Developing and testing a miniature fiber-coupled scintillator for in-core neutron counting in CROCUS

Abstract

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.

Design of a 150-miniature detectors 3D core-mapping system for the CROCUS reactor

“…The first prototype of a miniature neutron detector was developed and successfully tested at EPFL in collaboration with PSI in 2018 [6], [7]. This first development version was composed by a 1 mm 2 × 0.2 mm ZnS: 6 LiF(Ag) screen [9] positioned at the tip of a 2-mm core optical fiber [10].…”

“…This first development version was composed by a 1 mm 2 × 0.2 mm ZnS: 6 LiF(Ag) screen [9] positioned at the tip of a 2-mm core optical fiber [10]. The blue light produced (with peak emission at 450 nm [9]) after a thermal neutron interaction with 6 Li in the screen is transported through the optical fiber to a silicon photomultiplier (SiPM) [11], where each photon event is translated into an electrical signal after proper processing. Neutron events are then discriminated from background on the basis of the photondensity variation in the stream of photon pulses.…”

“…In addition, CROCUS is a readily available reactor with low technological uncertainties [5], where it would be possible to accurately position ~150 neutron detectors within the whole core volume upon the design of a proper mechanical supporting structure. At the same time, the Laboratory for Reactor Physics and Systems Behaviour (LRS) at EPFL in collaboration with the Paul Scherrer Institut (PSI) had developed a prototype of a miniature detector with an active volume below 1 mm 3 [6], [7]. The detection technology is based on the well-proven coupling of a miniature ZnS: 6 LiF(Ag) scintillator to a state-of-the-art silicon photomultiplier (SiPM) via jacketed optical fibers.…”

“…At the same time, the Laboratory for Reactor Physics and Systems Behaviour (LRS) at EPFL in collaboration with the Paul Scherrer Institut (PSI) had developed a prototype of a miniature detector with an active volume below 1 mm 3 [6], [7]. The detection technology is based on the well-proven coupling of a miniature ZnS: 6 LiF(Ag) scintillator to a state-of-the-art silicon photomultiplier (SiPM) via jacketed optical fibers. After optimization in dimensions, processing electronics, and acquisition scheme, such detectors suit the needs in terms of size and scalability for the setup of a 3D mapping system in CROCUS.…”

Delayed gamma fraction determination in the zero power reactor CROCUS

“…CROCUS has a recent history of neutron detector developments, such as fission chambers in current mode [20,21], fiber based miniature scintillators for neutron detection [22,23], or diamond based semiconductor detectors [24]. Gamma detection capabilities were added in the form of the LEAF system, an array of four scintillation based gamma spectrometers [16,25].…”