Development of GEM-based fast neutron detectors

A. Muraro1, G. Croci1,2, C. Cazzaniga3, G. Claps4, M. Cavenago5, G. Grosso1, F. Murtas4,6,, E. Perelli Cippo1, M. Rebai2,3, R. Pasqualotto7, M. Tardocchi1 and G. Gorini2,3 GEM based fast neutron detector for fusion and spallation sources experiments1Istituto 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)

OUTLINE2Why and how to use GEM-based detectors to detect fast neutrons

FAST NEUTRON DETECTORSProjectsLarge area detector (35 x 20 cm2) performance

ConclusionsWHY AND HOW TO USE GEMS TO DETECT NEUTRONS3Main GEMs advantagesVery high rate capability (MHz/mm2) 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 shapesRadiation hardnessLow sensitivity to gamma rays (with appropriate gain)

GEM are intrinsecally charged particles detectorsThe fast neutrons GEM detectors are equipped with a cathode composed by a polyethylene and an aluminium layer:The polyethylene layer serves as neutron converter: neutrons are converted in protons through elastic scattering on hydrogenThe aluminium layer is used in order to give at the detectors a directionality property

Mainframe Projects4CNSEM (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 beamletsnGEM (fast neutrons GEM) prototypes51 Analogue Prototype (nGEM-S-1)100 cm2 active areaCathode: Aluminium (40 m) + Polyethylene (60 m) The prototype has confirmed the directionality Property required by the CNESM diagnostic (Test @ FNG)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)2 Full-Size SPIDER prototype(nGEM-FS-1-2)S1:Cathode: Aluminium (50 m) + Polyethylene (100 m) 20 x 35 cm2 active areaS2:Cathode: Aluminium (50 m) + Polypropylene(2 mm) 20 x 35 cm2 active area

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

Construction of the Full-size prototype of the CNESM system for SPIDER

Drift gap4 mmTransfer1 gap2 mmTransfer2 gap2 mmInduction gap2mmCathodeAluminium (50 m) + Polypropylene(2 mm)Active area 20 x 35 cm2 active area

Readout anode configuration256 PADs (16x16 PADs)PAD dimensions13x22mm2 This detector represent at the moment the largest area GEM based fast neutrons detector ever realized6Test made on the full-size prototype:Test at ROTAX (ISIS)Main purposes:Evaluate the detector working point and the capability of gamma background rejection.Evaluate the detector stabilityEvaluate the efficency of the detector on all over the detector surface

7Working point determination

Gas mixture Ar/CO2 (70%-30%) Gas flow 5 l/hEdrift = 0.75 kV/cm ETransfer1 = 1.5 kV/cm ETransfer2 = 3 kV/cmEinduction = 3.865 kV/cmThe detector is insensitive to gamma rays at VGEM < 900 Working point in fast neutron detection VGEM=870V

Beam profile measurement

The measured FWHMx = 41.1 mm and FWHMy = 34.1 mm are compatible with the technical specifications of the ROTAX beam.StabilityIrradiation time 24 h with FPGAs motherboards outside of the beamNon-HV area (half of the detector far from the HV contacts)

HV area (half of the detector closer to the high voltage contacts)

Without T,P,U parameters control Stability =7.6% (non-HV) and 7.9% (HV) The nGEM counting rate of both areas follows the time evolution of the proton beam current demonstrating the possibility to on-line monitor the neutron beam intensity10Uniformity test results

Counting rate mapUniformity distributionUniformity mapStepx=22 mm=x-dimension of the pad Stepy=13 mm=y-dimension of the pad.

SPIDER-like analysis:Data to fit

In SPIDER we expected to see somthing like this (for 6 beamlets)...Beams spacing:40 mm along x and 22 along yas expected in SPIDERSPIDER-like analysis: Fit results

a0=MAX [count]a1 = X [mm]a2 = y [mm]a3 = x [mm]a4 = y [mm]Beam 0299.24817.472014.513256.448848.9141Beam 1284.14417.472014.5132102.84849.1036Beam 2284.67717.472014.513253.2270101.164Beam 3304.99917.472014.5132100.733101.799Beam 4269.80117.472014.513256.783873.0534Beam 5237.17617.472014.513296.399273.5918The detector is suitable for reconstructing the SPIDER beamVibrational Test

Test carried out in order to evaluate the possibility of discharges between two GEM foils due to the vibration of the GEM foils themselves in the operational conditions.Tests carried out on each couple of GEM foilsFrequency=5 Hz; Accelleration=010 m/s2Frequency=15 Hz;Acceleration=025 m/s2Frequency=40 Hz;Acceleration=040 m/s2Acceleration=10 m/s2; Frequency=01000 HzAcceleration=15 m/s2; Frequency=01000 HzAcceleration=20 m/s2; Frequency=2001000 Hz

All the tests gave negative resultsConclusionsThe full size prototype of nGEM detector for the CNESM syestem was successfully built and testedThe uniformity of the detector is better than 15%The detector is highly insensitive to the gamma ray backgroundThe time stability shows that the detector is suitable for on-line beam monitorsThe SPIDER-like analysis has shown that with this detector we are able to reconstruct the SPIDER beam structureThe vibrational tests have shown that the detector can works fine in environment with the presence of vibrationsDirectionality Property17

17 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.