1. 10 B-based Multi-Grid Detectors as an alternative to 3 He In-beam test on the IN6 ToF spectrometer 10 B-based Multi-Grid Detectors as an alternative to 3 He ILL Mathieu Ferraton, Jean-Claude Buffet, JeF Clergeau, Jonathan Correa, Sylvain Cuccaro, Bruno Guérard, Francesco Pitcitelli, Patrick Van Esch ESS Carina Höglund, Richard Hall Wilton, Anton Khaplanov 14/10/2013 Special Workshop on Neutron Detection with MPGDs

2. Outline Basic principles and design of the IN6 Multi-Grid prototype – mechanics – electronics In-beam measurement on the IN6 ToF spectrometer – Neutron efficiency and comparison with 3 He tubes – Gamma sensitivity – Background – Time resolution – Scattering in aluminum Construction of an IN5 Multi-Grid prototype 10 B-based Multi-Grid Detectors as an alternative to 3 He 14/10/2013 Special Workshop on Neutron Detection with MPGDs

3. Basic principles of the Multi-Grid and design of the IN6 prototype 10 B-based Multi-Grid Detectors as an alternative to 3 He 14/10/2013 Special Workshop on Neutron Detection with MPGDs

4. Principle and Mechanical Design Base elements are grids (1 grid = frame + blades) – 1 frame is mounted with 14 blades coated on each side with 10 B 4 C (1 µm thick) -> 15 cells in depth direction (1 cm each) – 1 grid = 15 × 4 cells (2 × 2 × 1 cm) -> 60 cells – grid are stacked to form 60 gas proportional counters neutrons Al substrate (0.5 mm) neutrons 10 B 4 C layers (1 µm) Fission fragments The IN6 prototype : 96 frames arranged as 6 stacks of 16 frames (sensitive area 0.15 m²) 60 anode wires per stack (total 360 wires) Position Sensitive Detector -> coincidence mode : – Frames : vertical position – Wires : horizontal position and depth Gas mixture : 1 bar Ar/CO 2 (90/10) flushed neutrons Amplification gas Neutron conversion in 10 B : 15 cm 8 cm 2 cm 10 B-based Multi-Grid Detectors as an alternative to 3 He 14/10/2013 Special Workshop on Neutron Detection with MPGDs

5. Readout Electronics : reduction of channel number 96 frames + 360 wires = 456 channels Reduction of channel number : – 30 wires connected together via resistive networks -> charge division allows determination of the targeted wire 12 resistive networks × 2 ends = 24 readout channels – 3 frames connected together horizontally 96 / 3 = 32 readout channels -> 56 readout channels to determine position and depth (reduction by a factor 8) R 2R RRR anode wires Charge Preamplifier RR Charge division card 10 B-based Multi-Grid Detectors as an alternative to 3 He 14/10/2013 Special Workshop on Neutron Detection with MPGDs

6. Readout Electronics : data acquisition Cathode signal (grids) Anode signal (resistive networks) Charge division (or energy) card MCC Time signal (from IN6 chopper) iSDNPC Actual position of the beam Result of the CoG algorithm x (cm) 0 MCC (Multi Channel Controller) Coincidence reconstruction Position resolution improvement trough CoG algorithm -> ½ grid (1 cm), ¼ grid (0.5 cm) 10 B-based Multi-Grid Detectors as an alternative to 3 He 14/10/2013 Special Workshop on Neutron Detection with MPGDs

7. In-beam measurement on the IN6 time-of-flight spectrometer at ILL 10 B-based Multi-Grid Detectors as an alternative to 3 He 14/10/2013 Special Workshop on Neutron Detection with MPGDs

8. Interest of this measurement Direct comparison between 10 B Multi-Grid detector and 3 He tubes performances More observables available thanks to the timing information from the IN6 chopper Low background instrument 10 B-based Multi-Grid Detectors as an alternative to 3 He 14/10/2013 Special Workshop on Neutron Detection with MPGDs

9. The IN6 instrument ToF spectrometer in the range of thermal neutrons Available incident wavelengths : 4.1, 4.6, 5.1, 5.6 Å Detection system : 337 3 He tubes (sensitive area 4 m²) Total background : 4.3 Hz 3 He tubes (3 bar, diameter 3.2 cm, length 30 cm) 3 weeks measurement @5.1 Å with users’ samples 24 h dedicated beam time @4.1 Å and 4.6 Å 10 B-based Multi-Grid Detectors as an alternative to 3 He 14/10/2013 Special Workshop on Neutron Detection with MPGDs

10. Neutron efficiency : comparison with IN6 3 He tubes : higher intrinsic efficiency (see theoretical curves) 10 B Multi-Grid : less dead spaces thanks to optimized geometry Bragg peaks in the detector -> position spectra match 3 He tubes’ (average on top and bottom 3 He tubes rows) -> better position resolution in the prototype (higher granularity) Ratio of integrated rates in Bragg peaks : - 4.1 Å : rate( 10 B) / rate( 3 He) = 1.08 - 4.6 Å : rate( 10 B) / rate( 3 He) = 0.97 -> measured efficiencies are similar in both detectors @4.1 and 4.6 Å !! YGarnet @4.1 ÅSi @4.6 Å 10 B-based Multi-Grid Detectors as an alternative to 3 He 14/10/2013 Special Workshop on Neutron Detection with MPGDs 53% efficiency was measured @2.5 Å

11. Time spectrum NaI Time spectrum multigrid plateau measurement Gamma sensitivity 3 He (gas) 10 B (solid) gammas and fission fragments well separated gamma and fission fragment spectra widely overlapping Multi-Grid detector : -> a similar time structure is observed for large values of the high voltage (above 900 V) -> no sign of this structure down from 900V NaI scintillator : -> Strong gamma background from IN6 (see spectrum) -> well defined time structure 10 B-based Multi-Grid Detectors as an alternative to 3 He 14/10/2013 Special Workshop on Neutron Detection with MPGDs

12. Background measurement 4.4 Hz flat background (30 times higher than IN6’s) was observed (no time structure) -> independent of the IN6 instrument Energy structure of the background (Pulse Height Spectrum measured on wires) : – significant rate up to more than 3 MeV (excluding neutrons and gammas) – background energy spectrum overlaps fission fragments spectrum (no threshold discrimination possible) ZnO@5.1 Å How do we explain this background ? PHS measured on wires 10 B-based Multi-Grid Detectors as an alternative to 3 He 14/10/2013 Special Workshop on Neutron Detection with MPGDs

13. Source of the background Aluminum is known to be contaminated by alpha emitters ( 238 U, 232 Th and daughters) – corresponding measured rate : 0.05 to 0.25 cm -2.h -1 – our detector is made of 360 Al tubes, 192 cm² each -> expected total rate between 0.96 Hz and 4.8 Hz (whole detector) -> the measured 4.4 Hz background rate is coherent with these values Two solutions have been investigated : - Ni layer ( > 23 µm) deposited on the Al surface - ultra-pure Al (U,Th concentration < 1 ppb) Background PHS on wires (no B 4 C coating) alpha energy (from MC simulation) -> Background reduced by a factor of 20 10 B-based Multi-Grid Detectors as an alternative to 3 He 14/10/2013 Special Workshop on Neutron Detection with MPGDs

14. Time resolution : effect of time correction Measured ToF depends on the interaction depth in the detector ToF value of interest is at the entrance window of the detector -> ToF value has to be corrected for depth Wavelength (Å) FWHM (µs) ( 10 B - 3 He) / 3 He 3 He 10 B 4,145,350,712,0% 4,645,953,015,5% 5,157,365,213,7% 10 B@4.6 Å δ depth = 1cm → δ ToF = 8.6 µs (@4.6 Å) Elastic neutrons 10 B-based Multi-Grid Detectors as an alternative to 3 He 14/10/2013 Special Workshop on Neutron Detection with MPGDs

15. Time resolution : effect of scattering in Al Base of the elastic peak is widening toward low energy for 4.1 Å and 4.6 Å data No such effect @5.1 Å YGarnet@4.1 Å Si@4.6 Å ZnO@5.1 Å -> consequence of scattering in aluminum (Bragg cutoff @4.67 Å) 10 B-based Multi-Grid Detectors as an alternative to 3 He 14/10/2013 Special Workshop on Neutron Detection with MPGDs

16. 4.6 Å z = 6 4.6 Å z = 12 Dedicated experiment (24 h beamtime), Bragg peaks in the detector (higher rate and peak/background ratio, well defined angular position) – 2 wavelengths and 2 samples (YGarnet@4.1 Å, Si@4.6 Å) – Bragg scattering angles : 4.1 Å : θ = 122° 4.6 Å : θ = 160° Neutron scattering in aluminum Some neutrons have negative speed (traveling from the back to the front of the detector) -> Bragg scattering at the back of the detector (thick exit Al window to cross) θ Cd shielding Only neutrons with λ < 4.67 Å (Bragg cutoff) induce coherent Bragg-scattering (σ incoherent << σ coherent ) Shielding the back of the detector will suppress the observed bump in time-of-flight spectra 10 B-based Multi-Grid Detectors as an alternative to 3 He 14/10/2013 Special Workshop on Neutron Detection with MPGDs

17. Summary of the results The Multi-Grid concept has confirmed the good expected performances (efficiency comparable with IN6’s, low gamma-sensitivity, good time-of-flight resolution) Scattering in aluminum was observed The question of alpha emission in aluminum was raised -> It is now time to go to the next step! 10 B-based Multi-Grid Detectors as an alternative to 3 He 14/10/2013 Special Workshop on Neutron Detection with MPGDs

18. The IN5 Multi-Grid prototype The construction phase of a much larger prototype has started (size of an IN5 module, 1/12 of the instrument’s detector) – Sensitive area will be 3 * 0.8 m² 1024 grids (8 stacks of 128 grids), ~17500 blades 480 wires, 3 m long (~1.5 km of wire) – 1500 readout channel (to be reduced to 288) 10 B-based Multi-Grid Detectors as an alternative to 3 He 14/10/2013 Special Workshop on Neutron Detection with MPGDs

19. The IN5 Multi-Grid prototype Design, inner mechanics and assembly at ILL –Final design now settled on –Vessel under fabrication by the German company ProBeam (electron-beam welding) Detector’s components supplied by ESS and Linkoping University –1024 Ni-plated frames –17500 two-side 10 B 4 C-coated blades (35 m²) -> Assembly will start in January 2014 10 B-based Multi-Grid Detectors as an alternative to 3 He 14/10/2013 Special Workshop on Neutron Detection with MPGDs

20. Thank you for attention !! 10 B-based Multi-Grid Detectors as an alternative to 3 He 14/10/2013 Special Workshop on Neutron Detection with MPGDs