- Email: [email protected]
Construction of the ALICE silicon pixel detector and prototype performance in test beam
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 560 (2006) 61–66 www.elsevier.com/locate/nima
Construction of the ALICE silicon pixel detector and prototype performance in test beam V. Manzari Istituto Nazionale di Fisica Nucleare, Sez. di Bari, via E. Orabona, 4, 70126 Bari, Italy On behalf of the Silicon Pixel Detector Project Available online 22 December 2005
Abstract The Silicon Pixel Detector (SPD) forms the two innermost layers of the Inner Tracking System (ITS) of the ALICE experiment at the CERN LHC. The SPD consists of 120 detector modules (half-staves) on two barrel layers with 107 pixel cells and a total silicon surface of 0.2 m2. The SPD is severely constrained in terms of material budget, dimension and volume of the whole system. The SPD design, the main system components and the current status of the project are overviewed. The automated equipment and the procedure for the assembly of the half-staves are illustrated. The results of the data analysis of a test beam performed with single assemblies at the SPS in 2003 are reported. It is shown that efficiency and spatial resolution are in good agreement with the design specifications and satisfy the ALICE requirements. r 2005 Elsevier B.V. All rights reserved. PACS: 25.75. q; 29.40.Gx; 29.40.Wk Keywords: Silicon pixel detector; Silicon tracker; ALICE; LHC
1. Introduction ALICE [1] is a general purpose detector to study ultrarelativistic nucleus–nucleus collisions at LHC. ALICE will explore a new regime of matter, increasing by a large factor both the volume and the energy density of the interacting fireball achieved at the CERN SPS and at RHIC. As in most of the collider experiments, the ALICE subdetector closest to the interaction point is an array of silicon detectors named Inner Tracking System (ITS) and shown in Fig. 1. The ITS consists of 6 cylindrical coaxial layers of silicon detectors. The configuration of the ITS has been optimized for track finding and impact parameter resolution. The two innermost layers make use of Silicon Pixel Detectors (SPD), two intermediate layers are made out of Silicon Drift Detectors (SDD) and the two outer layers, at larger radii, of double-side Silicon Strip Detectors (SSD) [2].
Tel.: +39 080 5443288; fax: +39 080 5442470.
E-mail address: [email protected]. 0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.11.222
The ALICE pixel detector is based on a hybrid technology: the basic detector element, named ladder, is a matrix of reversed biased p+n junctions flip–chip bonded to five readout chips. Each individual cell of the sensor matrix is connected via a Sn–Pb solder ball to a cell of the same size on the front-end chip. This technique has been pioneered by WA97 [3] and successfully used in NA57 [4] and more recently NA60 [5], all experiments being part of the heavy ion fixed target physics program at the CERN SPS. 2. The ALICE SPD The very high particle density expected in the nucleus–nucleus interactions at the LHC energy requires the use of a true two-dimensional detector close to the interaction point, combining very good space accuracy with high double track resolution. The SPD provides ALICE with a good secondary vertexing capability for charm and beauty decays as well as hyperon decays. The SPD will allow a track impact parameter resolution below 50 mm, as confirmed by the
ARTICLE IN PRESS 62
V. Manzari / Nuclear Instruments and Methods in Physics Research A 560 (2006) 61–66
Fig. 1. A schematic view of the ALICE experimental apparatus and the two-layer silicon pixel detector.
performance results reported in the following, and will improve the tracking of low pT particles and the momentum resolution. The constraints on physics performance are very demanding in terms of material budget and geometrical dimensions. The SPD components have been developed during several years of R&D activity, tested and qualified both in laboratory tests and in test beams. The basic building block of the SPD is the half-stave, schematically shown in Fig. 2. The two SPD layers are made of 120 half-staves arranged in 10 sectors around the beam pipe. Each half-stave contains two detector ladders (350 mm thick), one low mass Al-polyimide multi-layer flex (pixel bus), one Multi-Chip Module (MCM) and one grounding foil. The half-staves are mounted onto a 200 mm thick carbon fiber support sector embedding the cooling pipes which run underneath the half-staves. The total power dissipated by the SPD electronics is about 1.5 kW. The cooling system must maintain the SPD at an approximately constant temperature of 24 1C, as required by the high sensitivity of the adjacent SDD to temperature variations. For this purpose an evaporative C4F10-based cooling system has been developed. The system has proven to be very efficient and capable of assuring the required performance in all conditions. The total material budget of the SPD is kept below 1% X0 for each SPD layer. 2.1. The half-stave assembly In this paragraph, a brief description of the half-stave components is presented (a recent general overview can be found in [6]) and the half-stave assembly procedure is described.
Fig. 2. The half-stave layout.
The front-end pixel chip ALICE1LHCb is a mixed analog–digital signal chip produced in commercial 6 metal layer 0.25 mm CMOS process, made radiation tolerant by the design layout [7]. It contains 8192 cells, 50 425 mm2, arranged in 256 rows 32 columns. The pixel chips are probed on the wafer to identify the good-dies suitable for bump-bonding. The SPD ladder is an assembly of a silicon sensor matrix of 256 160 cells bump-bonded to five readout ALICE1LHCb front-end chips. Our bump-bonding vendor is VTT,1 which makes use of solder Pb–Sn bumps. The native thickness of the sensor is 200 mm while the readout chips are thinned down to 150 mm from 725 mm after the bump deposition. Before being mounted in a halfstave the ladders are tested with a 90Sr source: in order to be accepted the number of missing pixels must be less than 1% [8]. Each half-stave is readout and controlled by auxiliary ASICs placed on the MCM located at the end of the
1
VTT Center for Microelectronics, Espoo, Finland, http://www.vtt.fi
ARTICLE IN PRESS V. Manzari / Nuclear Instruments and Methods in Physics Research A 560 (2006) 61–66
module. A detailed picture of the read-out architecture and components is given in Ref. [9]. The multilayer Al/kapton flex provides data, control and power lines between the readout chips and the MCM. Two copper extender buses carry power lines to the front-end chips and to the auxiliary electronics housed in the MCM, while all data and control signals to and from the counting room are transmitted via optical fibers also connected to an optical module housed in the MCM. The assembly procedure of the half-stave has been worked out and extensively tested with dummy components first [10] and more recently with real working components. The micrometric alignment of the half-stave components is performed with a Mitutoyo coordinate measuring machine Crysta Apex 9166 equipped with tools and jigs developed and built in house for the proper vacuum based handling of the half-stave components. The selected glue is the bicomponent epoxy adhesive Eccobond 45 produced by Emerson and Cuming:2 It is electrically insulating with a good thermal conductivity. The Eccobond 45 can be cured at room temperature and the flexibility can be determined by the amount of the Catalyst 15 used. The construction of the half-stave is a three steps procedure: the first two, sketched in Fig. 3, concern the alignment and the gluing of the components, the third consists of the wire-bonding to establish the electrical connections among them. During the first construction step the two ladders and the MCM are aligned with respect to each other and then glued onto the grounding foil. The grounding foil is a very thin Al/kapton foil, 25+50 mm thick respectively, which allows a proper grounding of the half-stave and the thermal coupling between the front-end chips and the cooling ducts embedded in the carbon fiber support sector. In the second gluing step the multi-layer flex (pixel bus) is aligned and glued onto the two ladders and the MCM. The third and final step of the half-stave construction is the ultrasonic wire-bonding of the multi-layer flex to the front-end chips and the MCM. There are about 1100 bonds per half-stave. The alignment constraints and the proper gluing are crucial with respect to the reliability and robustness of the wire-bonds: special care is taken to ensure a uniform distribution of the glue, which has to be well confined in order to avoid any contamination of the bonding pads. The tight dimensional tolerances of the SPD require that the thickness and the planarity of the half-stave are kept under control very carefully during every production step. The required thickness of the glue layer is obtained by scraping the excess with respect to an adhesive mask making use of the coordinate measuring machine. Acceptable results in terms of strength of the assembly, minimum achievable thickness and reproducibility has been proven to correspond to a glue layer of 130 mm.
2
http://www.emersoncuming.com/
63
Fig. 3. The two step gluing of the half-stave assembly.
Fig. 4. A fully working prototype of the half-stave.
Before being mounted on the carbon fiber support sector the half-stave functionality is tested extensively also by means of a radioactive source. The first two fully working half-stave prototypes have been recently built, see Fig. 4, and they will be tested in a high-energy particle beam at the CERN SPS in November 2004. 2.2. The two layer barrel Two half-staves are joint together and then mounted onto a carbon fiber support sector to form the two SPD layers. Each carbon fiber sector carries four staves for the outer layer and two for the inner layer, respectively (Fig. 5). The main steps of the sector assembly procedure, described in Ref. [11], are the following: 1. The carbon fiber sector is equipped with the cooling ducts; 2. a thin layer of thermal grease is dispensed onto the sector surface where the half-staves will be placed: it ensures a good thermal coupling between the half-stave and the cooling pipe; 3. the two half-staves are aligned with respect to each other to form a stave and then positioned onto the carbon fiber sector.
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V. Manzari / Nuclear Instruments and Methods in Physics Research A 560 (2006) 61–66
Fig. 5. A schematic drawing of two sectors around the beam pipe: each sector carries on four staves on the outer layer and two staves on the inner layer.
The aligned stave is finally glued to the carbon-fiber sector using a UV curable glue: to ensure a good mechanical stability. The upper part of the stave is fixed by means of carbon fiber clips, which also act as a protection for the wire bonding. Each half-stave is tested as it is mounted onto the support sector. In fact the compactness of the SPD is such that the re-working of a underperforming half-stave is very risky in terms of probability to damage the neighbor halfstave. Once a sector is fully equipped mechanical and functional extensive tests are performed. Five sectors are then mounted together on an external carbon fiber support, to form a half-barrel, and tested. The whole SPD consists of two half-barrels. 3. Test beam prototype characterization The performance of SPD single assemblies has been studied in several test beam experiments carried out at the CERN SPS. Several pixel assemblies have been exposed both to a hadron and an ion beam. Here we report the results of the analysis from 120 GeV/c proton/pion beam data collected in October 2003. Fig. 6 shows the layout of the set up: a single chip assembly under test (middle plane) has been positioned in between two tracking stations. Each tracking station (minibus) consists of two single chip assemblies, placed one after the other along the beam. To improve the measurement of the transverse position of the incident tracks, one of the two assemblies in each minibus was rotated by 901, in order to provide the same accuracy in both coordinates. The trigger is set by the coincidence of the signals from an incoming beam particle traversing 4 scintillator counters. The hits on each plane are processed by a cluster finding algorithm, where a cluster is defined either as a single fired pixel or a group of fired pixels where each pixel shares at least one side with an adjacent fired pixel. Due to the shorter pixel size in the y-coordinate (50 mm), the cluster is
Fig. 6. Layout of the test beam at the CERN SPS in October 2003.
on average much more developed in this direction and it is found to be about 1.5 cells at a threshold of about 3000 electrons. Before the track reconstruction, the relative misalignments of the detectors in the x–y directions are taken into account. The internal alignment for each of the two minibuses has been achieved by using the correlations of the cluster coordinates on each of the two component detectors. This procedure assumes that clusters on the planes are only due to the beam track (no noisy hits) and that the track itself crosses the detectors at normal incidence: both assumptions are very reasonable. Fig. 7 shows the correlation and the difference between cluster ycoordinates of the two planes in the first minibus before the alignment correction is applied. The following step is to produce the relative alignment of the two minibuses. This is again achieved by cluster coordinate correlations. Once tracking planes are preliminarily aligned, a least squares fit procedure is applied to reconstruct the tracks both in the xz and yz projections, z being the coordinate along the beam. The residual distributions on the test planes (difference between predicted impact and nearest cluster positions) can be calculated: centering these distributions provides the alignment of the test plane. An iterative procedure, which excludes one plane at a time from the fit, allows the fine tuning of the alignment constants also for the tracking detectors. In Fig. 8 the residual distributions of
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V. Manzari / Nuclear Instruments and Methods in Physics Research A 560 (2006) 61–66
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events with a reconstructed track crossing the test plane, has been found to be in excess of 99% at a threshold of about 3000 electrons. The plot in Fig. 9 shows the detection efficiency as a function of the threshold. A wide plateau is present well before the usual operating range of 200 DAC units corresponding to about 3000 electrons. The detector performance has been studied as a function of the threshold and the track impact angle. A detailed discussion of the results from data analysis can be found in Ref. [12].
Fig. 8. Residual distributions from single pixel clusters only.
4. Conclusions reconstructed tracks for both coordinates are shown when only single pixel clusters are selected. To evaluate the intrinsic precision of the detector under test from the residual distribution, the tracking errors have been estimated by means of a simulation which takes into account the telescope geometry, the multiple scattering and the residual tracking plane misalignment. With this procedure the estimate tracking precision is about 10 mm in both transverse coordinates. When only single-pixel clusters are selected, the intrinsic spatial precision of the detector under test from the widths of the measured residual distribution, when the tracking precision is taken into account, is 120.170.5 mm in xcoordinate and 11.070.5 mm in y-coordinate. The measured spatial precision results to be slightly better in both coordinates with respect to the pitch/O12 value as expected for a digital device (122.7 and 14.4 mm in x and y coordinates respectively). The track impact parameter distribution shows that single pixel clusters are generated by tracks crossing a pixel cell in a region narrower than its size. Depending on the threshold setting, tracks impacting the cell close enough to the boundary regions will fire double pixel cluster. The detector efficiency, defined as the ratio between the number of events where a cluster correlated to a reconstructed track is detected and the total number of
The SPD provides the ALICE experiment with a high granularity detector for high-performance tracking and secondary vertexing capability. The construction procedure of the half-stave has been developed with dummy components and recently qualified in a test beam with the first fully working prototypes. One key element in the production of such a complex device as the SPD is the testing and qualification of the components at each phase of the construction. The detector performance has been evaluated from data collected in a beam test at the CERN SPS with single assemblies.
References [1] ALICE Collaboration, J. Phys. G 30 (2004). [2] ALICE Collaboration, ALICE Technical Design Report of the Inner Tracking System, CERN/LHCC 99-12, ALICE TDR 4. [3] H. Beker, et al., Nucl. Instr. and Meth. A 332 (1993) 188. [4] V. Manzari, et al., Nucl. Phys. A 661 (1999) 716c. [5] K. Banicz, et al., Nucl. Instr. and Meth. A 539 (2005) 137. [6] V. Manzari, et al., J. Phys. G 30 (2004) S1091. [7] W. Snoyes, et al., Nucl. Instr. and Meth. A 465 (2001) 176; F. Faccio, et al., Proceedings of the Fourth Workshop on Electronics for LHC Experiments, Rome, September 21–25, 1998, CERN/ LHCC/98-36, 105.
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[8] P. Riedler, et al., Nucl. Instr. and Meth. A 501 (2003) 111; P. Riedler, et al., Proceedings of Vertex 2003, Lake Windermere, UK, September 14–19, 2003, Nucl. Instr. and Meth. A, to be published. [9] A. Kluge, et al., Nucl. Instr. and Meth., this proceedings. [10] M. Caselle, et al., Nucl. Instr. and Meth. A 518 (2004) 297.
[11] S. Moretto, et al., Proceedings of IEEE Nuclear Science Symposium and Medical Imaging Conference, Rome, Italy, October 16–22, 2004. [12] G.E. Bruno, et al., Study of the ALICE Silicon Pixel Detector performance in a beam test at the SPS, ALICE Internal Note, ALICE-INT 2005-007.
Nuclear Instruments and Methods in Physics Research A 560 (2006) 61–66 www.elsevier.com/locate/nima
Construction of the ALICE silicon pixel detector and prototype performance in test beam V. Manzari Istituto Nazionale di Fisica Nucleare, Sez. di Bari, via E. Orabona, 4, 70126 Bari, Italy On behalf of the Silicon Pixel Detector Project Available online 22 December 2005
Abstract The Silicon Pixel Detector (SPD) forms the two innermost layers of the Inner Tracking System (ITS) of the ALICE experiment at the CERN LHC. The SPD consists of 120 detector modules (half-staves) on two barrel layers with 107 pixel cells and a total silicon surface of 0.2 m2. The SPD is severely constrained in terms of material budget, dimension and volume of the whole system. The SPD design, the main system components and the current status of the project are overviewed. The automated equipment and the procedure for the assembly of the half-staves are illustrated. The results of the data analysis of a test beam performed with single assemblies at the SPS in 2003 are reported. It is shown that efficiency and spatial resolution are in good agreement with the design specifications and satisfy the ALICE requirements. r 2005 Elsevier B.V. All rights reserved. PACS: 25.75. q; 29.40.Gx; 29.40.Wk Keywords: Silicon pixel detector; Silicon tracker; ALICE; LHC
1. Introduction ALICE [1] is a general purpose detector to study ultrarelativistic nucleus–nucleus collisions at LHC. ALICE will explore a new regime of matter, increasing by a large factor both the volume and the energy density of the interacting fireball achieved at the CERN SPS and at RHIC. As in most of the collider experiments, the ALICE subdetector closest to the interaction point is an array of silicon detectors named Inner Tracking System (ITS) and shown in Fig. 1. The ITS consists of 6 cylindrical coaxial layers of silicon detectors. The configuration of the ITS has been optimized for track finding and impact parameter resolution. The two innermost layers make use of Silicon Pixel Detectors (SPD), two intermediate layers are made out of Silicon Drift Detectors (SDD) and the two outer layers, at larger radii, of double-side Silicon Strip Detectors (SSD) [2].
Tel.: +39 080 5443288; fax: +39 080 5442470.
E-mail address: [email protected]. 0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.11.222
The ALICE pixel detector is based on a hybrid technology: the basic detector element, named ladder, is a matrix of reversed biased p+n junctions flip–chip bonded to five readout chips. Each individual cell of the sensor matrix is connected via a Sn–Pb solder ball to a cell of the same size on the front-end chip. This technique has been pioneered by WA97 [3] and successfully used in NA57 [4] and more recently NA60 [5], all experiments being part of the heavy ion fixed target physics program at the CERN SPS. 2. The ALICE SPD The very high particle density expected in the nucleus–nucleus interactions at the LHC energy requires the use of a true two-dimensional detector close to the interaction point, combining very good space accuracy with high double track resolution. The SPD provides ALICE with a good secondary vertexing capability for charm and beauty decays as well as hyperon decays. The SPD will allow a track impact parameter resolution below 50 mm, as confirmed by the
ARTICLE IN PRESS 62
V. Manzari / Nuclear Instruments and Methods in Physics Research A 560 (2006) 61–66
Fig. 1. A schematic view of the ALICE experimental apparatus and the two-layer silicon pixel detector.
performance results reported in the following, and will improve the tracking of low pT particles and the momentum resolution. The constraints on physics performance are very demanding in terms of material budget and geometrical dimensions. The SPD components have been developed during several years of R&D activity, tested and qualified both in laboratory tests and in test beams. The basic building block of the SPD is the half-stave, schematically shown in Fig. 2. The two SPD layers are made of 120 half-staves arranged in 10 sectors around the beam pipe. Each half-stave contains two detector ladders (350 mm thick), one low mass Al-polyimide multi-layer flex (pixel bus), one Multi-Chip Module (MCM) and one grounding foil. The half-staves are mounted onto a 200 mm thick carbon fiber support sector embedding the cooling pipes which run underneath the half-staves. The total power dissipated by the SPD electronics is about 1.5 kW. The cooling system must maintain the SPD at an approximately constant temperature of 24 1C, as required by the high sensitivity of the adjacent SDD to temperature variations. For this purpose an evaporative C4F10-based cooling system has been developed. The system has proven to be very efficient and capable of assuring the required performance in all conditions. The total material budget of the SPD is kept below 1% X0 for each SPD layer. 2.1. The half-stave assembly In this paragraph, a brief description of the half-stave components is presented (a recent general overview can be found in [6]) and the half-stave assembly procedure is described.
Fig. 2. The half-stave layout.
The front-end pixel chip ALICE1LHCb is a mixed analog–digital signal chip produced in commercial 6 metal layer 0.25 mm CMOS process, made radiation tolerant by the design layout [7]. It contains 8192 cells, 50 425 mm2, arranged in 256 rows 32 columns. The pixel chips are probed on the wafer to identify the good-dies suitable for bump-bonding. The SPD ladder is an assembly of a silicon sensor matrix of 256 160 cells bump-bonded to five readout ALICE1LHCb front-end chips. Our bump-bonding vendor is VTT,1 which makes use of solder Pb–Sn bumps. The native thickness of the sensor is 200 mm while the readout chips are thinned down to 150 mm from 725 mm after the bump deposition. Before being mounted in a halfstave the ladders are tested with a 90Sr source: in order to be accepted the number of missing pixels must be less than 1% [8]. Each half-stave is readout and controlled by auxiliary ASICs placed on the MCM located at the end of the
1
VTT Center for Microelectronics, Espoo, Finland, http://www.vtt.fi
ARTICLE IN PRESS V. Manzari / Nuclear Instruments and Methods in Physics Research A 560 (2006) 61–66
module. A detailed picture of the read-out architecture and components is given in Ref. [9]. The multilayer Al/kapton flex provides data, control and power lines between the readout chips and the MCM. Two copper extender buses carry power lines to the front-end chips and to the auxiliary electronics housed in the MCM, while all data and control signals to and from the counting room are transmitted via optical fibers also connected to an optical module housed in the MCM. The assembly procedure of the half-stave has been worked out and extensively tested with dummy components first [10] and more recently with real working components. The micrometric alignment of the half-stave components is performed with a Mitutoyo coordinate measuring machine Crysta Apex 9166 equipped with tools and jigs developed and built in house for the proper vacuum based handling of the half-stave components. The selected glue is the bicomponent epoxy adhesive Eccobond 45 produced by Emerson and Cuming:2 It is electrically insulating with a good thermal conductivity. The Eccobond 45 can be cured at room temperature and the flexibility can be determined by the amount of the Catalyst 15 used. The construction of the half-stave is a three steps procedure: the first two, sketched in Fig. 3, concern the alignment and the gluing of the components, the third consists of the wire-bonding to establish the electrical connections among them. During the first construction step the two ladders and the MCM are aligned with respect to each other and then glued onto the grounding foil. The grounding foil is a very thin Al/kapton foil, 25+50 mm thick respectively, which allows a proper grounding of the half-stave and the thermal coupling between the front-end chips and the cooling ducts embedded in the carbon fiber support sector. In the second gluing step the multi-layer flex (pixel bus) is aligned and glued onto the two ladders and the MCM. The third and final step of the half-stave construction is the ultrasonic wire-bonding of the multi-layer flex to the front-end chips and the MCM. There are about 1100 bonds per half-stave. The alignment constraints and the proper gluing are crucial with respect to the reliability and robustness of the wire-bonds: special care is taken to ensure a uniform distribution of the glue, which has to be well confined in order to avoid any contamination of the bonding pads. The tight dimensional tolerances of the SPD require that the thickness and the planarity of the half-stave are kept under control very carefully during every production step. The required thickness of the glue layer is obtained by scraping the excess with respect to an adhesive mask making use of the coordinate measuring machine. Acceptable results in terms of strength of the assembly, minimum achievable thickness and reproducibility has been proven to correspond to a glue layer of 130 mm.
2
http://www.emersoncuming.com/
63
Fig. 3. The two step gluing of the half-stave assembly.
Fig. 4. A fully working prototype of the half-stave.
Before being mounted on the carbon fiber support sector the half-stave functionality is tested extensively also by means of a radioactive source. The first two fully working half-stave prototypes have been recently built, see Fig. 4, and they will be tested in a high-energy particle beam at the CERN SPS in November 2004. 2.2. The two layer barrel Two half-staves are joint together and then mounted onto a carbon fiber support sector to form the two SPD layers. Each carbon fiber sector carries four staves for the outer layer and two for the inner layer, respectively (Fig. 5). The main steps of the sector assembly procedure, described in Ref. [11], are the following: 1. The carbon fiber sector is equipped with the cooling ducts; 2. a thin layer of thermal grease is dispensed onto the sector surface where the half-staves will be placed: it ensures a good thermal coupling between the half-stave and the cooling pipe; 3. the two half-staves are aligned with respect to each other to form a stave and then positioned onto the carbon fiber sector.
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V. Manzari / Nuclear Instruments and Methods in Physics Research A 560 (2006) 61–66
Fig. 5. A schematic drawing of two sectors around the beam pipe: each sector carries on four staves on the outer layer and two staves on the inner layer.
The aligned stave is finally glued to the carbon-fiber sector using a UV curable glue: to ensure a good mechanical stability. The upper part of the stave is fixed by means of carbon fiber clips, which also act as a protection for the wire bonding. Each half-stave is tested as it is mounted onto the support sector. In fact the compactness of the SPD is such that the re-working of a underperforming half-stave is very risky in terms of probability to damage the neighbor halfstave. Once a sector is fully equipped mechanical and functional extensive tests are performed. Five sectors are then mounted together on an external carbon fiber support, to form a half-barrel, and tested. The whole SPD consists of two half-barrels. 3. Test beam prototype characterization The performance of SPD single assemblies has been studied in several test beam experiments carried out at the CERN SPS. Several pixel assemblies have been exposed both to a hadron and an ion beam. Here we report the results of the analysis from 120 GeV/c proton/pion beam data collected in October 2003. Fig. 6 shows the layout of the set up: a single chip assembly under test (middle plane) has been positioned in between two tracking stations. Each tracking station (minibus) consists of two single chip assemblies, placed one after the other along the beam. To improve the measurement of the transverse position of the incident tracks, one of the two assemblies in each minibus was rotated by 901, in order to provide the same accuracy in both coordinates. The trigger is set by the coincidence of the signals from an incoming beam particle traversing 4 scintillator counters. The hits on each plane are processed by a cluster finding algorithm, where a cluster is defined either as a single fired pixel or a group of fired pixels where each pixel shares at least one side with an adjacent fired pixel. Due to the shorter pixel size in the y-coordinate (50 mm), the cluster is
Fig. 6. Layout of the test beam at the CERN SPS in October 2003.
on average much more developed in this direction and it is found to be about 1.5 cells at a threshold of about 3000 electrons. Before the track reconstruction, the relative misalignments of the detectors in the x–y directions are taken into account. The internal alignment for each of the two minibuses has been achieved by using the correlations of the cluster coordinates on each of the two component detectors. This procedure assumes that clusters on the planes are only due to the beam track (no noisy hits) and that the track itself crosses the detectors at normal incidence: both assumptions are very reasonable. Fig. 7 shows the correlation and the difference between cluster ycoordinates of the two planes in the first minibus before the alignment correction is applied. The following step is to produce the relative alignment of the two minibuses. This is again achieved by cluster coordinate correlations. Once tracking planes are preliminarily aligned, a least squares fit procedure is applied to reconstruct the tracks both in the xz and yz projections, z being the coordinate along the beam. The residual distributions on the test planes (difference between predicted impact and nearest cluster positions) can be calculated: centering these distributions provides the alignment of the test plane. An iterative procedure, which excludes one plane at a time from the fit, allows the fine tuning of the alignment constants also for the tracking detectors. In Fig. 8 the residual distributions of
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V. Manzari / Nuclear Instruments and Methods in Physics Research A 560 (2006) 61–66
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events with a reconstructed track crossing the test plane, has been found to be in excess of 99% at a threshold of about 3000 electrons. The plot in Fig. 9 shows the detection efficiency as a function of the threshold. A wide plateau is present well before the usual operating range of 200 DAC units corresponding to about 3000 electrons. The detector performance has been studied as a function of the threshold and the track impact angle. A detailed discussion of the results from data analysis can be found in Ref. [12].
Fig. 8. Residual distributions from single pixel clusters only.
4. Conclusions reconstructed tracks for both coordinates are shown when only single pixel clusters are selected. To evaluate the intrinsic precision of the detector under test from the residual distribution, the tracking errors have been estimated by means of a simulation which takes into account the telescope geometry, the multiple scattering and the residual tracking plane misalignment. With this procedure the estimate tracking precision is about 10 mm in both transverse coordinates. When only single-pixel clusters are selected, the intrinsic spatial precision of the detector under test from the widths of the measured residual distribution, when the tracking precision is taken into account, is 120.170.5 mm in xcoordinate and 11.070.5 mm in y-coordinate. The measured spatial precision results to be slightly better in both coordinates with respect to the pitch/O12 value as expected for a digital device (122.7 and 14.4 mm in x and y coordinates respectively). The track impact parameter distribution shows that single pixel clusters are generated by tracks crossing a pixel cell in a region narrower than its size. Depending on the threshold setting, tracks impacting the cell close enough to the boundary regions will fire double pixel cluster. The detector efficiency, defined as the ratio between the number of events where a cluster correlated to a reconstructed track is detected and the total number of
The SPD provides the ALICE experiment with a high granularity detector for high-performance tracking and secondary vertexing capability. The construction procedure of the half-stave has been developed with dummy components and recently qualified in a test beam with the first fully working prototypes. One key element in the production of such a complex device as the SPD is the testing and qualification of the components at each phase of the construction. The detector performance has been evaluated from data collected in a beam test at the CERN SPS with single assemblies.
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