GEM gas detectors for Soft Xray imaging in

GEM gas detectors for Soft X-ray imaging in Fusion devices with neutron-gamma background and polycapillary lenses D. Pacella, ENEA-Frascati F. Murtas 2, S. Dabagov 2, 4, L. Gabellieri 1, A. Romano 1, D. Mazon 3 , D. Hampai and FTU technical team 1 Euratom-ENEA Association, C. R. Frascati, Via E. Fermi, 45 - 00044 Frascati, Rome, Italy Nazionale di Fisica Nucleare, Via E. Fermi 40, 00044 Frascati, Rome Italy 3 Association Euratom-CEA, CEA Cadarache, DSM/IRFM 13108 St Paul lez durance Cedex, France 4 P. N. Lebedev Physical Institute RAS, Leninsky Pr. 53, 119991 Moscow, Russia 2 Istituto

Thinking to a Burning plasma……… (for long time we can only ……think it!) We need to cope with a high radiative background: neutrons (0 -14 Me. V) , gamma-rays, Hard X-ray We worked along two directions Detection of X-ray in presence of a strong background of 1 neutrons and gamma 2 X-ray optics to collect radiation and transport it far away X-ray optics Triple GEM gas Detector with 2 -D pixel read-out 2 -D C-MOS imager (Medipix 2) or a combination of both in the future

1 X-ray detection with n and gamma background Triple GEM gas detector, active area 10 cm x 10 cm, 2 -D pixel read-out (128), integrated front-end electronics (CARIOCA microchip)

Photon counting mode per each pixel CARIOCA chip gain Pulse amplifier and shaper threshold detector discriminator Analog signals counts MCA spectrum Lower gain pulse Higher gain Threshold scan Summing all the pulses over threshold counter counts DAQ Digital outputs Threshold (m. V) X-ray 6 Ke. V threshold Pulse amplitude

Tests at Frascati Neutron Generator (FNG) Frascati Neutron Generator (ENEA): D-D (2. 5 Me. V) and D-T (14 Me. V) neutrons 14 Me. V neutrons Max Intensity 1011 n/s over 4 π Maximum flux 1. 2 107 n/s cm 2 over the detector Detector response is linear even at high gain Disturbances on the front end electronics occur only at this maximum flux

Discrimination of neutron, gamma and X-rays Counts vs gain detector Counts vs threshold ϒ n+ϒ n neutrons ϒ X n X-ray With the gain we can enable the detection of the different radiations (pulses above threshold) or suppress them (pulses below threshold) At fixed gain, the pulse amplitude distribution (see scan in threshold) is different for the various sources of radiation: almost flat for neutrons, very peaked at low amplitude for gamma, with changeable derivative as function of gain for X-ray

Processes occuring into the detector g X n (2. 5 Me. V) (10 -4) Window (mylar 15 m) 7 10 -4 R=35 m >0. 1 p (~ 1 Me. V) DE/E ~ 50 ke. V (10 -6) Ar + CO 2 n’ 3 mm GEM 1 (50 m) 1 mm GEM 2 (50 m) 5 10 -4 R=16 m 2 mm Pixels (3 mm) (g) ~ 10 -4 (X) >0. 1 (n 2. 5) ~ 10 -4 (n 14) ~ 10 -5 GEM 3 (50 m) 1 mm read-out All these processes have to be simulated with a Monte Carlo The components of the detector can be defined to enhance or minimize the detection of the various source of radiation

2 -D X-ray imaging with neutron and gamma background 2 -D X-ray image of a source through an oblique slit 2 -D image of a X-ray through a slit 2 -D X-ray image + neutron and gamma background 9000 counts 7500 counts 2000 counts Flux neutrons= 2 106 n/s Flux gammas=6 106 ϒ/s X source slit detector Neutron source Spatial information of the detected neutron is conserved

The potentiality has been proved (NSTX, 2001 -2004). Now a full investigation will be done at Tore Supra (from 2011) NSTX 2001 -2004 A previous version (single GEM, lower active area) was developed a Frascati (ENEA) in collaboration with INFN-Pisa (R. Bellazzini and his group) in 2000 and then installed at NSTX (USA) from 2001 to 2004 Tore Supra July 2011 The present version (triple-Gem, 128 lines of sight) is now Installed at Tore Supra (see presentation of D. Mazon)

Neutron fluxes expected at ITER F. Moro et al. , Fusion Engineering and Design, 84 (2009) Flux gammas=10 6 -107 g/s Flux ~ 108 n/s cm 2 (14 Me. V) Flux (first wall) ~ 1014 n/s cm 2 Flux ~ 108 n/s cm 2 (improved detector) Flux ( FNG) ~ 107 n/s cm 2 Flux gammas=6 106 g/s X X A. Araujo et al. , Brazilian Journal of Physics Vol. 40 (2010) n, g bare detector at FNG (14 Me. V) Improvement of one order of magnitude does not seem prohibitive (non plastic window, absorber for protons, optimization of the fields, radiation hard electronics, shielding. . )

2 lens X-ray polycapillary lenses for imaging and tomography Detector Medipix 2: 256 x 256 pixels 55 m x 55 m 15 mm x 15 mm area

3. 8 mm spot size (full lens) spot size (half lens) 750 m 80 m 500 m

Imaging property of the full lens

illa r d “Broa lf tor (ha c e l l o ”c lens) view Arr a yp det ect or Possible schemes for MCF plasmas tomography 2 -3 ˚ ‘Narrow view’ collector (half lens) ~ mrad Tomography array Array bent fib ers detector

CONCLUSIONS • X-ray detection has been studied in presence of a high radiative (neutron and gamma) background • The different physical processes have been estimated, but accurate simulations will be required • Thanks to the adjustable detector gain and threshold, the different contributions could be discriminated and evaluated • The detector could be designed to tailor it on the detection of the desidered radiation • Higher limit for the background radiation are envisaged • No tests have been done so far to study aging and degradation at long term • Very preliminary tests on X-ray polycapillary optics are encouraging. They could allow the transport of X-ray radiation in configuration imaging and/or tomography Detector for high radiative environments and long distances SXR imaging with polycapillaries could be useful in other field