I. Physikalisches Institut, RWTH-Aachen, Germany. J. VANDENHIRTZ.

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1 THE AMS INFRARED TRACKER ALIGNMENT SYSTEM - FROM STS91 TO ISS W. WALLRAFF AND V. VETTERLE I. Physikalisches Institut, RWTH-Aachen, Germany wallraff@physik.rwth-aachen.de J. VANDENHIRTZ LemnaTec GmbH, Schumanstraße 18 Würselen D52146 Germany joerg@lemnatec.de We report on AMS tracker alignment control in space using artificial laser produced straight tracks (flight data AMS-1, laboratory tests AMS-2) as well as precisely measured high momentum cosmics tracks. 1 AMS experiment 1.1 AMS-1 The large acceptance Antimatter Spectrometer (AMS) experiment 1 2 has been operated successfully on the NASA STS91 shuttle flight (2-June June-98, AMS-1). It will be redeployed, including major upgrades, for a 1 day data taking mission (AMS-2) on the International Space Station late in 24; see talk by R. Battiston at this conference. 1.2 AMS Si-tracker Tracker Alignment System TAS AMS particle tracking is based on 8/6 (AMS-2/1) planes of double-sided Si detectors providing a maximum detectable rigidity (MDR) of 3(5) GV by measuring the sagitta of the tracks in a.9 T superconducting (.12 T permanent NdFeB) magnet. The sagitta can be determined with an accuracy of 22(25) μm. In AMS the position stability of the tracking elements is controlled using nearly straight tracks. Fig. 1a shows the laser beams and their measured profiles (recorded in space and transmitted to ground on June 4th 1998) in the AMS-1 configuration. From an analysis of the residuals for > 4 GV tracks individual ladder displacements have been derived 34 (for principle see fig. 1b, results fig. 4). TAS_Como_21.v1a: submitted to World Scientific on November 3, 211

2 AMS Laser & Cosmics alignment Plane 1 Si ladders Plane 6 [cm] 6 transparent Si with SiN x anti reflective coating 4 2 B IR Laser Beams simultaneous 2D IR beam profile measurements with double sided Si detectors (x-pitch 28 µm, y-pitch 11 µm) STS91 data IR Laser delivery system 5 m observed hit in plane 5 hit derived from fit using all but plane 5 Laser beam B plane 1 stiff particle track plane 2 plane 3 plane 4 plane 5 plane 6 a) b) PhysicsAC-I dx dy Figure 1. a) AMS 1 Si tracker and the Tracker Alignment System. The insert shows laser profiles observed while AMS was in orbit. b) ladder displacement measurement with cosmic tracks (curvature greatly exaggerated, 1 GV sagitta.5/3 mm for AMS-1/2). 1.3 TAS technical aspects Artificial tracks are produced by 182 nm Laser radiation. Si is highly transparent at 182 nm, provided the natural reflectivity (n Si = 3.3) can be reduced and shadowing by the metallization of the readout strips can be kept small. AMS alignment sensors are antireflective coated and use 1 μm wide readout strips in the Laser impact areas. Thus single layer transparency can be as high as 5%. It has been shown (AMS-1) that a Laser ray can be recorded in 6 Si layers in sequence 4. The AMS-2 tracker (8 planes Si, SC magnet) will be equipped with 2 sets of 1 laser rays each, that traverse the Si in 2 opposite directions (fig. 2b) and do overlap in the central planes. These rays are detected by generating electron hole pairs in the fully depleted Si particle detectors 46. Signals from the alignmentrays are recorded exactly like thecharged particle tracks. 182 nm Laser radiation is generated with high efficiency in DBR-Laser diodes coupled to monomode optical fibres that deliver - via miniature projection optics - low divergence circular rays into the tracker (fig. 2a). At the photon intensities readily available from Laser diodes (> 1 8 / pulse) signals exceeding that of 1 mips can be produced in the Si layer (thickness 3 μm) close to the projection optics. At adequate Laser intensities this approach allows high precision (< 2 μm) tracker stability tests in very short time (< 1 s). The fully operational system (2 beams) weighs less than 5 kg. TAS_Como_21.v1a: submitted to World Scientific on November 3, 212

3 3DviewAlgnmt_2a_mc.nb by 3G3 2/14/ :1:8 AMS-2 Tracker Alignment System basic components ISS operations DC power trigger laser diode( 5 / box) switched current sources ( at maximum 2 active any time) rep. rate < 2Hz, amplitude < 2 ma, width < 5 ns T = 15 C active for 3 sec, laser run (< 5s) (2s (5 s) before (after) laser run, Laser temperature control ( 1/box) coaxial cable monomode fibre coupler (5/box) Diode Boxes (1 of 2 ) monomode fibre (5 / box) AMS-2 Si-tracker & laser alignment rays 5 pairs check {x,y} in the central area (3 x 1 mm 2 ) of all 8 detector planes 5 x plate 1 center ladders shown only (plane 1,8 ladders 6,,12) {plane 2,,7 ladders 5,,1} -5 5 a) AMS DAQ dedicated TAS electronics box CAN IR signals [x, y] ( - 2 mips) Si detector planes (1-8) FC connection passive splitter alignment rays ( 2 x 2 of 2 x 1 ) P opt.5 mw at beam port max λ 182 nm beam ports beam ports tracker support structure b) plate 2 plate 3 plate 4 plate 5-5 y z ams2:21:tas_4q1:dlr_21xi1:laserbms&drvaachen_24x1_p3.ai Figure 2. AMS 2 Tracker Alignment System. a) laser radiation generation, b) basic configuration of 2 x 5 laser ray pairs. 2 TAS and AMS tracker stability 2.1 STS91 flight Overall the AMS-1 tracker 7 has been extraordinarily stable. Over the whole flight - including lift-off and landing - all tracking elements were found at their expected positions within ± 15 μm. Laser measurements (once per 3 orbits (on manual command)) were confirmed by observation with stiff (almost straight) tracks comparing extrapolated tracker hits with actually measured ones (fig. 4). Correcting for the time evolution of the small but finite displacements results in an approximately 2corrections are important only for the high rigidity tail of the cosmic ray spectra observable by AMS(p > 8 GV). The excursions observed are probably due thermal effects because they correlate with changes in flight attitude hence heating by the sun. TAS_Como_21.v1a: submitted to World Scientific on November 3, 213

4 Effective IR (182nm) Transmission (Integral ±5 Sigma) AMS-2 detectors and prototypes :1813h1.dat :1:34 Module:AR_hq_4a_mc.nb IR beam 7 as of: 821, 9, 13, 11, 21, 9< as seen by CCD by: W. Wallraff on: macpbk32c2 6 5 Transmission [%] AMS-2 5 S2 36 µm traces AMS-2 13AR 2 1 µm traces AR detectors with narrow traces are a factor of 3 more transparent than standard ones T(x) T(y) Aufnahme Nr. AMS-2 wil be equipped with 2 sets of 5 laser alignment beams overlapping in at least 2 planes a) b) pixno y pixno y IR beam behind 1 3µm Si as seen by CCD :k1717h2_2_3.dat :1:36 Module:AR_hq_ 4a_mc.nb 7 as of: 821, 9, 13, 11, 21, 9< by: W. Wallraff on: macpbk32c pixno x 45% transparency pixno x ams2:21:tas_4q1:dlr_21xi1:dlr_3_efftrnsmssn_v2.ai Figure 3. a) AMS-2 Si sensor transparency, standard and with antireflective coating. b) high quality coatings eliminate front back interferences and distortions of the laser beam while passing through a sensor. 2.2 prospects Based on AMS-1 experience a tracker stability verification along 1 lines and with better than 4 μm accuracy can be expected from short (< 1 s) runs 4 times per orbit. This measurementover an area of 3 x 1 mm 2 in the center of the acceptance is complemented by minimizing for high rigidity tracks over the full acceptance the pulls in the redundant trackfit with 8 points through the rather smooth and very stable AMS-2 B-field. The method of position control of Si trackers with artificial laser generated straight tracks has not only applications for space experiments. A similar system has recently been studied for implementation in the large area Si tracker of the CMS experiment 5 to be installed at LHC. TAS_Como_21.v1a: submitted to World Scientific on November 3, 214

5 measured displacement [µm] STS91 time line position pull plane 5 Cosmic Alignment dy pl 5 pre -flight position at pad after -flight position at MPPF Laser Alignmenty : 2 : 4 : 6 : 8 : met [dd hh:mm] a) b) position pulls STS91 alignment ladders dy without alignment c) dy with alignment dy pl1 dy pl2 dy pl3 dy pl4 dy pl5 dy pl GMT [ hours since : ] 4, June 6, June 8, June 1, June 12, June dy/[µm] Figure 4. AMS 1 tracker stability during the STS 91 space flight; a) time line of y displacements (?B), from stiff cosmic tracks; squares indicate Laser data. Frequency distributions of observed displacements in the AMS-1 (Laser) alignment ladders before b) and after correction c) observed for high momentum cosmic rays during the STS91 spaceflight; details are given in the references 3 and 4. Acknowledgments NASA, DoE and DLR have generously supported this work. We like to thank the AMS collaboration and the Si tracker team for their cooperation. The meeting at Villa Olmo has proven again to be highly useful, many thanks to the organizers. References 1. U. Becker, ICRC XXVI, icc 1574, Salt Lake City (1999). 2. W. Wallraff, JHEP-PREP-hep21/211, Budapest (21). 3. W. Wallraff et al., ICRC XXVII OG 11, 2197, Hamburg (21). 4. J. Vandenhirtz Ein Infrarot Laser Positions Kontroll System für das AMS Experiment, PhD thesis RWTH-Aachen (July 21). 5. B. Wittmer The Laser Alignment System for the CMS Silicon Microstrip Tracker, PhD thesis RWTH-Aachen (November 21). 6. Weihua Gu Characterization of the CMS Pixel Detectors, PhD thesis RWTH-Aachen (October 21). 7. J. Alcaraz, et al.; A Silicon microstrip tracker in space: Experience with the AMS Silicon tracker on STS-91, Nuovo Cimento 112A, 1325 (1999). TAS_Como_21.v1a: submitted to World Scientific on November 3, 215