Design of Novel Coatings for Neutron Detection Carina Höglund1,2 Björn Alling2, Jens Birch2, Lars Hultman2, Mewlude Imam1,2, Jens Jensen2, Henrik Pedersen2, Irina Stefanescu3, Karl Zeitelhack3, Richard Hall-Wilton1 1 European Spallation Source, Sweden 2 Thin Film Physics Division, IFM Linköping University, Sweden 3 TU Munich, Germany

Background Building the ESS 3He crisis The world’s best source of neutrons for the study of materials First neutrons in 2019 on 7 instruments 22 instruments by 2025 3He crisis 280 m2 active detector area  Most need alternatives to 3He  Thin film neutron converters LiU – ESS – ILL collaboration 2

Thin films from Linköping University Thin Film Physics division in Linköping: ~60 persons (26 PhD students) Spin-offs include Ion Bond, Thin Film Electronics, Impact Coatings, n-Works Materials Research (~250 persons) Interdisciplinary, involving industry and institutes Energy materials Hard and low-friction coatings Neutron-detecting films Power electronics & wide-band semiconductors Gas and chemical sensors Conducting polymers State-of-the-art laboratories Integrating experiments, theory, and modeling Active in patenting - results reach the market 3

Alternative neutron absorbers Absorbing material Thermal neutron absorption cross section Natural isotope abundance Disadvantages 3He 5 333 barn 0.00014 at.% Not available Expensive 6Li 940 barn 7.6 at.% Low efficiency Highly reactive 10B 3837 barn 19.9 at.% 157Gd 254 000 barn 15.65 at.% Poor n-g discrimination Reaction paths: 3He + n → 3H + 1p + g 6Li + n → 3H + a 10B + n → 7Li + a + g (94 %) 7Li + a (6 %) 157Gd + n → 158Gd + g + e- First choice! Under investigation 4

The 10B detector principle 10B has a neutron absorption of 70% compared to 3He at l = 1.8 Å natB contains 80 at.% 11B and 20 at.% 10B 10B + n → 7Li + a + g (94%) 10B + n → 7Li + a (6%) Charged products emitted back to back Anode wire / electric field to amplify Collect signal from ionization process Incident neutron CF4 gas 10B 7Li a Substrate Choose 10B4C due to: Easy to handle in dc magnetron sputtering Excellent wear resistance, thermal and chemical stability Electrically conducting High quality 10B coatings with good adhesion are a key ingredient! 5

DC magnetron sputtering of 10B4C (PVD) CemeCon CC800/9 deposition machine: 10B4C Si 1 mm Almost 80 at.% 10B Impurities H +N + O only ~1 at.% Good adhesion onto Al, Si, Al2O3, etc Thicknesses up to above 4 mm Very low residual stress (0.09 GPa at 1 mm) Densities of 2.45 g/cm3 (bulk 2.52 g/cm3) No damage by neutron radiation 2-sided substrates Large area (>5 m2/week)  Prototypes! Patent SE 535 805 C2 C. Höglund et al., J. Appl. Phys. 111 104908 (2012) 6

Chemical vapor deposition of B4C (CVD) Not a line of sight technique  Complex geometries Can reduce stress  Better adhesion Demands on a CVD process: Low temperature, < 600 °C No corrosive gases Easy up scaling 10B-enriched version of the B-precursor must be available Organoboranes: Very reactive – low process temperature! - Trimethyl borane (TMB): B(CH3)3 - Triethyl borane (TEB): B(C2H5)3 - Tributyl borane (TBB): B(C4H9)3 Al-substrates Coated with PVD H. Pedersen, C. Höglund et al., Chem. Vap. Dep. 18, 221 (2012) 7

Thermally activated CVD Growth parameters: T = 400-600 °C Pressure 50 mbar RF Coil BxC deposited at 600 °C in Ar Films with 0.5-1 mm adhere well to both Al and Si (100) Deposition rate 0.4-1 mm/h B/C close to 4 at 600 °C 5 at.% H at 600 °C Susceptor Insulation TEB Carrier gas (H2 or Ar) Quartz tube  Find a way to lower deposition temperature! H. Pedersen, C. Höglund et al., Chem. Vap. Dep. 18, 221 (2012) 8

Plasma assisted CVD Usually first choice for low temperature CVD Decompose source gas with energetic species in the plasma instead of thermal energy Allows for a low overall process temperature Use TMB and TEB We are up and running now! 9

Increasing efficiency compared to flat cathodes CVD PVD Grooved cathodes with 10B are expected to have an efficiency superior to flat plates Possible with 30% efficiency gain Very good agreement between GEANT4 predictions and measurements! I. Stefanescu, C. Höglund, et al., NIM A 727, 109 (2013) 10

Theoretical modeling of neutron detecting films First-principles Density Functional Theory can reveal… Phase stability Electrical character Atomic structure and defects Mechanical properties … of existing and potential future neutron detector materials Crystalline and amorphous B4C TM1−xGdxN (TM = Ti, Zr, Hf) First-principles DFT Ti  Phase separation Zr, Hf  Readily mix B. Alling, C. Höglund et al., Appl. Phys. Lett. 98, 241911 (2011) 11

GdN compounds with PVD Problems: Gd is known to oxidize severely  Can we find a stable and conducting compound? A GdN thin film of ~200 nm is fully oxidized within 12 h Possible solution: Stabilize GdN by alloying with a chemically, mechanically, and/or thermally more stable nitride! 12

Collaborations Samples have also been coated for: ILL  4+ prototypes, IN6 demonstrator, planning for IN5 demonstrator Samples have also been coated for: Czech Technical University, Sintef / Mid Sweden University, University of Milano Bicocca & INFN, Technical University Munich, etc… B. Guerard et al., Neutron News 23:4, 20 (2012) A. Khaplanov et al., Proc. of ICANS XX (2012) J. Correa et al., IEEE TNS, 60, 871 (2013) B. Guerard et al., NIM A 720, 116 (2013) I. Stefanescu et al., NIM A 727, 109 (2013) A. Khaplanov et al., NOPD submitted I. Stefanescu et al., J. Instr., submitted F. Krejci et al., 14th ICAPTT proc, subm. etc. Contract for new deposition system dedicated for 10B4C coatings production signed last week!  From May we have a huge capacity for 10B4C coatings! Interested in samples? Come and discuss with me!  13

Conclusions and outlook Alternatives to 3He detectors are urgently needed The next generation of neutron detectors will contain 10B thin films 10B4C thin films can be PVD deposited with ~80 at.% of 10B Elevated temperatures and high deposition rates yield good adhesion The 10B4C coatings are not damaged by neutrons The process can easily be up-scaled for large area neutron detectors Thermal CVD and PACVD for complex structures is under development Technology demonstration 4 multi-grid prototypes, incl. IN6 segment  50% detection efficiency! Grooved cathodes can raise the efficiency by 30% Several collaborations show promising prototype results Up-scale the process New deposition system (dedicated for 10B4C) is available in ~6 months! 2019  First 7 instruments at ESS need >3000 m2 coatings 2025  15 more instruments at ESS need >5000 m2 coatings 14

The collaborating team Carina Höglund * Richard Hall-Wilton Anton Khaplanov ** Kalliopi Kanaki Scott Kolya … Jonathan Correa Francesco Piscitelli Bruno Guerard Patrick van Esch Thierry Bigault Jean-Claude Buffet Mathieu Ferraton … Mewlude Imam*** Jens Birch Lars Hultman Henrik Pedersen Björn Alling Annop Ektarawong Jens Jensen Irina Stefanescu Karl Zeitelhack * stationed at Linköping University ** stationed at ILL *** 50% paid by ESS 15

Thank you for your attention!