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G. Croci 1,2, C. Cazzaniga 3, G. Claps 4, M. Cavenago 5, G. Grosso 1, F. Murtas 4,6, S. Puddu 6, A. Muraro 1, E. Perelli Cippo 1, M. Rebai 2,3, R. Pasqualotto 7, M. Tardocchi 1 and G. Gorini 2,3 Development of GEM-based neutron beam monitors 1 Istituto di Fisica del Plasma, IFP-CNR - Milano (IT) 2 INFN, Sezione di Milano-Bicocca (IT) 3 Dipartimento di Fisica, Universit di Milano-Bicocca (IT) 4 INFN LNF - Frascati (IT) 5 INFN LNL - Legnaro(IT) 6 CERN Geneva (CH) 7 Consorzio RFX Padova (IT)

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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 cm 2 ) THERMAL NEUTRON DETECTORS Mainframe projects Prototypes construction Performances on neutron beams Conclusions and Future Perspectives 2

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WHY AND HOW TO USE GEMS TO DETECT NEUTRONS GEMs offer the following advantages Very high rate capability (MHz/mm 2 ) 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) GEM detectors born for tracking and triggering applications (detection of charged particles) In order to detect neutral particles you need a converter Fast Neutrons: Polyethylene converter + Aluminium Neutrons are converted in protons through elastic scattering on hydrogen Thermal Neutrons: 10 Boron converter Neutrons are detected using the productus (alpha,Li) from nuclear reaction 10 B(n,alpha)7Li 3

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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 4

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

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Mainframe Projects CNSEM (Close Contact Neutron Surface Emission Mapping) diagnostic for ITER NBI Prototypes (SPIDER & MITICA) Beam monitor for ChipIr @ ISIS and ESS E d =100keV nGEM neutron Detector Aim: Reconstruct Deuterium beam profile from neutron beam profile. Angular resolution and directionality property needed ChipIr CAD model at ISIS-TS2 ESS Model Aim: Construct large area, real-time and high rate beam monitors for fast neutron lines Deuterium Beam (100 Kev) Neutron Flux 10 10 n/cm 2 s Deuterium Beam composition: 5x16 beamlets See G.Gorini Talk 6

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nGEM (fast neutrons GEM) prototypes 1 Analogue Prototype (nGEM-S-1) 100 cm 2 active area Cathode: Aluminium (40 m) + Polyethylene (60 m) 2 Small area Digital Prototypes (10x10 cm 2 nGEM-S-2/3) nGEM-S-2 Cathode: Aluminium (40 m) + Polyethylene (60 m) Gas Ar/CO 2 & Ar/CO 2 /CF 4 nGEM-S-3 (same cathode as full size prototype) Cathode: Aluminium (50 m) + Polyethylene (100 m) 1 Full-Size SPIDER prototype (nGEM-FS-1) Cathode: Aluminium (50 m) + Polyethylene (100 m) 20 x 35 cm 2 active area 4 Prototypes of nGEM have been built and tested so far with Gas Mixture Ar/CO 2 & Ar/CO 2 /CF 4 7

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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: 10 11 n/s (14 MeV) 10 9 n/s (2.5 MeV) ISIS Rutherford Appleton Laboratory Didcot (Uk) Spectrum from Thermal to 800 MeV Flux: Thermal ( 1MeV): 6*10 5 n/cm 2 s nTOF CERN Geneva (Ch) Spectrum from a few meV to several GeV Flux 10 5 n/cm 2 /pulse 8

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2.5 MeV neutron Test at FNG (Frascati Neutron Generator ENEA) Deuterium beam Deuterium target nGEM detector Analog Prototype nGEM-S-2 See P. Valente Talk 9

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Directionality Property 10 Neutron Flux 10 8 n/cm 2 s (measured by in-site NE213 scintillator). The optimized aluminium thickness that allows to discard protons emitted at an angle > 45is 40 m (determined by MCNP Simulations) Each pulse height spectrum was normalized considering the total number of neutrons generated by the neutron gun measured by the NE213 scintillator. n p p Al gas CH 2 n pp G. Croci et Al, JINST C03010 2012 Results confirm that nGEM is fully able to discard protons emitted at >45. 10

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Neutron flux Linearity nGEM-S-2 Very important feature for a beam monitor Neutron Flux up to 10 8 n/cm 2 /s Counts over the full area scales linearly with neutron flux Efficiency (@ 2.5 MeV) = 2*10 -5 V GEM = 1020 V 2.5 MeV neutrons (Ar/CO2/CF4 gas mixture) Detector working point and gamma rays background rejection Counting rate Vs chamber gain: up to 890 V the chamber is sensitive to fast neutron but not to gamma rays (Ar/Co2 70%/30% gas mixture) ISIS FNG 11

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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 2D Beam profiles and intensity in real time Neutron beam monitorig during the shutter opening nGEM-S-2 12

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Vesuvio Beam 2D Measurement Y direction cut X direction cut 2D Fast Neutron Intensity Map FWHM = 34 mm FWHM = 36 mm G. Croci et Al, NIM A 720, 144-48 OFFLINE Analysis 13

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Detector Counting Rate Stability in time Counting stability Neutron flux = 10 5 /n/cm2 nGEM counting rate exactly follows the ISIS beam Measured% of counting rate variation with time = 4.7 % Stability is a very important feature for a beam monitor G. Croci et Al, NIM A 720, 144-48 14

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Fast Neutron time line Rate measurement scan on time delay from beam T 0 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 E n >2MeVE n

n Low efficiency detector 1% is sufficient since the neutron flux is very high (>10 6 n/cm 2 s) bGEM prototype of thermal neutron beam monitor Triple GEM detector equipped with an aluminum cathode coated with 1m of B 4 C: first bGEM prototype Exploit the 10 B(n,) 7 Li reaction in order to detect thermal neutrons B 4 C coated aluminium cathode mounted on its support B 4 C coated aluminium cathode assembled inside the bGEM chamber layout Detector Schematics 22

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e CN CH N CH 3 N2N2 CH 2 e Atoms, Radicals Molecules, Ions and Electrons powered electrode grounded electrode Time-average voltage profile across electrodes in rf discharge Plasma deposition area B 4 C target RF plasma sputtering system for B 4 C coating at IFP-CNR (Milano,Italy) Gas Injection Courtesy of E. Vassallo (IFP-CNR ) 23

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Thermal neutron measurements as a function of detector gain (wp and -rejec) at ISIS-Vesuvio A wide plateau is present for 820 V

Future Perspectives A new larger area nGEM neutron detector for MITICA (the evolution of SPIDER) is under design and will be developed next year A new high efficiency (>50%) thermal neutrons GEM- based detector - based on a 3D cathode of thin lamellas - for future spallation neutron sources has been designed and is currently been built. Results will be presented in the next months. This detector can represent a valid alternative to 3 He detectors 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 29

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Relationship with the industry HVGEM : MPElettronica Rome (Italy) CARIOCA Chips: Artel SRL Florence (Italy) MB-FPGA: Athenatek Rome (Italy) GEM FRAMES: Meroni & Longoni Milan (Italy) GEM Foils: CERN Detector construction: LNF-INFN (Frascati) and IFP-CNR (Milano) 30

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Spare Slides 31

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Filters in the beam line: effect on nGEM counting rate MaterialCountrate (Hz) %Expected if fast neutrons (6 MeV) Expected if thermal neutrons Expected if gamma rays No Material1307100 // Lead (5 cm) 4423437 %15 %7.3 % Cadmium (1 mm) 12089398%0%0%97% Polyethylene (15 cm) 139109%9%0%29% Aluminium (2.5 cm) 8586573%79%75% 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 CH 2 : the observed decrease is compatible with the fact that we are detecting fast neutrons G. Croci et Al, NIM A 720, 144-48

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Material Filters in the beam (Imaging with bGEM) CH 2 L- Shaped Cd Neutrons are scattered Neutrons are absorbed 33