AbstractâA GEM-Based TPC is a very promising option for the central tracker of the PANDA experiment. PANDA is a spectrometer which will detect charged ...
2009 IEEE Nuclear Science Symposium Conference Record
N15-2
A GEM-Based TPC Prototype for PANDA M. Vandenbroucke for the GEM-TPC Collaboration Technische Universit¨at M¨unchen, Physik Department, 85748 Garching, Germany
Abstract—A GEM-Based TPC is a very promising option for the central tracker of the PANDA experiment. PANDA is a spectrometer which will detect charged and neutral particles emitted over the full solid angle from 2 × 107 p¯ p annihilations per second to perform high-precision studies in the field of nonperturbative QCD. The central tracker of the spectrometer has to provide good momentum resolutions and at the same time have a very small material budget in order to minimize secondary interactions or photon conversion. A TPC ideally fulfills these requirements, and, in addition, provides particle identification by measuring the specific energy loss. The GEM technology, used for gas amplification, provides an intrinsic suppression of ion backflow usually done by an ion gate for a wire chamber, inappropriate here because of the continuous nature of the beam. A small GEM-TPC prototype was built and characterized with cosmic muons. An average resolution of 200 μm had been achieved using the PASA/ALTRO readout electronics. In order to perform more detailed investigations of the detector performance in various conditions, a tracking telescope was set up at the electron beam at ELSA, Bonn, Germany. The characterization of the external tracking telescope has begun in December 2008 and the first electron tracks was observed in the TPC.
I. I NTRODUCTION The PANDA (Antiproton Annihilation at Darmstadt) experiment is an internal target experiment at the High Energy Storage Ring (HESR) for antiprotons at the new Facility for Antiproton and Ion Research (FAIR) at Darmstadt, Germany. A Time Projection Chamber (TPC) is proposed as the central tracker due to its excellent tracking and particle identification capabilities [1]. A spatial resolution of ∼150 μm in the rΦ projection and ∼1 mm in z resulting in a momentum resolution of the order of a few percent over a range of 0.1 to 8 GeV and a low material budget are needed by the rich physics program of PANDA. The continuous antiproton beam structure at HESR, however, makes the use of a traditional ion gate, used to avoid space charge accumulation, impractical. Due to their intrinsic ion back-flow suppression, the use of GEM structures [2] (Gas Electron Multiplier) for gas amplification opens up the possibility to operate the TPC in an ungated mode without accumulating excessive space charge. This talk will report on the results of systematic tests of a small-size GEM-TPC with cosmic muons and, using a new front-end chip, with electrons at the ELSA accelerator, where a dedicated test bench has been set up. A larger prototype with 73 cm drift length, whose design will be reviewed, is currently under construction and is expected to be finished by the end of the year. This work is supported by the 6th Framework Program of the EU (Contracts No. RII3-CT-2004-506078, I3 Hadron Physics, and No. 515873-DS, DIRAC-secondary-Beams), the German Bundesministerium f¨ur Bildung und Forschung, the Maier-Leibnitz-Labor der LMU und TU M¨unchen, and the DFG Cluster of Excellence ‘Origin and Structure of the Universe’.
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Fig. 1: The new readout plane (Left) and the GEM-TPC test chamber with L-shape Front-end Electronics (FE) cards (right)
II. T HE GEM-TPC TEST CHAMBER AND COSMIC CHARACTERISATION
A small GEM-TPC was built and characterised with cosmic muons [3]. The test chamber of 20 cm radius, and 8 cm drift length, operates with an Ar/CO2 (70/30) gas mixture. On the readout plate, an active area of 10 × 10 cm2 was filled with rectangular pads of 1.0 × 6.2 mm2 and covered by 3 GEM foils for electron amplification. On the top of the GEM stack, a 20 cm diameter copper plate (with a 10 × 10 cm2 hole) was mounted to shield the inactive region and uniform the electric drift field. This field is homogenised inside the drift volume by cylindrical 3 mm copper strips printed on both side of a 125 m thick insulating polyimide foil. The default voltage of the GEM stack was tuned to create an induction field of 3.75 kV/cm between GEM foils, 50 kV/cm in the GEM holes and a drift field of 250 V/cm. This setting have been tuned for the GEM detector of the COMPASS (COmmon Muon Proton Apparatus for Structure and Spectroscopy) experiment at CERN [7] which uses an intense beam of hadrons. This voltage configuration has been proven to be optimised for avoiding discharges and protect the detector. Copper X-rays (8 keV Kα ) were used to measure a gain of ∼ 4000. The PASA front-end pre-amplifier/shaper amplified charge signals which were digitised with the 10 bit, 10 MHz ALTRO ADC modules [4]. An inverter card was needed because of the negative polarity of the GEM signals. The whole setup reached a noise level (the root mean square of the baseline fluctuation) of ∼1900 e− . The setup recorded a data set consisting of ∼ 13000 tracks where 4100 events was taken with the default HV settings. As a result, a spatial resolution along the short side of the readout pads of 140 μm for drift distances shorter than 10 mm, and 200 μm in average was obtained. Tracks were reconstructed for different orientations of the chamber, penetrating the chamber
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Fig. 2: Beam telescope of the ELSA test bench. Blue line: electron beam, SI: silicon, GM: GEM detector and trig: trigger scintillators. The white dots are the references points for the photogrammetry
either in parallel to the readout plane or perpendicular to it. A rather strong increase of the resolution with drift distance was observed, which is attributed to the high threshold imposed by the PASA/ALTRO readout electronics. Indeed the amplitude of signals is lowered by diffusion and cut by a high threshold; a better electronics is needed to continue this characterisation. III. I MPROVEMENT OF THE TEST CHAMBER The test chamber was upgraded before its installation in the test setup. A new readout plane was designed with hexagonal pads of 1.25 mm and 1.5 mm outer radius. Simulation shown the hexagonal pattern homogenates the spatial resolution and the optimal resolution should be reached for the 1.5 mm pads. For smaller pads, the spatial resolution is dominated by diffusion. For a better monitoring of the drift velocity, temperature sensors were added on the readout plate, inside the gas volume. The main improvement of the test chamber consists in the use of a readout based on the AFTER (Asic For TPC Electronic Readout) chip developed by CEA Saclay for the T2K experiment [5]. This 72 channels ASIC contains an analog memory, based on a Switched Capacitor Array (SCA), of 511 cells which are read by a custom-made pipelined ADC sampling at 10 to 50 MHz. The full front-end electronics reaches a good noise performance of 800 e− at an input capacitance of 10 pF as shown on Fig. 3. The chip has been characterised in detail by studying its noise performance under various conditions, and by external injection of a known charge. A linear gain of 5.5 f C/ADCch has been observed over the full range of the AFTER-T2K chip. The relatively low consumption of the four ASIC front-end card of 3 W allows us to use it without cooling. Moreover the L-shape of those cards, design to put the chips outside of the beam, takes away the heat from the chamber avoiding a temperature gradient in drift volume which would have modify the electron drift velocity and decrease performance of the detector. IV. T HE TEST BENCH AT ELSA To perform a complete characterisation of the test chamber, free of bias, it has been decided to use an external tracking on a test beam. We set up a tracking telescope on the electron accelerator ELSA in Bonn, Germany. The test bench is situated at the bent electron beam of the photon tagging system near the
Fig. 3: Noise pattern of the 76 channels of one AFTER-T2K chip mounted on a L shape card. Channels 0 to 4 and 71 to 75, that have a lower noise, are not connected to the pad plane. The 4 lower noise channels are FPN (Fix Pattern Noise) channels which are not connected outside the T2K to correct common mode noise.
Crystal Barrel experiment. The expected energy of the beam is around 400 MeV in the trigger acceptance. The setup consists of 4 single sided silicon strip detectors and 4 GEM planes, triggered by four 2 cm2 plastic scintillators, see Fig. 2. Detectors are read using the readout chain of the COMPASS experiment based on the DATE data acquisition software framework [8]. Both GEM and silicon detectors are read with the APV-25s chip [9], digitalised with a custom made ADC [10]. The acceptance of the trigger scintillator provide a trigger rate of ∼ 200 Hz that we lower to ∼ 150 Hz to conserve a sufficient dead time between events to read the 16 chips of one ADC card for the TPC. The GEM strip detectors have an active area of 10.24 × 10.24 cm2 and a pitch of 400 μm. Each GEM strip detector has 512 channels divided in two projections perpendicular to eachother. The two projections of the silicon strip detectors contain 384 strips 50 μm reparted on an active area of 1.92 × 1.92 cm2 . Because of the low energy of the electron beam, multiple scattering has to be taken in account. Indeed a first arrangement of the bench with one scintillator and the TPC inside the tracking telescope scattered too much the electron beam and track reconstruction was no longer possible [11]. The radiation length have been calculated for each detector, see table I. TABLE I: Radiation length of the telescope detectors Detector Scintillator GEM Silicon GEM-TPC Drift end plate GEM-TPC New drift end plate
X/X0 1.436% 1.13% 0.32% 1.334% 0.34%
Projected on the pad plane of the TPC, the projection of the tracks, scattered by the material in the telescope, cover a width of ∼ 90 μm which will bias the measured resolution. To further lower the ammount of multiple scattering in the drift end plate, a new one has been produced reducing the amount of material by a factor of 4 and this width to ∼ 60 μm. A rough alignment of the test bench have been performed using a photogrammetry software called Photomodler [12].
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From a set of pictures taken from different angles and reference points (see Fig. 2), a 3-D model is created with a precision in the order of millimetre. This geometry is used as an input of the Millipede [6] package. By the minimisation of the χ2 contribution for all tracks and hits for both tracks and alignment parameters, we perform an alignment of the test bench in the ∼ 100 μm range. To reconstruct tracks within the telescope, a fast hough transform have been implemented on the XZ and YZ projection [13]. This reconstruction is done assuming straight line tracks.
(a) Beam profile observed in the cham-(b) Distribution of signals in the TPC ber along the sample space (at 20 MHz)
V. R ESULTS FROM THE TRACKING TELESCOPE The tracking telescope was successfully commissioned in December 2008 and has started to record data. First signals have been observed in the GEM-TPC and the data analysis has been started. As we use only 4 detectors, the tracking is totally dominated by the silicon detectors, giving very small residuals for them as presented in table II. Indeed since the weighting parameter in the track √ fitting algorithm depends on the expected resolution (pitch/ 12), the tracks are fixed in respect to the silicon detectors which then have very small residuals between their signals and the reconstructed track.
(b) Y
Fig. 4: Correlation between tracks of the telescope and cluster of the test chamber
The first analysis of the residuals of the TPC has shown a resolution corresponding to the size of the area covered by one chip. This result is explained by the presence of a large crosstalk inside one chip area as shown on Fig. 5. This pattern can be explain either by the geometry distribution of channel on the L-shape card or/and by a bad synchronisation between the digitalisation and the front-end chips; further investigation should fix this issue in the near future. Then, systematics studies using different angles, gas and gains are planned. A cosmics trigger has been installed paralleled to the chamber to compare resolution with the result of the previous characterisation with cosmic muons.
TABLE II: Residuals of telescope detectors (biased) [11] Detector GEM 1 X GEM 1 Y GEM 2 X GEM 2 Y Silicon 1 X Silicon 1 Y Silicon 2 X Silicon 2 Y
(a) X
Residuals (biased) 192.2 μm 190.1 μm 212.4 μm 183.5 μm 1.8 μm 1.8 μm 2.0 μm 2.8 μm
VI. F IRST ANALYSIS OF THE TPC DATA AND FURTHER TESTS
The test chamber took data during two runs of one week each during summer 2009. The 24 readout chips can now be read along the full 511 samples range using a zero suppression algorithm on the ADC card. For each electronic channel, the mean value and the RMS are stored to suppress signals below the noise threshold (here at 3σ). A beam profile has been observed in the chamber, see Fig. 4a and signals have been observed along the expected drift time, see Fig. 4b. Signals are present on 284 samples taken at 20 MHz, i.e. a drfit time of 14.2 μs as expected. The decrease of signals is due to a geometry effect (here only one chip is read). As a preliminary study of the consistency of the setup, especially of the synchronisation, the correlation between tracks reconstructed by the beam telescope and signals of the test chamber has been studies. The correlation projected on x and y axis (z is along the beam axis) is shown on Fig. 4. The correlations are observed and an advanced study can be performed.
Fig. 5: Autocorrelation of signals of the TPC. A crosstalk between channels and their symmetric according to the median channel is observed
VII. F URTHER PROTOTYPES In parallel of the test chamber studies, two larger prototypes have been design to prove the feasibility of a continuous sampling GEM-TPC in a high rate environment.
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These cylindrical chambers have an outer radius of 30 cm, an inner radius of 10 cm and a 73 cm of drift length, Fig. 6 shows a 3-D CAD view of the prototypes. Both of these detectors have 10296 hexagonal pads connected to 42 frontend cards. There are two electronics foreseen, one based on the AFTER-T2K, as for the test chamber, and the other one based on the self triggered n-XYTER ASIC [15] originally developed for neutron detection. For the AFTER-T2K electronics, the front-end cards are a shorten version of the L-shape cards and the first prototype of those reaches a good noise performance of ∼ 750e− . Each detector has a GEM stack of 3 GEM foils and possibly a fourth one for additional ion back-flow suppression. These foils are segmented into 8 iris-shaped sectors connected through high-ohmic resistors to minimised the probability of discharges. Six GEM foils have already been tested at TUM and have shown good performance. The field cages are made of a self-supported sandwich structure of Rohacell core and Kapton insulated layers which are currently build at the detector lab of GSI.
R EFERENCES [1] The PANDA collaboration Technical Progress Reports for PANDA, (2005) http://www-panda.gsi.de/archive/public/panda tpr.pdf [2] F. Sauli, GEM, Charge transfer and charge broadening of GEM structures in high magnetic fields, Nucl. Instr. Meth. A386, 531, (1997) [3] Q. Weitzel et al.,Development of a High-Rate GEM-Based TPC for PANDA, IEEE Nucl. Sci. Symp. Conf. Rec. (2007) [4] L. Musa et al., The ALICE TPC Front End Electronic, Proceedings of the IEEE Nucl. Sci. Symp., Portland, (2003) [5] P. Baron et al., AFTER, an ASIC for the Readout of the Large T2K Time Projection Chambers Nuclear Science, IEEE Transactions on Volume 55, Issue 3, (2008) [6] V. Blobel and C. Kleinwort, A new method for high-precision alignment of track detectors (Aug 2002) [7] S. Uhl, Construction and Commissioning of the PixelGEM Tracking System for the COMPASS Experiment Master’s thesis, 2008 [8] L. Schmitt et al., The DAQ of the COMPASS experiment, IEEE Transactions on Nuclear Science 51 (2004) 439-444 [9] L. Jones, APV 25 User Guide Version 2.2 [10] Alexander Mann, Igor Konorov and Stephan Paul, A Versatile Sampling ADC System for On-Detector Applications and the AdvancedTCA Crate Standard, published in 15th IEEE-NPSS Real-Time Conference, 2007 [11] S. Dorheim,Track Reconstruction in a Setup for the CHaracterization of a GEM-TPC at ELSA Master’s Thesis, 2009 [12] http://www.photomodeler.com/ [13] F. Boehmer et al., Simulations of a High-Rate TPC for PANDAIEEE NPSS conference 2009, in preparation [14] A. Winnebeck, PHD-thesis, Universitat Bonn, 2009 [15] A.S. Brogna et al., The n-XYTER Reference Manual 2008
Fig. 6: 3-D CAD view of the TPC for FOPI and CB ELSA. Thanks to this self-supported structure, these chambers will be inserted into the detectors of the FOPI experiment at GSI and Crystal Barrel at ELSA. At FOPI, among other, the TPC will allow a vertex reconstruction in x,y+z in the order of the millimetre and at ELSA it will be inserted between the 1230 crystals to upgrade the detector [14]. VIII. O UTLOOK The upgraded test chamber and its tracking telescope are taking data to provide the first complete characterisation of the small-size GEM-TPC. The external tracking and the very good noise performance of the readout electronic will allow a precise measurement of spatial resolutions, drift velocity and charge accumulation inside the TPC. Since the same electronics will be available for the larger prototypes, their characterisation on the test bench at ELSA will be much faster and easier. The working test bench will provide a privileged place for advanced studies such as the monitor of the ion back-flow and others innovative techniques for GEM-TPC. These measurements will provide a base for operating the two chambers in real physics experiment, and, at term, a starting point for the design of the GEM-TPC for the Panda experiment.
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