

In this work we revisit the problem of the restricted count rate capability of thermal neutron detectors utilizing the ZnS scintillator. The effect of the long afterglow of the scintillator on the detector performance at high counting rates is studied. A count rate capability substantially above values currently regarded as feasible is demonstrated.

In the work presented here we built and evaluated a single-channel neutron detection unit consisting of a ZnS:$^6$LiF scintillator with embedded WLS fibers readout by a SiPM. The unit has a sensitive volume of 2.4 x 2.8 x 50 mm$^3$; 12 WLS fibers of diameter 0.25 mm are uniformly distributed over this volume and are coupled to a 1 x 1 mm$^2$ active area SiPM. We report the following performance parameters: neutron detection efficiency $\sim 65\,$% at $1.2\,��$, background count rate $< 10^{-3}$ Hz, gamma-sensitivity with $^{60}$Co source $< 10^{-6}$, dead time $\sim 20\,��$s, multi-count ratio $< 1\,$%. All these parameters were achieved up to the SiPM dark count rate of $\sim 2\,$MHz. We consider such detection unit as an elementary building block for realization of one-dimensional multichannel detectors for applications in the neutron scattering experimental technique. The dimensions of the unit and the number of embedded fibers can be varied to meet the specific application requirements. The upper limit of $\sim 2\,$MHz on the SiPM dark count rate allows to use SiPMs with larger active areas if required., 10 pages, 10 figures

In this new concept of 2-dimensional scintillation detector, the detector consists of an almost gapless assembly of individual and optically isolated ZnS/6LiF bars with embedded and uniformly distributed WLS fibers running from one end of the bar to the other. The longitudinal position is determined from the left-right asymmetry of the number of photons detected on both sides of the fibers while the transverse position is simply given by the transverse position of the bar. Special Y11(400)M WLS fibers with different attenuation lengths ranging from 13 cm to 30 cm have been developed by Kuraray for this specific application. Two different detection units have been built. Their performance in a neutron beam are presented and compared with simulation results.

Recently we reported on a high efficiency, low gamma-sensitivity, high count rate capable 1D neutron detector consisting of ZnS:6LiF scintillator readout with WLS-fibers coupled to SiPMs which showed a performance comparable with a 3He-based detector. In this work we discuss a possible extension of the developed technology to large area 2D detectors with high count rate capability suitable, for example, for the new generation of neutron powder diffraction instruments planned at the European Spallation Source.

A scalable 16-ch thermal neutron detection system has been developed in the framework of the upgrade of a neutron diffractometer . The detector is based on a ZnS: 6 LiF scintillator with embedded WLS fibers which are read out with SiPMs. In this paper, we present the 16-ch module, the dedicated readout electronics, a direct comparison between the performance of the diffractometer obtained with the current 3He detector and with the 16-ch detection module, and the channel-to-channel uniformity.

We present a digital signal processing system based on a photon counting approach which we developed for a thermal neutron detector consisting of ZnS(Ag):6LiF scintillating layers read out with WLS fibers and SiPMs. Three digital filters have been evaluated: a moving sum, a moving sum after differentiation and a digital CR-RC^4 filter. The performances of the detector with these filters are presented. A full analog signal processing using a CR-RC^4 filter has been emulated digitally. The detector performance obtained with this analog approach is compared with the one obtained with the best performing digital approach.