Novel MPGD Based Neutron Detectors for the European Spallation Source 13.11.2014 D. Pfeiffer1,2, F. Resnati2, R. Hall-Wilton1,3, G. Iakovidis2,4, K. Kanaki1, T. Kittelmann1, E. Olivieri2, L. Ropelewski2, H. Schindler2, I. Stefanescu1, P. Thuiner2,5, R. Veenhof2,6 1European Spallation Source ESS AB, SE-221 00 Lund, Sweden 2CERN, CH-1211 Geneva 23, Switzerland 3Mid-Sweden University, SE-851 70 Sundsvall, Sweden 4National Technical University of Athens, 10682 Athens, Greece 5Vienna University of Technology, 1040 Vienna, Austria 6Uludag University, 16059 Nilufer-Bursa, Turkey Dorothea Pfeiffer

Content New spallation sources like the ESS require neutron detectors with so far unprecedented position resolution, rate capabilities and detection efficiencies Due to the He3 crisis and the increased requirements, new detector concepts with new neutron converters are needed As part of the R&D effort, three topics will be presented here Improving position resolution: Measurements with Boron-GEM and uTPC analysis Simulation of gas detectors for neutron detection: Geant4/Garfield interface High efficiency detector: Gd-GEM simulations and first measurements 13.11.2014 Dorothea Pfeiffer

Boron-GEM measurements Triple and Single GEM (*) detectors with Al cathodes coated with 1 μm 10B4C and x/y strip readout (2*256 strips, 400 um pitch) Parts of the cathode not coated or covered with copper tape to stop α/Li+ 8 mm drift space with E=1kv/cm and detector gain of ~200 (*) M. Titov and L. Ropelewski, Micro-pattern gaseous detector technologies and RD51 collaboration, Modern Physics Letters A 28 (2013) 1340022 13.11.2014 10B4C coated Al cathode Dorothea Pfeiffer GEM with cathode and x/y readout

Boron-GEM spectra Measurements of 370 MBq 241 AmBe source with PE shield 10B4C has a cross section of 3835 barns for thermal neutrons n + 10B -> 7Li*(0.84 MeV) + a (1.47 MeV) + g (0.48 MeV) (93%) Q=2.3 MeV n + 10B -> 7Li (1.16 MeV) + a (1.78 MeV) (7%) Q=2.79 MeV Within the energy resolution of the detectors, the simulated spectrum could be reconstructed 13.11.2014 Geant4 simulation of deposited Energy in 8 mm drift and 1 um of 10B4C Dorothea Pfeiffer Measured spectrum with SRS, APV-25 and single GEM Measured spectrum with MCA and triple GEM

uTPC analysis 13.11.2014 Dorothea Pfeiffer

uTPC analysis At a drift field of 1 kV/cm, electrons in ArCO2 (70%/30%) have a drift velocity of about 4 cm/μs The drift speed is slow in comparison with the speed of the alpha particle ~ m/μs Hence in a detector with 8 mm drift, the signal of charges created close to the cathode is detected about 200 ns after the signal of charges created close to the first GEM Idea: Use this time difference in combination with the 27 25 ns time bins of the APV-25 readout chip to create a software TPC or uTPC(*) that can do tracking Determine the beginning of the track and use this value instead of the center of mass to improve position resolution 13.11.2014 Dorothea Pfeiffer (*) G. Iakovidis, The Micromegas project for the ATLAS upgrade, Journal of Instrumentation 8 (2013) C12007

uTPC Boron: track and time structure of α/Li+ 13.11.2014 Start of track Start of track Track x Track y Drift time of each signal is defined by means of a simple constant fraction discriminator Dorothea Pfeiffer Digitized raw waveforms x Digitized raw waveforms y

Boron-GEM hit distribution 13.11.2014 Triple GEM hit distribution, 10B4C coated cathode covering 50% of detector Single GEM hit distribution, 10B4C coated cathode covered with copper tape Dorothea Pfeiffer To evaluate the position resolution, regions with a pronounced difference in the number of hits were analyzed The position derived from the reconstructed start of the track was compared to the center of mass approach

Boron-GEM position resolution 13.11.2014 Triple GEM Single GEM Dorothea Pfeiffer Distribution of the reconstructed x coordinate using a center-of-mass-based technique (in blue) and the TPC analysis (in red). The position resolution extracted from the fit of the ERF functions is σ= 1 mm for the center-of-mass-based reconstruction and σ= 330 um (left) and σ= 200 μm (right) for the start of track Position resolution goal of σ =200 μm met

Geant4 Simulation Framework Thomas Kittelmann et al., http://arxiv.org/abs/1311.1009 13.11.2014 ESS Geant4 simulation framework already includes description of neutron diffraction in polycrystals, plugin freely available for noncommercial purposes at http://cern.ch/nxsg4 For simulations of neutron gas detectors it is desirable to use Geant4 and Garfield++ in the same simulation to correctly describe neutron capture, primary ionization, drift, amplification and induction of signal in one simulation Description of developed Geant4/Garfield interface can be found http://garfieldpp.web.cern.ch/garfieldpp/examples/geant4-interface/ Dorothea Pfeiffer

Geant4 Gd Simulations 25 meV neutrons 13.11.2014 Scoring of electrons that cross boundary between converter and drift Drift backwards 0.25 – 50 um Converter Drift forwards Dorothea Pfeiffer Geant4 simulations to evaluate different converter materials and thicknesses Natural Gd, 155 Gd, 157 Gd, Gd2O3 and enriched Gd2O3 were simulated Neutron capture in Gd leads to γ cascade and sometimes conversion e- Simulations carried out with Geant4.10, G4NDL4.4 and flag G4NEUTRONHP_USE_ONLY_PHOTONEVAPORATION (final state data for gammas is not used)

Gd optimum converter thickness 44 % 155 Gd: 9 um optimal 13.11.2014 35 % 157 Gd: 3 um optimal 19 % Dorothea Pfeiffer Nat. Gd: 6 um optimal

Gd simulation and coatings 13.11.2014 Oxides lead to results comparable to metals for the number of captured neutrons and conversion electrons created, but as insulator need a thin conductive layer Contrary to what is found in the literature, 155 Gd has a higher percentage of conversion electrons per captured neutron and a higher efficiency than 157 Gd (to be verified by measurements) Collaboration ongoing between ESS, CERN TE-VSC and Linkoping University to develop and improve Gd coatings Dorothea Pfeiffer

Gd-GEM measurements 13.11.2014 Dorothea Pfeiffer First measurements in backwards configuration with drift space of 3 mm and gain of 4000

Primary ionization spectrum 13.11.2014 Dorothea Pfeiffer

uTPC Gd: γ and e- track and time structure 13.11.2014 Start of track γ particle (55 Fe) electron For the tracking of electrons, a more elaborate algorithm is needed than for alphas Dorothea Pfeiffer Time structure γ Time structure electron

Gd-GEM comparision Gd/normal cathode 13.11.2014 241 AmBe unshielded 241 AmBe lead shielded Dorothea Pfeiffer Gd Gd

Summary first Gd measurements Detector principle is working Higher rate in part of detector with Gd cathode Higher number of electron tracks starting at Gd side Difference more pronounced when gammas from 241 AmBe source are shielded, and thus neutrons form a larger part of the source flux Detailed performance measurements planned (neutron beam) 13.11.2014 Dorothea Pfeiffer

13.11.2014 Dorothea Pfeiffer

13.11.2014 Backup slides Dorothea Pfeiffer

Neutron capture and conversion efficiencies 13.11.2014 Dorothea Pfeiffer

Spectra of conversion electrons 13.11.2014 natural Gd 157 Gd Mean: 67 keV Mean: 60 keV Creation converter Dorothea Pfeiffer 157 Gd natural Gd Mean: 54 keV Arrival drift Mean: 69 keV