I. Stefanescu 1,2, K. Zeitelhack 2, R. Hall-Wilton 1, C. Höglund 1,3, I. Defendi 2, M. Zee 2, L. Hultman 3, J. Birch 3, R. Schneider 4 1 European Spallation Source ESS AB, Lund, Sweden 2 MLZ/FRM2, Technische Universität München, Garching, Germany 3 Department of Physics, Chemistry and Biology (IFM), Linköping University, Linköping, Sweden 4 Mesytec GmbH & Co. KG, Germany A Novel 10 B-Based Detector with Stacked Macrostructured Cathodes and Resistive Wire Readout for Neutron Scattering Applications

The whereabouts 2 Project started over three years ago at the FRM2 research reactor in Munich. Framework: German In-Kind contribution to the ESS Design Upgrade Phase. Task: investigate the feasibility of replacing the He-3 tubes in large-area neutron detectors by a technology based on the B-10 solid converter. Some financial support through the NMI-3 FP7-II program.

The concept 33 3D regular pattern can be created by employing milling, extrusion, forming, etc. What we do: create a 3D pattern on the surface of the substrates upon which the Boron layer is deposited. Use these “macrostructured” surfaces as cathodes in a traditional MWPC. Stack several MWPCs (>10) in order to reach the desired efficiency (50% for thermal neutrons). Sense wire planes  mm 7 Li(α) α( 7 Li) θ=45° It exploits the increase of the detection efficiency with the neutron incidence angle. ε ～ 12% ε ～ 28% λ=4.7 Å x 22.5° n θ ε abs  R α,Li /sin(θ) Al 10 B n ε abs  R α,Li A design with Boron layers mounted at an angle with respect to the normal would be the obvious choice. A large detector volume is required in order to cover the desired solid angle. n + 10 B  α + 7 Li (σ th = 3840 b) n

GEANT4/GARFIELD simulations Optimal shape of the groove, conformality of the coating, and electric field properties in 1 atm of Ar/CO 2 gas (70-30) extensively investigated with the GEANT4 and GARFIELD simulation codes. 4 uniform coating, ideal t side = t top * sin(θ/2) Θ is the important parameter for shape optimization. θ optimal height: maximum efficiency and weak dependence on the conformality of the coating (calculations for = 4.7 Å, θ=45°) non-uniform coating, realistic t side = t top * (x/h), 0 ≤ x ≤ h Height of the groove h is the important parameter for shape optimization. h “Macrostructured cathode” I. Stefanescu et al., NIMA727 (2013) 109.

The proof-of-concept measurements 5 Proof-of-concept measurements performed at the TREFF instrument at FRM2 ( = 4.7 Å). Flat Macrostructured I. Stefanescu et al., NIMA727 (2013) 109. C.Höglund, JAP 111 (2012) I. Stefanescu et al., JINST 8, P12003(2013). Built a test detector with an active area of 10 cm x 10 cm, operated in continuous flow of Ar/CO 2 (70/30). Test various coating techniques (magnetron sputtering, electron-beam evaporation), designs of the 3D pattern, and conformality of the coating. Used to study the efficiency of stacks of up to five MWPCs with macrostructured cathodes against stacks incorporating the same number of counters but with a flat surface. Coatings by magnetron sputtering at the Linköping University, Sweden. Results show that the use of macrostructure cathodes leads to a reduction by 30% the number of counters required to reach a certain efficiency.

The FRM2/ESS B-10 demonstrator 6 The construction of a realistic size B-10 demonstrator based on this concept started mid The current version of the B-10 demonstrator incorporates a stack of two MWPCs with an active area of 40 cm x 40 cm (4 Boron layers). The goal is to have a stack of 5 counters. Operated in continuous flow of Ar/CO 2 (80/20). A cathode plane consists of eight 5 cm x 40 cm macrostructured plates made by extrusion and coated with 1.4 μm 10 B 4 C by magnetron sputtering at Linköping University. Anode wire planes made by winding Stableohm 875 resistive wire of 17 μm diameter around Aluminum frames. Wire pitch is 5 mm, wire length is 40 cm, wire resistance is 3 kOhm. Anode-cathode distance =6.5 mm. Wires glued to the frames with Stycast Electrical connections were made with conductive glue. The 2-counter detector stack The cathode plane made of 8 macrostructured plates Making the wire plane The FRM2/ESS demonstrator

The readout electronics 7 The concept of macrostructured cathodes can be used with any type of amplification and readout structures that can cover large-areas at a reasonable cost. We used a novel readout scheme* based of resistive wires electrically connected in pairs of two such that two independent, interleaved chains are created. X Y ≈ 72 wires per plane (per counter) at 5 mm pitch. One counter in the detector stack needs only 6 preamplifier channels for 2D readout. *Thanks to R. Schneider, Mesytec GmbH & Co. KG, Germany. Mesytec Front-End custom- made boards

Tests of the demonstrator with the FRM2 moderated 252 Cf-source 8 Demonstrator exposed to the moderated neutrons emitted by the 252 Cf source available in the FRM2 Detector Lab. The summed signals from groups of 12 wires were readout with commercial electronics. Gain curves used by Mesytec to design the Front-End readout boards. PHS used to determine the charge deposited in 1 atm of Ar/CO 2 gas Photograph of the B-10 demonstrator taking data with the moderated Cf source at FRM2.

Tests of the B-10 demonstrator with collimated neutron beam at 9 Efficiency measured at the TREFF instrument at FRM2 with 4.7 Å neutron beam by reading out the signal from the 12 most central wires with a Mesytec MRS2000 and a commercial MCA. Very good agreement between the measured and calculated values, as demonstrated in earlier measurements with the small test detector. Photograph of the B-10 demonstrator taking data at the TREFF instrument at FRM2. beam Thickness of the coating (1.4 μm) is optimal for a stack of 5 counters (our goal).

10 Mesytec delivered Front-End boards in July 2014, first tests of the complete electronics chain performed in August. Mesytec Front-End (custom made)  Mesytec 16-ch preamplifier MPR-16  Mesytec shaper MSCF-16  Philips 7164 Peak-sensing ADC (12 bit)  Camda DAQ. Data acquired in list mode and used offline to the develop the algorithm for 2D position reconstruction (MATLAB software). Preliminary results showed that unfortunately, some wires stopped working. At closer inspection we found out that the conductive glue is the cause for this problem. Tests of the electronics with the FRM2 moderated 252 Cf-source Mesytec Front-End readout board 2D image of a 252 Cf source recorded with the FRM2/ESS B-10 demonstrator.

11 Position resolution measurements with collimated neutron beam at No calibration mask used, but placed the demonstrator on a XY moving table and scanned the surface of the demonstrator in small steps with a 2.1(H) mm x 2.6 (V) mm beam spot. Data analysis in progress, results shown here are preliminary. Each wire requires own calibration. The physical limit of the position resolution is dominated by the range of the 1.47 MeV α-particle in 1 atm of Ar/CO 2 gas (70/30) which is ~7 mm. Values on top the the peaks represents the FWHM of the Gauss distribution used to fit the peak. Resolution along the wire expected for this technology: ~8 mm, i.e., 1% of the length of the wire chain (80 cm). Sum of scans along the wires. X Y beam Wire number Position along the wire, mm Results of scans across the wires in steps of 2.5 mm (1/2 of wire pitch) Neutron position response measurement along the wire.

12 Conclusions and perspectives We proposed a detector concept based on stacked MWPCs with B-10 coated macrostructured cathodes and resistive wire readout. The validity of the concept demonstrated in proof-of-concept measurements and computer simulations. Last 12 months were dedicated to the construction and testing of a realistic size demonstrator (active area 40 cm x 40 cm) with moderate position resolution based on the proposed concept. First in-beam tests with the 2-counter demonstrator (4 Boron layers, 1.4 μm) and resistive wire readout indicate ~30% efficiency at 4.7 Å and position resolution around 5 mm x 8 mm. Testing of the current version of the demonstrator will continue, start designing the full version of the demonstrator, which will be more application-oriented. Macrostructured detector design shown to work. It is a scalable, cheap, and provides a He-3 alternative neutron detector with good performance.

13 Back-up slides from here

14 MWPC with macrostructured parallel plates Drift time  0.7 μs MWPC with flat parallel plates wires Ar/CO 2 (70/30) 5 mm Drift time  0.4 μs Drift of charges in an 1” tube filled with 6 atm of 3 He is  4.5 μs. Electron drift lines calculated with GARFIELD

15 Results from proof-of-concept measurements Very good agreement between the GEANT4 predictions and measurements; Weaker electric field near bottom causing a reduction in the gas gain, but the effects on the charge collection efficiency are not significant. h = 5 mm h = 2.1 mm

Results from GEANT4 simulations 16 experimental