MicroPattern Gas Detectors with Pixel Read-Out

1. MicroPattern Gas Detectors with Pixel Read-Out Luca Latronico INFN - Pisa A work by: R.Bellazzini, A. Brez, L. Baldini, L. Latronico, G. Spandre, M. Massai, F. Angelini, M. Minuti, E. Costa, P. Soffitta, D. Pacella 9th Pisa Meeting Frontier Detectors for Frontier Physics La Biodola May 29, 2003

2. coming to Elba with a big ferry you had a 1D-low resolution view …… nice, but not too exciting … … but sampling the surface of the island with powerful tools … … things got much more interesting A familiar look at the talk

3. Lesson • imaging with high resolution require: • sensitive medium – GAS • 2D fine PIXEL read-out Topic of the talk • Two smart applications of micropattern detectors with pixel read-out: • X-ray polarimetry in astrophysics - pixels to reconstruct a photoelectron track in the space domain • Time resolved imaging of plasma X-ray emission - pixels to reconstruct the plasma dynamics in the time domain

4. X-ray Astrophysical Polarimetry – why is it so important cosmic x-ray sources will produce polarized radiation unless they are truly spherical and the magnetic field orientations are random • emission processes themselves(cyclotron, synchrotron, non-thermal bremmstrahlung) • scattering on aspherical accreting plasmas: disks, blobs, columns • vacuum polarization and birefringence through extreme magnetic fields Polarimetry would add to energy and time two further observable quantities (the amount and the angle of polarization) constraining any model and interpretation: a theoretical/observational break-through (Mészáros, P. et al. 1988) In 35 years only one positive detection of X-ray Polarization: the Crab (Novick et al. 1972, Weisskopf et al.1976, Weisskopf et al. 1978)THE TECHNIQUES WERE THE LIMIT!

5. Polarimetry: AXP Polarimetry: the Missing Piece of the Puzzle Imaging: Chandra Timing: RXTE Spectroscopy: AstroE2, Constellation-X, Chandra

6. The photoelectric effect is very sensitive to photon polarization Photoelectric cross section Projecting on the plane orthogonal to the propagation direction…

7. The X-ray polarimeter GEM as amplifier Trigger from GEM Ne/DME 80/20 8-layer read-out board Pixel size 0.1-0.2 mm Area 2.4 x 2.4 mm2 (512-1024 pixels) angle and amount of polarization is computed from the angular distribution of the photoelectron tracks

8. Large-angle scattering Auger electron Bragg peak Real photoelectrons tracks from unpolarized radiation The initial part of the track, with a low ionization density, evolves into a clear Bragg peak, while the photoelectron direction is randomized by Coulomb scattering. 5,0 keV photoelectron, 870 eV Auger electron For the first time the few KeV photoelectrons in gas are seen not as an indistinct blob of charge but as a real track for which we can compute different moments

9. I - the direction of emission of the photoelectron is reconstructed by finding the major and minor principal axes (M2max, M2min) of the charge distribution on the pixels. The major principal axis is identified as the photoemission direction. II - the third moment (M3) of the asymmetric charge distribution is computed. It liesalong the major axis on the side, with respect to the barycentre, where the charge release is smaller (i.e. at the beginning of the track) The absorption point is obtained going back from the barycentre, along the major axis on the direction of M3, of a distance L   M2max (larger boxes == larger energy losses) Photoelectron track reconstruction

10. Basic algorithm Improved algorithm Angular reconstruction The reconstruction of the conversion point can be exploited to improve angular accuracy, rejecting the final part of the track, which is blurred by Coulomb scattering (real events, 8.0 keV polarized radiation)

11. 5.9 KeV unpolarized source 5.4 KeV polarized source MDP scales as: for bright sources for faint sources Angular distribution Modulation factor m = (Cmax – Cmin)/ (Cmax + Cmin) ˜50% at 6 KeV

12. Absorption point reconstruction Scatter plot of the barycenters relative to the reconstructed impact point 5.9 KeV unpolarized source 5.4 KeV 100% linearly polarized source No rotation of the detector is needed!

13. Timing 20ns FWHM e- signal with 2 mm gap Imaging 500 mm holes, s=70mm Spectroscopy Polarimetry linearity m up to 50% DE/E = 20%FVHM

14. 20 keV photons INFN-style detector 6 keV photons Goddard Space Flight Center AXP : a photo-electric polarimetry mission 1-10 KeV energy range - 3mirrors of 4.5m f.l. - polarimeter in the focus Current implementation • CMOS full custom pixel array read-out! • Advantages: asynchronous, fast, low noise, honeycomb array design • Prototype specs: • exagonal array of 1024/2048 100 mm2 pixels with preamplifier/shaper, S/H • shaping time ~ few mseconds • external trigger for parallel S/H on all the channels • ADC after S/H :external , flash • read-out time: not more than  0.1-0.2  ms • ENC: 200 e- ( very small  detector capacitance) TFT readout

15. Tangential view Plasma diagnostic The problem: high rate time-resolved imaging of the hottest part of the plasma (1-10KeV) which is surrounded by a large halo of colder plasma • imaging capability • energy discrimination in the 1-10 KeV range to isolate core emission • 106g/s mm2 • High frame rate to follow ms scale fluctuations 0.6o Pinhole 3cm in the core ~ 1 detector pixel

16. The detector Kapton thickness 50mm Triangular geometry f 65mm, pitch 90mm GEM size 2.5X2.5cm2 divided in 4 decoupled regions. Printed circuit board 128 2mm2 pixels Parallel read-out 128 independent chans/counters

17. The electronics 128 free-running counters global rate ~ 1GHz VME acquisition system Detector and front end amplifier pulser Host PC gate latching Cables 15 m pad Charge preamp Threshold Pulse shaper discriminator Counter w memory CPU ethernet Unipolar pulses 50 ns width 0.01 - 1 V proportional to X-ray energy 104 -106 electrons 20 ns ECL pulses 50 ns width Energy discrimination logic output 10 ns width Fast counter (up to 100 Mhz) Noise 53 electrons Latch freq up to 1 Mhz

18. The full system

19. Detector calibration: pixels equalization All the pixels have the same spectral response (by 2%) with an X ray source of 1- 10 keV

20. Imaging at high frame rate X-rays through a copper shutter revolving around a fast drill 10 KHz sampling, 150 frames, 15ms 100 Hz CCD-like sampling, 150 frames, 1.5 s time-averaged image – no motion is seen

21. INFN, Pisa NSTX National Spherical Tokamak eXperiment FTU Frascati Tokamak Upgrade

22. Plasma center activity H H-MODE shot # 107314 1KHz frame rate

23. High rate imaging of the plasma center The plasma center is affected by strong oscillations in soft X-ray emission. This effect disappears at r ~ 20 cm USXR_H UP USXR_V TOP 1KHz frame rate

24. Shot #108670 H-mode 5MW bp = 0.7 :excellent agreement t = 0.35 s t = 0.45 s Shot #108729 H-mode 5MW bp = 1.2 :excellent disagreement t = 0.23 s t = 0.40 s t = 0.50 s Plasma core images and comparison with models powerful diagnostics suggest reviewing current models

25. Summary for the Plasma Imaging X-ray CounterS • a novel system has been checked and validated • it combines spatial imaging capability with energy discrimination • energy range: 1- 10 keV adjustable • high time resolution (up to 100 Khz, up to 1Gg/s resolved over the whole detector ) • extremely high dynamic range (300) • large flexibility in imaging: zoom and changes of the spot • Large S/N, pure statistical noise in photon counting mode • good detection efficiency

26. Conclusions • complete decoupling of the amplifying stage from the charge collection plane allows choice of a pixel read-out • GEM amplifier provides very fast electron signal • pixel dimensions can be optimized depending on the application: • fine pitch to reconstruct topology of photoelectron from polarized astrophysical sources • large pitch to allow very high counting rate of large plasma volumes