- Email: [email protected]
The LHCb vertex detector
Nuclear Instruments and Methods in Physics Research A 435 (1999) 65}73
The LHCb vertex detector T. Bowcock *, C. Parkes University of Liverpool, Liverpool, UK European Laboratory for Particle Physics (CERN), CH-1211 Geneva 23, Switzerland
Abstract LHCb is a dedicated LHC experiment for precision measurements of CP-violation and rare decays in the B meson sector. Achieving excellent resolution on the production and decay vertices of b-hadrons is vital to this programme. The design of the vertex detector is reviewed, the development programme outlined, and results from the "rst prototype detectors are provided. 1999 Elsevier Science B.V. All rights reserved. Keywords: LHC experiment; Vertex detector
1. Introduction
2. LHCb overview
The study of CP violation in the B system, through rare decay asymmetries, is a recognised priority of particle physics over the next two decades. LHCb is an experiment recently approved for operation at the LHC which will pursue this programme with very high statistical precision. Due to the large b production cross-section in pp collisions at (s"14 TeV, a #ux of 10 bb pairs will be created per year at the chosen LHCb running luminosity of 2;10 cm\ s\. This luminosity was selected to so that the average pp collision has a single interaction; multiple interaction collisions will be explicity rejected.
The LHCb experiment is a single-arm spectrometer with a polar angle coverage of approximately 10}300 mrad. This geometry is motivated by the forward peaked structure of the bb cross-section at high energies. The detector layout is shown in Fig. 1: the interaction region is on the left of the plot and contained within the vertex detector. In addition to the vertex detector the other critical components of the design are the RICH detectors and the trigger system.
* Corresponding author. E-mail addresses: [email protected] (T. Bowcock), [email protected] (C. Parkes)
E RICH * Pion Kaon discrimination is an essential attribute of a B physics experiment: in many important "nal states the signal and background share the same topology but di!er in particle composition. The RICH system aims to identify charged particles over the range 1}150 GeV/c. E Trigger * A robust four level trigger system has been designed for LHCb. The system must reduce the LHC interaction rate of 40 MHz to the
0168-9002/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 0 4 1 2 - X III. VERTEX DETECTORS AT HADRON COLLIDERS
66
T. Bowcock, C. Parkes / Nuclear Instruments and Methods in Physics Research A 435 (1999) 65}73
Fig. 1. The LHCb experiment seen from above.
data recording rate of 200 Hz. Events with B mesons can be distinguished from other inelastic pp interactions by the presence of secondary vertices, and particles with high transverse momentum (p ). The "rst and second level triggers 2 make use of the p property. The vertex detector 2 plays a critical role in the second, third and fourth level triggers where displaced vertices are used. The second level trigger is discussed below in Section 3.5. The fourth trigger level explicity reconstructs "nal states to select events associated with speci"c b-hadron decay modes.
struction impact parameter determination and vertex "nding capabilities are required to reject combinatorial background. In particular, B studies demand excellent proper time resolution: a resolution of 43$2fs is obtained in the BPD\p\ channel. After one year of running this will allow exclusion at 95% CL of x up to 91. The base-line design presented here is described in further detail in Ref. [1]. Where physics results are quoted they are based on a full GEANT3 simulation of the LHCb detector used in conjuction with the PYTHIA event generator. 3.2. Geometry
3. Vertex detector 3.1. Introduction The ability to be sensitive to the displacement, from the primary vertex, of B meson decay vertices and hence to be able to calculate the proper lifetime of the meson, is a fundamental requirement of the LHCb experiment. The role of the vertex detector in the second level triggering is discussed below. The o!-line recon-
The vertex detector is comprised of a sequence of stations placed transverse to the beam direction. A total of 17 stations are envisaged, displaced along 1 m. The number and distribution of the stations is chosen to ensure that the majority of tracks seen downstream in t he spectrometer will traverse at least 3 vertex detector planes, and to account for the &5 cm longitudinal spread in the interaction region: only 5% of tracks in the BPp>p\ channel traverse less than three
T. Bowcock, C. Parkes / Nuclear Instruments and Methods in Physics Research A 435 (1999) 65}73
stations. The vertex detector layout is illustrated in Fig. 2. During stable LHC operation the detectors will be lowered to within &1 cm of the beam. This
67
arrangement ensures the critical "rst measured point on a track will be close to the primary vertex. The design envisages the detectors being located in a secondary vacuum inside a 100 lm aluminium Rf-"eld. A wake-"eld suppressor is also foreseen. A diagram of the vacuum vessel is provided in Fig. 3. However, during LHC beam injection the material must be retracted by 3 cm. This is facilitated by the construction of the complete assembly in two halves which can be moved apart vertically. The two halves have a longitudinal o!set of 2 cm, and a small overlap region. It is expected that they will be positioned to an accuracy of 5 lm. 3.3. Detector design
Fig. 2. The distribution of vertex detector stations along the beampipe direction.
Each station will consist of two detectors rings to read pseudo-orthogonal co-ordinates. One detector will have circular strips measuring the R coordinate, while the other detector reads the
coordinate. The measuring detector strips are tilted from a pure radial design by 53, with
Fig. 3. Side view of the vacuum tank for the vertex detector, only the top half of the system is shown.
III. VERTEX DETECTORS AT HADRON COLLIDERS
68
T. Bowcock, C. Parkes / Nuclear Instruments and Methods in Physics Research A 435 (1999) 65}73
Fig. 4. The strip layout for the r and measuring detectors.
successive stations being rotated such that a $53 stereo angle is developed. This detector design is illustrated in Fig. 4. Each 1803 half station is constructed from three detectors, these three detectors each span 613: the overlap between these components will assist in the internal alignment of the disc. It is estimated that the internal alignment accuracy will be about 5 lm. The detectors will be read out using a doublemetal technology, as pioneered by DELPHI [2], in which the signal from a given strip is routed out over the top of the other structures via 1 lm thick aluminium strip isolated from the rest of the detector by a thin SiO dielectric layer. Thus, the readout electronics can be placed outside the acceptance, thereby minimising the multiple scattering. The layout of the routing lines to the readout electronics is constrained by the need to minimise cross-talk and capacitance while preserving the strip order in the readout chain. Maintaining the geometrical strip order facilitate the correction of local common base-line shifts. 3.4. Readout system The front-end electronics are mounted at approximately 7 cm from the beam axis. Hence the
radiation tolerance of the chips must exceed several humdred krad for one year. If operation is required for the expected lifetime of the detector a rad-hard technology must be used. The SCT128A chip possesses a fast bipolar analogue frontend and CMOS analogue pipeline and is fabricated in a DMILL technology. However, this chip does not meet all the requirements of the vertex detector thus a substantial e!ort is being made to design a new chip based on our experience with the HELIX and SCT128A. Analogue, rather than binary, readout has been chosen since it allows for a better monitoring and control of the radiation damage in the detector. Furthermore, it provides better precision of the reconstructed hits and better discrimination between hits and clusters due to electronic noise. The data is stored in the front end bu!er until a decision is obtained from the "rst level triggering. Analogue information from the 200 00 channels is transmitted on 7000 twisted-pair cables through the vacuum tank to the readout electronics at a distance of about 10 m from the detector. The data will then be digitised. Considering the expected signal amplitude range from our front-end chip (500 MeV), pedestal and noise variations, and the required resolution (2 MeV), we anticipate using
T. Bowcock, C. Parkes / Nuclear Instruments and Methods in Physics Research A 435 (1999) 65}73
69
a nine-bit ADC. Following digitisation the data is split into two paths: E In the "rst path, the data are preprocessed and sent to the second level trigger system (see Section 3.5). The preprocessing performs pedestal subtraction, common baseline shift correction and the cluster search. E In the second path, the data are stored in a buffer, pending a decision from the second level trigger. On receiving an accept signal from the trigger the data are transmitted over 60 m, by optical link, to processing electronics. At this stage a more sophisticated zero-suppression algorithm is performed in DSPs.
Fig. 5. Performance of the displaced vertex trigger algorithm.
3.5. Secondary vertex trigger The second level vertex trigger aims to separate B events from minimum bias events by using the signature of displaced secondary vertices. The input event rate is 1 MHz, this corresponds to a data collection rate of 2 GB/s. The algorithm aims for an average execution time of 250 ls. A major advantage of the R} geometry of the vertex detector is the assitance it provides in performing a fast second level trigger. Using only the R measuring detectors tracks are constructed in the R}Z projection and can be combined to produce two track vertices. The position of the primary vertex may then be determined. detector information is then invoked for all tracks that have a significant impact parameter with respect to the primary vertex. Finally, secondary vertices are found that are separated signi"cantly from the primary interaction point. The performance of this algorithm is shown in Fig. 5 for a range of B meson decay channels signi"cant for CP violation studies. This algorithm has been benchmarked and could be performed on &120 1000 MIP processors. 3.6. Radiation environment The particle #ux in a given detector varies as a function of the radial distance from the beam pipe (r) with a roughly 1/r dependence. The
expected radiation doses in the detector are shown in Fig. 6. The radiation environment means that the choice of detector technology has to be made with great care. Despite the relative ease of access to the vertex detector compared with ATLAS and CMS, premature failure of the detectors could mean up to a year of data being lost. Thus, a detailed understanding of the degradation of the performance due to radiation is critical. This implies the choice of substrate thickness, resistivity, doping, feature design and operating temperatures has to be made carefully. An LHCb test beam and irradiation programme is underway to provide the relevant data. 3.7. Test beam study Prototype n-strip detectors were designed and fabricated by Hamamatsu Photonics. The routing of the second metal layer represented a substantial challenge. The "nal design is shown in Fig. 7. A total of 12 prototype detectors were equipped with (slow shaping) VA2 readout electronics. The detectors and electronics were mounted into a telescope (see Fig. 8) comprising of 3 partial disks each equipped with an r-measuring and a phi-measuring detectors. Mounted directly in front of the telescope were a series of thin targets. The targets provided a source of secondary vertices which could be used to check the method of software
III. VERTEX DETECTORS AT HADRON COLLIDERS
70
T. Bowcock, C. Parkes / Nuclear Instruments and Methods in Physics Research A 435 (1999) 65}73
Fig. 6. Total particle #uence at two station of the vertex detector as a function of the radius after 1 yr of LHC running.
Fig. 7. Routing of the second metal (readout) layer to the electronics. The top section of the plot shows the routing for the
measuring detectors, the lower section the routing for the r-measuring detectors.
alignment in the "nal experiment and the trigger algorithm. Data were taken with the primary beam (120 GeV pions) passing through the targets and in normal incidence to the detectors with no target present. 3.7.1. Preliminary results The average noise over the detectors is shown in Fig. 9; one ADC count is equivalent to approximately 250 electrons. The average signal/noise ratio from each of the detectors is shown in Figs. 10 and 11. Comparing the pulse heights in each cluster with the noise in a single channel signal/noise ratios of about 50:1 are obtained. Residuals from "ts to charged particle passing through the detectors show that resolutions of order 7 lm are obtained from the inner regions with 40 lm pitch. These preliminary results will soon be superseded with a full analysis of the test beam data that should yield information on: charge collection e$ciency as a function of the geometry, resolution and an un-
Fig. 8. Photograph of LHCb telescope.
derstanding of the performance in terms of the capacitances in the double metal detectors. 3.8. Irradiation plans In late 1998 an irradiation of these detectors to about 3;10 p/cm is planned with 20 MeV
T. Bowcock, C. Parkes / Nuclear Instruments and Methods in Physics Research A 435 (1999) 65}73
71
a slightly di!erent geometry. It is advantageous for the physics performance to have the "rst measuring layer as close to the interaction point as is possible. In discussion with the LHC machine experts it is possible to foresee a detector operating with a minimum active radius of 8 mm from the beam. A reduced inner radius also allows a smaller outer radius whilst maintaining the same angular coverage. The p strip prototypes have an outer active radius of 40 mm. The smaller overall dimensions of the sensor area permits the construction of a 1803 module on a single wafer, leading to a substantial simpli"cation in the "nal design. The p-strip also allows a somewhat narrower pitch than the n-strip design as it is not necessary to use isolating p-stops. Full testing of both n-strip and p-strip detectors under severe radiation conditions will enable us to pick the appropriate technology (pstrip, n-strip or double sided) for use in the LHCb detector. 3.10. Module design and thermal modelling
Fig. 9. Noise in ADC counts for all detectors in the telescope. For clarity only every 8th channel is shown.
protons. Study of these detectors in a telescope equipped with fast readout electronics (SCT128A) will enable us to study the charge collection e$ciency of the irradiated detectors. Of importance to the LHCb experiment is the e!ect of the non-uniform irradiation. When the detectors are non-uniformly irradiated part of the n-bulk detectors will be operating in a regime where N "0, i.e. just before type inversion. In the region where N "0 the depletion voltage drops to 0. Although this in itself is not a problem it has been suggested that this can also lead to distortions in the "elds and a systematic misdetermination of cluster positions. For the LHCb experiment where the clusters are used in the trigger this could pose a serious problem. 3.9. P-strip design In addition to the n-strip detectors the LHCb experiment are also prototyping p-strip detectors of
The "nal LHCb module will be comprised of the active material and the hybrid containing the electronics both encapsulated within a thin walled r.f. shield in vacuum. The detectors need to be kept at the correct operating temperature in order to optimize performance. However insu$cient cooling can lead not only to a degradation of lifetime and performance but also to a catastrophic overheating (thermal runaway). LHCb has modelled the performance of severely damaged detectors using results from the ATLAS collaboration [3] and parametrizations provided by the ROSE collaboration [3]. Two packages were used to simulate the thermal performance of these detectors. (i) A full 3D "nite element analysis (ANSYS) of a detailed module. In this package a simple and pessimistic estimate of radiation damage e!ects was implemented. (ii) A 2D "nite element program. This description contained a fully time dependent parameterization of the irradiation induced current. The results indicate that for both the n-strip prototype currently under test } cooled by a module containing a thermal spine } and for the spineless p-strip design that the detectors are operating far from the threshold of thermal runaway. However the analysis does show
III. VERTEX DETECTORS AT HADRON COLLIDERS
72
T. Bowcock, C. Parkes / Nuclear Instruments and Methods in Physics Research A 435 (1999) 65}73
Fig. 10. Average signal/noise ratio in each phi detector in the telescope in groups of 32 strips. The ordinate is the S/N ratio and the abscissa is the group number.
that the inner 2 mm of the (smaller) p-strip detector will have a depletion voltage of over 400 V after one year of running despite being only 220 lm thick. It is suspected that this may lead to loss of charge collection e$ciency (CCE). Tests will be conducted with the prototype detectors to verify (after irradiation) the CCE versus voltage characteristics of the two options.
3.11. Project milestones The following major project milestones are foreseen: E A full half station containing both R and
measuring detectors will be bonded to frontend electronics running at LHC beam crossing
T. Bowcock, C. Parkes / Nuclear Instruments and Methods in Physics Research A 435 (1999) 65}73
73
Fig. 11. Average signal/noise ratio in each r-detector in the telescope in groups of 32 strips. The ordinate is the S/N ratio and the abscissa is the group number.
E E E E
speed of 40 MHz. This will be constructed and tested during the year 2000. In 2001 the vertex detector design will be "nalized, and documented in a Technical Design Report. Construction of the vertex detector will have commenced by the start of 2003. Comprehensive testing of the full assembly is anticipated in 2004 prior to the detector insertion. LHCb aims to be ready for the start of LHC running, which is planned for 2005.
Acknowledgements We are grateful for the assistance of our colleagues in the LHCb vertex detector group in the preparation of this presention and report.
References [1] LHCb Technical proposal, CERN/LHCC 98-4. [2] V. Chabaud et al., Nucl. Instr. and Meth. A 368 (1996) 314. [3] P.P. Allport et al., Nucl. Instr. and Meth. A 418 (1998) 110.
III. VERTEX DETECTORS AT HADRON COLLIDERS
The LHCb vertex detector T. Bowcock *, C. Parkes University of Liverpool, Liverpool, UK European Laboratory for Particle Physics (CERN), CH-1211 Geneva 23, Switzerland
Abstract LHCb is a dedicated LHC experiment for precision measurements of CP-violation and rare decays in the B meson sector. Achieving excellent resolution on the production and decay vertices of b-hadrons is vital to this programme. The design of the vertex detector is reviewed, the development programme outlined, and results from the "rst prototype detectors are provided. 1999 Elsevier Science B.V. All rights reserved. Keywords: LHC experiment; Vertex detector
1. Introduction
2. LHCb overview
The study of CP violation in the B system, through rare decay asymmetries, is a recognised priority of particle physics over the next two decades. LHCb is an experiment recently approved for operation at the LHC which will pursue this programme with very high statistical precision. Due to the large b production cross-section in pp collisions at (s"14 TeV, a #ux of 10 bb pairs will be created per year at the chosen LHCb running luminosity of 2;10 cm\ s\. This luminosity was selected to so that the average pp collision has a single interaction; multiple interaction collisions will be explicity rejected.
The LHCb experiment is a single-arm spectrometer with a polar angle coverage of approximately 10}300 mrad. This geometry is motivated by the forward peaked structure of the bb cross-section at high energies. The detector layout is shown in Fig. 1: the interaction region is on the left of the plot and contained within the vertex detector. In addition to the vertex detector the other critical components of the design are the RICH detectors and the trigger system.
* Corresponding author. E-mail addresses: [email protected] (T. Bowcock), [email protected] (C. Parkes)
E RICH * Pion Kaon discrimination is an essential attribute of a B physics experiment: in many important "nal states the signal and background share the same topology but di!er in particle composition. The RICH system aims to identify charged particles over the range 1}150 GeV/c. E Trigger * A robust four level trigger system has been designed for LHCb. The system must reduce the LHC interaction rate of 40 MHz to the
0168-9002/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 0 4 1 2 - X III. VERTEX DETECTORS AT HADRON COLLIDERS
66
T. Bowcock, C. Parkes / Nuclear Instruments and Methods in Physics Research A 435 (1999) 65}73
Fig. 1. The LHCb experiment seen from above.
data recording rate of 200 Hz. Events with B mesons can be distinguished from other inelastic pp interactions by the presence of secondary vertices, and particles with high transverse momentum (p ). The "rst and second level triggers 2 make use of the p property. The vertex detector 2 plays a critical role in the second, third and fourth level triggers where displaced vertices are used. The second level trigger is discussed below in Section 3.5. The fourth trigger level explicity reconstructs "nal states to select events associated with speci"c b-hadron decay modes.
struction impact parameter determination and vertex "nding capabilities are required to reject combinatorial background. In particular, B studies demand excellent proper time resolution: a resolution of 43$2fs is obtained in the BPD\p\ channel. After one year of running this will allow exclusion at 95% CL of x up to 91. The base-line design presented here is described in further detail in Ref. [1]. Where physics results are quoted they are based on a full GEANT3 simulation of the LHCb detector used in conjuction with the PYTHIA event generator. 3.2. Geometry
3. Vertex detector 3.1. Introduction The ability to be sensitive to the displacement, from the primary vertex, of B meson decay vertices and hence to be able to calculate the proper lifetime of the meson, is a fundamental requirement of the LHCb experiment. The role of the vertex detector in the second level triggering is discussed below. The o!-line recon-
The vertex detector is comprised of a sequence of stations placed transverse to the beam direction. A total of 17 stations are envisaged, displaced along 1 m. The number and distribution of the stations is chosen to ensure that the majority of tracks seen downstream in t he spectrometer will traverse at least 3 vertex detector planes, and to account for the &5 cm longitudinal spread in the interaction region: only 5% of tracks in the BPp>p\ channel traverse less than three
T. Bowcock, C. Parkes / Nuclear Instruments and Methods in Physics Research A 435 (1999) 65}73
stations. The vertex detector layout is illustrated in Fig. 2. During stable LHC operation the detectors will be lowered to within &1 cm of the beam. This
67
arrangement ensures the critical "rst measured point on a track will be close to the primary vertex. The design envisages the detectors being located in a secondary vacuum inside a 100 lm aluminium Rf-"eld. A wake-"eld suppressor is also foreseen. A diagram of the vacuum vessel is provided in Fig. 3. However, during LHC beam injection the material must be retracted by 3 cm. This is facilitated by the construction of the complete assembly in two halves which can be moved apart vertically. The two halves have a longitudinal o!set of 2 cm, and a small overlap region. It is expected that they will be positioned to an accuracy of 5 lm. 3.3. Detector design
Fig. 2. The distribution of vertex detector stations along the beampipe direction.
Each station will consist of two detectors rings to read pseudo-orthogonal co-ordinates. One detector will have circular strips measuring the R coordinate, while the other detector reads the
coordinate. The measuring detector strips are tilted from a pure radial design by 53, with
Fig. 3. Side view of the vacuum tank for the vertex detector, only the top half of the system is shown.
III. VERTEX DETECTORS AT HADRON COLLIDERS
68
T. Bowcock, C. Parkes / Nuclear Instruments and Methods in Physics Research A 435 (1999) 65}73
Fig. 4. The strip layout for the r and measuring detectors.
successive stations being rotated such that a $53 stereo angle is developed. This detector design is illustrated in Fig. 4. Each 1803 half station is constructed from three detectors, these three detectors each span 613: the overlap between these components will assist in the internal alignment of the disc. It is estimated that the internal alignment accuracy will be about 5 lm. The detectors will be read out using a doublemetal technology, as pioneered by DELPHI [2], in which the signal from a given strip is routed out over the top of the other structures via 1 lm thick aluminium strip isolated from the rest of the detector by a thin SiO dielectric layer. Thus, the readout electronics can be placed outside the acceptance, thereby minimising the multiple scattering. The layout of the routing lines to the readout electronics is constrained by the need to minimise cross-talk and capacitance while preserving the strip order in the readout chain. Maintaining the geometrical strip order facilitate the correction of local common base-line shifts. 3.4. Readout system The front-end electronics are mounted at approximately 7 cm from the beam axis. Hence the
radiation tolerance of the chips must exceed several humdred krad for one year. If operation is required for the expected lifetime of the detector a rad-hard technology must be used. The SCT128A chip possesses a fast bipolar analogue frontend and CMOS analogue pipeline and is fabricated in a DMILL technology. However, this chip does not meet all the requirements of the vertex detector thus a substantial e!ort is being made to design a new chip based on our experience with the HELIX and SCT128A. Analogue, rather than binary, readout has been chosen since it allows for a better monitoring and control of the radiation damage in the detector. Furthermore, it provides better precision of the reconstructed hits and better discrimination between hits and clusters due to electronic noise. The data is stored in the front end bu!er until a decision is obtained from the "rst level triggering. Analogue information from the 200 00 channels is transmitted on 7000 twisted-pair cables through the vacuum tank to the readout electronics at a distance of about 10 m from the detector. The data will then be digitised. Considering the expected signal amplitude range from our front-end chip (500 MeV), pedestal and noise variations, and the required resolution (2 MeV), we anticipate using
T. Bowcock, C. Parkes / Nuclear Instruments and Methods in Physics Research A 435 (1999) 65}73
69
a nine-bit ADC. Following digitisation the data is split into two paths: E In the "rst path, the data are preprocessed and sent to the second level trigger system (see Section 3.5). The preprocessing performs pedestal subtraction, common baseline shift correction and the cluster search. E In the second path, the data are stored in a buffer, pending a decision from the second level trigger. On receiving an accept signal from the trigger the data are transmitted over 60 m, by optical link, to processing electronics. At this stage a more sophisticated zero-suppression algorithm is performed in DSPs.
Fig. 5. Performance of the displaced vertex trigger algorithm.
3.5. Secondary vertex trigger The second level vertex trigger aims to separate B events from minimum bias events by using the signature of displaced secondary vertices. The input event rate is 1 MHz, this corresponds to a data collection rate of 2 GB/s. The algorithm aims for an average execution time of 250 ls. A major advantage of the R} geometry of the vertex detector is the assitance it provides in performing a fast second level trigger. Using only the R measuring detectors tracks are constructed in the R}Z projection and can be combined to produce two track vertices. The position of the primary vertex may then be determined. detector information is then invoked for all tracks that have a significant impact parameter with respect to the primary vertex. Finally, secondary vertices are found that are separated signi"cantly from the primary interaction point. The performance of this algorithm is shown in Fig. 5 for a range of B meson decay channels signi"cant for CP violation studies. This algorithm has been benchmarked and could be performed on &120 1000 MIP processors. 3.6. Radiation environment The particle #ux in a given detector varies as a function of the radial distance from the beam pipe (r) with a roughly 1/r dependence. The
expected radiation doses in the detector are shown in Fig. 6. The radiation environment means that the choice of detector technology has to be made with great care. Despite the relative ease of access to the vertex detector compared with ATLAS and CMS, premature failure of the detectors could mean up to a year of data being lost. Thus, a detailed understanding of the degradation of the performance due to radiation is critical. This implies the choice of substrate thickness, resistivity, doping, feature design and operating temperatures has to be made carefully. An LHCb test beam and irradiation programme is underway to provide the relevant data. 3.7. Test beam study Prototype n-strip detectors were designed and fabricated by Hamamatsu Photonics. The routing of the second metal layer represented a substantial challenge. The "nal design is shown in Fig. 7. A total of 12 prototype detectors were equipped with (slow shaping) VA2 readout electronics. The detectors and electronics were mounted into a telescope (see Fig. 8) comprising of 3 partial disks each equipped with an r-measuring and a phi-measuring detectors. Mounted directly in front of the telescope were a series of thin targets. The targets provided a source of secondary vertices which could be used to check the method of software
III. VERTEX DETECTORS AT HADRON COLLIDERS
70
T. Bowcock, C. Parkes / Nuclear Instruments and Methods in Physics Research A 435 (1999) 65}73
Fig. 6. Total particle #uence at two station of the vertex detector as a function of the radius after 1 yr of LHC running.
Fig. 7. Routing of the second metal (readout) layer to the electronics. The top section of the plot shows the routing for the
measuring detectors, the lower section the routing for the r-measuring detectors.
alignment in the "nal experiment and the trigger algorithm. Data were taken with the primary beam (120 GeV pions) passing through the targets and in normal incidence to the detectors with no target present. 3.7.1. Preliminary results The average noise over the detectors is shown in Fig. 9; one ADC count is equivalent to approximately 250 electrons. The average signal/noise ratio from each of the detectors is shown in Figs. 10 and 11. Comparing the pulse heights in each cluster with the noise in a single channel signal/noise ratios of about 50:1 are obtained. Residuals from "ts to charged particle passing through the detectors show that resolutions of order 7 lm are obtained from the inner regions with 40 lm pitch. These preliminary results will soon be superseded with a full analysis of the test beam data that should yield information on: charge collection e$ciency as a function of the geometry, resolution and an un-
Fig. 8. Photograph of LHCb telescope.
derstanding of the performance in terms of the capacitances in the double metal detectors. 3.8. Irradiation plans In late 1998 an irradiation of these detectors to about 3;10 p/cm is planned with 20 MeV
T. Bowcock, C. Parkes / Nuclear Instruments and Methods in Physics Research A 435 (1999) 65}73
71
a slightly di!erent geometry. It is advantageous for the physics performance to have the "rst measuring layer as close to the interaction point as is possible. In discussion with the LHC machine experts it is possible to foresee a detector operating with a minimum active radius of 8 mm from the beam. A reduced inner radius also allows a smaller outer radius whilst maintaining the same angular coverage. The p strip prototypes have an outer active radius of 40 mm. The smaller overall dimensions of the sensor area permits the construction of a 1803 module on a single wafer, leading to a substantial simpli"cation in the "nal design. The p-strip also allows a somewhat narrower pitch than the n-strip design as it is not necessary to use isolating p-stops. Full testing of both n-strip and p-strip detectors under severe radiation conditions will enable us to pick the appropriate technology (pstrip, n-strip or double sided) for use in the LHCb detector. 3.10. Module design and thermal modelling
Fig. 9. Noise in ADC counts for all detectors in the telescope. For clarity only every 8th channel is shown.
protons. Study of these detectors in a telescope equipped with fast readout electronics (SCT128A) will enable us to study the charge collection e$ciency of the irradiated detectors. Of importance to the LHCb experiment is the e!ect of the non-uniform irradiation. When the detectors are non-uniformly irradiated part of the n-bulk detectors will be operating in a regime where N "0, i.e. just before type inversion. In the region where N "0 the depletion voltage drops to 0. Although this in itself is not a problem it has been suggested that this can also lead to distortions in the "elds and a systematic misdetermination of cluster positions. For the LHCb experiment where the clusters are used in the trigger this could pose a serious problem. 3.9. P-strip design In addition to the n-strip detectors the LHCb experiment are also prototyping p-strip detectors of
The "nal LHCb module will be comprised of the active material and the hybrid containing the electronics both encapsulated within a thin walled r.f. shield in vacuum. The detectors need to be kept at the correct operating temperature in order to optimize performance. However insu$cient cooling can lead not only to a degradation of lifetime and performance but also to a catastrophic overheating (thermal runaway). LHCb has modelled the performance of severely damaged detectors using results from the ATLAS collaboration [3] and parametrizations provided by the ROSE collaboration [3]. Two packages were used to simulate the thermal performance of these detectors. (i) A full 3D "nite element analysis (ANSYS) of a detailed module. In this package a simple and pessimistic estimate of radiation damage e!ects was implemented. (ii) A 2D "nite element program. This description contained a fully time dependent parameterization of the irradiation induced current. The results indicate that for both the n-strip prototype currently under test } cooled by a module containing a thermal spine } and for the spineless p-strip design that the detectors are operating far from the threshold of thermal runaway. However the analysis does show
III. VERTEX DETECTORS AT HADRON COLLIDERS
72
T. Bowcock, C. Parkes / Nuclear Instruments and Methods in Physics Research A 435 (1999) 65}73
Fig. 10. Average signal/noise ratio in each phi detector in the telescope in groups of 32 strips. The ordinate is the S/N ratio and the abscissa is the group number.
that the inner 2 mm of the (smaller) p-strip detector will have a depletion voltage of over 400 V after one year of running despite being only 220 lm thick. It is suspected that this may lead to loss of charge collection e$ciency (CCE). Tests will be conducted with the prototype detectors to verify (after irradiation) the CCE versus voltage characteristics of the two options.
3.11. Project milestones The following major project milestones are foreseen: E A full half station containing both R and
measuring detectors will be bonded to frontend electronics running at LHC beam crossing
T. Bowcock, C. Parkes / Nuclear Instruments and Methods in Physics Research A 435 (1999) 65}73
73
Fig. 11. Average signal/noise ratio in each r-detector in the telescope in groups of 32 strips. The ordinate is the S/N ratio and the abscissa is the group number.
E E E E
speed of 40 MHz. This will be constructed and tested during the year 2000. In 2001 the vertex detector design will be "nalized, and documented in a Technical Design Report. Construction of the vertex detector will have commenced by the start of 2003. Comprehensive testing of the full assembly is anticipated in 2004 prior to the detector insertion. LHCb aims to be ready for the start of LHC running, which is planned for 2005.
Acknowledgements We are grateful for the assistance of our colleagues in the LHCb vertex detector group in the preparation of this presention and report.
References [1] LHCb Technical proposal, CERN/LHCC 98-4. [2] V. Chabaud et al., Nucl. Instr. and Meth. A 368 (1996) 314. [3] P.P. Allport et al., Nucl. Instr. and Meth. A 418 (1998) 110.
III. VERTEX DETECTORS AT HADRON COLLIDERS