Development of GEM-based neutron beam monitors

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 Development of GEM-based neutron beam monitors1Istituto di Fisica del Plasma, IFP-CNR - Milano (IT)2INFN, Sezione di Milano-Bicocca (IT)3Dipartimento di Fisica, Universit di Milano-Bicocca (IT)4INFN LNF - Frascati (IT)5INFN LNL - Legnaro(IT)6CERN Geneva (CH)7Consorzio RFX Padova (IT)

OUTLINEWhy and how to use GEM-based detectors to detect neutrons

FAST NEUTRON DETECTORSMainframe projectsPrototypes constructionPerformances on neutron beamsLarge area detector (35 x 20 cm2)

THERMAL NEUTRON DETECTORSMainframe projectsPrototypes constructionPerformances on neutron beams

Conclusions and Future Perspectives

2WHY AND HOW TO USE GEMS TO DETECT NEUTRONSGEMs offer the following advantagesVery high rate capability (MHz/mm2) suitable for high flux neutron beams like at ESSSubmillimetric space resolution (suited to experiment requirements)Time resolution from 5 ns (gas mixture dependent)Possibility to be realized in large areas and in different shapesRadiation hardnessLow 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 converterFast Neutrons: Polyethylene converter + Aluminium Neutrons are converted in protons through elastic scattering on hydrogenThermal Neutrons: 10Boron converter Neutrons are detected using the productus (alpha,Li) from nuclear reaction 10B(n,alpha)7Li

3FAST NEUTRON BEAM MONITORSDetails about triple GEM detector, HV-GEM Power Supply, CARIOCA chips and FPGA-Board have been already shown by G. Claps talk4Complete GEM detector systemHVGEMHV Filters3 GEM detectorwith padded anode FPGABoardLNF128 chDAQ PC12 V PSCharged particles X Ray GammasNeutrons

Current Monitor2D monitor with pads readoutPossibility to set time slices from 5 ns up to 1 s5Mainframe ProjectsCNSEM (Close Contact Neutron Surface Emission Mapping) diagnostic for ITER NBI Prototypes (SPIDER & MITICA) Beam monitor for ChipIr @ ISIS and ESS

Ed=100keVnGEM neutron DetectorAim: 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 linesDeuterium Beam (100 Kev)Neutron Flux 1010 n/cm2s

Deuterium Beam composition: 5x16 beamletsSee G.Gorini Talk6nGEM (fast neutrons GEM) prototypes1 Analogue Prototype (nGEM-S-1)100 cm2 active areaCathode: Aluminium (40 m) + Polyethylene (60 m)

2 Small area Digital Prototypes (10x10 cm2 nGEM-S-2/3)nGEM-S-2Cathode: Aluminium (40 m) + Polyethylene (60 m) Gas Ar/CO2 & Ar/CO2/CF4nGEM-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 cm2 active area

4 Prototypes of nGEM have been built and tested so far with Gas Mixture Ar/CO2 & Ar/CO2/CF4

7Test @ Neutron FacilitiesDirectionality Property nGEM-S-1 (Analogue)High Voltage Scan (efficiency scan) All prototypesLinearity w.r.t neutron flux nGEM-S-2Beam Profile Measurements All Digital prototypesGamma Background sensitivity All prototypesFast neutron time-line (ISIS beam time profile reconstruction) nGEM-S-2Counting stability All digital prototypesImaging nGEM-S-2/3

FNGEnea Frascati(Italy)

2.5 MeV neutrons14 Mev neutronsMax Flux: 1011 n/s (14 MeV)109 n/s (2.5 MeV)ISIS Rutherford Appleton LaboratoryDidcot (Uk)

Spectrum from Thermal to 800 MeVFlux: Thermal ( 1MeV): 6*105n/cm2s

nTOF CERNGeneva(Ch)

Spectrum from a few meV to several GeV

Flux 105 n/cm2/pulse82.5 MeV neutron Test at FNG (Frascati Neutron Generator ENEA)

Deuterium beamDeuterium targetnGEM detector

Analog PrototypenGEM-S-2See P. Valente Talk9

Directionality Property10 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 > 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. nppAlgasCH2nppG. Croci et Al, JINST C03010 2012

Results confirm that nGEM is fully able to discard protons emitted at >45.10Neutron flux Linearity nGEM-S-2Very important feature for a beam monitorNeutron Flux up to 108 n/cm2/sCounts over the full area scales linearly with neutron fluxEfficiency (@ 2.5 MeV) = 2*10-5

VGEM = 1020 V2.5 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)ISISFNG11Real-time 2D beam map measurementsMonitor 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 intensityin real timeNeutron beam monitorig during the shutter opening

nGEM-S-212Vesuvio Beam 2D Measurement

Y direction cutX direction cut2D Fast Neutron Intensity Map

FWHM = 34 mmFWHM = 36 mmG. Croci et Al, NIM A 720, 144-48OFFLINEAnalysis13Detector Counting Rate Stability in time

Counting stabilityNeutron flux = 105/n/cm2nGEM counting rate exactly follows the ISIS beamMeasured% of counting rate variation with time = 4.7 %Stability is a very important feature for a beam monitorG. Croci et Al, NIM A 720, 144-4814Fast Neutron time lineRate 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

En>2MeVEn106n/cm2 s)

bGEM prototype of thermal neutron beam monitorTriple GEM detector equipped with an aluminum cathode coated with 1m of B4C: first bGEM prototypeExploit the 10B(n,)7Li reaction in order to detect thermal neutrons

B4C coated aluminium cathode mounted on its supportB4C coated aluminium cathode assembled inside the bGEM chamber layoutDetector Schematics22

eCNCHNCH3N2CH2eAtoms, RadicalsMolecules , Ionsand Electrons

powered electrodegrounded electrodeTime-average voltage profile across electrodes in rf dischargePlasma deposition areaB4C targetRF plasma sputtering system for B4C coating at IFP-CNR (Milano,Italy)Gas InjectionCourtesy of E. Vassallo (IFP-CNR)23Thermal neutron measurements as a function of detector gain (wp and -rejec) at ISIS-VesuvioA wide plateau is present for 820 V 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 3He 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

29Relationship with the industryHVGEM : MPElettronica Rome (Italy)CARIOCA Chips: Artel SRL Florence (Italy)MB-FPGA: Athenatek Rome (Italy)GEM FRAMES: Meroni & Longoni Milan (Italy)GEM Foils: CERNDetector construction: LNF-INFN (Frascati) and IFP-CNR (Milano)30Spare Slides31Filters in the beam line: effect on nGEM counting rateMaterialCountrate (Hz)% Expected if fast neutrons (6 MeV)Expected if thermal neutronsExpected if gamma raysNo Material1307100//////Lead (5 cm)4423437 %15 %7.3 %Cadmium (1 mm)12089398%0%

97%

Polyethylene (15 cm)139109%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 gammasCd: 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, 144-48Material Filters in the beam (Imaging with bGEM)

CH2L-Shaped CdNeutrons are scatteredNeutrons are absorbed33