1. Novel MPGD Based Neutron Detectors for the European Spallation Source
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 Lund, Sweden
2CERN, CH-1211 Geneva 23, Switzerland
3Mid-Sweden University, SE Sundsvall, Sweden
4National Technical University of Athens, Athens, Greece
5Vienna University of Technology, 1040 Vienna, Austria
6Uludag University, Nilufer-Bursa, Turkey
Dorothea Pfeiffer

2. 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
Dorothea Pfeiffer

3. 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)
10B4C coated Al cathode
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GEM with cathode and x/y readout

4. 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
Geant4 simulation of deposited Energy in 8 mm drift and 1 um of 10B4C
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Measured spectrum with SRS, APV-25 and single GEM
Measured spectrum with MCA and triple GEM

5. uTPC analysis
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6. 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 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
Dorothea Pfeiffer
(*) G. Iakovidis, The Micromegas project for the ATLAS upgrade, Journal of Instrumentation 8 (2013) C12007

7. uTPC Boron: track and time structure of α/Li+
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
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Digitized raw waveforms x
Digitized raw waveforms y

8. Boron-GEM hit distribution
Triple GEM hit distribution,
10B4C coated cathode covering
50% of detector
Single GEM hit distribution,
10B4C coated cathode covered with copper tape
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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

9. Boron-GEM position resolution
Triple GEM
Single GEM
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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

10. Geant4 Simulation Framework
Thomas Kittelmann et al.,
ESS Geant4 simulation framework already includes description of neutron diffraction in polycrystals, plugin freely available for noncommercial purposes at
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
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11. Geant4 Gd Simulations
25 meV neutrons
Scoring of electrons that cross boundary between converter and drift
Drift backwards
0.25 – 50 um
Converter
Drift forwards
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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)

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

13. Gd simulation and coatings
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
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14. Gd-GEM measurements
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First measurements in backwards configuration with drift space of 3 mm and gain of 4000

15. Primary ionization spectrum
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16. uTPC Gd: γ and e- track and time structure
Start of track
γ particle (55 Fe)
electron
For the tracking of electrons, a more elaborate algorithm is needed than for alphas
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Time structure γ
Time structure electron

17. Gd-GEM comparision Gd/normal cathode
241 AmBe unshielded
241 AmBe lead shielded
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18. 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)
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19.
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20.
Backup slides
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21. Neutron capture and conversion efficiencies
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22. Spectra of conversion electrons
natural Gd
157 Gd
Mean: 67 keV
Mean: 60 keV
Creation converter
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157 Gd
natural Gd
Mean: 54 keV
Arrival drift
Mean: 69 keV