bgv fibre module: test beam report
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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
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 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% and εhit = 97.8%.
BGV fibre module test beam results
CONTENTS
3.2 Beam profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4 Track alignment 8
4.2 Measurement of the tracks with the fibre telescope . . . . . . . . . . . . . . . . . . . . 8
4.3 Extrapolation of the track onto the BGV module . . . . . . . . . . . . . . . . . . . . . . 9
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
BGV fibre module test beam results
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 and profile 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 each four 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 to facilitate 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 the tracking 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 with Beetle ASIC [2] at the front-end and TELL1 data acquisition interface [3] at the back-end. Through the read-out chain, the analogue signals are deformed due to electronics limitation. Raw data produced by the detector undergoes a chain of signal corrections before clustering. The clusters are used for tracking and reconstruction of the LHC beam profile. The read-out and the corrections of 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 test beam at SPS in November 2015. The goal of the measurement campaign was to determine hit resolution and hit detection efficiency of the module. The particle tracks were reconstructed using a telescope based on short scintillating fibres with five tracking stations. This document presents the 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 and occupies channels 0 to 1023 whereas the plane with 2-rotated fibres is the TU side and goes from channel 1024 to 2047. The module was mounted on an X-Y moving table (see figure 2) allowing to inject the beam at different positions. The table presents however a drift in the vertical direction once its position is set. The drift is of the order of 100µm for the duration of the data acquisition runs. A correction for the drift is discussed in more details in section 4.4.
Figure 2: Scheme of the X-Y moving table on which the BGV module (zoom) and the LHCb
SciFi modules are installed.
Figure 3: Picture of the setup installed at the November test beam at CERN with the SciFi telescope placed upstream of the BGV SciFi
module.
The setup includes a telescope used to reconstruct the track of particles. It is made of five tracking stations with X and Y measurement. It was placed approximately 20 cm upstream of the BGV module. Figure 3 displays a picture of the setup. The telescope is read out with VATA64 and USBboard electronics [7, 8]. BGV and telescope events are synchronised which enables the measurement 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 they cross 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 crosses fibre mats with a total thickness of X/X0 ≈ 3%. The RMS of transverse deviations due to multiple scattering 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 deviations below 1µm between the two layers. Multiple scattering is therefore negligible for the foreseen measurements 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, a charged 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 a crossing 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 a weighted 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 channels with signal above a specified seed threshold. Second, the algorithm builds the clusters including one neighbouring 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
p lit
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 called the light yield.
• The mean cluster position is calculated as the average of the cluster channels weighted by their 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 in figure 6 for all data acquisition runs. In each run, the beam produces clusters in two regions of the module 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 each module side are the same because the fibres are almost parallel. Figure 7 shows the number of clusters per event detected by the module. The majority of the events contains one cluster on each side. Events with less clusters indicate inefficiencies. Some events contain many more clusters (up to 300). They originate from delta electrons emitted in the setup.
Cluster channel 0 500 1000 1500 2000
N um
be r
of E
nt rie
Cluster mean position
Figure 6: Cluster position for all data acquisition runs. Red vertical dashed lines
correspond to the limits between 128-channel SiPM arrays.
Mean 2.207
RMS 3.433
N um
0 5 20 200
Figure 7: Number of clusters per event. The majority of the events contain two clusters, one
each side. Some events contain a very large number 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 injection along 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 runs from 13 and 18 PE demonstrates that the quality of the fibre mat, the mirror glueing or the SiPM alignment 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. The RS and TU fibre mats are not exactly the same. On the TU side, the optical cut of the fibres is not perpendicular 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
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
Run 8 Run 9
Run 10 Run 11
Run 12 Run 13
Run 14 Run 15
(a) RS side.
Cluster sum per channel [p.e.] 0 5 10 15 20 25 30 35 40
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
Run 8 Run 9
Run 10 Run 11
Run 12 Run 13
Run 14 Run 15
(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 an impact 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
0
10000
20000
30000
40000
50000
60000
70000
80000
Cluster sum superimposed for all runs
Figure 9: Cluster sum for all runs. The light yield is 15.6 and 14.6 PE for the RS and TU
side respectively.
Light yield [PE] 12.5 13 13.5 14 14.5 15 15.5 16 16.5 17 17.5
C lu
st er
s iz
Cluster size vs light yield
Figure 10: Correlation between the mean cluster size and the light yield. No significant difference is observed between the two sides.
When the light yield is increased, the signal in the neighbouring channels is also increased and the probability that it exceeds the threshold is higher. On average, the cluster size is therefore positively correlated with the light yield. Figure 10 shows that the correlation between the mean cluster 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 between the 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 test and the impact point of the track which is provided by another independent device. Over many events, 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 their measurement. The residual distribution is therefore a Gaussian with sigma:
σresidual = Ç
σ2 hit +σ
2 track (1)
In practice, the residual is the only parameter that can be measured. The track resolution σtrack must 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 detailed analysis of the performance of the telescope is the subject of a separate document [9]. The layers of the telescope are mechanically aligned with respect to each other to a level of 100µm. An offline 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 found when σresidual of all layers has reached a minimum value. In order to ensure the convergence, the algorithm 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 is displayed in figure 11. The central peak follows accurately a Gaussian function. However, tails are present on the sides. They arise from multiple scattering. The distribution is fitted using a Crystal Ball 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
-4 -2 0 2 4
residual -0.4 -0.2 0 0.2 0.4
E ve
nt s
/ndf = 6.219312χ
Figure 11: Residual distribution of the central plane of the telescope fitted with a Crystal Ball function.
The residual in the central layer of the telescope is σresidual = 36.8µm. In an ideal case where all layers have the same hit resolution σhit and are equally spaced, one can calculate analytically the contribution of σhit and σtrack to σresidual [10]. The result is σtrack = 16.5µm in the centre of the telescope and σhit = 32.9µm.
The actual telescope has however not this ideal configuration. First, the layers are not equally spaced (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 the telescope.
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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 the impact point of the track is estimated to be:
RS side: σtrack = 44.2± 0.8µm TU side: σtrack = 40.5± 0.8µm
The impact point is calculated in the coordinate system of the telescope. It must be converted into the coordinate system of the BGV module in order to measure the residual. The origin of the coordinate systems are shifted in space with ~r = (Xoffset, Yoffset, Zoffset) and their axis are rotated with small angles αX, αY, αZ. An offline procedure adjusts these parameters in order to align the coordinate systems to a sub-micrometre level. The procedure runs iteratively on each parameter and finds the optimal value by minimising σresidual. Examples of this alignment procedure for the parameters Zoffset and αZ are displayed in figure 12.
[mm]offsetZ 100 200 300 400 500 600
S ig
m a
of r
es id
ua l [
m m
offset Alignment of the distance Z
(a) Dependence of σresidual on the alignement parameter Zoffset 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
S ig
m a
of r
es id
ua l [
m m
ZαAlignment of the angle
(b) Dependence of σresidual on the alignement parameter αZ which corresponds to the angle between the Z-axis of the telescope and the BGV module coordinate systems.
Figure 12: Illustration of the procedure used to align the telescope and the BGV module coordinate systems. 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 telescope was observed. This results in a wide residual distribution with a non-gaussian central peak. The residual as a function of the event number is shown in figure 13a. For a small number of events, the residual 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 drift is measured with the mean of the Gaussian function and is seen to be non-linear. Discontinuities in 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 for the measurement of resolution.
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Event number 0 50 100 150 200 250 310×
R es
id ua
l [ m
Drift before correction
(a) Before correction. A drift of approximately 0.2 mm is visible. The discontinuity arises from a short interval dur- ing which the beam was interrupted and the table contin- ued to drift.
Event number 0 50 100 150 200 250 310×
R es
id ua
l [ m
(b) After correction. The residual is constantly centered around zero.
Figure 13: Residual as a function of event number for one of the data acquisition run. The residual distributions 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 as reference for the measurement of resolution. Averaged over all runs, σresidual is:
RS side: σresidual = 58.4± 1.2µm TU side: σresidual = 59.0± 2.3µm
where the uncertainty includes the standard deviation over all runs and the errors of the fit. Using the estimated track resolution σtrack from section 4.3, the hit resolution σhit for each side is:
RS side: σhit = 38.2± 2.8µm TU side: σhit = 42.9± 3.9µm
The hit resolution of the two sides of the module is not equal. It is approximately 12% worse for the TU side. This arises from the optical cut of the fibres which is tilted with a 2-angle for this side. In figure 14, it is also seen that the residual distribution comprises tails suppressed by a factor of 100 compared to the central Gaussian. They can be interpreted as large angle multiple scattering. Furthermore, the tails are not symmetric which is due to an insufficient correction of raw data [4].
In figure 10, it was noticed that one run has an unexpectedly large average cluster size (2.94 versus 2.60 - 2.70) due to a non-perfect mirror glueing or SiPM positioning. As a consequence, the residual is also seen to be larger than for the other runs. σresidual is 67.5µm which corresponds to σhit = 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 optimal thresholds are 2.5/1.5/4.5 PE. With higher thresholds, the residual increases because signal in neighbours 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.
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Residual [mm] -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2-10
-5
0
5
10 Pull distribution
Residual [mm] -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4
E ve
nt s
-5
0
5
10 Pull distribution
Residual [mm] -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4
E ve
nt s
(b) TU side.
Figure 14: Residual distribution for one of the runs with cluster thresholds of 2.5/1.5/4.5 PE, fitted with a Crystal Ball function.
Seed threshold [PE] 1.5 2 2.5 3 3.5 4 4.5
m ]
µ [
Module side
RS TU
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 point than the seed distance.
The hit detection efficiency εhit is calculated as the ratio between the number efficient events and the total number of events. It strongly depends on the seed distance, on the cluster thresholds and on the light yield.
Figures 16 and 17 show the dependence on the seed distance with fixed seed/neighbour/sum thresholds of 2.5/1.5/4.5 PE. The efficiency is relatively flat in all the beam region except at the position of non-sensitive areas (discussed in more details in section 7). With a Gaussian residual distribution of sigma σresidual, the expected efficiency at a seed distance of 1 ·σresidual is 68% which is consistent with the measurement. However, at increased seed distance, figure 17 shows that the efficiency 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 to 1.25 mm (≈ 20 ·σresidual) in the following.
BGV x position [mm] -204 -202 -200 -198 -196 -194 -192
E ffi
ci en
Efficiency (RS side)
Figure 16: Hit detection efficiency in the beam region for different seed distances. The
purple (blue) hatched zone is the exact position of a dead channel (gap).
Seed distance [mm] -110 1 10
E ffi
ci en
Efficiency for different seed distances
Figure 17: Average hit detection efficiency as a function of seed distance. The average
excludes the non-efficient regions such as gaps or dead channels.
The hit detection efficiency as a function of cluster thresholds is shown in figure 18 and 19 for one run. For the highest thresholds (4.5/3.5/6.5 PE) the efficiency is in the order of 90% whereas as it reaches approximately 99% for the lowest thresholds (1.5/0.5/2.5 PE). At 2.5/1.5/4.5 PE, in average 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
E ffi
ci en
Cluster thresholds [p.e.]
1.5 / 0.5 / 2.5
2.0 / 1.0 / 4.0
2.5 / 1.5 / 4.5
3.0 / 2.0 / 5.0
3.5 / 2.5 / 5.5
4.0 / 3.0 / 6.0
4.5 / 3.5 / 6.5
Efficiency (RS side)
Figure 18: Hit detection efficiency in the beam region for different cluster thresholds. The purple (blue) hatched zone is the exact
position of a dead channel (gap).
Seed threshold [PE] 1.5 2 2.5 3 3.5 4 4.5
E ffi
ci en
Efficiency for different seed thresholds
Figure 19: Average hit detection efficiency as a function of seed threshold. The average
excludes the non-efficient regions such as gaps or dead channels.
Figure 20 presents the efficiency as a function of light yield for different cluster thresholds. For each 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 partially due to the lower light yield. However, figure 21 demonstrates that, at the same light yield, the efficiency is smaller on the TU side than on the RS one. The tilted optical cut of the fibre mat is therefore responsible for a small reduction in hit detection efficiency (between 0.2 and 0.5%).
Light yield [PE] 10 11 12 13 14 15 16 17 18
E ffi
ci en
cy
0.86
0.88
0.9
0.92
0.94
0.96
0.98
1
Cluster thresholds [p.e.] 1.5 / 0.5 / 2.5 2.0 / 1.0 / 4.0 2.5 / 1.5 / 4.5 3.0 / 2.0 / 5.0 3.5 / 2.5 / 5.5 4.0 / 3.0 / 6.0 4.5 / 3.5 / 6.5
Efficiency as a function of light yield (RS side)
(a) RS side.
Light yield [PE] 10 11 12 13 14 15 16 17 18
E ffi
ci en
cy 0.86
0.88
0.9
0.92
0.94
0.96
0.98
1
Cluster thresholds [p.e.] 1.5 / 0.5 / 2.5 2.0 / 1.0 / 4.0 2.5 / 1.5 / 4.5 3.0 / 2.0 / 5.0 3.5 / 2.5 / 5.5 4.0 / 3.0 / 6.0 4.5 / 3.5 / 6.5
Efficiency as a function of light yield (TU side)
(b) TU side.
Figure 20: Efficiency as a function of light yield for different cluster thresholds.
Light yield [PE] 13 13.5 14 14.5 15 15.5 16 16.5 17
E ffi
ci en
Comparison of the efficiency of the two layers
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 of 6.5 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 schematically in 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 at the 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 this non-sensitive area is therefore not precisely defined.
Pieces of fibre mat
Dead fibres from cutting
(400 μm)
(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 deliver no 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 comprising 64 channels each. The two dies are aligned with each other on the same package with a gap of 250µm in-between.
Gap between two SiPMs: Two 128-channel SiPMs are aligned on the fibre mat with a total gap of 400µm between the sensitive areas. Half of these gaps are aligned on the gap between the 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 detection efficiency. 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 of a dead channel and of a gap between SiPM dies are similar because of their same width (figure 23a and 23b). The efficiency drops to approximately 30% in the centre of the area and is completely recovered 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 SiPM gap. Second, for the last gap which is only an SiPMs gap and no mat gap, the efficiency increases rapidly away from the centre.
The cluster thresholds can be adjusted in order to optimise the efficiency in the gaps. Figure 24 shows the efficiency profile as a function of the thresholds for one example of each type of SiPM gap. 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
E ffi
ci en
(a) Dead channel (250µm).
Relative position [mm] -0.3 -0.2 -0.1 0 0.1 0.2 0.3
E ffi
ci en
(b) Gap between two SiPM dies (250µm).
Relative position [mm] -0.3 -0.2 -0.1 0 0.1 0.2 0.3
E ffi
ci en
(c) Gap between two SiPMs (400µm).
Figure 23: Efficiency in all non-sensitive areas where the beam was injected with cluster thresholds of 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%. In the 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
E ffi
ci en
1 Efficiency in a dead channel for different cluster thresholds
Cluster thresholds [PE] 1.5 / 0.5 / 2.5 2.0 / 1.0 / 4.0 2.5 / 1.5 / 4.5 3.0 / 2.0 / 5.0 3.5 / 2.5 / 5.5 4.0 / 3.0 / 6.0 4.5 / 3.5 / 6.5
(a) Dead channel (250µm).
Relative position [mm] -0.3 -0.2 -0.1 0 0.1 0.2 0.3
E ffi
ci en
cy
0
0.1
0.2
0.3
0.4
0.5
0.6
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0.9
1 Efficiency in a gap between SiPM dies for different cluster thresholds
Cluster thresholds [PE] 1.5 / 0.5 / 2.5 2.0 / 1.0 / 4.0 2.5 / 1.5 / 4.5 3.0 / 2.0 / 5.0 3.5 / 2.5 / 5.5 4.0 / 3.0 / 6.0 4.5 / 3.5 / 6.5
(b) Gap between two SiPM dies (250µm).
Relative position [mm] -0.3 -0.2 -0.1 0 0.1 0.2 0.3
E ffi
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cy
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0.1
0.2
0.3
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1 Efficiency in a gap between SiPMs for different cluster thresholds
Cluster thresholds [PE] 1.5 / 0.5 / 2.5 2.0 / 1.0 / 4.0 2.5 / 1.5 / 4.5 3.0 / 2.0 / 5.0 3.5 / 2.5 / 5.5 4.0 / 3.0 / 6.0 4.5 / 3.5 / 6.5
(c) Gap between two SiPMs (400µm).
Figure 24: Efficiency in three examples of non-sensitive area for different cluster thresholds. The hatched zones represent the areas not covered by photodetectors.
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BGV fibre module test beam results
8 CONCLUSION
The hit resolution of the BGV SciFi module (4 fibre layers) depends on the cluster thresholds. The optimal thresholds are 2.5/1.5/4.5 PE and lead toσhit = 38.2±2.8µm andσhit = 42.9±3.9µm for the RS and the TU side respectively. Excluding the non-sensitive areas in the SciFi module, the hit detection efficiency is εhit = 98.3% and εhit = 97.8% at these thresholds and using a seed distance of 1.25 mm. In the non-sensitive areas such as SiPM dead channels or die gaps (250µm), 50% of the clusters are recovered in the neighbouring channels. In larger non-sensitive areas such as gaps between two SiPMs (400µm), it drops to 10 to 20%.
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REFERENCES
[1] P. Hopchev et al. A Beam Gas Vertex detector for beam size measurement in the LHC, CERN, Geneva, 2014.
[2] S. Löchner, M. Schmelling. The Beetle Reference Manual, Max-Planck-Institute for Nuclear Physics, Heidelberg, Germany, 2006.
[3] G. Haefeli et al. The LHCb DAQ interface board TELL1, Nuclear Instruments and Methods in Physics Research, 2006.
[4] O. Girard, L. An, A. Kuonen, H. Li, G. Haefeli. BGV fibre module read-out, signal correction and clustering, EPFL, Lausanne, 2016.
[5] LHCb Collaboration. LHCb Tracker Upgrade Technical Design Report, CERN, Geneva, 2014.
[6] S. Giani, G. Haefeli, C. Joram, M. Tobin, Z. Xu. Digitisation of SiPM signals for the LHCb Upgrade SciFi 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 the context 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.
[10] F. Ragusa. An Introduction to Charged Particles Tracking, Italo-Hellenic School of Physics, Lecce, 2006.
[11] P. Berclaz. Analysis of particle telescope, EPFL, Lausanne, 2015.
O. Girard, L. An, A. Kuonen, G. Haefeli 18
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
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 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% and εhit = 97.8%.
BGV fibre module test beam results
CONTENTS
3.2 Beam profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4 Track alignment 8
4.2 Measurement of the tracks with the fibre telescope . . . . . . . . . . . . . . . . . . . . 8
4.3 Extrapolation of the track onto the BGV module . . . . . . . . . . . . . . . . . . . . . . 9
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
BGV fibre module test beam results
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 and profile 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 each four 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 to facilitate 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 the tracking 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 with Beetle ASIC [2] at the front-end and TELL1 data acquisition interface [3] at the back-end. Through the read-out chain, the analogue signals are deformed due to electronics limitation. Raw data produced by the detector undergoes a chain of signal corrections before clustering. The clusters are used for tracking and reconstruction of the LHC beam profile. The read-out and the corrections of 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 test beam at SPS in November 2015. The goal of the measurement campaign was to determine hit resolution and hit detection efficiency of the module. The particle tracks were reconstructed using a telescope based on short scintillating fibres with five tracking stations. This document presents the 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 and occupies channels 0 to 1023 whereas the plane with 2-rotated fibres is the TU side and goes from channel 1024 to 2047. The module was mounted on an X-Y moving table (see figure 2) allowing to inject the beam at different positions. The table presents however a drift in the vertical direction once its position is set. The drift is of the order of 100µm for the duration of the data acquisition runs. A correction for the drift is discussed in more details in section 4.4.
Figure 2: Scheme of the X-Y moving table on which the BGV module (zoom) and the LHCb
SciFi modules are installed.
Figure 3: Picture of the setup installed at the November test beam at CERN with the SciFi telescope placed upstream of the BGV SciFi
module.
The setup includes a telescope used to reconstruct the track of particles. It is made of five tracking stations with X and Y measurement. It was placed approximately 20 cm upstream of the BGV module. Figure 3 displays a picture of the setup. The telescope is read out with VATA64 and USBboard electronics [7, 8]. BGV and telescope events are synchronised which enables the measurement 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 they cross 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 crosses fibre mats with a total thickness of X/X0 ≈ 3%. The RMS of transverse deviations due to multiple scattering 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 deviations below 1µm between the two layers. Multiple scattering is therefore negligible for the foreseen measurements 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, a charged 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
O. Girard, L. An, A. Kuonen, G. Haefeli 4
BGV fibre module test beam results
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 a crossing 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 a weighted 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 channels with signal above a specified seed threshold. Second, the algorithm builds the clusters including one neighbouring 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
p lit
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 called the light yield.
• The mean cluster position is calculated as the average of the cluster channels weighted by their 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 in figure 6 for all data acquisition runs. In each run, the beam produces clusters in two regions of the module 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 each module side are the same because the fibres are almost parallel. Figure 7 shows the number of clusters per event detected by the module. The majority of the events contains one cluster on each side. Events with less clusters indicate inefficiencies. Some events contain many more clusters (up to 300). They originate from delta electrons emitted in the setup.
Cluster channel 0 500 1000 1500 2000
N um
be r
of E
nt rie
Cluster mean position
Figure 6: Cluster position for all data acquisition runs. Red vertical dashed lines
correspond to the limits between 128-channel SiPM arrays.
Mean 2.207
RMS 3.433
N um
0 5 20 200
Figure 7: Number of clusters per event. The majority of the events contain two clusters, one
each side. Some events contain a very large number 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 injection along 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 runs from 13 and 18 PE demonstrates that the quality of the fibre mat, the mirror glueing or the SiPM alignment 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. The RS and TU fibre mats are not exactly the same. On the TU side, the optical cut of the fibres is not perpendicular 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
0
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0.07
Run 8 Run 9
Run 10 Run 11
Run 12 Run 13
Run 14 Run 15
(a) RS side.
Cluster sum per channel [p.e.] 0 5 10 15 20 25 30 35 40
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
Run 8 Run 9
Run 10 Run 11
Run 12 Run 13
Run 14 Run 15
(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 an impact 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
0
10000
20000
30000
40000
50000
60000
70000
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Cluster sum superimposed for all runs
Figure 9: Cluster sum for all runs. The light yield is 15.6 and 14.6 PE for the RS and TU
side respectively.
Light yield [PE] 12.5 13 13.5 14 14.5 15 15.5 16 16.5 17 17.5
C lu
st er
s iz
Cluster size vs light yield
Figure 10: Correlation between the mean cluster size and the light yield. No significant difference is observed between the two sides.
When the light yield is increased, the signal in the neighbouring channels is also increased and the probability that it exceeds the threshold is higher. On average, the cluster size is therefore positively correlated with the light yield. Figure 10 shows that the correlation between the mean cluster 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 between the 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 test and the impact point of the track which is provided by another independent device. Over many events, 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 their measurement. The residual distribution is therefore a Gaussian with sigma:
σresidual = Ç
σ2 hit +σ
2 track (1)
In practice, the residual is the only parameter that can be measured. The track resolution σtrack must 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 detailed analysis of the performance of the telescope is the subject of a separate document [9]. The layers of the telescope are mechanically aligned with respect to each other to a level of 100µm. An offline 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 found when σresidual of all layers has reached a minimum value. In order to ensure the convergence, the algorithm 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 is displayed in figure 11. The central peak follows accurately a Gaussian function. However, tails are present on the sides. They arise from multiple scattering. The distribution is fitted using a Crystal Ball 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
-4 -2 0 2 4
residual -0.4 -0.2 0 0.2 0.4
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nt s
/ndf = 6.219312χ
Figure 11: Residual distribution of the central plane of the telescope fitted with a Crystal Ball function.
The residual in the central layer of the telescope is σresidual = 36.8µm. In an ideal case where all layers have the same hit resolution σhit and are equally spaced, one can calculate analytically the contribution of σhit and σtrack to σresidual [10]. The result is σtrack = 16.5µm in the centre of the telescope and σhit = 32.9µm.
The actual telescope has however not this ideal configuration. First, the layers are not equally spaced (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 the telescope.
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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 the impact point of the track is estimated to be:
RS side: σtrack = 44.2± 0.8µm TU side: σtrack = 40.5± 0.8µm
The impact point is calculated in the coordinate system of the telescope. It must be converted into the coordinate system of the BGV module in order to measure the residual. The origin of the coordinate systems are shifted in space with ~r = (Xoffset, Yoffset, Zoffset) and their axis are rotated with small angles αX, αY, αZ. An offline procedure adjusts these parameters in order to align the coordinate systems to a sub-micrometre level. The procedure runs iteratively on each parameter and finds the optimal value by minimising σresidual. Examples of this alignment procedure for the parameters Zoffset and αZ are displayed in figure 12.
[mm]offsetZ 100 200 300 400 500 600
S ig
m a
of r
es id
ua l [
m m
offset Alignment of the distance Z
(a) Dependence of σresidual on the alignement parameter Zoffset 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
S ig
m a
of r
es id
ua l [
m m
ZαAlignment of the angle
(b) Dependence of σresidual on the alignement parameter αZ which corresponds to the angle between the Z-axis of the telescope and the BGV module coordinate systems.
Figure 12: Illustration of the procedure used to align the telescope and the BGV module coordinate systems. 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 telescope was observed. This results in a wide residual distribution with a non-gaussian central peak. The residual as a function of the event number is shown in figure 13a. For a small number of events, the residual 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 drift is measured with the mean of the Gaussian function and is seen to be non-linear. Discontinuities in 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 for the measurement of resolution.
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Event number 0 50 100 150 200 250 310×
R es
id ua
l [ m
Drift before correction
(a) Before correction. A drift of approximately 0.2 mm is visible. The discontinuity arises from a short interval dur- ing which the beam was interrupted and the table contin- ued to drift.
Event number 0 50 100 150 200 250 310×
R es
id ua
l [ m
(b) After correction. The residual is constantly centered around zero.
Figure 13: Residual as a function of event number for one of the data acquisition run. The residual distributions 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 as reference for the measurement of resolution. Averaged over all runs, σresidual is:
RS side: σresidual = 58.4± 1.2µm TU side: σresidual = 59.0± 2.3µm
where the uncertainty includes the standard deviation over all runs and the errors of the fit. Using the estimated track resolution σtrack from section 4.3, the hit resolution σhit for each side is:
RS side: σhit = 38.2± 2.8µm TU side: σhit = 42.9± 3.9µm
The hit resolution of the two sides of the module is not equal. It is approximately 12% worse for the TU side. This arises from the optical cut of the fibres which is tilted with a 2-angle for this side. In figure 14, it is also seen that the residual distribution comprises tails suppressed by a factor of 100 compared to the central Gaussian. They can be interpreted as large angle multiple scattering. Furthermore, the tails are not symmetric which is due to an insufficient correction of raw data [4].
In figure 10, it was noticed that one run has an unexpectedly large average cluster size (2.94 versus 2.60 - 2.70) due to a non-perfect mirror glueing or SiPM positioning. As a consequence, the residual is also seen to be larger than for the other runs. σresidual is 67.5µm which corresponds to σhit = 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 optimal thresholds are 2.5/1.5/4.5 PE. With higher thresholds, the residual increases because signal in neighbours 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.
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Residual [mm] -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2-10
-5
0
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10 Pull distribution
Residual [mm] -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4
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10 Pull distribution
Residual [mm] -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4
E ve
nt s
(b) TU side.
Figure 14: Residual distribution for one of the runs with cluster thresholds of 2.5/1.5/4.5 PE, fitted with a Crystal Ball function.
Seed threshold [PE] 1.5 2 2.5 3 3.5 4 4.5
m ]
µ [
Module side
RS TU
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 point than the seed distance.
The hit detection efficiency εhit is calculated as the ratio between the number efficient events and the total number of events. It strongly depends on the seed distance, on the cluster thresholds and on the light yield.
Figures 16 and 17 show the dependence on the seed distance with fixed seed/neighbour/sum thresholds of 2.5/1.5/4.5 PE. The efficiency is relatively flat in all the beam region except at the position of non-sensitive areas (discussed in more details in section 7). With a Gaussian residual distribution of sigma σresidual, the expected efficiency at a seed distance of 1 ·σresidual is 68% which is consistent with the measurement. However, at increased seed distance, figure 17 shows that the efficiency 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 to 1.25 mm (≈ 20 ·σresidual) in the following.
BGV x position [mm] -204 -202 -200 -198 -196 -194 -192
E ffi
ci en
Efficiency (RS side)
Figure 16: Hit detection efficiency in the beam region for different seed distances. The
purple (blue) hatched zone is the exact position of a dead channel (gap).
Seed distance [mm] -110 1 10
E ffi
ci en
Efficiency for different seed distances
Figure 17: Average hit detection efficiency as a function of seed distance. The average
excludes the non-efficient regions such as gaps or dead channels.
The hit detection efficiency as a function of cluster thresholds is shown in figure 18 and 19 for one run. For the highest thresholds (4.5/3.5/6.5 PE) the efficiency is in the order of 90% whereas as it reaches approximately 99% for the lowest thresholds (1.5/0.5/2.5 PE). At 2.5/1.5/4.5 PE, in average 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
E ffi
ci en
Cluster thresholds [p.e.]
1.5 / 0.5 / 2.5
2.0 / 1.0 / 4.0
2.5 / 1.5 / 4.5
3.0 / 2.0 / 5.0
3.5 / 2.5 / 5.5
4.0 / 3.0 / 6.0
4.5 / 3.5 / 6.5
Efficiency (RS side)
Figure 18: Hit detection efficiency in the beam region for different cluster thresholds. The purple (blue) hatched zone is the exact
position of a dead channel (gap).
Seed threshold [PE] 1.5 2 2.5 3 3.5 4 4.5
E ffi
ci en
Efficiency for different seed thresholds
Figure 19: Average hit detection efficiency as a function of seed threshold. The average
excludes the non-efficient regions such as gaps or dead channels.
Figure 20 presents the efficiency as a function of light yield for different cluster thresholds. For each 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 partially due to the lower light yield. However, figure 21 demonstrates that, at the same light yield, the efficiency is smaller on the TU side than on the RS one. The tilted optical cut of the fibre mat is therefore responsible for a small reduction in hit detection efficiency (between 0.2 and 0.5%).
Light yield [PE] 10 11 12 13 14 15 16 17 18
E ffi
ci en
cy
0.86
0.88
0.9
0.92
0.94
0.96
0.98
1
Cluster thresholds [p.e.] 1.5 / 0.5 / 2.5 2.0 / 1.0 / 4.0 2.5 / 1.5 / 4.5 3.0 / 2.0 / 5.0 3.5 / 2.5 / 5.5 4.0 / 3.0 / 6.0 4.5 / 3.5 / 6.5
Efficiency as a function of light yield (RS side)
(a) RS side.
Light yield [PE] 10 11 12 13 14 15 16 17 18
E ffi
ci en
cy 0.86
0.88
0.9
0.92
0.94
0.96
0.98
1
Cluster thresholds [p.e.] 1.5 / 0.5 / 2.5 2.0 / 1.0 / 4.0 2.5 / 1.5 / 4.5 3.0 / 2.0 / 5.0 3.5 / 2.5 / 5.5 4.0 / 3.0 / 6.0 4.5 / 3.5 / 6.5
Efficiency as a function of light yield (TU side)
(b) TU side.
Figure 20: Efficiency as a function of light yield for different cluster thresholds.
Light yield [PE] 13 13.5 14 14.5 15 15.5 16 16.5 17
E ffi
ci en
Comparison of the efficiency of the two layers
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 of 6.5 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 schematically in 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 at the 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 this non-sensitive area is therefore not precisely defined.
Pieces of fibre mat
Dead fibres from cutting
(400 μm)
(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 deliver no 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 comprising 64 channels each. The two dies are aligned with each other on the same package with a gap of 250µm in-between.
Gap between two SiPMs: Two 128-channel SiPMs are aligned on the fibre mat with a total gap of 400µm between the sensitive areas. Half of these gaps are aligned on the gap between the 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 detection efficiency. 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 of a dead channel and of a gap between SiPM dies are similar because of their same width (figure 23a and 23b). The efficiency drops to approximately 30% in the centre of the area and is completely recovered 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 SiPM gap. Second, for the last gap which is only an SiPMs gap and no mat gap, the efficiency increases rapidly away from the centre.
The cluster thresholds can be adjusted in order to optimise the efficiency in the gaps. Figure 24 shows the efficiency profile as a function of the thresholds for one example of each type of SiPM gap. 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
E ffi
ci en
(a) Dead channel (250µm).
Relative position [mm] -0.3 -0.2 -0.1 0 0.1 0.2 0.3
E ffi
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(b) Gap between two SiPM dies (250µm).
Relative position [mm] -0.3 -0.2 -0.1 0 0.1 0.2 0.3
E ffi
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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 thresholds of 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%. In the gap between SiPMs, the 2% efficiency present over a large portion of the gap remains with low
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BGV fibre module test beam results
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
E ffi
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1 Efficiency in a dead channel for different cluster thresholds
Cluster thresholds [PE] 1.5 / 0.5 / 2.5 2.0 / 1.0 / 4.0 2.5 / 1.5 / 4.5 3.0 / 2.0 / 5.0 3.5 / 2.5 / 5.5 4.0 / 3.0 / 6.0 4.5 / 3.5 / 6.5
(a) Dead channel (250µm).
Relative position [mm] -0.3 -0.2 -0.1 0 0.1 0.2 0.3
E ffi
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1 Efficiency in a gap between SiPM dies for different cluster thresholds
Cluster thresholds [PE] 1.5 / 0.5 / 2.5 2.0 / 1.0 / 4.0 2.5 / 1.5 / 4.5 3.0 / 2.0 / 5.0 3.5 / 2.5 / 5.5 4.0 / 3.0 / 6.0 4.5 / 3.5 / 6.5
(b) Gap between two SiPM dies (250µm).
Relative position [mm] -0.3 -0.2 -0.1 0 0.1 0.2 0.3
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1 Efficiency in a gap between SiPMs for different cluster thresholds
Cluster thresholds [PE] 1.5 / 0.5 / 2.5 2.0 / 1.0 / 4.0 2.5 / 1.5 / 4.5 3.0 / 2.0 / 5.0 3.5 / 2.5 / 5.5 4.0 / 3.0 / 6.0 4.5 / 3.5 / 6.5
(c) Gap between two SiPMs (400µm).
Figure 24: Efficiency in three examples of non-sensitive area for different cluster thresholds. The hatched zones represent the areas not covered by photodetectors.
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BGV fibre module test beam results
8 CONCLUSION
The hit resolution of the BGV SciFi module (4 fibre layers) depends on the cluster thresholds. The optimal thresholds are 2.5/1.5/4.5 PE and lead toσhit = 38.2±2.8µm andσhit = 42.9±3.9µm for the RS and the TU side respectively. Excluding the non-sensitive areas in the SciFi module, the hit detection efficiency is εhit = 98.3% and εhit = 97.8% at these thresholds and using a seed distance of 1.25 mm. In the non-sensitive areas such as SiPM dead channels or die gaps (250µm), 50% of the clusters are recovered in the neighbouring channels. In larger non-sensitive areas such as gaps between two SiPMs (400µm), it drops to 10 to 20%.
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REFERENCES
[1] P. Hopchev et al. A Beam Gas Vertex detector for beam size measurement in the LHC, CERN, Geneva, 2014.
[2] S. Löchner, M. Schmelling. The Beetle Reference Manual, Max-Planck-Institute for Nuclear Physics, Heidelberg, Germany, 2006.
[3] G. Haefeli et al. The LHCb DAQ interface board TELL1, Nuclear Instruments and Methods in Physics Research, 2006.
[4] O. Girard, L. An, A. Kuonen, H. Li, G. Haefeli. BGV fibre module read-out, signal correction and clustering, EPFL, Lausanne, 2016.
[5] LHCb Collaboration. LHCb Tracker Upgrade Technical Design Report, CERN, Geneva, 2014.
[6] S. Giani, G. Haefeli, C. Joram, M. Tobin, Z. Xu. Digitisation of SiPM signals for the LHCb Upgrade SciFi 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 the context 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.
[10] F. Ragusa. An Introduction to Charged Particles Tracking, Italo-Hellenic School of Physics, Lecce, 2006.
[11] P. Berclaz. Analysis of particle telescope, EPFL, Lausanne, 2015.
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