G. Claps

nGEM neutron diagnostic concept for high power deuterium beams

The ITER neutral beam test facility under construction in Padova will host two experimental devices: SPIDER, a 100 kV negative H/D RF source, and MITICA, a full scale, 1 MeV deuterium beam injector. Detection of fusion neutrons will be used as a means to resolve the horizontal beam intensity profile. The neutron detection system will be placed right behind the SPIDER beam dump, as close to the neutron emitting surface as possible thus providing the map of the neutron emission on the beam dump surface. The system uses nGEM neutron detectors. These are Gas Electron Multiplier detectors equipped with a cathode that also serves as neutron-proton converter foil. The cathode is designed to ensure that most of the detected neutrons at a point of the nGEM surface are emitted from the corresponding 40 × 22 mm2 beamlet footprint on the dump front surface. The nGEM readout pads (area 20 × 22 mm2) will record a useful count rate of 5 kHz providing a time resolution of temporal structures in the neutron fluency rate to be measured of better than 1 s. The directional response of the nGEM to neutrons is a key to reducing the scattering contribution to the measured neutron flux. First results achieved using small size nGEM prototypes shows that these detectors own the directionality property and that experimental results are in agreement with MCNP simulation. In addition they have a neutron efficiency detection of about 10−5 and their response scales linearly following the intensity of the neutron flux.

Triple GEM gas detectors as real time fast neutron beam monitors for spallation neutron sources

A fast neutron beam monitor based on a triple Gas Electron Multiplier (GEM) detector was developed and tested for the ISIS spallation neutron source in U.K. The test on beam was performed at the VESUVIO beam line operating at ISIS. The 2D fast neutron beam footprint was recorded in real time with a spatial resolution of a few millimeters thanks to the patterned detector readout.

Measurements of gamma-ray sensitivity of a GEM based detector using a coincidence technique

Gas Electron Multiplier based detectors have been recently used for neutron measurement at spallation sources and for fusion relevant applications. The present work is meant to characterize the detector sensitivity to ?-ray background which often can represent the main background source associated to neutron measurements. The ?-ray sensitivity has been measured as a function of detector gain and it has been found to be lower than 10?7at the typical gain used for neutrons detection. Coincidence measurements, giving the possibility to select mono-energetic photons and to suppress background events, allowed to measure the absolute sensitivity at a well defined ?-ray energy and at low interaction rates. The mechanism of interaction and energy deposition of ?-rays in the detector has been studied using MCNPX Monte Carlo simulations.

Diffraction measurements with a boron-based GEM neutron detector

The research of reliable substitutes of 3He detectors is an important task for the affordability of new neutron scattering instrumentation for future spallation sources like the European Spallation Source. GEM (Gas Electron Multiplier)-based detectors represent a valid alternative since they can combine high-rate capability, coverage of up to area and good intrinsic spatial resolution (for this detector class it can be better than 0.5 mm). The first neutron diffraction measurements performed using a borated GEM detector are reported. The detector has an active area of and is equipped with a borated cathode. The GEM detector was read out using the standard ISIS Data Acquisition System. The comparison with measurements performed with standard 3He detectors shows that the broadening of the peaks measured on the diffractogram obtained with the GEM is 20–30% wider than the one obtained by 3He tubes but the active area of the GEM is twice that of 3He tubes. The GEM resolution is improved if half of its active area is considered. The signal-to-background ratio of the GEM is about 1.5 to 2 times lower than that of 3He. This measurement proves that GEM detectors can be used for neutron diffraction measurements and paves the way for their use at future neutron spallation sources.

A GEM-based thermal neutron detector for high counting rate applications

Among other neutron detector systems proposed as a possible substitute for 3He tubes, GEM-based ones have shown appealing characteristics, when coupled with suitable neutron-converter cathodes. In this paper, we present the results of a GEM-based neutron detector in a high-flux environment (the ORPH?E reactor in Saclay), especially in terms of maximum rate capability and linearity. Recorded data show that the detector can manage neutron counting rates in the order of 50 ×? 106 counts/sec cm2 while maintaining a reasonable linearity and with no sign of instability.

3He-free triple GEM thermal neutron detector

A novel type of thermal neutron detector based on the gas electron multiplier (GEM) technology is presented in the framework of the research and development activity on the 3He replacement for neutron detection. The device relies on a series of boron-coated alumina sheets placed perpendicularly to the incident neutron beam direction. The detector, named side-on GEM (S-GEM), was tested on beam at the High Flux Isotope Reactor at the Oak Ridge National Laboratory (US) to assess its performance in terms of beam position resolution, efficiency and signal-to-background (S/B) ratio as compared to a 10 bar 3He tube for sub-thermal neutrons. Using 3 mm wide PADs, a sub-millimeter position resolution was obtained. The achieved efficiency is about 30% with a quite good S/B ratio. The obtained results demonstrate the effectiveness of the proposed detector configuration to achieve a good spatial resolution and, in the perspective, a higher thermal neutron efficiency, comparable to 3He tubes typically used for diagnostic in nuclear reactors. The main issues to be addressed to reach the goal, mostly related to boron coating procedures and characterization, are also pointed out.

Neutron beam imaging with GEM detectors

Neutron GEM-based detectors represent a new frontier of devices in neutron physics applications where a very high neutron flux must be measured such as future fusion experiments (e.g. ITER Neutral beam Injector) and spallation sources (e.g. the European Spallation source). This kind of detectors can be properly adapted to be used both as beam monitors but also as neutron diffraction detectors that could represent a valid alternative for the 3He detectors replacement. Fast neutron GEM detectors (nGEM) feature a cathode composed by one layer of polyethylene and one of aluminium (neutron scattering on hydrogen generates protons that are detected in the gas) while thermal neutron GEM detectors (bGEM) are equipped with a borated aluminium cathode (charged particles are generated through the 10B(n,α)7Li reaction). GEM detectors can be realized in large area (1 m2) and their readout can be pixelated. Three different prototypes of nGEM and one prototype of bGEM detectors of different areas and equipped with different types of readout have been built and tested. All the detectors have been used to measure the fast and thermal neutron 2D beam image at the ISIS-VESUVIO beamline. The different kinds of readout patterns (different areas of the pixels) have been compared in similar conditions. All the detectors measured a width of the beam profile consitent with the expected one. The imaging property of each detector was then tested by inserting samples of different material and shape in the beam. All the samples were correctly reconstructed and the definition of the reconstruction depends on the type of readout anode. The fast neutron beam profile reconstruction was then compared to the one obtained by diamond detectors positioned on the same beamline while the thermal neutron one was compared to the imaged obtained by cadmium-coupled x-rays films. Also efficiency and the gamma background rejection have been determined. These prototypes represent the first step towards the realization of new neutron beam monitors for fusion experiments and spallation sources.

Investigation on thermal neutron detectors based on the Gas Electron Multiplier technology

A thermal neutron detector based on the Gas Electron Multiplier technology is presented. It is configured to let neutron beam to impinge on the detector laterally, in order to allow neutron beam interaction with a series of borated glass layers placed along the neutron path inside the device. The detector was tested on beam at ISIS spallation neutron source and at the TRIGA reactor at ENEA-Centro Ricerche Casaccia.

Progress on the realization of a new GEM based neutron diagnostic concept for high flux neutron beams

Fusion reactors will need high flux neutron detectors to diagnose the deuterium-deuterium and deuterium-tritium. A candidate detection technique is the Gas Electron Multiplier (GEM). New GEM based detectors are being developed for application to a neutral deuterium beam test facility. The proposed detection system is called Close-contact Neutron Emission Surface Mapping (CNESM). The diagnostic aims at providing the map of the neutron emission due to interaction of the deuterium beam with the deuterons implanted in the beam dump surface. This is done by placing a detector in close contact, right behind the dump. CNESM uses nGEM detectors, i.e. GEM detectors equipped with a cathode that also serves as neutron-proton converter foil. After the realization and test of several small area prototypes, a full size prototype has been realized and tested with laboratory sources. Test on neutron beams are foreseen for the next months.

A new GEM based neutron diagnostic concept for high flux neutron beams

Fusion-fission hybrid reactors will need high flux neutron detectors to diagnose the deuterium-tritium fusion plasmas as well as the fission reactions. A candidate detection technique is the Gas Electron Multiplier (GEM). New GEM based detectors are being developed for application to a neutral deuterium beam test facility. The proposed detection system is called Close-contact Neutron Emission Surface Mapping (CNESM). The diagnostic aims at providing the map of the neutron emission due to interaction of the deterium beam with the deuterons implanted in the beam dump surface. This is done by placing a detector in close contact, right behind the dump. CNESM uses nGEM detectors, i.e. GEM detectors equipped with a cathode that also serves as neutron-proton converter foil. A small size prototype of nGEM detector has been built and is ready to be tested.