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-221 00 Lund, Sweden2CERN, CH-1211 Geneva 23, Switzerland3Mid-Sweden University, SE-851 70 Sundsvall, Sweden4National Technical University of Athens, 10682 Athens, Greece5Vienna University of Technology, 1040 Vienna, Austria6Uludag University, 16059 Nilufer-Bursa, Turkey

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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

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Boron-GEM measurements

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10B4C coated Al cathode

• 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

GEM with cathode and x/y readout

Boron-GEM spectra

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• 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 driftand 1 um of 10B4C

Measured spectrum with SRS, APV-25 and single GEM Measured spectrum with MCA and triple GEM

uTPC analysis

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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

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(*) G. Iakovidis, The Micromegas project for the ATLAS upgrade, Journal of Instrumentation 8 (2013) C12007

uTPC Boron: track and time structure of /Liα +

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Digitized raw waveforms x Digitized raw waveforms y

Track x Track y

Start of track Start of track

Drift time of each signal is defined by means of a simple constant fraction discriminator

Boron-GEM hit distribution

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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

Triple GEM hit distribution,10B4C coated cathode covering50% of detector

Single GEM hit distribution,10B4C coated cathode covered with copper tape

Boron-GEM position resolution

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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

Triple GEM Single GEM

Geant4 Simulation Framework

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• 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/

Thomas Kittelmann et al., http://arxiv.org/abs/1311.1009

Geant4 Gd Simulations

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Converter

Drift backwards

0.25 – 50 um

25 meV neutronsScoring of electrons that cross boundary between converter and drift

Drift forwards

• 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

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44 %

155 Gd: 9 um optimal

157 Gd: 3 um optimal 35 %

19 %

Nat. Gd: 6 um optimal

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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Gd-GEM measurements

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14First measurements in backwards configuration with drift space of 3 mm and gain of 4000

Primary ionization spectrum

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uTPC Gd: and eγ - track and time structure

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Time structure γ Time structure electron

γ particle (55 Fe) electron

Start of track

For the tracking of electrons, a more elaborate algorithm is needed than for alphas

Gd-GEM comparision Gd/normal cathode

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241 AmBe unshielded 241 AmBe lead shielded

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)

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Backup slides

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Neutron capture and conversion efficiencies

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Spectra of conversion electrons

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natural Gd

natural Gd

157 Gd

157 Gd

Mean: 67 keV

Mean: 69 keV

Mean: 60 keV

Mean: 54 keV

Creationconverter

Arrivaldrift