1. New High-Resolution Gadolinium-GEM Neutron Detectors for the NMX Instrument at ESSs D. Pfeiffer 1,2, F. Resnati 1,2, R. Hall-Wilton 1,4, J. Birch 3, M. Etxegarai 3, C. Hoglund 1,3, I. Llamas-Jansa 5, E. Oksanen 1, E. Oliveri 2, L. Robinson 1, L. Ropelewski 2, S. Schmidt 1,3, Ch. Streli 6, P. Thuiner 6 1 European Spallation Source ESS AB, Sweden, 2 CERN, CH-1211 Geneva 23, Switzerland, 3 Linkoping University, IFM SE-58183 Linkoping, Sweden, 4 Mid-Sweden University, SE-85170 Sundsvall, Sweden, 5 Institute of Energy Technology IFE, NO-2007 Kjeller, Norway, 6 Vienna University of Technology, 1040 Vienna, Austria Region σ [μm]+/- error [μm] 1 (Tape) 24820 2 (Tape) 34122 3 (Tape) 32021 4 (Tape) 24217 5 (Beam) 154246 6 (Beam) 194958 7 (Beam) 205355 8 (Beam) 231272 9 (Beam) 192454 10 (Beam) 2417100 Gas Electron Multipliers (GEMs) with solid converters GEM detectors are Micro Pattern Gaseous Detectors (MPGDs) and were developed at CERN by F. Sauli in 1996 3. GEMs (figure 4) are studied and continuously further developed by RD51 4, a strong R&D collaboration of ca. 500 scientists and 100 institutes world wide. Since GEM detectors with solid neutron converters are a promising option for NMX due to their stable operation under high gain and high rates, the ESS joined the RD51 collaboration in 2013. NMX will be equipped with 3 detectors with a surface of 60 cm x 60 cm mounted on robotic arms (figure 1). Depending on the needed neutron detection efficiency, either 10 B 4 C or Gd can be used. With enriched Gd a detection efficiency for thermal neutrons of up to 45% can be obtained. If 10 B 4 C is used, the neutron capture produces an α or Li ion that is then detected in the gas volume 5. In the case of Gd, conversion electrons with energies between 20keV and 200 keV and a γ cascade are created. Measurement at the R2D2 beam line at IFE Measurements were carried out at the R2D2 beam line at the JEEP II research reactor at the Institute for Energy Technology (IFE) in Kjeller/Norway 6. The R2D2 beam line is the joint ESS-IFE beam line for detector tests. The system was optimized to deliver mono- energetic neutrons of 2Å, with a maximum flux of 11 kHz at a beam size of 2cm x 2 cm. The beam line was equipped with a beam monitor, two movable Cd-loaded collimating slits, and a calibrated 3He tube (figure 6). A Triple-GEM detector with a 250 μm thick natural Gd cathode was tested in the configuration show in figure 5, and a neutron detection efficiency of 9% was obtained. The drift length was 10 mm, and the drift field 700 V/cm. The detector was operated at a gain of 5000, and flushed with Ar/CO2 (70%/30%) with a flow of 10 stl/h. The anode was segmented into two orthogonal sets of strips, 400 μ m wide, that covered a surface of 10 cm x 10 cm. Each strip was connected to an APV25 7 hybrid preamplifier, the signals were digitized with the SRS 8, and acquired with DATE 9 and AMORE 10 software. The μ TPC concept Ionizing particles crossing a portion of the gas volume create electrons and ions that move in opposite directions separated by the electric field. The speed of α particles and conversion electrons is much larger than the electron drift velocity, therefore the primary charge can be assumed to be released instantaneously along their path. The electron cloud moves rigidly at a constant speed due to the uniform electric field. Neglecting the small amount of diffusion, the electron cloud preserves its original track shape along the drift. Results and outlook In presence of a sharp edge like the one created by the Copper tape, a position resolution of < 350 μ m is obtained with a Gd Triple-GEM. This is slightly worse than the < 200 μ m obtained with a Boron Single-GEM 12. The position resolution of both detectors using the μ TPC method fulfills the requirements for NMX and is about 6 times as good as a center-of-gravity approach (figure 12). The resolution looking at the edge of the beam is limited by the divergence of the collimation system and the scattering of the neutrons in the readout board. The detector development for NMX is right on track, next steps include tests with enriched Gd cathodes to improve the efficiency and different readout materials to reduce scattering. Position resolution The x view of one typical track is shown in figures 7 and 8. Whereas tracks created by α particles are straight, the conversion electrons leave curled tracks. Since the converter is on the cathode, the beginning of the track has always the largest drift time. This concept forms the base of the micro Time Projection Chamber (μTPC) 11. To find the strip with the largest drift time of the signal, different algorithms can be used. In case of 10 B 4 C, the waveform in each strip has only one maximum, so that a constant fraction discriminator can be used (figure 9c). For the Gd, the waveform can have several local maxima, in which case the time of the last local maximum (figure 9a) should be used. The start of the track is then the strip with the latest time for the last local maximum. The algorithms 9b and 9d lead to slightly worse results. Figure 11 shows the reconstructed x/y hit distribution for a collimated beam of 3mm x 10 cm. The detector was rotated by 7 degrees. Two set of edges can be seen: the edges due to the collimation of the beam and the edges created with 50 μm of Copper tape that stops the majority of conversion electrons from entering the gas volume. The NMX instrument The European Spallation Source (ESS) 1 in Lund (Sweden) is foreseen to be operational in 2019 and will become the world's most powerful thermal neutron source. One of the planned 22 neutron scattering instruments is the macromolecular crystallography (figure 2) instrument NMX 2. NMX requires an excellent neutron detection efficiency, high-rate capabilities, time resolution to resolve spatial overlap (figure 3), and an unprecedented spatial resolution in the order of a few hundred micrometers. 1) NMX 2) Chrystal 3) Overlap 5) GEM with Gd converter 4) GEM foil 6) R2D2 beamline at IFE with Gd-GEM 8) Track by Gd conversion electron7) Track by 10 B 4 C alpha 9) Waveform on one strip and different start-of-track algorithms 10) Gd cathode11) Hit distribution with analyzed positions 12) Distribution of reconstructed y-coordinate in region 3 a) last local maximum b) global maximum c) discriminator rising flank d) discriminator falling flank 1 European Spallation Source ESS AB, http://europeanspallationsource.se/. 2 S. Peggs et al, ESS Technical Design Report, ESS-2013-0001 (2013). 3 F. Sauli, GEM: A new concept for electron amplification in gas detectors, Nuclear Instruments and Methods in Physics Research A 386 (1997) 531. 4 M. Titov and L. Ropelewski, Micro-pattern gaseous detector technologies and RD51 collaboration,Modern Physics Letters A 28 (2013) 1340022. 5 C. Hoglund et al., B4C thin films for neutron detection, Journal of Applied Physics 111 (2012) 104908. 6 Institute for Energy Technology IFE, http://ife.no/. 7 M. French et al, Design and Results from the APV25, a Deep Submicron CMOS Front-End Chip for the CMS Tracker, Nuclear Instruments and Methods in Physics Research A 466 (2001) 359-365. 8 S. Martoiu et al, Development of the scalable readout system for micro-pattern gas detectors and other applications, Journal of Instrumentation 8 (2013) C03015. 9 F. Carena et al, The ALICE data acquisition system, Nuclear Instruments and Methods in Physics Research A 741 (2014) 130-162. 10 F. Roukoutakis and B. von Haller, Commissioning of the ALICE-LHC online data quality monitoring framework, Nuclear Instruments and Methods in Physics Research A 603 (2009) 446-450. 11 G. Iakovidis, The Micromegas project for the ATLAS upgrade, Journal of Instrumentation 8 (2013) C12007. 12 D. Pfeiffer et al., The uTPC method: improving the position resolution of neutron detectors based on MPGDs, Journal of Instrumentation 10 (2015) P04004.