Development of GEM-based fast neutron detectors

1. Development of GEM-based fast neutron detectors G. Croci1,2, C. Cazzaniga3, G. Claps4, M. Cavenago5, G. Grosso1, F. Murtas4,6, S. Puddu6, A. Muraro1, E. Perelli Cippo1, M. Rebai2,3, R. Pasqualotto7, M. Tardocchi1 and G. Gorini2,3 1Istituto di Fisica del Plasma, IFP-CNR - Milano (IT) 2INFN, Sezione di Milano-Bicocca (IT) 3Dipartimento di Fisica, Università di Milano-Bicocca (IT) 4INFN – LNF - Frascati (IT) 5INFN – LNL - Legnaro(IT) 6CERN – Geneva (CH) 7Consorzio RFX – Padova (IT)

2. OUTLINE • Why and how to use GEM-based detectors to detect neutrons • FAST NEUTRON DETECTORS • Mainframe projects • Prototypes construction • Performances on neutron beams • Large area detector (35 x 20 cm2) • Conclusions and Future Perspectives

3. WHY AND HOW TO USE GEMS TO DETECT NEUTRONS • GEM detectors born for tracking and triggering applications (detection of charged particles) • In order to detect fast neutrons you need a converter • Fast Neutrons: Polyethylene converter + Aluminium • Neutrons are converted in protons through elastic scattering on hydrogen • GEMs offer the following advantages • Very high rate capability (MHz/mm2) suitable for high flux neutron beams like at ESS • Submillimetric space resolution (suited to experiment requirements) • Time resolution from 5 ns (gas mixture dependent) • Possibility to be realized in large areas and in different shapes • Radiation hardness • Low sensitivity to gamma rays (with appropriate gain)

4. FAST NEUTRON BEAM MONITORS Details about triple GEM detector, HV-GEM Power Supply, CARIOCA chips and FPGA-Board have been already shown by G. Claps talk

5. Complete GEM detector system Charged particles X Ray Gammas Neutrons 12 V PS HVGEM HV Filters 3 GEM detector with padded anode FPGA Board LNF 128 ch DAQ PC Current Monitor 2D monitor with pads readout Possibility to set time slices from 5 ns up to 1 s

6. GEM applications as fast neutrons detectors (1) CNSEM (Close Contact Neutron Surface Emission Mapping) diagnostic for ITER NBI Prototypes (SPIDER & MITICA) Deuterium Beam (100 Kev) Neutron Flux 1010 n/cm2s Ed=100keV Deuterium beam impants on the Cu Beam dump  Generation of 2.5 MeV fusion neutrons from reaction with successive beam Deuterium Beam composition (not uniform): 5x16 beamlets Aim: Reconstruct Deuterium beam profile from neutron beam profile. • GEM detector are suited for this measurment • Large Area (compared to e.g. Standard neutron detector (NE213) • 2D On-line map reconstruction • High rate detector • Angular resolution and directionality property (keep information on deuterons direction) nGEM neutron Detector

7. GEM applications as fast neutrons detectors (2) Beam monitor for ChipIr @ ISIS and ESS pulsed neutron spallation sources ESS Model See M. Rebai’s talk Aim: Construct large area, real-time and high rate beam monitors for fast neutron lines ChipIr CAD model at ISIS-TS2

8. nGEM (fast neutrons GEM) prototypes 4 Prototypes of nGEM have been built and tested so far with Gas Mixture Ar/CO2 & Ar/CO2/CF4 • 1 «Analogue» Prototype (nGEM-S-1) • 100 cm2 active area • Cathode: Aluminium (40 μm) + Polyethylene (60 μm) • 2 Small area Digital Prototypes (10x10 cm2 – nGEM-S-2/3) • nGEM-S-2 • Cathode: Aluminium (40 μm) + Polyethylene (60 μm) • Gas Ar/CO2 & Ar/CO2/CF4 • nGEM-S-3 (same cathode as full size prototype) • Cathode: Aluminium (50 μm) + Polyethylene (100 μm) • 1 Full-Size SPIDER prototype (nGEM-FS-1) • Cathode: Aluminium (50 μm) + Polyethylene (100 μm) • 20 x 35 cm2 active area

9. Test @ Neutron Facilities • Directionality Property  nGEM-S-1 (Analogue) • High Voltage Scan (efficiency scan)  All prototypes • Linearity w.r.t neutron flux  nGEM-S-2 • Beam Profile Measurements  All Digital prototypes • Gamma Background sensitivity  All prototypes • Fast neutron time-line (ISIS beam time profile reconstruction) nGEM-S-2 • Counting stability  All digital prototypes • Imaging  nGEM-S-2/3 FNG Enea Frascati (Italy) 2.5 MeV neutrons 14 Mev neutrons Max Flux: 1011 n/s (14 MeV) 109 n/s (2.5 MeV) ISIS – Rutherford Appleton Laboratory Didcot (Uk) Spectrum from Thermal to 800 MeV Flux: Thermal (<100 meV): 7*105 Fast (> 1MeV): 6*105 n/cm2s nTOF – CERN Geneva (Ch) Spectrum froma few meV to several GeV Flux 105 n/cm2/pulse

10. 2.5 MeV neutron Test at FNG (Frascati Neutron Generator – ENEA) Deuterium target nGEM detector Deuterium beam Analog Prototype nGEM-S-2 See P. Valente Talk

11. 11 Directionality Property (FNG) • Detection only of neutrons that keep the deuteron direction information (SPIDER)  need to discard protons emitted at an angle wrt neutron direction • Neutron Flux ≃ 108 n/cm2 s (measured by in-site NE213 scintillator). • The optimized aluminium thickness that allows to discard protons emitted at an angle > 45°is 40 μm (determined by MCNP Simulations) p p n gas p Al p CH2 n Results confirm that nGEM is fully able to discard protons emitted at θ>45°. G. Croci et Al, JINST C03010 2012

12. Neutron flux Linearity Detector working point and gamma rays background rejection FNG ΔVGEM = 1020 V 2.5 MeV neutrons (Ar/CO2/CF4 gas mixture) ISIS • nGEM-S-2 • Very important feature for a beam monitor • Neutron Flux up to 108 n/cm2/s • Counts over the full area scales linearly with neutron flux • Efficiency (@ 2.5 MeV) = 2*10-5 Counting rate Vschamber gain: up to 890 V the chamber is sensitive to fast neutron but not to gamma rays (Ar/Co2 70%/30% gas mixture)

13. Real-time 2D beam map measurements Monitor for a fast neutron beam with energies ranging from a few meV to 800 MeV Tested at neutron beam of the Vesuvio facility at RAL-ISIS nGEM-S-2 Neutron beam monitorig during the shutter opening 2D Beam profiles and intensity in real time

14. Vesuvio Beam 2D Measurement 2D Fast Neutron Intensity Map OFFLINE Analysis X direction cut Y direction cut FWHM = 36 mm FWHM = 34 mm G. Croci et Al, NIM A 720, 144-48

15. Detector Counting Rate Stability in time Counting stability • Neutron flux = 105/n/cm2 • nGEM counting rate exactly follows the ISIS beam • Statistical accuracy 5% with time resolution of 1 s • Very important feature for a beam monitor G. Croci et Al, NIM A 720, 144-48

16. nTOF Online 2D Beam Measurement Instantaneous counts Constant: 3640 ± 40 Mean1: 5.9 ± 1.8 cm Mean2: 5.5 ± 1.8 cm y x y Constant:515 ± 15 s Mean1: 6.0 ± 1.8 cm Mean2: 5.4 ± 1.7 cm x Cumulative counts nTOF beam was correctly reconstructed

17. Scan in energy at nTOF The FPGA can detect neutrons vs a delay in time allowing to make a time (i.e. Neutron energy) scan thatallows the efficiency vs energy to be measured (uncertainty ~1% ) . 3 MeV 10 MeV 2 e-4 100 keV

18. First nGEM full size prototype for SPIDER GEM Stretching and Framing GEM Foil HV Test Cathode Stretching and Framing 35 cm 20 cm At the moment it is the largest area GEM-based fast neutron detector!!!! Assembly 256 Pads

19. First test @ ISIS Vesuvio In this case the MBFPGA is put outside of the neutron beam using flat LVDS cables to carry the signal out. This decreases neutron induced soft errors in the FPGA. Using this setup the prototype run for several days without any inconvenience

20. Preliminary results ISIS beam 2D profile normalized to current: 12x22 mm2 pad area; half detector shown Data analysis in progress

21. Conclusions • GEM-based fast neutrons beam monitors have been successfully realized and tested. They provide: • Real-time neutron beam profile with a portable system (HV System + CARIOCAS & MBFPGA LNF) • Measurements with the necessary space resolution (pad dimension) • Time resolution of 100 ns for fast neutron measurements • Complete Gamma ray background rejection • Stability in time • First «large area» detector built and results are under study

22. Future Perspectives • A new larger area nGEM neutron detector for MITICA (the evolution of SPIDER) is under design and will be developed next year • We are working on a new GEMINI chip which will be able to increase the number of channels. The new chip can manage 32 channels, in comparison to the 8 channels of CARIOCA. This new GEMINI chip will be used to upgrade all these detectors

23. Spare Slides

24. En>2MeV En<2MeV Fast Neutron time line Rate measurement scan on time delay from beam T0 using GEM detector with 100 ns gate. Comparison with proton ISIS current impinging on the target (double structure)  nGEM is able to see the double proton structure G. Croci et Al, JINST P07021 2012

25. Filters in the beam line: effect on nGEM counting rate Lead: the observed decrease is compatible with the hypothesis that the fast neutron beam is scattered by the lead block and that the detector is non sensitive to gammas Cd: the observed decrease is compatible with the thesis that we are not detecting thermal neutrons CH2: the observed decrease is compatible with the fact that we are detecting fast neutrons G. Croci et Al, NIM A 720, 144-48