1. A. Muraro 1, G. Croci 1,2, C. Cazzaniga 3, G. Claps 4, M. Cavenago 5, G. Grosso 1, F. Murtas 4,6,, E. Perelli Cippo 1, M. Rebai 2,3, R. Pasqualotto 7, M. Tardocchi 1 and G. Gorini 2,3 GEM based fast neutron detector for fusion and spallation sources experiments 1 Istituto di Fisica del Plasma, IFP-CNR - Milano (IT) 2 INFN, Sezione di Milano-Bicocca (IT) 3 Dipartimento di Fisica, Università di Milano-Bicocca (IT) 4 INFN – LNF - Frascati (IT) 5 INFN – LNL - Legnaro(IT) 6 CERN – Geneva (CH) 7 Consorzio RFX – Padova (IT)

2. OUTLINE 2 Why and how to use GEM-based detectors to detect fast neutrons FAST NEUTRON DETECTORS Projects Large area detector (35 x 20 cm 2 ) performance Conclusions

3. WHY AND HOW TO USE GEMS TO DETECT NEUTRONS 3 Main GEMs advantages Very high rate capability (MHz/mm 2 ) 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 are intrinsecally charged particles detectors The 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 hydrogen The aluminium layer is used in order to give at the detectors a directionality property

4. Mainframe Projects 4 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

5. nGEM (fast neutrons GEM) prototypes 5 1 «Analogue» Prototype (nGEM-S-1) 100 cm 2 active area Cathode: Aluminium (40 μm) + Polyethylene (60 μm) The prototype has confirmed the directionality Property required by the CNESM diagnostic (Test @ FNG) 2 Small area Digital Prototypes (10x10 cm 2 – nGEM-S-2/3) nGEM-S-2 Cathode: Aluminium (40 μm) + Polyethylene (60 μm) Gas Ar/CO 2 & Ar/CO 2 /CF 4 nGEM-S-3 (same cathode as full size prototype) Cathode: Aluminium (50 μm) + Polyethylene (100 μm) 2 Full-Size SPIDER prototype(nGEM-FS-1-2) S1:Cathode: Aluminium (50 μm) + Polyethylene (100 μm) 20 x 35 cm 2 active area S2:Cathode: Aluminium (50 μm) + Polypropylene(2 mm) 20 x 35 cm 2 active area 5 Prototypes of nGEM have been built and tested so far with Gas Mixture Ar/CO 2 & Ar/CO 2 /CF 4

6. Construction of the Full-size prototype of the CNESM system for SPIDER Drift gap  4 mm Transfer 1 gap  2 mm Transfer 2 gap  2 mm Induction gap  2mm Cathode  Aluminium (50 μm) + Polypropylene(2 mm) Active area  20 x 35 cm 2 active area Readout anode configuration  256 PADs (16x16 PADs) PAD dimensions  13x22mm 2 This detector represent at the moment the largest area GEM based fast neutrons detector ever realized

7. Test 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 stability Evaluate the efficency of the detector on all over the detector surface

8. Working point determination Gas mixture  Ar/CO 2 (70%-30%) Gas flow  5 l/h E drift = 0.75 kV/cm E Transfer1 = 1.5 kV/cm E Transfer2 = 3 kV/cm E induction = 3.865 kV/cm The detector is insensitive to gamma rays at V GEM < 900 Working point in fast neutron detection  V GEM =870V

9. Beam profile measurement The measured FWHMx = 41.1 mm and FWHMy = 34.1 mm are compatible with the technical specifications of the ROTAX beam.

10. Stability Irradiation time≈ 24 h with FPGAs motherboards outside of the beam Non-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 intensity

11. Uniformity test results Counting rate map Uniformity distribution Uniformity map Step x =22 mm=x-dimension of the pad Step y =13 mm=y-dimension of the pad.

12. 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 y  as expected in SPIDER

13. SPIDER-like analysis: Fit results a0=MAX [count] a1 = σ X [mm] a2 = σ y [mm] a3 = μ x [mm] a4 = μ y [mm] Beam 0 299.24817.472014.513256.448848.9141 Beam 1 284.14417.472014.5132102.84849.1036 Beam 2 284.67717.472014.513253.2270101.164 Beam 3 304.99917.472014.5132100.733101.799 Beam 4 269.80117.472014.513256.783873.0534 Beam 5 237.17617.472014.513296.399273.5918 The detector is suitable for reconstructing the SPIDER beam

14. Vibrational 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 foils Frequency=5 Hz; Accelleration=0÷10 m/s 2 Frequency=15 Hz;Acceleration=0÷25 m/s 2 Frequency=40 Hz;Acceleration=0÷40 m/s 2 Acceleration=10 m/s 2 ; Frequency=0÷1000 Hz Acceleration=15 m/s 2 ; Frequency=0÷1000 Hz Acceleration=20 m/s 2 ; Frequency=200÷1000 Hz All the tests gave negative results

15. Conclusions The full size prototype of nGEM detector for the CNESM syestem was successfully built and tested The uniformity of the detector is better than 15% The detector is highly insensitive to the gamma ray background The time stability shows that the detector is suitable for on-line beam monitors The SPIDER-like analysis has shown that with this detector we are able to reconstruct the SPIDER beam structure The vibrational tests have shown that the detector can works fine in environment with the presence of vibrations

17. Directionality Property 17 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 > 45°is 40 μm (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 θ>45°.