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The transition radiation detector in the CBM experiment at FAIR
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
The transition radiation detector in the CBM experiment at FAIR✩ Philipp Kähler a ,∗, Florian Roether b , for the CBM collaboration a b
Institut für Kernphysik, Universität Münster, Wilhelm-Klemm-Str. 9, 48149 Münster, Germany Institut für Kernphysik, Goethe-Universität, Max-von-Laue-Str. 1, 60438 Frankfurt am Main, Germany
ARTICLE
INFO
Keywords: FAIR CBM Transition radiation detector TRD High rate
ABSTRACT As the civil works for the SIS100 accelerator have started at FAIR/GSI and the design of the Compressed Baryonic Matter (CBM) experiment and its detector subsystems have progressed to an almost complete level, also the production of most of the detectors and components is starting soon. Commissioning of larger test systems (‘‘FAIR Phase 0’’) and last detector characterisations are being conducted. In this paper, the design of the MWPC-based Transition Radiation Detector (TRD) for the very high particle rates of the CBM experiment is shown. We present details of the self-triggered front-end electronics of the TRD and general aspects of the read-out. Furthermore, results on the energy resolution of the TRD and first insights into recent measurements of TRD-MWPCs in the high-rate field of the CERN Gamma Irradiation Facility are presented.
1. The Compressed Baryonic Matter experiment at FAIR The Compressed Baryonic Matter (CBM) experiment is an upcoming fixed-target experiment at the Facility for Antiproton and Ion Research (FAIR) accelerator SIS100.1 At collision energies from 2.8 to 5 𝐴GeV in the centre-of-mass, interaction rates at the target of up to 10 MHz will be reached for the heavy collision systems. The magnet and detector subsystems of CBM can be seen in Fig. 1. In the heavy-ion collisions of the experiment, it is expected to reach net-baryon densities of 5 to 8𝜌0 , with 𝜌0 being the normal nuclear density. Thereby, the CBM experiment has the potential to contribute to the understanding of QCD matter at neutron-star densities. According to transport calculations, the dense QCD regime is reached for a comparably long time of ≥ 5 fm/𝑐 [1]. Due to the high interaction rates, we expect access to rare probes of the medium like e.g. excitation functions of multi-strange hyperons near the phase boundary in a new level of precision. Detailed information on the physics programme of CBM have been summarised recently in [2]. Particular contributions of the Transition Radiation Detector, as deduced from system simulations, will, e.g., be electron identification for electron momenta above 2 GeV/𝑐 in di-electron measurements and charged fragment identification, which is in particular relevant for hypernuclei measurements [3]. 2. The Transition Radiation Detector in CBM The design for the Transition Radiation Detector (TRD) in CBM is based on irregular PE-foam radiators and Xe-filled MWPCs. Four
TRD layers will be installed. The inner detector design is sketched in Fig. 2, wherein also the working principle of the detector can be retraced: passing electrons are not only depositing energy in the MWPC by ionisation of the detector gas, but also produce transition radiation photons in the radiator material — increasing with momentum, and significantly starting at electron momenta between 1 and 2 GeV/𝑐. Electron identification is mainly based on the registration of these photons in the MWPC in addition to the electron energy loss, and thereby enables the suppression of the dominant pion background in hadronic collisions, since the pions are not reaching sufficient 𝛾 factors for transition radiation generation. The radiator in our detector will be built of PE foam and thus be of the irregular type. While the spectrum of transition radiation from one interface of materials with different dielectric constants can be directly calculated, a calculation of the resulting spectrum from an irregular distribution of such interfaces turned out to be challenging [4]. From testbeam campaigns with particle identification reference detectors and accompanying radiator simulations, the PE foam has been chosen due to its overall highest efficiency (performance, detector design). It is optimised for comparatively soft transition radiation with dominant photon energies below 15 keV. For efficient absorption of the transition radiation photons, the MWPCs of the detector will be operated with Xe/CO2 in a mixing ratio of 85:15. The entrance region of the MWPC will be built from polyimide foils, as partly visible in Fig. 3, which are coated with a thin aluminium layer to also serve as negative electrode for the drift field. The chambers are to be operated at a maximum differential overpressure of 1 mbar.
✩ This work was supported by BMBF, Germany grants 05P16PMFC1 and 05P15RFFC1 and by the GSI F&E programme. ∗ Corresponding author. E-mail addresses: [email protected] (P. Kähler), [email protected] (F. Roether). 1 SIS: Schwerionen-Synchrotron, ger. for Heavy-Ion Synchrotron.
https://doi.org/10.1016/j.nima.2019.162727 Received 31 March 2019; Accepted 6 September 2019 Available online xxxx 0168-9002/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: P. Kähler, F. Roether, The transition radiation detector in the CBM experiment at FAIR, Nuclear Inst. and Methods in Physics Research, A (2019) 162727, https://doi.org/10.1016/j.nima.2019.162727.
P. Kähler and F. Roether
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 1. Sketch of the CBM experiment at FAIR-SIS100, with the beam pipe coming from left-hand side. A magnetic field for momentum measurement will be applied only to the active volumes of the Silicon Tracking System (STS) and the Micro-Vertex Detector (MVD), and also the target will be installed inside the experiment’s magnet. Shown here is the so-called ‘‘electron setup’’ of the experiment, while for the ‘‘muon setup’’, the MuCh system takes the position of the RICH detector.
Fig. 4. Effective photon absorption probability for the chosen MWPC design, when being operated with Xe/CO2 (85:15). Materials of the entrance region of the chamber are included such that only the useable absorption in the detector gas is shown here.
photon absorption probability of the entrance region is shown in Fig. 4. It is obtained by multiplying the photon absorption probability in the detector gas with the transmission probability through the entrance window. A special requirement in CBM is the stability of the detectors in high particle rates. Therefore, the TRD-MWPC is foreseen in a fast design with a thin drift region of 5 mm and a symmetrical amplification region of 3.5+3.5 mm. The chambers will be operated at a gas gain of about 2000 and with a drift field of 100 V/cm, leading to an overall signal collection time between 250 and 300 ns [3]. 3. Detector read-out 3.1. TRD Read-out and SPADIC as TRD front-end electronic
Fig. 2. Functional sketch of a CBM-TRD module. The MWPC part is constructed with a symmetrical amplification region of 3.5+3.5 mm and a drift region of 5 mm. The PE foam foil radiator contacts directly to the entrance window to achieve high TR efficiency, while the window is aluminised on the inside to also serve as electrode for the drift field.
The TRD will be read-out by a segmented cathode-pad plane, in which mirror charge is induced from the ionisation avalanches at an anode wire of the MWPC. The pad width is adjusted such that for the given field geometry a 10:80:10 charge distribution is reached on three adjacent pads, given a charged particle track centred to a cathode pad [5]. This way, a good precision of position reconstruction can be achieved. With pad widths of 6.7 to 6.8 mm, a position resolution of 0.3 mm is shown to be reachable [3]. Connected to the cathode pads, the digitisation of the TRD will be done by SPADIC (Self-triggered Pulse Amplification and Digitisation asIC) chips, which are highly integrated front-ends consisting of 2 units à 16 channel each [6]. Analogue and synthesised digital part are combined on one die with 180 nm structure size. The inputs are designed for a charge range of up to 75 fC, matching the expected signal heights generated by the used MWPCs. After a 1st order shaper with a peaking time of 120 ns, translating the input signals of the MWPC to standard pulse shapes obeying proportionality of integral and maximum value to the amount of incoming charge, the shaped pulse of each input channel is digitised in a continuously running 9 bit ADC. The sampling frequency is derived from a clock in the DAQ system. For TRD readout, a sampling frequency of 160 MHz is planned. Still in the SPADIC, a so-called hit logic is continuously checking the incoming samples against a trigger condition. The trigger condition is freely programmable and may consist of absolute or relational conditions required from subsequent ADC samples. If the trigger condition is fulfilled, a hit message is generated and sent via the e-link interface. The number of samples and their position relative to the trigger condition can be configured. If the trigger condition is fulfilled again during an ongoing message building, a new set of samples will be
Fig. 3. Photograph of the lower half of the entrance region of a MWPC for the TRD. Width of 95 cm. The white structure is the part of the PE-foam radiator which is directly attached to the chamber. One grid cell is left free for visibility of the aluminised polyimide entrance window (gold-yellow colour). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
To stabilise the position of the flexible entrance window with a maximum expansion below 1 mm, a grid of carbon ledges with 0.8 ×15 mm2 is glued to the entrance foil. We note that calculations have confirmed sufficient gain stability even for the maximally expected distortion of the entrance window, while anyhow just the drift region is directly affected. The ledges of the carbon grid are visible in Fig. 3 as black lines, while the lighter lines are filaments fixing the part of the radiator foam which is directly attached to the chamber. The effective overall 2
Please cite this article as: P. Kähler, F. Roether, The transition radiation detector in the CBM experiment at FAIR, Nuclear Inst. and Methods in Physics Research, A (2019) 162727, https://doi.org/10.1016/j.nima.2019.162727.
P. Kähler and F. Roether
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 5. Functional elements of the CBM readout as it is foreseen for the TRD of the experiment, while the multiplicity from left to right is omitted for clarity. After signal digitisation and message building in the SPADIC (Self-triggered Pulse Amplification and Digitisation asIC), the e-links from up to 21 SPADIC boards get GBT-multiplexed on the ROB (Read-out Board) and in a common stream transmitted optically to a FLES (First-Level Event Selector) entry node. Link reception from multiple ROBs is done by boards in these entry nodes, which we call CRI (Common Readout Interface). On the CRI FPGA as well as on CPU level of the entry nodes, different levels of data reduction by feature-extraction and event selection can be enabled.
sent. Thereby, this readout has no intrinsic deadtime, while of course the reconstruction precision of multiple charge depositions (pile-up) decreases with time distances approaching the signal collection time and analogue shaping time — and/or hits close in space. To enable comfortable trigger conditions, representing effective thresholds just below energy deposition of minimum-ionising particles, and to enable efficient reconstruction, the chip features so-called forced-neighbour readout, which means the transmission of ADC samples also from pads adjacent to a self-trigger, fully matching in recording time. Concerning the analogue baseline, a trending average is calculated in the chip for every channel and is being transmitted for every self- or neighbour-triggered message. Due to the absence of external triggers, the exact time information of every charge message is crucial. The time structure is optimised for low overhead: each message includes an 8 bit timestamp, while the next timing level is a 6 bit overflow counter generated only for each group of channels. A third, general timing is added in a data concentration level on one of the subsequent boards. Altogether, the time information represents below 3% of the data stream in case of high rates.
Fig. 6. Average trigger rates in kHz for minimum-bias Au+Au collisions at 10 𝐴GeV, for an interaction rate of 10 MHz, for the TRD layer with the largest rate values. Calculated using the UrQMD event generator and GEANT3.
3.2. Data acquisition chain and data reduction A sketch of the functional elements of the data acquisition for the TRD can be seen in Fig. 5. Following to the SPADIC front-ends, the data will be multiplexed still in the detector layer with GBT-based Readout Boards (ROBs). In the subsequent FPGA layer, data reduction by feature extraction is foreseen. Methods for charge determination and time reconstruction with a precision above the ADC sampling are currently in development and will be tested on real detector data. 4. Expected trigger rates and structural scaling Simulations with UrQMD [7], representing Au+Au collisions at 10 𝐴GeV with minimum-bias trigger have been used to prognose the resulting trigger rates propagated to the detector plane, while interactions with all foreseen materials have been included with GEANT3 [8]. From these simulation, a trigger rate distribution per detector channel is derived. In feedback with these simulations, the sizes of the cathode pads have been chosen such that the local trigger are balanced to meet a reasonable occupancy of the read-out electronics. Finally, four cathodepad lengths from 1.2 up to 8.0 cm have been chosen, and two different module sizes of 55 × 55 cm2 and 99 × 99 cm2 as can be seen in Fig. 6. The colour axis of this plot shows the resulting local trigger rates. The modules of each second TRD layer are rotated by 90 degrees to achieve equal position resolution in both dimensions.
Fig. 7. Energy resolution measured with a SPADIC readout chip on an MWPC obeying the final HV geometry. Different reconstruction methods to determine the charge deposition on a pad are compared. Clusterisation of neighboured pads following in each case. A clear dependence on the gas gain (anode voltage) can be seen.
Furthermore, measurements with a 55 Fe source have been recorded to determine the energy resolution of the system of MWPC and readout electronics. Fig. 7 shows the resolution, achieved by Gaussian fitting to the K-line emission of the source. The different curves are comparing methods to define the charge value recorded on a cathode pad: ‘‘MaxADC’’ takes only the maximum ADC value of the shaped pulses, while the other two maximum-based methods are including two or four more samples around the maximum. The intention is a stabilisation against phase shifts of the analogue pulse against the sampling frequency. The ‘‘integral’’ method sums over all recorded 32 samples. The comparatively better results for the ‘‘MaxADC’’ methods hint on modes with frequencies lower than, but of similar order as the sampling frequency, which is currently being investigated. We note that the achieved energy resolutions met all requirements on the detector.
5. Detector tests in lab situations and at beam facilities 5.1. Detection efficiency and energy resolution It was possible to demonstrate an overall electron detection efficiency of (98.5 ± 2.0)%, with the large uncertainty being understood from issues in an older front-end electronics version, and thereby verifying the overall principal functionality of the proposed read-out. 3
Please cite this article as: P. Kähler, F. Roether, The transition radiation detector in the CBM experiment at FAIR, Nuclear Inst. and Methods in Physics Research, A (2019) 162727, https://doi.org/10.1016/j.nima.2019.162727.
P. Kähler and F. Roether
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
per detector hit in this measurement against the expected values for operation in CBM is ongoing. Also the determination of the muon detection efficiency from simultaneously recorded muon beam passing the detectors is being prepared. References [1] I.C. Arsene, L.V. Bravina, W. Cassing, Y.B. Ivanov, A. Larionov, J. Randrup, V.N. Russkikh, V.D. Toneev, G. Zeeb, D. Zschiesche, Dynamical phase trajectories for relativistic nuclear collisions, Phys. Rev. C 75 (2007) 34902, http://dx.doi.org/ 10.1103/PhysRevC.75.034902. [2] T. Ablyazimov, A. Abuhoza, R. Adak, et al., Challenges in QCD matter physics – the scientific programme of the Compressed Baryonic Matter experiment at FAIR, Eur. Phys. J. A 53 (2017) 60, http://dx.doi.org/10.1140/epja/i2017-12248-y. [3] C. Blume, C. Bergmann, D. Emschermann, The Transition Radiation Detector of the CBM Experiment at FAIR : Technical Design Report for the CBM Transition Radiation Detector (TRD), Facility for Antiproton and Ion Research in Europe, Darmstadt, 2018, http://dx.doi.org/10.15120/GSI-2018-01091. [4] A. Andronic, J. Wessels, Transition radiation detectors, Nucl. Instrum. Methods Phys. Res. A 666 (2012) 130–147, http://dx.doi.org/10.1016/j.nima.2011.09.041. [5] E. Mathieson, Cathode charge distributions in multiwire chambers, Nucl. Instrum. Methods Phys. Res. A 270 (2) (1988) 602–603, http://dx.doi.org/10.1016/01689002(88)90736-X. [6] T. Armbruster, P. Fischer, M. Krieger, I. Perić, Multi-channel charge pulse amplification, digitization and processing asic for detector applications, in: 2012 IEEE Nuclear Science Symposium and Medical Imaging Conference Record (NSS/MIC), 2012, pp. 697–702, http://dx.doi.org/10.1109/NSSMIC.2012.6551195. [7] S. Bass, M. Belkacem, M. Bleicher, et al., Microscopic models for ultrarelativistic heavy ion collisions, Prog. Part. Nucl. Phys. 41 (1998) 255–369, http://dx.doi. org/10.1016/S0146-6410(98)00058-1. [8] R. Brun, F. Bruyant, M. Maire, A. McPherson, P. Zanarini, GEANT3, CERN-DD-EE-84-1, 1987. [9] D. Pfeiffer, G. Gorine, H. Reithler, B. Biskup, A. Day, A. Fabich, J. Germa, R. Guida, M. Jaekel, F. Ravotti, The radiation field in the Gamma Irradiation Facility GIF++ at CERN, Nucl. Instrum. Methods Phys. Res. A 866 (2017) 91–103, http://dx.doi.org/10.1016/j.nima.2017.05.045.
Fig. 8. Currents measured in the HV supplies for anode and cathode wire layers, plotted here with respect to the counting rate on the detector. Linear fits are applied as linearity check.
5.2. Operation at high rates To characterise the MWPC behaviour in a high radiation load, recently measurements at the CERN Gamma Irradiation Facility (GIF) [9] have been conducted. MWPCs for the CBM-TRD have been exposed to different gamma irradiation levels. Fig. 8 shows the measured currents at the amplification and drift wires of the chamber vs. the hit rates as seen in the front-end electronics. The scaling of both is compatible with linearity, which we interpret as the absence of space-charge effects in the MWPC and thereby as stable gas gain and stable ion backflow under the measured loads. A comparison of the amounts of ionisation
4
Please cite this article as: P. Kähler, F. Roether, The transition radiation detector in the CBM experiment at FAIR, Nuclear Inst. and Methods in Physics Research, A (2019) 162727, https://doi.org/10.1016/j.nima.2019.162727.
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
The transition radiation detector in the CBM experiment at FAIR✩ Philipp Kähler a ,∗, Florian Roether b , for the CBM collaboration a b
Institut für Kernphysik, Universität Münster, Wilhelm-Klemm-Str. 9, 48149 Münster, Germany Institut für Kernphysik, Goethe-Universität, Max-von-Laue-Str. 1, 60438 Frankfurt am Main, Germany
ARTICLE
INFO
Keywords: FAIR CBM Transition radiation detector TRD High rate
ABSTRACT As the civil works for the SIS100 accelerator have started at FAIR/GSI and the design of the Compressed Baryonic Matter (CBM) experiment and its detector subsystems have progressed to an almost complete level, also the production of most of the detectors and components is starting soon. Commissioning of larger test systems (‘‘FAIR Phase 0’’) and last detector characterisations are being conducted. In this paper, the design of the MWPC-based Transition Radiation Detector (TRD) for the very high particle rates of the CBM experiment is shown. We present details of the self-triggered front-end electronics of the TRD and general aspects of the read-out. Furthermore, results on the energy resolution of the TRD and first insights into recent measurements of TRD-MWPCs in the high-rate field of the CERN Gamma Irradiation Facility are presented.
1. The Compressed Baryonic Matter experiment at FAIR The Compressed Baryonic Matter (CBM) experiment is an upcoming fixed-target experiment at the Facility for Antiproton and Ion Research (FAIR) accelerator SIS100.1 At collision energies from 2.8 to 5 𝐴GeV in the centre-of-mass, interaction rates at the target of up to 10 MHz will be reached for the heavy collision systems. The magnet and detector subsystems of CBM can be seen in Fig. 1. In the heavy-ion collisions of the experiment, it is expected to reach net-baryon densities of 5 to 8𝜌0 , with 𝜌0 being the normal nuclear density. Thereby, the CBM experiment has the potential to contribute to the understanding of QCD matter at neutron-star densities. According to transport calculations, the dense QCD regime is reached for a comparably long time of ≥ 5 fm/𝑐 [1]. Due to the high interaction rates, we expect access to rare probes of the medium like e.g. excitation functions of multi-strange hyperons near the phase boundary in a new level of precision. Detailed information on the physics programme of CBM have been summarised recently in [2]. Particular contributions of the Transition Radiation Detector, as deduced from system simulations, will, e.g., be electron identification for electron momenta above 2 GeV/𝑐 in di-electron measurements and charged fragment identification, which is in particular relevant for hypernuclei measurements [3]. 2. The Transition Radiation Detector in CBM The design for the Transition Radiation Detector (TRD) in CBM is based on irregular PE-foam radiators and Xe-filled MWPCs. Four
TRD layers will be installed. The inner detector design is sketched in Fig. 2, wherein also the working principle of the detector can be retraced: passing electrons are not only depositing energy in the MWPC by ionisation of the detector gas, but also produce transition radiation photons in the radiator material — increasing with momentum, and significantly starting at electron momenta between 1 and 2 GeV/𝑐. Electron identification is mainly based on the registration of these photons in the MWPC in addition to the electron energy loss, and thereby enables the suppression of the dominant pion background in hadronic collisions, since the pions are not reaching sufficient 𝛾 factors for transition radiation generation. The radiator in our detector will be built of PE foam and thus be of the irregular type. While the spectrum of transition radiation from one interface of materials with different dielectric constants can be directly calculated, a calculation of the resulting spectrum from an irregular distribution of such interfaces turned out to be challenging [4]. From testbeam campaigns with particle identification reference detectors and accompanying radiator simulations, the PE foam has been chosen due to its overall highest efficiency (performance, detector design). It is optimised for comparatively soft transition radiation with dominant photon energies below 15 keV. For efficient absorption of the transition radiation photons, the MWPCs of the detector will be operated with Xe/CO2 in a mixing ratio of 85:15. The entrance region of the MWPC will be built from polyimide foils, as partly visible in Fig. 3, which are coated with a thin aluminium layer to also serve as negative electrode for the drift field. The chambers are to be operated at a maximum differential overpressure of 1 mbar.
✩ This work was supported by BMBF, Germany grants 05P16PMFC1 and 05P15RFFC1 and by the GSI F&E programme. ∗ Corresponding author. E-mail addresses: [email protected] (P. Kähler), [email protected] (F. Roether). 1 SIS: Schwerionen-Synchrotron, ger. for Heavy-Ion Synchrotron.
https://doi.org/10.1016/j.nima.2019.162727 Received 31 March 2019; Accepted 6 September 2019 Available online xxxx 0168-9002/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: P. Kähler, F. Roether, The transition radiation detector in the CBM experiment at FAIR, Nuclear Inst. and Methods in Physics Research, A (2019) 162727, https://doi.org/10.1016/j.nima.2019.162727.
P. Kähler and F. Roether
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 1. Sketch of the CBM experiment at FAIR-SIS100, with the beam pipe coming from left-hand side. A magnetic field for momentum measurement will be applied only to the active volumes of the Silicon Tracking System (STS) and the Micro-Vertex Detector (MVD), and also the target will be installed inside the experiment’s magnet. Shown here is the so-called ‘‘electron setup’’ of the experiment, while for the ‘‘muon setup’’, the MuCh system takes the position of the RICH detector.
Fig. 4. Effective photon absorption probability for the chosen MWPC design, when being operated with Xe/CO2 (85:15). Materials of the entrance region of the chamber are included such that only the useable absorption in the detector gas is shown here.
photon absorption probability of the entrance region is shown in Fig. 4. It is obtained by multiplying the photon absorption probability in the detector gas with the transmission probability through the entrance window. A special requirement in CBM is the stability of the detectors in high particle rates. Therefore, the TRD-MWPC is foreseen in a fast design with a thin drift region of 5 mm and a symmetrical amplification region of 3.5+3.5 mm. The chambers will be operated at a gas gain of about 2000 and with a drift field of 100 V/cm, leading to an overall signal collection time between 250 and 300 ns [3]. 3. Detector read-out 3.1. TRD Read-out and SPADIC as TRD front-end electronic
Fig. 2. Functional sketch of a CBM-TRD module. The MWPC part is constructed with a symmetrical amplification region of 3.5+3.5 mm and a drift region of 5 mm. The PE foam foil radiator contacts directly to the entrance window to achieve high TR efficiency, while the window is aluminised on the inside to also serve as electrode for the drift field.
The TRD will be read-out by a segmented cathode-pad plane, in which mirror charge is induced from the ionisation avalanches at an anode wire of the MWPC. The pad width is adjusted such that for the given field geometry a 10:80:10 charge distribution is reached on three adjacent pads, given a charged particle track centred to a cathode pad [5]. This way, a good precision of position reconstruction can be achieved. With pad widths of 6.7 to 6.8 mm, a position resolution of 0.3 mm is shown to be reachable [3]. Connected to the cathode pads, the digitisation of the TRD will be done by SPADIC (Self-triggered Pulse Amplification and Digitisation asIC) chips, which are highly integrated front-ends consisting of 2 units à 16 channel each [6]. Analogue and synthesised digital part are combined on one die with 180 nm structure size. The inputs are designed for a charge range of up to 75 fC, matching the expected signal heights generated by the used MWPCs. After a 1st order shaper with a peaking time of 120 ns, translating the input signals of the MWPC to standard pulse shapes obeying proportionality of integral and maximum value to the amount of incoming charge, the shaped pulse of each input channel is digitised in a continuously running 9 bit ADC. The sampling frequency is derived from a clock in the DAQ system. For TRD readout, a sampling frequency of 160 MHz is planned. Still in the SPADIC, a so-called hit logic is continuously checking the incoming samples against a trigger condition. The trigger condition is freely programmable and may consist of absolute or relational conditions required from subsequent ADC samples. If the trigger condition is fulfilled, a hit message is generated and sent via the e-link interface. The number of samples and their position relative to the trigger condition can be configured. If the trigger condition is fulfilled again during an ongoing message building, a new set of samples will be
Fig. 3. Photograph of the lower half of the entrance region of a MWPC for the TRD. Width of 95 cm. The white structure is the part of the PE-foam radiator which is directly attached to the chamber. One grid cell is left free for visibility of the aluminised polyimide entrance window (gold-yellow colour). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
To stabilise the position of the flexible entrance window with a maximum expansion below 1 mm, a grid of carbon ledges with 0.8 ×15 mm2 is glued to the entrance foil. We note that calculations have confirmed sufficient gain stability even for the maximally expected distortion of the entrance window, while anyhow just the drift region is directly affected. The ledges of the carbon grid are visible in Fig. 3 as black lines, while the lighter lines are filaments fixing the part of the radiator foam which is directly attached to the chamber. The effective overall 2
Please cite this article as: P. Kähler, F. Roether, The transition radiation detector in the CBM experiment at FAIR, Nuclear Inst. and Methods in Physics Research, A (2019) 162727, https://doi.org/10.1016/j.nima.2019.162727.
P. Kähler and F. Roether
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 5. Functional elements of the CBM readout as it is foreseen for the TRD of the experiment, while the multiplicity from left to right is omitted for clarity. After signal digitisation and message building in the SPADIC (Self-triggered Pulse Amplification and Digitisation asIC), the e-links from up to 21 SPADIC boards get GBT-multiplexed on the ROB (Read-out Board) and in a common stream transmitted optically to a FLES (First-Level Event Selector) entry node. Link reception from multiple ROBs is done by boards in these entry nodes, which we call CRI (Common Readout Interface). On the CRI FPGA as well as on CPU level of the entry nodes, different levels of data reduction by feature-extraction and event selection can be enabled.
sent. Thereby, this readout has no intrinsic deadtime, while of course the reconstruction precision of multiple charge depositions (pile-up) decreases with time distances approaching the signal collection time and analogue shaping time — and/or hits close in space. To enable comfortable trigger conditions, representing effective thresholds just below energy deposition of minimum-ionising particles, and to enable efficient reconstruction, the chip features so-called forced-neighbour readout, which means the transmission of ADC samples also from pads adjacent to a self-trigger, fully matching in recording time. Concerning the analogue baseline, a trending average is calculated in the chip for every channel and is being transmitted for every self- or neighbour-triggered message. Due to the absence of external triggers, the exact time information of every charge message is crucial. The time structure is optimised for low overhead: each message includes an 8 bit timestamp, while the next timing level is a 6 bit overflow counter generated only for each group of channels. A third, general timing is added in a data concentration level on one of the subsequent boards. Altogether, the time information represents below 3% of the data stream in case of high rates.
Fig. 6. Average trigger rates in kHz for minimum-bias Au+Au collisions at 10 𝐴GeV, for an interaction rate of 10 MHz, for the TRD layer with the largest rate values. Calculated using the UrQMD event generator and GEANT3.
3.2. Data acquisition chain and data reduction A sketch of the functional elements of the data acquisition for the TRD can be seen in Fig. 5. Following to the SPADIC front-ends, the data will be multiplexed still in the detector layer with GBT-based Readout Boards (ROBs). In the subsequent FPGA layer, data reduction by feature extraction is foreseen. Methods for charge determination and time reconstruction with a precision above the ADC sampling are currently in development and will be tested on real detector data. 4. Expected trigger rates and structural scaling Simulations with UrQMD [7], representing Au+Au collisions at 10 𝐴GeV with minimum-bias trigger have been used to prognose the resulting trigger rates propagated to the detector plane, while interactions with all foreseen materials have been included with GEANT3 [8]. From these simulation, a trigger rate distribution per detector channel is derived. In feedback with these simulations, the sizes of the cathode pads have been chosen such that the local trigger are balanced to meet a reasonable occupancy of the read-out electronics. Finally, four cathodepad lengths from 1.2 up to 8.0 cm have been chosen, and two different module sizes of 55 × 55 cm2 and 99 × 99 cm2 as can be seen in Fig. 6. The colour axis of this plot shows the resulting local trigger rates. The modules of each second TRD layer are rotated by 90 degrees to achieve equal position resolution in both dimensions.
Fig. 7. Energy resolution measured with a SPADIC readout chip on an MWPC obeying the final HV geometry. Different reconstruction methods to determine the charge deposition on a pad are compared. Clusterisation of neighboured pads following in each case. A clear dependence on the gas gain (anode voltage) can be seen.
Furthermore, measurements with a 55 Fe source have been recorded to determine the energy resolution of the system of MWPC and readout electronics. Fig. 7 shows the resolution, achieved by Gaussian fitting to the K-line emission of the source. The different curves are comparing methods to define the charge value recorded on a cathode pad: ‘‘MaxADC’’ takes only the maximum ADC value of the shaped pulses, while the other two maximum-based methods are including two or four more samples around the maximum. The intention is a stabilisation against phase shifts of the analogue pulse against the sampling frequency. The ‘‘integral’’ method sums over all recorded 32 samples. The comparatively better results for the ‘‘MaxADC’’ methods hint on modes with frequencies lower than, but of similar order as the sampling frequency, which is currently being investigated. We note that the achieved energy resolutions met all requirements on the detector.
5. Detector tests in lab situations and at beam facilities 5.1. Detection efficiency and energy resolution It was possible to demonstrate an overall electron detection efficiency of (98.5 ± 2.0)%, with the large uncertainty being understood from issues in an older front-end electronics version, and thereby verifying the overall principal functionality of the proposed read-out. 3
Please cite this article as: P. Kähler, F. Roether, The transition radiation detector in the CBM experiment at FAIR, Nuclear Inst. and Methods in Physics Research, A (2019) 162727, https://doi.org/10.1016/j.nima.2019.162727.
P. Kähler and F. Roether
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per detector hit in this measurement against the expected values for operation in CBM is ongoing. Also the determination of the muon detection efficiency from simultaneously recorded muon beam passing the detectors is being prepared. References [1] I.C. Arsene, L.V. Bravina, W. Cassing, Y.B. Ivanov, A. Larionov, J. Randrup, V.N. Russkikh, V.D. Toneev, G. Zeeb, D. Zschiesche, Dynamical phase trajectories for relativistic nuclear collisions, Phys. Rev. C 75 (2007) 34902, http://dx.doi.org/ 10.1103/PhysRevC.75.034902. [2] T. Ablyazimov, A. Abuhoza, R. Adak, et al., Challenges in QCD matter physics – the scientific programme of the Compressed Baryonic Matter experiment at FAIR, Eur. Phys. J. A 53 (2017) 60, http://dx.doi.org/10.1140/epja/i2017-12248-y. [3] C. Blume, C. Bergmann, D. Emschermann, The Transition Radiation Detector of the CBM Experiment at FAIR : Technical Design Report for the CBM Transition Radiation Detector (TRD), Facility for Antiproton and Ion Research in Europe, Darmstadt, 2018, http://dx.doi.org/10.15120/GSI-2018-01091. [4] A. Andronic, J. Wessels, Transition radiation detectors, Nucl. Instrum. Methods Phys. Res. A 666 (2012) 130–147, http://dx.doi.org/10.1016/j.nima.2011.09.041. [5] E. Mathieson, Cathode charge distributions in multiwire chambers, Nucl. Instrum. Methods Phys. Res. A 270 (2) (1988) 602–603, http://dx.doi.org/10.1016/01689002(88)90736-X. [6] T. Armbruster, P. Fischer, M. Krieger, I. Perić, Multi-channel charge pulse amplification, digitization and processing asic for detector applications, in: 2012 IEEE Nuclear Science Symposium and Medical Imaging Conference Record (NSS/MIC), 2012, pp. 697–702, http://dx.doi.org/10.1109/NSSMIC.2012.6551195. [7] S. Bass, M. Belkacem, M. Bleicher, et al., Microscopic models for ultrarelativistic heavy ion collisions, Prog. Part. Nucl. Phys. 41 (1998) 255–369, http://dx.doi. org/10.1016/S0146-6410(98)00058-1. [8] R. Brun, F. Bruyant, M. Maire, A. McPherson, P. Zanarini, GEANT3, CERN-DD-EE-84-1, 1987. [9] D. Pfeiffer, G. Gorine, H. Reithler, B. Biskup, A. Day, A. Fabich, J. Germa, R. Guida, M. Jaekel, F. Ravotti, The radiation field in the Gamma Irradiation Facility GIF++ at CERN, Nucl. Instrum. Methods Phys. Res. A 866 (2017) 91–103, http://dx.doi.org/10.1016/j.nima.2017.05.045.
Fig. 8. Currents measured in the HV supplies for anode and cathode wire layers, plotted here with respect to the counting rate on the detector. Linear fits are applied as linearity check.
5.2. Operation at high rates To characterise the MWPC behaviour in a high radiation load, recently measurements at the CERN Gamma Irradiation Facility (GIF) [9] have been conducted. MWPCs for the CBM-TRD have been exposed to different gamma irradiation levels. Fig. 8 shows the measured currents at the amplification and drift wires of the chamber vs. the hit rates as seen in the front-end electronics. The scaling of both is compatible with linearity, which we interpret as the absence of space-charge effects in the MWPC and thereby as stable gas gain and stable ion backflow under the measured loads. A comparison of the amounts of ionisation
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Please cite this article as: P. Kähler, F. Roether, The transition radiation detector in the CBM experiment at FAIR, Nuclear Inst. and Methods in Physics Research, A (2019) 162727, https://doi.org/10.1016/j.nima.2019.162727.