BGV fibre module: Test beam report Olivier Girard a , Liupan An b , Axel Kuonen a , Guido Haefeli a a Ecole polytechnique fédérale de Lausanne (EPFL), Switzerland b Tsinghua University, China 17 January 2017 Internal Note Revision: 1 Reference: LPHE Note 2016-04 Abstract A tracking module of the Beam Gas Vertex (BGV) detector was tested in the experimental setup of the LHCb SciFi tracker test beam at SPS. A telescope was placed upstream the module in order to reconstruct the track of particles. A detailed analysis of the hit resolution and hit detection efficiency of the module as well as the influence of the cluster thresholds is presented. With optimal thresholds, the hit resolution is measured to be σ hit = 38.2 ± 2.8 μm and σ hit = 42.9 ± 3.9 μm for the two sides of the module, whereas the efficiency is " hit = 98.3% and " hit = 97.8%.
Transcript
BGV fibre module:Test beam report
Olivier Girard a, Liupan An b, Axel Kuonen a, Guido Haefeli a
a Ecole polytechnique fédérale de Lausanne (EPFL), Switzerlandb Tsinghua University, China
A tracking module of the Beam Gas Vertex (BGV) detector was tested in the experimentalsetup of the LHCb SciFi tracker test beam at SPS. A telescope was placed upstream the modulein order to reconstruct the track of particles. A detailed analysis of the hit resolution and hitdetection efficiency of the module as well as the influence of the cluster thresholds is presented.With optimal thresholds, the hit resolution is measured to be σhit = 38.2± 2.8µm and σhit =42.9 ± 3.9µm for the two sides of the module, whereas the efficiency is εhit = 98.3% andεhit = 97.8%.
4.4 Correction for the drift of the table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5 Measurement of hit resolution 10
6 Measurement of hit detection efficiency 12
7 Hit detection efficiency in non-sensitive areas 13
8 Conclusion 15
References 18
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1 INTRODUCTION
The Beam Gas Vertex (BGV) detector aims to provide non-disruptive real-time beam size measure-ments at the LHC. It uses the beam-gas imaging technique to measure individual beam size andprofile with 5% resolution within 1 minute. The demonstrator detector, seen in figure 1, was in-stalled at point 4 in November 2015. It comprises a gas tank and two tracking stations with eachfour scintillating fibre (SciFi) modules. [1]
Figure 1: Picture of the BGV detector after installation at the LHC in November 2015.
Scintillating plastic fibres of 250µm diameter are the sensitive material of the tracker modules.They are arranged in mats of 4 or 5 layers forming one detection plane. The SciFi modules con-tain two detection planes that are rotated by an angle of 2◦ with respect to each other in order tofacilitate pattern recognition. To reduce multiple scattering, the amount of material in the accep-tance region of the detector was limited. The SciFi modules with 4 layers of fibre are placed in thetracking station near the gas tank and the ones with 5 layers in the far station. [1]
The read-out of the SciFi modules is based on the read-out of silicon strip detectors at LHCb withBeetle ASIC [2] at the front-end and TELL1 data acquisition interface [3] at the back-end. Throughthe read-out chain, the analogue signals are deformed due to electronics limitation. Raw dataproduced by the detector undergoes a chain of signal corrections before clustering. The clustersare used for tracking and reconstruction of the LHC beam profile. The read-out and the correctionsof raw data are the subject of a separate document [4].
A 4-layer BGV SciFi module was tested in the experimental setup of the LHCb SciFi tracker testbeam at SPS in November 2015. The goal of the measurement campaign was to determine hitresolution and hit detection efficiency of the module. The particle tracks were reconstructed usinga telescope based on short scintillating fibres with five tracking stations. This document presentsthe analysis procedure and the main results.
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2 EXPERIMENTAL SETUP
By convention, in the BGV module, the detection plane with straight fibres is called the RS side andoccupies channels 0 to 1023 whereas the plane with 2◦-rotated fibres is the TU side and goes fromchannel 1024 to 2047. The module was mounted on an X-Y moving table (see figure 2) allowingto inject the beam at different positions. The table presents however a drift in the vertical directiononce its position is set. The drift is of the order of 100µm for the duration of the data acquisitionruns. A correction for the drift is discussed in more details in section 4.4.
Figure 2: Scheme of the X-Y moving table onwhich the BGV module (zoom) and the LHCb
SciFi modules are installed.
Figure 3: Picture of the setup installed at theNovember test beam at CERN with the SciFitelescope placed upstream of the BGV SciFi
module.
The setup includes a telescope used to reconstruct the track of particles. It is made of fivetracking stations with X and Y measurement. It was placed approximately 20 cm upstream of theBGV module. Figure 3 displays a picture of the setup. The telescope is read out with VATA64and USBboard electronics [7, 8]. BGV and telescope events are synchronised which enables themeasurement of hit detection efficiency and resolution of the BGV module.
The beam is composed of a mixture of MIP-like 180 GeV/c pions, protons and muons. As theycross the setup, they undergo multiple scattering. The deflection angle is well described by a Gaus-sian function with tails for large scattering angles. Inside the telescope, the beam only crossesfibre mats with a total thickness of X/X0 ≈ 3%. The RMS of transverse deviations due to multiplescattering is estimated to be below 4µm in the telescope and 6µm extrapolated to the BGV mod-ule. The BGV module represents a total thickness of X/X0 ≈ 1% leading to transverse deviationsbelow 1µm between the two layers. Multiple scattering is therefore negligible for the foreseenmeasurements given the expected resolution of SciFi modules (> 30µm).
3 CLUSTER ANALYSIS
3.1 SIGNAL FORMATION AND CLUSTERING
The signal formation in a fibre module is illustrated in figure 4. Through a scintillation process, acharged particle generates photons which are collected directly or via mirror reflection by multi-channel silicon photo-multipliers (SiPMs). SiPMs are based on silicon diodes working in avalanche
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mode (above the breakdown voltage VBD) and are therefore intrinsic amplifiers. The gain is pro-portional to the over-voltage ∆V = Vbias − VBD and is typically G ∼ 3.5 · 106 (at ∆V = 3.5 V).
Figure 4: Signal formation in a SciFi module. Scintillation photons are generated in the fibres by acrossing particle and detected by several pixels of the SiPM. The signal of each channel of the SiPM is
the sum of the signal of all pixels within this channel. The crossing point is calculated with aweighted mean of the signals. [5]
A clustering algorithm [6], illustrated in figure 5, is applied to the data in order to suppress noise(zero-suppression) and calculate the crossing point of particles. The algorithm first selects channelswith signal above a specified seed threshold. Second, the algorithm builds the clusters including oneneighbouring channel on the left and on the right if they have signal above the neighbour threshold.Third, the cluster is finally accepted if the total signal inside the cluster exceeds the sum threshold.
Clusters
sum thresholdRemoved by
SiPM channels
thresholdSeed
thresholdNeighbour
Sig
nal am
plit
ude
Figure 5: Illustration of the clustering algorithm.
The clusters have the following properties:
• The cluster sum is the total signal in the cluster and is generally expressed in photoelectrons(PE). It is expected to be Landau distributed because it follows the distribution of energy
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deposit in a thin layer of matter. The most probable value (MPV) of this distribution is calledthe light yield.
• The mean cluster position is calculated as the average of the cluster channels weighted bytheir signal. It is the best approximation for the crossing point of the particle.
• The cluster size is the number of channels composing the cluster.
3.2 BEAM PROFILE
The beam profile is measured by the distribution of the mean position of the clusters. It is shown infigure 6 for all data acquisition runs. In each run, the beam produces clusters in two regions of themodule because it crosses the two detection layers. The beam is approximately 40 channels (1 cm)wide in the direction perpendicular to the fibres. The shape of the beam profiles measured on eachmodule side are the same because the fibres are almost parallel. Figure 7 shows the number ofclusters per event detected by the module. The majority of the events contains one cluster on eachside. Events with less clusters indicate inefficiencies. Some events contain many more clusters (upto 300). They originate from delta electrons emitted in the setup.
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Figure 6: Cluster position for all dataacquisition runs. Red vertical dashed lines
correspond to the limits between 128-channelSiPM arrays.
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Figure 7: Number of clusters per event. Themajority of the events contain two clusters, one
each side. Some events contain a very largenumber of clusters due to delta electrons.
3.3 LIGHT YIELD AND CLUSTER SIZE
The cluster sum distributions are shown in figure 8 for each run and each module side separately.In each run, the beam was injected at a different transverse position. The position of injectionalong the fibres was not measured. However, the light yield is expected to vary by less than 10%if the beam is injected close to the mirror or close to the SiPMs. The variation between the runsfrom 13 and 18 PE demonstrates that the quality of the fibre mat, the mirror glueing or the SiPMalignment with respect to the mat is not equal at each position.
The cluster sum for all runs together is displayed in figure 9 for each side of the module. TheRS and TU fibre mats are not exactly the same. On the TU side, the optical cut of the fibres is notperpendicular to the fibres but tilted with a 2◦-angle. Consequently, the light reflection, refraction
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Cluster sum per channel [p.e.]0 5 10 15 20 25 30 35 40
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Cluster sum for different runs (RS side)
(a) RS side.
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(b) TU side.
Figure 8: Cluster sum for each run and for the RS side (left) and the TU side (right).
and angular distribution at the interface between the fibres and the SiPM is different. This has animpact on the light yield which is seen to be smaller on the TU side.
Cluster sum [p.e.]0 10 20 30 40 50 60 70
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Figure 9: Cluster sum for all runs. The lightyield is 15.6 and 14.6 PE for the RS and TU
Figure 10: Correlation between the meancluster size and the light yield. No significantdifference is observed between the two sides.
When the light yield is increased, the signal in the neighbouring channels is also increased andthe probability that it exceeds the threshold is higher. On average, the cluster size is thereforepositively correlated with the light yield. Figure 10 shows that the correlation between the meancluster size and the light yield is diffuse and no difference is seen between the two module sides.However, for one of the runs on the RS side, the cluster size is significantly larger than the expec-tation from the other measurements. This probably arises from the presence of an air gap betweenthe fibre mat and the mirror or the SiPM.
4 TRACK ALIGNMENT
4.1 DEFINITION OF RESIDUAL AND HIT RESOLUTION
The residual is the distance between the position of the hit measured on the device under testand the impact point of the track which is provided by another independent device. Over manyevents, the distribution of residuals is the convolution of the distribution of hit positions and track
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positions. Both are supposed Gaussian with a sigma σhit and σtrack equal to the resolution on theirmeasurement. The residual distribution is therefore a Gaussian with sigma:
σresidual =Ç
σ2hit +σ
2track (1)
In practice, the residual is the only parameter that can be measured. The track resolution σtrackmust be as good as possible in order to enhance the sensitivity on the hit resolution σhit.
4.2 MEASUREMENT OF THE TRACKS WITH THE FIBRE TELESCOPE
The fibre telescope was optimised to provide track measurement with high resolution. The detailedanalysis of the performance of the telescope is the subject of a separate document [9]. The layersof the telescope are mechanically aligned with respect to each other to a level of 100µm. Anoffline alignment procedure allows to align the layers to a sub-micrometre level. In this procedure,every X (Y) layer has one degree of freedom: a shift in X (Y). The best alignment shifts are foundwhen σresidual of all layers has reached a minimum value. In order to ensure the convergence, thealgorithm runs iteratively on the inner layers with fixed outer layers and then the opposite.
The residual distribution of the central layer of the telescope after this alignment procedure isdisplayed in figure 11. The central peak follows accurately a Gaussian function. However, tails arepresent on the sides. They arise from multiple scattering. The distribution is fitted using a CrystalBall function which enables to discriminate between a central Gaussian and two exponential tails.The sigma of the Gaussian is used for σresidual.
residual-0.4 -0.2 0 0.2 0.4
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0.011±alphal = 1.865
0.011±alphar = 1.921
0.000080±mean = -0.0011535
0.14±nl = 4.85
0.096±nr = 3.917
0.000067±sigma = 0.036186
/ndf = 6.219312χ
Figure 11: Residual distribution of the central plane of the telescope fitted with a Crystal Ballfunction.
The residual in the central layer of the telescope is σresidual = 36.8µm. In an ideal case whereall layers have the same hit resolution σhit and are equally spaced, one can calculate analyticallythe contribution of σhit and σtrack to σresidual [10]. The result is σtrack = 16.5µm in the centre ofthe telescope and σhit = 32.9µm.
The actual telescope has however not this ideal configuration. First, the layers are not equallyspaced (10 cm between the outer layers and 15 cm between the central layer and the outer layers).Second, the hit resolution is not equal for all layers because of the different number of fibre layers(5 or 6) and the different fibre mat quality and mechanical alignment. Nevertheless, a simulation[11] showed similar results for the track resolution in the ideal and the actual configuration of thetelescope.
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4.3 EXTRAPOLATION OF THE TRACK ONTO THE BGV MODULE
The track is extrapolated to the position of the BGV module. Using [10], the resolution of theimpact point of the track is estimated to be:
The impact point is calculated in the coordinate system of the telescope. It must be convertedinto the coordinate system of the BGV module in order to measure the residual. The origin of thecoordinate systems are shifted in space with ~r = (Xoffset, Yoffset, Zoffset) and their axis are rotatedwith small angles αX, αY, αZ. An offline procedure adjusts these parameters in order to align thecoordinate systems to a sub-micrometre level. The procedure runs iteratively on each parameterand finds the optimal value by minimising σresidual. Examples of this alignment procedure for theparameters Zoffset and αZ are displayed in figure 12.
[mm]offsetZ100 200 300 400 500 600
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ma
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esid
ual [
mm
]
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offsetAlignment of the distance Z
(a) Dependence of σresidual on the alignement parameterZoffset which corresponds to the distance between the tele-scope and the BGV module on the beam line.
]° [Zα-1 -0.5 0 0.5 1 1.5 2
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esid
ual [
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ZαAlignment of the angle
(b) Dependence of σresidual on the alignement parameterαZ which corresponds to the angle between the Z-axis ofthe telescope and the BGV module coordinate systems.
Figure 12: Illustration of the procedure used to align the telescope and the BGV module coordinatesystems. The best alignment is found when the residual reaches a minimum. The position of the
minimum is found with a fit to a parabolic function.
4.4 CORRECTION FOR THE DRIFT OF THE TABLE
For some runs, a vertical drift of the support table of the BGV module with respect to the telescopewas observed. This results in a wide residual distribution with a non-gaussian central peak. Theresidual as a function of the event number is shown in figure 13a. For a small number of events, theresidual distribution is centred around a fixed value. However, over time, the centre of the distri-bution moves which means that the BGV is moving with respect to the telescope. The average driftis measured with the mean of the Gaussian function and is seen to be non-linear. Discontinuitiesin the average drift indicate temporary interruption of the beam while the table continued drifting.The average drift is fitted with a set of linear functions that are used for correction (figure 13b).After correction, the total residual distribution exhibits a central gaussian peak which is used forthe measurement of resolution.
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Event number0 50 100 150 200 250 310×
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idua
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m]
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Drift before correction
(a) Before correction. A drift of approximately 0.2 mm isvisible. The discontinuity arises from a short interval dur-ing which the beam was interrupted and the table contin-ued to drift.
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(b) After correction. The residual is constantly centeredaround zero.
Figure 13: Residual as a function of event number for one of the data acquisition run. The residualdistributions are fitted by slice of 1000 events with a Gaussian function. The average drift (blue
points) is measured with the mean of the Gaussian function and fitted with a set of linear functions(white line).
5 MEASUREMENT OF HIT RESOLUTION
An example of residual distribution is displayed in figure 14 with cluster thresholds of 2.5/1.5/4.5 PE.It is fitted with a Crystal Ball function where the sigma of the Gaussian part σresidual is taken asreference for the measurement of resolution. Averaged over all runs, σresidual is:
where the uncertainty includes the standard deviation over all runs and the errors of the fit. Usingthe estimated track resolution σtrack from section 4.3, the hit resolution σhit for each side is:
The hit resolution of the two sides of the module is not equal. It is approximately 12% worsefor the TU side. This arises from the optical cut of the fibres which is tilted with a 2◦-angle forthis side. In figure 14, it is also seen that the residual distribution comprises tails suppressed by afactor of 100 compared to the central Gaussian. They can be interpreted as large angle multiplescattering. Furthermore, the tails are not symmetric which is due to an insufficient correction ofraw data [4].
In figure 10, it was noticed that one run has an unexpectedly large average cluster size (2.94versus 2.60 - 2.70) due to a non-perfect mirror glueing or SiPM positioning. As a consequence, theresidual is also seen to be larger than for the other runs. σresidual is 67.5µm which corresponds toσhit = 51.0µm (34% increase compared to the nominal hit resolution).
The influence of the cluster thresholds on the residual is shown in figure 15. The optimalthresholds are 2.5/1.5/4.5 PE. With higher thresholds, the residual increases because signal inneighbours is lost which degrades the resolution on the cluster position. With lower thresholds,noise in the neighbouring channel can be included which also alters the cluster position.
Figure 14: Residual distribution for one of the runs with cluster thresholds of 2.5/1.5/4.5 PE, fittedwith a Crystal Ball function.
Seed threshold [PE]1.5 2 2.5 3 3.5 4 4.5
m]
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Residual as a function of seed threshold
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Residual as a function of seed threshold
Figure 15: Width of the residual distribution as a function of seed threshold.
6 MEASUREMENT OF HIT DETECTION EFFICIENCY
For each side of the BGV module, events are classified in two categories:
Efficient: when a cluster is found below a certain seed distance from the impact point of the track.
Inefficient: when no cluster is found or when one is found but further away from the impact pointthan the seed distance.
The hit detection efficiency εhit is calculated as the ratio between the number efficient events andthe total number of events. It strongly depends on the seed distance, on the cluster thresholds andon the light yield.
Figures 16 and 17 show the dependence on the seed distance with fixed seed/neighbour/sumthresholds of 2.5/1.5/4.5 PE. The efficiency is relatively flat in all the beam region except at theposition of non-sensitive areas (discussed in more details in section 7). With a Gaussian residualdistribution of sigma σresidual, the expected efficiency at a seed distance of 1 ·σresidual is 68% whichis consistent with the measurement. However, at increased seed distance, figure 17 shows that theefficiency does not levels off at 100% which is caused by, first, inefficiencies and, second, multiple
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scattering. In order to minimise the contribution of multiple scattering, the seed distance is set to1.25 mm (≈ 20 ·σresidual) in the following.
BGV x position [mm]-204 -202 -200 -198 -196 -194 -192
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Figure 16: Hit detection efficiency in thebeam region for different seed distances. The
purple (blue) hatched zone is the exactposition of a dead channel (gap).
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Figure 17: Average hit detection efficiency asa function of seed distance. The average
excludes the non-efficient regions such as gapsor dead channels.
The hit detection efficiency as a function of cluster thresholds is shown in figure 18 and 19 forone run. For the highest thresholds (4.5/3.5/6.5 PE) the efficiency is in the order of 90% whereasas it reaches approximately 99% for the lowest thresholds (1.5/0.5/2.5 PE). At 2.5/1.5/4.5 PE, inaverage over all runs, εhit is:
RS side: εhit = 98.3% TU side: εhit = 97.8%
BGV x position [mm]-204 -202 -200 -198 -196 -194 -192
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Figure 18: Hit detection efficiency in thebeam region for different cluster thresholds.The purple (blue) hatched zone is the exact
position of a dead channel (gap).
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Figure 19: Average hit detection efficiency asa function of seed threshold. The average
excludes the non-efficient regions such as gapsor dead channels.
Figure 20 presents the efficiency as a function of light yield for different cluster thresholds. Foreach set of thresholds, the efficiency grows linearly with the light yield except at small thresholds
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where the light yield has no impact on the efficiency. The lower efficiency on the TU side is partiallydue to the lower light yield. However, figure 21 demonstrates that, at the same light yield, theefficiency is smaller on the TU side than on the RS one. The tilted optical cut of the fibre mat istherefore responsible for a small reduction in hit detection efficiency (between 0.2 and 0.5%).
Figure 21: Efficiency as a function of light yield for cluster thresholds of 2.5/1.5/4.5 PE.
7 HIT DETECTION EFFICIENCY IN NON-SENSITIVE AREAS
Each detection plane (26 cm wide) of the BGV module is composed of four pieces of fibre mat of6.5 cm width placed next to each other. They are read out by eight 128-channel SiPM arrays. Non-sensitive areas of the mats or the SiPMs are source of inefficiency. They are shown schematicallyin figure 22.
For the mats, the non-sensitive areas are spread over all channels due to broken fibres, non-perfect mirror glueing or dirt on the optical cuts. Some are located at specific positions such as atthe contact between the pieces of fibre mat where a gap is present (see figure ??). Mats may have
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in addition defects and broken fibres at the edge owing to the longitudinal cut. The width of thisnon-sensitive area is therefore not precisely defined.
Pieces of fibre mat
Gap between twoadjacent mats
Dead fibres from cutting
SiPM die gap(250 μm)
Gap between twoadjacent SiPMs
(400 μm)
Gap between twoadjacent SiPMs
(400 μm)
SiPM arrays
SiPM dies
Figure 22: Location of the main non-sensitive areas of the mats and the SiPMs of the fibre module.
On the other side, the optical surface of the fibre mats is not fully covered by photodetectors.These non-sensitive areas of the SiPMs are of three types:
Dead channel: Some SiPM channels are disconnected from the read-out electronics and deliverno signal. They induce a non-sensitive area of 250µm width.
Gap between SiPM dies: A 128-channel SiPM array is composed of two silicon dies comprising64 channels each. The two dies are aligned with each other on the same package with a gapof 250µm in-between.
Gap between two SiPMs: Two 128-channel SiPMs are aligned on the fibre mat with a total gapof 400µm between the sensitive areas. Half of these gaps are aligned on the gap betweenthe pieces of fibre mats (as shown in figure 22).
Injection of the beam in such areas enables to study in detail their impact on hit detectionefficiency. The efficiency in all non-sensitive areas where the beam was injected is shown in fig-ure 23. For each type of non-sensitive area, the efficiency profile is closely replicated. The effect ofa dead channel and of a gap between SiPM dies are similar because of their same width (figure 23aand 23b). The efficiency drops to approximately 30% in the centre of the area and is completelyrecovered 100µm away from the edge.
In the centre of 400µm gaps, the efficiency is 2% (figure 23c). Two different profiles are visible.First, for three of these gaps, which are the ones aligned at junction between pieces of fibre mats,the efficiency is 2% over 300µm and starts only being recovered 50µm from the edge of the SiPMgap. Second, for the last gap which is only an SiPMs gap and no mat gap, the efficiency increasesrapidly away from the centre.
The cluster thresholds can be adjusted in order to optimise the efficiency in the gaps. Figure 24shows the efficiency profile as a function of the thresholds for one example of each type of SiPMgap. In dead channels and die gaps, the efficiency integrated on the gap is 50% at 2.5/1.5/4.5 PE.
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Relative position [mm]-0.3 -0.2 -0.1 0 0.1 0.2 0.3
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(a) Dead channel (250µm).
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(b) Gap between two SiPM dies (250µm).
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(c) Gap between two SiPMs (400µm).
Figure 23: Efficiency in all non-sensitive areas where the beam was injected with cluster thresholdsof 2.5/1.5/4.5 PE. The hatched zones represent the areas not covered by photodetectors.
With the low thresholds, this grows to 75% whereas with the high thresholds, it drops to 30%. Inthe gap between SiPMs, the 2% efficiency present over a large portion of the gap remains with low
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thresholds. This demonstrates that in such gaps, only very large clusters are recovered.
Relative position [mm]-0.3 -0.2 -0.1 0 0.1 0.2 0.3
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cy
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1Efficiency in a dead channel for different cluster thresholds
Figure 24: Efficiency in three examples of non-sensitive area for different cluster thresholds. Thehatched zones represent the areas not covered by photodetectors.
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8 CONCLUSION
The hit resolution of the BGV SciFi module (4 fibre layers) depends on the cluster thresholds. Theoptimal thresholds are 2.5/1.5/4.5 PE and lead toσhit = 38.2±2.8µm andσhit = 42.9±3.9µm forthe RS and the TU side respectively. Excluding the non-sensitive areas in the SciFi module, the hitdetection efficiency is εhit = 98.3% and εhit = 97.8% at these thresholds and using a seed distanceof 1.25 mm. In the non-sensitive areas such as SiPM dead channels or die gaps (250µm), 50% ofthe clusters are recovered in the neighbouring channels. In larger non-sensitive areas such as gapsbetween two SiPMs (400µm), it drops to 10 to 20%.
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[3] G. Haefeli et al. The LHCb DAQ interface board TELL1, Nuclear Instruments and Methods inPhysics Research, 2006.
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[6] S. Giani, G. Haefeli, C. Joram, M. Tobin, Z. Xu. Digitisation of SiPM signals for the LHCb UpgradeSciFi tracker, CERN, Geneva, 2014.
[7] M. G. Bagliesi et al. A custom front-end ASIC for the readout and timing of 64 SiPM photosensors,Nuclear Physics B - Proceedings Supplements (Vol. 215, Issue1), 2011.
[8] G. Haefeli, A. Gong, L. Liang. USB readout board for SiPM and AMS2 Silicon Ladders in thecontext of the PEBS experiment, CERN, Geneva, 2009.
[9] L. An, O. Girard, G. Haefeli, A. Kuonen. Testbeam analysis for a scintillating fibre telescope, EPFL,Lausanne, 2016.
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