- Email: [email protected]
TRD detector development for the CBM experiment
Nuclear Instruments and Methods in Physics Research A 732 (2013) 375–379
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
TRD detector development for the CBM experiment M. Petriş a,n, M. Petrovici a, V. Cătănescu a, M. Târzilă a, V. Simion a, D. Bartoş a, I. Berceanu a, A. Bercuci a, G. Caragheorgheopol a, F. Constantin a, L. Rădulescu a, J. Adamczewski-Musch b, S. Linev b a b
Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest, Romania Gesellschaft für Schwerionenforschung, Darmstadt, Germany
art ic l e i nf o
a b s t r a c t
Available online 3 August 2013
A transition radiation detector (TRD) prototype based on a single multiwire proportional chamber coupled with a small drift region was developed for the innermost part of the CBM-TRD subsystem. It preserves the same gas thickness for transition radiation absorption as the double-sided TRD prototype for which a pion misidentification probability of better than 1% for a six layer configuration was obtained. In the same time it fulfills the requirement of a high geometrical efficiency of CBM-TRD stations. The readout electrode geometry with triangular shaped pads gives access to a two-dimensional position information with a single TRD layer. The detector was tested with a mixed electron/pion beam of 1–5 GeV/c momentum at the CERN PS accelerator. A pion misidentification probability of 1.18% for a six layer configuration based on this architecture was obtained. The two-dimensional position resolutions (along and across the pads) were measured. The pad signals were processed using a new front-end electronics called Fast Analog Signal Processor (FASP), designed for highcounting-rate environments. CADENCE simulations were used for further optimization of the FASP amplifier for operating this new architecture. & 2013 Elsevier B.V. All rights reserved.
Keywords: Gaseous detectors TRD prototype Electron/pion discrimination Front-end electronics
1. Introduction The Compressed Baryonic Matter (CBM) experiment [1] is a heavy-ion fixed target experiment at the future Facility for Antiproton and Ion Research (FAIR) [2], aiming to investigate the properties of nuclear matter at extreme conditions of temperature and density. The measurement of rare probes requires high beam intensities with reaction rates of up to 10 MHz. Therefore fast and radiation hard detectors with dedicated self-triggered front-end electronics are required to be developed. Moreover, the high particle track multiplicity (of about 1000 charged particles in central Au+Au collisions at 25 A GeV) needs to be handled by high-granularity detectors. A transition radiation detector (TRD) for identification of high-momentum electrons, with a pion misidentification probability of better than 1% for 90% electron efficiency, and tracking of all charged particles is considered to be part of the experimental set-up. The most forward angles of the CBM-TRD detector have to cope with counting rates of up to 100 kHz/cm2. A TRD prototype based on a single multiwire proportional chamber (MWPC) coupled with a small drift region was developed for the CBM-TRD subsystem. It preserves the same gas thickness for transition radiation absorption as the prototype reported in
n
Corresponding author. Tel.: +40 214042311. E-mail address: [email protected] (M. Petriş).
0168-9002/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2013.07.087
Ref. [3], for which an extrapolated pion misidentification probability of 0:7% for a six layer configuration was obtained, and fulfills the requirement of high geometrical efficiency of CBM-TRD stations. The readout electrode geometry with triangular shaped pads gives access to a two-dimensional position information with a single TRD layer [4].
2. Detector design and construction Although it has a very good e=π discrimination in a high-countingrate environment [3,5], the size of the double-sided TRD (based on two MWPCs readout by a common double-side pad structure electrode) is limited by the topology of the signal extraction in the same plane as the readout electrode. In order to overcome this problem we propose a standard TRD architecture of 2 4 mm amplification region coupled with a 4 mm drift zone. It has a gas thickness identical to the 4 3 mm double-sided TRD prototype. The size of the drift zone was appropriately chosen to minimize the drift time of the ionization clusters inside the active volume, while keeping the TR conversion efficiency as large as possible. Details on this architecture are presented in Fig. 1. The detector is closed on one side by the drift electrode, made from an aluminized Kapton foil of 25 μm thickness stretched on an 8 mm HF71 Rohacell plate, and on the other side by the readout electrode made from a 300 μm thickness printed circuit board. The cathode wire plane with 1.5 mm pitch separates the
376
M. Petriş et al. / Nuclear Instruments and Methods in Physics Research A 732 (2013) 375–379
amplification region from the drift zone. The cathode wires of 75 μm diameter are made from Cu/Be alloy. The anode wire plane with 3 mm pitch is situated in the middle of the amplification region. The anode wires of 20 μm diameter are made from gold-plated tungsten. The 72 triangular shaped pads of the readout electrode (Fig. 2) define an active area of 36 cm 8 cm. Each triangular pad of 4 cm2 ¼(1 cm 8 cm)/2 area is read out separately.
3.
55
Fe source test
The detector performance was first tested in the laboratory using an 55Fe X-ray source. An 80% Ar+20% CO2 gas mixture was flushed through the counter at atmospheric pressure. The anode signals were amplified by a charge-sensitive preamplifier followed by a shaping amplifier. For pad signal's processing a new front-end electronics called Fast Analog Signal Processor (FASP) was used [6,7]. The FASP amplifier was developed as dedicated electronics for processing the fast signal of the double-sided TRD prototype [5]. The ASIC chip was designed in AMS CMOS 0:35 μm technology.
Fig. 1. 3D view of the detector configuration.
It has a conversion gain of 6.2 mV/fC and a selectable shaping time of 20 ns or 40 ns. This first version of FASP has 8 analog channels each with two types of output: a fast output with a typical semiGaussian shape and a peak-sensing (called “flat top”) output. For the presented measurements the flat top output of FASP was used with 40 ns shaping time. An example of an anode signal spectrum is shown in Fig. 3a; for a pad signal readout the spectrum is presented in Fig. 3b. The obtained energy resolutions for 5.9 keV X-rays ðsE =EÞ were 8% using the anode signals, and 10% using the pad signals.
4. In-beam test 4.1. Experimental setup The detector prototype was tested with a mixed electron/pion beam of 1–5 GeV/c momentum at the T10 beam line of CERN's PS accelerator in a joint measurement campaign of the CBM collaboration. A description of the experimental setup can be found in Ref. [8]. The electrons and pions were selected using the information from a Cherenkov detector positioned upstream and from a Pb-glass calorimeter positioned downstream of the test setup. Three TRD detectors with the same geometry for the readout electrode were tested (Fig. 4): two versions of the Double-Sided TRD prototype (DSTRD-V1 and DSTRD-V2) and the TRD prototype reported in this paper, called Single-Sided TRD (SSTRD). Each Double-Sided TRD prototype is based on two MWPCs readout by a common double-sided pad structure electrode [3]. The performance of the two DSTRD versions was already reported in Ref. [5]. The SSTRD, flushed with an 80% Xe+20% CO2 gas mixture at atmospheric pressure, was operated with 1900 V anode voltage and 400 V drift voltage. A regular radiator of type 20/500/120 (20 μm foil thickness, 500 μm gap, 120 foils) was positioned in front of the drift electrode, as can be seen in Fig. 4. The signals of sixteen triangular pads were processed by two FASP modules, using the flat top outputs and the same 40 ns shaping time as in the source tests. They were digitized by 32-channel peak-sensing Mesytec ADCs. 4.2. Electron identification
Fig. 2. Photo of the readout electrode.
Fig. 3.
55
Using the information from the Cherenkov detector and the Pbglass calorimeter, the pulse height distributions for electrons and pions with 2 GeV/c momentum were obtained (Fig. 5a). The pion misidentification probability as a function of the number of TRD layers for 90% electron efficiency (Fig. 5b) was estimated by
Fe source spectrum: (a) using anode signals; (b) using pad signals.
M. Petriş et al. / Nuclear Instruments and Methods in Physics Research A 732 (2013) 375–379
377
applying the likelihood method in the Monte Carlo simulation [9]. The measured pulse height spectra of electrons and pions for a single layer were used as input for the simulation. Fig. 5b shows an extrapolated pion misidentification probability of ð1:18 70:07Þ% for a six layer configuration based on this architecture. This SSTDR chamber has the same gas thickness for TR absorption as the double-sided architecture with 3 mm anode–cathode distance for which a 0.8% pion misidentification probability was reported [5]. For the SSTRD prototype, however, the consecutive ionization clusters with a large drift time difference between them are not integrated by the present FASP version, as will be shown in Section 5 based on CADENCE simulations.
triangular pads in two coordinate systems: one parallel to the catheti of the triangle ðx; yÞ and the other tilted by the hypotenuse angle ðx′; y′Þ. Taking lines parallel to the y and y′ axes for each case through the reconstructed x and x′ coordinates, we obtained from their crossing the reconstructed coordinate along the pads. The ð6:29 7 0:09Þ mm position resolution along the pads (Fig. 6b) was obtained using as reference the position information from the DSTRD-V1 chamber which was rotated by 901 relative to the SSTRD prototype (see Fig. 4).
4.3. Position resolution
In order to optimize the operation of the SSTRD prototype with a new version of the FASP amplifier, the response of the front-end electronics has been simulated using CADENCE software [11]. Detector signals simulated with Garfield [12] are used as input for CADENCE simulations. The maximum drift time of an ionization cluster produced by a charged particle (hit) inside the gas volume, calculated with Garfield, was about 250 ns for the SSTRD prototype and about 100 ns for the double-sided prototype. Since the FASP amplifier was designed as dedicated front-end electronics for the double-sided prototype [5], the 40 ns shaping time (ST) of its first version was optimized for processing the fast signals delivered by this prototype. The linearity of the FASP output signal processed with 40 ns shaping time, with the charge induced by hits with the ionization clusters randomly distributed inside the drift volume in a drift time window (DTW) of 100 ns for the first case (triangle markers) and 250 ns for the second case (square markers) is shown in Fig. 7. With 40 ns shaping time set for the FASP, we see for the DoubleSided TRD prototype a very good proportionality and linearity of the signal with the input charge. The 5.97 mV/fC slope is very close to the conversion gain and the 2.43 mV offset is small. For the second case of a single MWPC coupled with a 4 mm drift region, the deviations from linearity and the fluctuations of the output amplitude become significant. The 5.17 mV/fC slope is lower than the conversion gain and an about 50 mV offset is observed. Fig. 8 shows the uniformity of the FASP response for the same fixed input charge of 65 fC induced by hits which produce the ionization clusters randomly distributed in the same time windows as used in Fig. 7, i.e. 100 ns for the DSTRD and 250 ns for the SSTRD. The 5 mV standard deviation (Stdev) around a mean of 392 mV output value (very close to the theoretical mean of 402 mV) is small for the double-sided architecture. For input charges induced by hits which produce the ionization clusters with drift times randomly distributed in the 100 ns time window, the average FASP output conserves the gain, the fluctuations being rather small. For the SSTRD geometry, the rather large 40 mV standard deviation around a mean
In the performed analysis the sixteen readout triangular pads are paired into eight rectangular pads of w¼10 mm width. The position reconstruction was made using the signals induced on clusters of two or three rectangular pads. Using the standard deviation of the Gauss function fitted to the pad response functions, the position of the track relative to the pad with maximum charge was calculated following the method described in Ref. [10]. The distribution of the difference between the reconstructed positions across the pads in the SSTRD prototype and in the DSTRD-V2 prototype, both with the same geometry of the readout electrode, is shown in Fig. 6a. A position resolution across the pads of ð327 7 4Þ μm was obtained supposing equal contributions of both chambers. For the position reconstruction along the pads, the algorithm described in Ref. [5] was followed. We paired the
Fig. 4. A photo of the TRD detectors in the experimental setup.
5. Electronics simulations
Fig. 5. (a) Energy loss spectra for 2 GeV/c electrons (thin line) and for pions (thick line); (b) Pion misidentification probability as a function of the number of TRD layers for 90% electron efficiency (the errors are at the level of the symbol size).
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M. Petriş et al. / Nuclear Instruments and Methods in Physics Research A 732 (2013) 375–379
Fig. 6. Distribution of the difference between the reconstructed positions in SSTRD and in DSTRD-V2: (a) across the pads; (b) along the pads.
Fig. 7. Linearity of the FASP response for hits having the ionization clusters randomly distributed in a DTW of 100 ns (triangles) for the DSTRD [5] and of 250 ns (squares) for the SSTRD, processed with 40 ns shaping time.
Fig. 8. Uniformity of the FASP response for hits with the same input charge of 65 fC for a distribution of the ionization clusters and ST the same as in Fig. 7.
of 289 mV shows that the FASP shaping time has to be optimized for the processing of such signals. The improvements in the linearity of the output signal, due to the increase of the shaping time from 40 ns to 80 ns and to 100 ns, for hits with ionization clusters randomly distributed in the 250 ns time window are shown in Fig. 9. For 100 ns shaping time, the 5.88 mV/fC slope of the linear fit approaches the conversion gain and the offset is reduced to 12 mV. Fig. 10 demonstrates the improvements by the increased shaping time on the fluctuations of the output values for hits with 65 fC input charge. The mean value of the distribution
Fig. 9. Linearity of the FASP response as a function of the input charge for hits having the ionization clusters randomly distributed in a drift time window of 250 ns for the SSTRD, processed with shaping times (ST) of 40 ns (circles), 80 ns (triangles), and 100 ns (squares).
Fig. 10. Uniformity of the FASP response for hits with the same input charge of 65 fC with a distribution of ionization clusters in a 250 ns DTW for the SSTRD, processed with the same shaping times as in Fig. 9.
increases to 371 mV for 100 ns shaping time, and the standard deviation of the distribution is reduced to 16 mV. Based on these results, a second version of the FASP amplifier is currently under development with an increased value of 100 ns for the shaping time. This value of the shaping time can still cope with the 100 kHz/cm2 counting rate requirements of the innermost zone of the CBM-TRD subsystem.
M. Petriş et al. / Nuclear Instruments and Methods in Physics Research A 732 (2013) 375–379
6. Conclusions A TRD prototype based on a single multiwire proportional chamber coupled with a small drift region (SSTRD) was developed for the CBM-TRD subsystem. A good pion misidentification probability of 1.18% was obtained for a six TRD layer configuration. Intrinsic position resolutions of 327 μm across the pads and of 6.3 mm along the pads were measured. The obtained performances of this prototype can still be improved by increasing the FASP shaping time to 100 ns, as it was shown by the performed simulations. For polar angles of the CBM-TRD subsystem where the counting rate could reach up to 100 kHz/cm2 a further TRD prototype, with the same inner geometry as SSTRD but with increased granularity (1 cm2 pad area), is already built and tested. Detailed investigations of the detector performance in high counting rate and multihit environment are foreseen for the near future. Acknowledgments We acknowledge V. Aprodu, L. Prodan and A. Radu for their skillful contributions to the construction of the detectors. This
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work was supported by EU-FP7/HP2-WP18 Grant no. 227431, EUFP7/HP3-WP19 Grant no. 283286, NASR/CAPACITATI 179EU Project and NASR/NUCLEU Project.
References [1] Compressed Baryonic Matter Experiment, Technical Status Report, January 2005 〈https://www-alt.gsi.de/onTEAM/dokumente/public/DOC-2005-Feb-447. html〉. [2] 〈http://www.fair-center.eu〉. [3] M. Petrovici, et al., Nuclear Instruments and Methods in Physics Research Section A 579 (2007) 961. [4] M. Petrovici, et al., Romanian Journal of Physics 56 (2011) 654. [5] M. Petriş, et al., Nuclear Instruments and Methods in Physics Research Section A 714 (2013) 17. [6] V. Cătănescu, et al., CBM Progress Report 2009, 47, GSI Darmstadt, 2010. [7] A. Caragheorgheopol, et al., CBM Progress Report 2010, 46, GSI Darmstadt, 2011. [8] D. Emschermann, C. Bergmann, CBM Progress Report 2010, 42, GSI Darmstadt, 2011. [9] M.L. Cherry, et al., Nuclear Instruments and Methods in Physics Research Section A 115 (1974) 141. [10] W. Blum, W. Riegler, L. Rolandi, Particle Detection with Drift Chambers, Springer, Berlin, Heidelberg, 2008. [11] Cadence 〈http://www.europractice.stfc.ac.uk/software/cadence.html〉. [12] 〈http://garfield.web.cern.ch/garfield/〉.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
TRD detector development for the CBM experiment M. Petriş a,n, M. Petrovici a, V. Cătănescu a, M. Târzilă a, V. Simion a, D. Bartoş a, I. Berceanu a, A. Bercuci a, G. Caragheorgheopol a, F. Constantin a, L. Rădulescu a, J. Adamczewski-Musch b, S. Linev b a b
Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest, Romania Gesellschaft für Schwerionenforschung, Darmstadt, Germany
art ic l e i nf o
a b s t r a c t
Available online 3 August 2013
A transition radiation detector (TRD) prototype based on a single multiwire proportional chamber coupled with a small drift region was developed for the innermost part of the CBM-TRD subsystem. It preserves the same gas thickness for transition radiation absorption as the double-sided TRD prototype for which a pion misidentification probability of better than 1% for a six layer configuration was obtained. In the same time it fulfills the requirement of a high geometrical efficiency of CBM-TRD stations. The readout electrode geometry with triangular shaped pads gives access to a two-dimensional position information with a single TRD layer. The detector was tested with a mixed electron/pion beam of 1–5 GeV/c momentum at the CERN PS accelerator. A pion misidentification probability of 1.18% for a six layer configuration based on this architecture was obtained. The two-dimensional position resolutions (along and across the pads) were measured. The pad signals were processed using a new front-end electronics called Fast Analog Signal Processor (FASP), designed for highcounting-rate environments. CADENCE simulations were used for further optimization of the FASP amplifier for operating this new architecture. & 2013 Elsevier B.V. All rights reserved.
Keywords: Gaseous detectors TRD prototype Electron/pion discrimination Front-end electronics
1. Introduction The Compressed Baryonic Matter (CBM) experiment [1] is a heavy-ion fixed target experiment at the future Facility for Antiproton and Ion Research (FAIR) [2], aiming to investigate the properties of nuclear matter at extreme conditions of temperature and density. The measurement of rare probes requires high beam intensities with reaction rates of up to 10 MHz. Therefore fast and radiation hard detectors with dedicated self-triggered front-end electronics are required to be developed. Moreover, the high particle track multiplicity (of about 1000 charged particles in central Au+Au collisions at 25 A GeV) needs to be handled by high-granularity detectors. A transition radiation detector (TRD) for identification of high-momentum electrons, with a pion misidentification probability of better than 1% for 90% electron efficiency, and tracking of all charged particles is considered to be part of the experimental set-up. The most forward angles of the CBM-TRD detector have to cope with counting rates of up to 100 kHz/cm2. A TRD prototype based on a single multiwire proportional chamber (MWPC) coupled with a small drift region was developed for the CBM-TRD subsystem. It preserves the same gas thickness for transition radiation absorption as the prototype reported in
n
Corresponding author. Tel.: +40 214042311. E-mail address: [email protected] (M. Petriş).
0168-9002/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2013.07.087
Ref. [3], for which an extrapolated pion misidentification probability of 0:7% for a six layer configuration was obtained, and fulfills the requirement of high geometrical efficiency of CBM-TRD stations. The readout electrode geometry with triangular shaped pads gives access to a two-dimensional position information with a single TRD layer [4].
2. Detector design and construction Although it has a very good e=π discrimination in a high-countingrate environment [3,5], the size of the double-sided TRD (based on two MWPCs readout by a common double-side pad structure electrode) is limited by the topology of the signal extraction in the same plane as the readout electrode. In order to overcome this problem we propose a standard TRD architecture of 2 4 mm amplification region coupled with a 4 mm drift zone. It has a gas thickness identical to the 4 3 mm double-sided TRD prototype. The size of the drift zone was appropriately chosen to minimize the drift time of the ionization clusters inside the active volume, while keeping the TR conversion efficiency as large as possible. Details on this architecture are presented in Fig. 1. The detector is closed on one side by the drift electrode, made from an aluminized Kapton foil of 25 μm thickness stretched on an 8 mm HF71 Rohacell plate, and on the other side by the readout electrode made from a 300 μm thickness printed circuit board. The cathode wire plane with 1.5 mm pitch separates the
376
M. Petriş et al. / Nuclear Instruments and Methods in Physics Research A 732 (2013) 375–379
amplification region from the drift zone. The cathode wires of 75 μm diameter are made from Cu/Be alloy. The anode wire plane with 3 mm pitch is situated in the middle of the amplification region. The anode wires of 20 μm diameter are made from gold-plated tungsten. The 72 triangular shaped pads of the readout electrode (Fig. 2) define an active area of 36 cm 8 cm. Each triangular pad of 4 cm2 ¼(1 cm 8 cm)/2 area is read out separately.
3.
55
Fe source test
The detector performance was first tested in the laboratory using an 55Fe X-ray source. An 80% Ar+20% CO2 gas mixture was flushed through the counter at atmospheric pressure. The anode signals were amplified by a charge-sensitive preamplifier followed by a shaping amplifier. For pad signal's processing a new front-end electronics called Fast Analog Signal Processor (FASP) was used [6,7]. The FASP amplifier was developed as dedicated electronics for processing the fast signal of the double-sided TRD prototype [5]. The ASIC chip was designed in AMS CMOS 0:35 μm technology.
Fig. 1. 3D view of the detector configuration.
It has a conversion gain of 6.2 mV/fC and a selectable shaping time of 20 ns or 40 ns. This first version of FASP has 8 analog channels each with two types of output: a fast output with a typical semiGaussian shape and a peak-sensing (called “flat top”) output. For the presented measurements the flat top output of FASP was used with 40 ns shaping time. An example of an anode signal spectrum is shown in Fig. 3a; for a pad signal readout the spectrum is presented in Fig. 3b. The obtained energy resolutions for 5.9 keV X-rays ðsE =EÞ were 8% using the anode signals, and 10% using the pad signals.
4. In-beam test 4.1. Experimental setup The detector prototype was tested with a mixed electron/pion beam of 1–5 GeV/c momentum at the T10 beam line of CERN's PS accelerator in a joint measurement campaign of the CBM collaboration. A description of the experimental setup can be found in Ref. [8]. The electrons and pions were selected using the information from a Cherenkov detector positioned upstream and from a Pb-glass calorimeter positioned downstream of the test setup. Three TRD detectors with the same geometry for the readout electrode were tested (Fig. 4): two versions of the Double-Sided TRD prototype (DSTRD-V1 and DSTRD-V2) and the TRD prototype reported in this paper, called Single-Sided TRD (SSTRD). Each Double-Sided TRD prototype is based on two MWPCs readout by a common double-sided pad structure electrode [3]. The performance of the two DSTRD versions was already reported in Ref. [5]. The SSTRD, flushed with an 80% Xe+20% CO2 gas mixture at atmospheric pressure, was operated with 1900 V anode voltage and 400 V drift voltage. A regular radiator of type 20/500/120 (20 μm foil thickness, 500 μm gap, 120 foils) was positioned in front of the drift electrode, as can be seen in Fig. 4. The signals of sixteen triangular pads were processed by two FASP modules, using the flat top outputs and the same 40 ns shaping time as in the source tests. They were digitized by 32-channel peak-sensing Mesytec ADCs. 4.2. Electron identification
Fig. 2. Photo of the readout electrode.
Fig. 3.
55
Using the information from the Cherenkov detector and the Pbglass calorimeter, the pulse height distributions for electrons and pions with 2 GeV/c momentum were obtained (Fig. 5a). The pion misidentification probability as a function of the number of TRD layers for 90% electron efficiency (Fig. 5b) was estimated by
Fe source spectrum: (a) using anode signals; (b) using pad signals.
M. Petriş et al. / Nuclear Instruments and Methods in Physics Research A 732 (2013) 375–379
377
applying the likelihood method in the Monte Carlo simulation [9]. The measured pulse height spectra of electrons and pions for a single layer were used as input for the simulation. Fig. 5b shows an extrapolated pion misidentification probability of ð1:18 70:07Þ% for a six layer configuration based on this architecture. This SSTDR chamber has the same gas thickness for TR absorption as the double-sided architecture with 3 mm anode–cathode distance for which a 0.8% pion misidentification probability was reported [5]. For the SSTRD prototype, however, the consecutive ionization clusters with a large drift time difference between them are not integrated by the present FASP version, as will be shown in Section 5 based on CADENCE simulations.
triangular pads in two coordinate systems: one parallel to the catheti of the triangle ðx; yÞ and the other tilted by the hypotenuse angle ðx′; y′Þ. Taking lines parallel to the y and y′ axes for each case through the reconstructed x and x′ coordinates, we obtained from their crossing the reconstructed coordinate along the pads. The ð6:29 7 0:09Þ mm position resolution along the pads (Fig. 6b) was obtained using as reference the position information from the DSTRD-V1 chamber which was rotated by 901 relative to the SSTRD prototype (see Fig. 4).
4.3. Position resolution
In order to optimize the operation of the SSTRD prototype with a new version of the FASP amplifier, the response of the front-end electronics has been simulated using CADENCE software [11]. Detector signals simulated with Garfield [12] are used as input for CADENCE simulations. The maximum drift time of an ionization cluster produced by a charged particle (hit) inside the gas volume, calculated with Garfield, was about 250 ns for the SSTRD prototype and about 100 ns for the double-sided prototype. Since the FASP amplifier was designed as dedicated front-end electronics for the double-sided prototype [5], the 40 ns shaping time (ST) of its first version was optimized for processing the fast signals delivered by this prototype. The linearity of the FASP output signal processed with 40 ns shaping time, with the charge induced by hits with the ionization clusters randomly distributed inside the drift volume in a drift time window (DTW) of 100 ns for the first case (triangle markers) and 250 ns for the second case (square markers) is shown in Fig. 7. With 40 ns shaping time set for the FASP, we see for the DoubleSided TRD prototype a very good proportionality and linearity of the signal with the input charge. The 5.97 mV/fC slope is very close to the conversion gain and the 2.43 mV offset is small. For the second case of a single MWPC coupled with a 4 mm drift region, the deviations from linearity and the fluctuations of the output amplitude become significant. The 5.17 mV/fC slope is lower than the conversion gain and an about 50 mV offset is observed. Fig. 8 shows the uniformity of the FASP response for the same fixed input charge of 65 fC induced by hits which produce the ionization clusters randomly distributed in the same time windows as used in Fig. 7, i.e. 100 ns for the DSTRD and 250 ns for the SSTRD. The 5 mV standard deviation (Stdev) around a mean of 392 mV output value (very close to the theoretical mean of 402 mV) is small for the double-sided architecture. For input charges induced by hits which produce the ionization clusters with drift times randomly distributed in the 100 ns time window, the average FASP output conserves the gain, the fluctuations being rather small. For the SSTRD geometry, the rather large 40 mV standard deviation around a mean
In the performed analysis the sixteen readout triangular pads are paired into eight rectangular pads of w¼10 mm width. The position reconstruction was made using the signals induced on clusters of two or three rectangular pads. Using the standard deviation of the Gauss function fitted to the pad response functions, the position of the track relative to the pad with maximum charge was calculated following the method described in Ref. [10]. The distribution of the difference between the reconstructed positions across the pads in the SSTRD prototype and in the DSTRD-V2 prototype, both with the same geometry of the readout electrode, is shown in Fig. 6a. A position resolution across the pads of ð327 7 4Þ μm was obtained supposing equal contributions of both chambers. For the position reconstruction along the pads, the algorithm described in Ref. [5] was followed. We paired the
Fig. 4. A photo of the TRD detectors in the experimental setup.
5. Electronics simulations
Fig. 5. (a) Energy loss spectra for 2 GeV/c electrons (thin line) and for pions (thick line); (b) Pion misidentification probability as a function of the number of TRD layers for 90% electron efficiency (the errors are at the level of the symbol size).
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M. Petriş et al. / Nuclear Instruments and Methods in Physics Research A 732 (2013) 375–379
Fig. 6. Distribution of the difference between the reconstructed positions in SSTRD and in DSTRD-V2: (a) across the pads; (b) along the pads.
Fig. 7. Linearity of the FASP response for hits having the ionization clusters randomly distributed in a DTW of 100 ns (triangles) for the DSTRD [5] and of 250 ns (squares) for the SSTRD, processed with 40 ns shaping time.
Fig. 8. Uniformity of the FASP response for hits with the same input charge of 65 fC for a distribution of the ionization clusters and ST the same as in Fig. 7.
of 289 mV shows that the FASP shaping time has to be optimized for the processing of such signals. The improvements in the linearity of the output signal, due to the increase of the shaping time from 40 ns to 80 ns and to 100 ns, for hits with ionization clusters randomly distributed in the 250 ns time window are shown in Fig. 9. For 100 ns shaping time, the 5.88 mV/fC slope of the linear fit approaches the conversion gain and the offset is reduced to 12 mV. Fig. 10 demonstrates the improvements by the increased shaping time on the fluctuations of the output values for hits with 65 fC input charge. The mean value of the distribution
Fig. 9. Linearity of the FASP response as a function of the input charge for hits having the ionization clusters randomly distributed in a drift time window of 250 ns for the SSTRD, processed with shaping times (ST) of 40 ns (circles), 80 ns (triangles), and 100 ns (squares).
Fig. 10. Uniformity of the FASP response for hits with the same input charge of 65 fC with a distribution of ionization clusters in a 250 ns DTW for the SSTRD, processed with the same shaping times as in Fig. 9.
increases to 371 mV for 100 ns shaping time, and the standard deviation of the distribution is reduced to 16 mV. Based on these results, a second version of the FASP amplifier is currently under development with an increased value of 100 ns for the shaping time. This value of the shaping time can still cope with the 100 kHz/cm2 counting rate requirements of the innermost zone of the CBM-TRD subsystem.
M. Petriş et al. / Nuclear Instruments and Methods in Physics Research A 732 (2013) 375–379
6. Conclusions A TRD prototype based on a single multiwire proportional chamber coupled with a small drift region (SSTRD) was developed for the CBM-TRD subsystem. A good pion misidentification probability of 1.18% was obtained for a six TRD layer configuration. Intrinsic position resolutions of 327 μm across the pads and of 6.3 mm along the pads were measured. The obtained performances of this prototype can still be improved by increasing the FASP shaping time to 100 ns, as it was shown by the performed simulations. For polar angles of the CBM-TRD subsystem where the counting rate could reach up to 100 kHz/cm2 a further TRD prototype, with the same inner geometry as SSTRD but with increased granularity (1 cm2 pad area), is already built and tested. Detailed investigations of the detector performance in high counting rate and multihit environment are foreseen for the near future. Acknowledgments We acknowledge V. Aprodu, L. Prodan and A. Radu for their skillful contributions to the construction of the detectors. This
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work was supported by EU-FP7/HP2-WP18 Grant no. 227431, EUFP7/HP3-WP19 Grant no. 283286, NASR/CAPACITATI 179EU Project and NASR/NUCLEU Project.
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