Mechanical stability of the CMS Tracker

measurements with the laser alignment system and particle tracks

Abstract

The CMS Tracker consists of about 210 m2 of silicon strip sensors assembled on various carbon fiber composite structures. The engineering design of the Tracker provides the mounting accuracy of tracker components of about 50 microns but allows some constrained movements. The mechanical stability of Tracker components during Tracker operation is monitored with a dedicated Laser Alignment System and using particle tracks from cosmic and collisions. The laser system monitors a relative movement of large substructures with an accuracy of a few microns in a few minutes interval while the alignment with tracks reconstructs the absolute position of individual detector modules with a similar accuracy but after days of data taking. During the long term operation at fixed temperature of $+4^{o}$C in years 2011--2013 the alignment of tracker components was stable within 10 microns. Temperature variations in the Tracker volume are found to cause the displacements of tracker structures of about 20 microns/$10^{o}$C that are recovered with the temperature restoration. The sufficient temperature measurements and the appropriate modelling of the temperature effects are important for the future upgrade of LHC detectors.

Results

The relative alignments and mechanical stability of the CMS Tracker has been monitored using the dedicated laser alignment system and particle tracks with an accuracy of a few microns. During operation at stable temperature of $+4^o$C in LHC run 1 the long term mechanical stability of the Tracker has been within 10 microns. All variations in alignment parameters are found to be related to temperature variations. For large scale structures treated as a rigid-body the displacements are in the order of 20 microns$10^o$C and are largely recoverable with temperature. \par The results of this study are important for operation in run 2 in colder conditions and for the following upgrades of the CMS Tracker. The operation of tracking detectors in varying temperature conditions requires not only careful choice of construction materials and mounting but also continuous monitoring of temperatures in important locations that can be used as input for the mechanical model.

Figures

Figure 1: CMS Tracker components. The Tracker Outer Barrel (TOB), Tracker End Cap Plus (TECP) and Minus (TECM) are supported inside Tracker Support Tube (TST) by two rails. The Tracker Inner Barrel (TIB) and Tracker Inner Disks (TID) are supported by the TOB. Red arrows show kinematic constraints of large structures in joints. Overall there are 15148 silicon strip detector modules of different designs with a total area of about 206 m2. The whole Tracker weight is about 2.2t that is distributed in four brackets connected to the CMS calorometers.

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Figure 2: Temperature in Silicon strip modules in different TOB, TIB layers and TEC, TID disks shown as a color code. The cooling plant was running at T=-5C. The white spots correspond to not operational detectors and the hot spots are the closed due to leaks cooling loops.

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Figure 3: Layout of Laser Alignment System. Eight laser beams inside alignment tubes are illuminating same detectors as used for tracking, overall 176 detectors. The beams connect TIB,TOB, TECP and TECM in global alignment of the large Tracker structures. The 32 laser beams are used for the internal alignment of TEC disks only, illuminate 288 detectors in TEC's.

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LAS reconstructs relative displacements of laser beam profile in each illuminated detector with respect to the reference position. These displacements are used to calculate variations in alignment parameters for large structures in the global fit procedure that uses TOB as a reference detector, LAS measures relative displacements of TIB and TEC'S with respect to TOB position in the reference time that can be defined at the beginning of stable operation. Because of strip orientations, LAS can reconstructs for the TIB Dx, Dy displacements and Rz, Rx, Ry rotations. For TEC's with the radial strips orientation, the LAS reconstructs the Dx, Dy displacements and the Rz rotations around z.

 

Figure 4: Examples of laser beam profiles (amplitude in ADC counts vs strip number) in TIB and TOB modules obtained in two different acquisition steps (green and blue lines). The dashed vertical lines show the reconstructed positions of beam center. The positions are reconstructed with an accuracy of about 2-4 microns using slopes at the halve amplitude.

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Laser beam profiles in TECP and TECM modules obtained in two different acquisition steps. For TECM the laser beam pass the largest distance and the profile suffers from diffraction.

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Figure 5: Longterm stability of TIB alignment parameters reconstructed with LAS during 2011-2013 datataking. Each point corresponds to the 5 min LAS acquisition interval. The error bars are the fit errors from the LAS alignment procedure. LAS data are available only when the Tracker is operational and included in the global data acquisition. The black line is the temperature of the cooling liquid in the return circuit, for the run1 T$_{r}\approx+6^{o}C$. The spikes in temperature correspond to the trips of cooling plants while small variations are the powerdown of the detector onboard electronics. All large variations in alignment parameters were associated to some temperature variations in the detector volume. During operation at stable temperature, the TIB parameters are within 10 microns.

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Figure 6: Longterm stability of TECP alignment parameters reconstructed with LAS during 2011-2013 datataking. During operation at stable temperature, the TECP parameters are within 15 microns.

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Figure 6: Longterm stability of TECM alignment parameters reconstructed with LAS during 2011-2013 datataking. During operation at stable temperature, the TECP parameters are within 20 microns.

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Figure 7: Variation of TIB alignment parameters after the power trip of the Tracker.The temperature of not powered Tracker is 10-15C lower, depending on position. The Tracker power was recovered after about1h and the LAS acquisition started about 20 min after the re-powering when the temperature in the Tracker is not stabilized yet. The observed displacement is about 10 microns. The amplitude of displacements depends upon the time delay between the actual temperature variations and the beginning of measurements.

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Figure 8: Variations of TIB alignment parameters after the trip of cooling plant that also caused the detectors power trip. The Tracker is warmed up for about 1h then cooling is restarted. After 2h the Tracker was powered and the LAS acquisition is started. These transient variations of alignment parameters related to relatively fast temperature variations can be observed only with LAS. Such periods are excluded for physics analysis.

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Figure 9: Stability of TIB alignment parameters during two weeks of continuous operation at fixed temperature of cooling plant at T=+4C. The spread of points for each parameter correspond to the LAS reconstraction accuracy.

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Figure 10: Stability of TECM alignment parameters at fixed temperature of cooling plant T=+4C with some longterm variations in Dy. .

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Figure 11: Longterm measurements of TIB alignment parameters with particle tracks using MILLEPEDE algorithms. The MILLEPEDE reconstruct the absolute displacements in the same parameters as used in LAS.

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Figure 12: Longterm measurements of TECP alignment parameters with particle tracks using MILLEPEDE algorithms.

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Figure 13: Longterm measurements of TECM alignment parameters with particle tracks using MILLEPEDE algorithms.

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Figure 14: Evolution of LAS TIB alignment parameters for T=+4 $\rightarrow$ -5 $\rightarrow$ -10 $\rightarrow$ +4C temperature transitions. The reference run is taken at 0C. The cosmic tracks were collected during the test and are used to calculate the alignment parameters with MILLEPEDE that are shown for the 0 $\rightarrow$ -10 transition(open polymarkers).

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Interpretation of the Tracker structures displacements during -10 $\rightarrow$ +4C transition  

-- ValeryZhukov - 2015-09-14

Topic attachments
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PDFpdf TECM_ATmp.pdf r1 manage 1199.2 K 2015-09-14 - 16:52 ValeryZhukov  
PDFpdf TECM_ATz1.pdf r1 manage 211.9 K 2015-09-14 - 16:49 ValeryZhukov  
PDFpdf TECP_ATmp.pdf r1 manage 1187.6 K 2015-09-14 - 16:52 ValeryZhukov  
PDFpdf TIB_AT_grin.pdf r1 manage 545.7 K 2015-09-14 - 16:53 ValeryZhukov  
PDFpdf TIB_ATmp.pdf r1 manage 1881.8 K 2015-09-14 - 16:52 ValeryZhukov  
PDFpdf TIB_ATz1.pdf r1 manage 600.7 K 2015-09-14 - 16:49 ValeryZhukov  
PDFpdf TIB_ATz2.pdf r1 manage 40.4 K 2015-09-14 - 16:49 ValeryZhukov  
PDFpdf TIB_ATz3com.pdf r1 manage 48.9 K 2015-09-14 - 16:49 ValeryZhukov  
PDFpdf laslayout.pdf r1 manage 59.9 K 2015-09-14 - 16:43 ValeryZhukov  
PDFpdf profTECM.pdf r1 manage 17.1 K 2015-09-14 - 16:45 ValeryZhukov  
PDFpdf profTECP.pdf r1 manage 14.9 K 2015-09-14 - 16:45 ValeryZhukov  
PDFpdf profTIB.pdf r1 manage 15.0 K 2015-09-14 - 16:45 ValeryZhukov  
PDFpdf profTOB.pdf r1 manage 14.9 K 2015-09-14 - 16:45 ValeryZhukov  
PDFpdf trackerlayoutz.pdf r1 manage 46.2 K 2015-09-14 - 16:43 ValeryZhukov  
PDFpdf tsil5m.pdf r1 manage 264.2 K 2015-09-14 - 16:43 ValeryZhukov  
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