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Gas based detector systems: MWPC, MSGC,GEM and others R. Bellazzini INFN – Pisa

Gas based detector systems: MWPC, MSGC,GEM and others R. Bellazzini INFN – Pisa International Advanced School Leonardo da Vinci. entrance window. Proportional counters. Depending on the voltage applied on the wire different mode of operation are possible: ionization proportional

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Gas based detector systems: MWPC, MSGC,GEM and others R. Bellazzini INFN – Pisa

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  1. Gas based detector systems: MWPC, MSGC,GEM and others R. Bellazzini INFN – Pisa International Advanced School Leonardo da Vinci

  2. entrance window Proportional counters • Depending on the voltage applied on the wire different mode of operation are possible: • ionization • proportional • limited proportionality • Geiger-Müller First came the CylinderProportional Counter: developed over a century ago it consists of a thin wire anode coaxially positioned in a gas-filled cylindrical cathode tube.

  3. The MultiWire Proportional Chamber (MWPC) • Disadvantages of counter tubes: • need of an entrance window for low energy particles • positional accuracy, obtained with an array of CT, of the order of centimetres. • In 1968 G.Charpak invented the MultiWire Proportional Chamber: an array of many anode wires, 12 mm apart, in a single gas volume enclosed between two metalised cathode planes. Each wire acts as an independent proportional counter with a position resolution of 0.2÷1.0 mm. 2D information on the interaction point is derived from the barycentres of the induced charge distribution on the cathode planes. Position information can be further improved by taking into account the drift time of the electrons towards anode wires (based on this principle are the Drift Chamber and the Time Projection Chamber)

  4. Increasing electric field ions electrons Avalanche electrons are collected very fast on the wire. The development of the anode signal is due essentially to the slow motion of ions towards cathodes. The MWPC Electric field lines in a MWPC. The effect of the slight shifting of a wire is clearly visible. It has no effect on the field around the wire. Electrons released in the gas volume by a ionizing particle or a X-ray drift towards the anode wires where the avalanche multiplication starts. The avalanche is localized in a restricted volume close to the wire. At high fluxes this can result in space charge effect: the built-up of slow positive ions modify the Electric field by making the effective wire diameter thicker gain decreases

  5. From MWPC to MicroStrip Gas Chamber (MSGC) • With the introduction of the MSGC several drawbacks of the MWPC are overcome. • in MWPC the position of the electrodes can be distorted by electrostatic and gravitational • forces. • in MSGC the electrodes position is well defined due to fixation on the substrate. • an entire sector of an MWPC is disabled when a wire breaks and short-cuts other electrodes. • an interrupted strip in a MSGC has, as worst consequence, a locally reduced position resolution. • a strong frame is needed in MWPC to maintain the tension force of the wires. • the possible distortion of the wire position limits the wire spacing to a minimum distance of • 1 mm,this limits position resolution and time shaping. Moreover, as the ions are not removed • quickly enough, the occurrence of space charge hampers full avalanche development at count • rates above 102÷103 counts/sec•mm2. • in a MSGC, the combination of very small inter-electrode distances and high electric field, • allows to attain fast charge collection ( 50 ns) and signal shaping (~ 20 ns). The fine detector • segmentation allows position resolution of 40 mm.

  6. The photo-lithographic process The lift-off technique

  7. The MSGC Cross-section of a MSGC Simulated current signals Electric field lines strongly concentrates on anode strips Signals 10 times faster with respect to MWPC signals

  8. due to the motion ofalong a linewith velocity Signal development on a multi-electrode system As for MWPC, the main signal contribution stems from the current induced by the ions motion. Green’s theorem states for a multi-electrode system the relation between potentials and charges for two state, before and after one or more potentials and charges are changed, in the form: where are the initial values and the values after the change . Assumeis a charge on an infinitesimal electrode and the sensing electrode is 1, thenwhere is caused by and by . The induced current: Where is the weighting field (determined by applying a unity potential to electrode 1 and zero to all the others). The total induced charge is:

  9. Residuals distribution Ne(25)-DME(75) Vcath= -530 V Vdrift= -3000 V  Spatial resolution = 30.5 ± 0.4 mm MSGC performances:spatial resolution

  10. Rate capability From MWPC To MSGC Rate capability increases 3 orders of magnitude Rate capability > 1 MHz/mm2 R. Bellazzini et al., Nucl. Instr. Meth.

  11. Energy resolution

  12. The discharge problem • Characteristics of a MSGC: • very good spatial resolution • high rate capability • very good energy resolution • very good two-track separation but, sometimes: possible discharges at high gain on exposure to heavily ionizing particles. When the total charge in the avalanche exeeds a value between 107÷108 electron-ions pairs (Raether's limit), an enhancement of the electricfield in front and behind the primary avalanche induces the fast growth of a long, filament-like streamer. In thehigh fields and narrow gaps typical of micro-pattern devices, this leads to discharge, with damaging effects onthe strips. Solution! Cathode edge passivation Advanced passivation Standardpassivation

  13. High intensity beam test for spark rate studies Telescope of32 MSGCstested at PSI in Nov99 16384 electronic channels

  14. Spark rate measurement 350-400 MeV/c p+/p beam. MIP spectrum withnon-negligibleHIP rate. 32 days exposure @LHC rate!(Max rate ~6 KHz/mm2) Spark rate 1/day/detector in the whole telescope Measured spark probability  4 10-13 4 channels (out of 16000!) lost in ~400 hrs run

  15. 2D reconstruction capability With the PSI MSGC telescope exploited 2D-Reconstruction capability with Pion beam R. Bellazzini et al., Nucl. Instr. Meth. A457 (2001) 22

  16. The MicroGap Chamber (MGC) The space charge effect, in the MSGC, is strictly connected to the presence of an insulating substrate. When a significant fraction of ions of the avalanche hits this surface, it causes a charge-up which reduces locally the electric field. In the MGC the insulating surface exposed to the gas is strongly reduced. Scanning electron microscope picture R. Bellazzini et al., Nucl. Instr. Meth. A335 (1993) 69 Electric field lines close to the anodes are even more intense than in a MSGC resulting in a very fast signal development.

  17. MWPC MSGC MGC Signal evolution from MWPC to MGC Signals Electric field

  18. MGC performances short term measurement of gain stability The MGC can withstand high rate of radiation (up to few MHz/mm2 ) without visible change in gain. No evidence of charging-up effect with time is observed

  19. The Gas Electron Multiplier (GEM) A new class of position sensitive MicroPattern Gas Detectors, robust and cheap, has been developed using the advanced printed circuit (PCB ) technology: GEM, Micro Groove and Well Detectors The GEM is a thin polymer foil (Kapton), metal coated on both sides, chemically pierced by a high density of holes. On application of a voltage gradient, electrons released on the top side drift into the hole, multiply in avalanche and transfer to the other side. Proportional gains above 103 are obtained in most common gases. F. Sauli, Nucl. Instr. Meth. A386 (1997) 531

  20. Cu-plated Kapton Copper etching Kapton etching Edge finish GEM manufacturing Typical geometry: 5 mm Cu on 50 mm Kapton 70 mm hole at 140 mm pitch

  21. The GEM Charge amplification and read-out take place on separate electrodes. The read-out PC board can be structered in a multi-pixel pattern to get full 2D imaging capability. R.Bouclier et al., Nucl. Instr. Meth. A396 (1997) 50

  22. The GEM The duration of the signal correspond to the drift time of the electrons in the transfer gap. Full width 20 ns (2 mm gap) Induced charge profile on strips 600 mm FWHM

  23. Multiple GEM structures Exposed to heavily ionizing particles all the MicroPattern Detectors can discharge at low gains. Triple GEM A. Bressan, Nucl. Instr. Meth. A424 (1999) 361 Cascades of GEM can provide equal gain at lower voltage so the discharge probability is reduced. S. Bachmann, CERN-EP/2001-151

  24. The MicroGroove Detector (MGD) A new type of 2D position sensitive gas proportional counter fabricated using the advanced PCB technology Advantages: 2D, robustness, big size, low cost Disadvantages: Lower field gradient, reduced speed R. Bellazzini et al., Nucl. Instr. Meth. A424 (1999) 444

  25. 4 cathodes OR signal (reversed polarity) cathodes 4 anodes OR signal (triggering) anodes MGD electric field and signals Equipotential and drift lines

  26. Ballistic deficit If the charge collection time is longer than the shaping time only a fraction (ballistic deficit) of the full avalanche charge is observed in the signal. MGD has the lowest ballistic deficit if compared to MSGC and MGC: MGC – b.d.= 90% MSGC – b.d. = 67% MGD – b.d. = 50% Lower b.c. implies higher gain when a fast electronic is used

  27. MGD performances: gain and energy resolution Gain up to 104 and morecan be easily reached with a good uniformity over the whole detector area . Full energy peak (5.4 keV) and Argon escape peak are clearly resolved . Energy resolution ~ 20% .

  28. Charging and rate capability Short-term gain stability for high intensity X-ray source. Only a small charging effects is observed at very high gain > 104. Rate capability: no drop in gain observed up to 1.2 kHz/mm2. Recovery after Kapton charging is very fast.

  29. The Well Detector Based on PCB technology, it consists of a thin Kapton foil, copper-clad on both sides. Charge amplifying micro-wells are etched into the first metal and Kapton layers. These end on a micro-strip pattern which is defined onto the second metal plane and is used for read-out. The pre-pregging technique is used to bond the Kapton foil to a 300 mm thick vetronite support. extremely resistant to mechanical shocks R. Bellazzini et al., Nucl. Instr. Meth. A423 (1999) 125

  30. The Well detector Electric field map of one cell. The focusing effect of the drift field reaches the maximum at ~ 4 kV/cm where full collection efficiency is observed. Microscope photographs of top and bottom of the wells.

  31. Jem-X: a MSGC astrophysical application The JEM-X Project on INTEGRAL ( a g-ray observatory satellite) Danish Space Research Institute To be launched in October 2002 by ESA The INTEGRAL spacecraft

  32. The Joint European X-ray Monitor (JEM-X) The X-Ray Monitor JEM-X supplements the main Integral instruments (Spectrometer andImager) and plays a crucial role in the detection and identification of the gamma-ray sources and in theanalysis and scientific interpretation of Integral gamma-ray data. • JEM-X will make observationssimultaneously with the main gamma-ray instruments and provide images with arcminute angular resolutionin the 3÷35 keV energy band. • The baseline photon detection system consists of two identical highpressure imaging MSGCs. Each detector unit views the sky through its coded aperture mask locatedat a distance of 3.2 mabove the detection plane. • Due to the broad spectral coverage and the ability to detect and resolve cyclotron lines JEM-X will study sources categories such as: • Active Galactic Nuclei • Accreting X-Ray Pulsars • X-Ray Trasients • Black Hole Candidates

  33. The Jem-X Detector The JEM-X Detector consists of 2 identical, high pressure, 2D-MSGCs. The gas filling is a mixture Xenon-Methane (Gain ~ 1500). The diameter is 250 mm, corresponding to a collecting area of 500cm2. 2D-MSGC plate Qualification model

  34. Jem-X : an X-ray monitor Response of JEM-X to a full area illumination Spectral sensitivity of the JEM-X qualification model Courtesy C. Budtz-Jorgensen (DSRI)

  35. GEM + Pixel Read-Out: recent applications The complete separation between the amplification structure and the pick-up electrodes allows full flexibility on the choice of read-out pattern  GEM+Micro Pixel electrode Recent applications: X-ray Polarimeter Time resolved plasma diagnostic

  36. Heitler W.,The Quantum Theory of Radiation X-ray Polarimetry with a MicroPattern Gas Detector Polarimetry can provide a general tool to explore the structure of compact sources and derive information on mass and angular momentum of supermassive objects. A new polarimeter, based on the photoelectric effect, using a MicroPattern Gas Detector coupled to a GEM has been developed. The device is highly efficient in the energy range 210 keV, particularly interesting for X-ray Astronomy. The photoelectric effect is a process very sensitive to photon polarization and with a large cross section at low energy. In the case of linearly polarized photons, the differential photoelectron cross sectionhas a maximum in the plane orthogonal to the direction of the incoming photon. The photoelectron is ejected with maximum probability in the direction of the photon electric field with a cos2 modulation.

  37. GEM+MicroPixel Read-out 8-layer read-out board Pixel size 0.2 mm Area 2.4 x 2.4 mm2 (128 pixels) Angle and amount of polarization is computed from the angular distribution of the photoelectron tracks, reconstructed by a finely segmented gas detector. Being fully 2D there no need to rotate the detector as in traditional polarimeters. The GEM and the drift plane are glued with two fiberglass spacers, respectively of 1.5 mm (transfer gap) and 6 mm (absorption gap) over the read-out plane

  38. First step - the direction of emission of the photoelectron is reconstructed by finding the major amd minor principal axes (M2max, M2min) of the charge distribution on the pixels. The major principal axis is identified as the photoemission direction. Second step - the third momentum (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 Photoelectron tracks reconstruction (two-step algorithm)

  39. Angular distribution of p.e. tracks Reconstructed emission angles of the photoelectron Distribution of the track barycentres with respect to the absorption point Unpolarized photons ~100 % Polarized Photons The modulation factor for 100% polarized radiation is = b/(2a+b). m 50% E. Costa, R.Bellazzini et al., Nature vol. 411 (2001) 662

  40. Position resolution Distribution of barycentres and absorption points of a collimated unpolarized radiation (5.4 keV) The collimator diameter is 50 mm (smaller than the pixel size) and the absorption points are concentrated in a very small spot of 70 mm (rms). Barycentres are instead distributed at some distance from the absorption points because of the large energy release at the end of the track.

  41. Comparison with traditional polarimeters Extra-galactic sources Degree of polarization Observing time to measure at 99% confidence level the degree of polarization of galactic and extra-galactic sources with traditional and MP polarimeters

  42. XEUS-1 : a possible application The tested prototypeat the focus of XEUS-1 (the X-ray Evolving Universe Spectroscopy mission) could perform polarimetry at % level on many bright AGN in about 1 day observation, in the energy range 2÷10 keV. XEUS consist of a Detector spacecraft with the focal plane instrumentation that receives cosmic X-rays focused by a Mirror spacecraft flying at 50 m in front of it.

  43. Parallel read-out Printed circuit board 128 pixels (2.5 x 2.5 cm2) Plasma imaging with Micro Pattern Gas Detectors Ultra-fast system for X ray imaging based on GEM New diagnostic device in soft X range (315 KeV) for magnetic fusion plasmas Successfully tested on the Frascati Tokamak Upgrade (FTU, Italy) and on the National Spherical Tokamak eXperiment (NSTX) at Princetown (US).

  44. The Frascati TOKAMAK Upgrade (FTU) The experimental set-up at the Enea Laboratories in Frascati The inner toroid Bt = 8 T, Ip = 1.6 MA R = 0.93 m a = 0.3 m

  45. High rate performaces Imaging at high rates (2MHz/pixel) Linearity of GEM current at very high counting rates. Image of a wrench placed close to the detector exposure time = 50 ms Counting rate linear up to 2 MHz/pixel (limited by electronics dead-time)

  46. Setup at the National Spherical Tokamak eXperiment (Princetown USA)

  47. 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 GEM USXR_H UP USXR_V TOP

  48. Plasma center activity 10 khz # 107316

  49. An All-sky X-ray Monitor (AXM) Mission The AXM spacecraft 31 GEM proportional counters provide a continuous monitoring of the entire X-ray sky, except for earth occultation and a small region around the sun. This coverage allow the study of a wide variety of short duration outbursts.

  50. The AXM camera assembly One of the 31 GEM cameras

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