Design and Structure of the Upgraded Silicon Vertex Detector at Belle

Transcription

1 Design and Structure of the Upgraded Silicon Vertex Detector at Belle G. R. Moloney 6, R. Abe 8, T. Abe 20, H. Aihara 14, Y. Asano 18, T. Aso 16, A. Bakich 11, T. Browder 2, M. C. Chang 12, Y. Chao 12, K. F. Chen 12, S. Chidzik 10, J. Dalseno 6, R. Dowd 6, J. Dragic 6, C. W. Everton 6, R. Fernholz 10, M. Friedl 19, H. Fujii 17, Z. W. Gao 12, A. Gordon 6, Y. N. Guo 21, J. Haba 17, K. Hara 9, T. Hara 9, Y. Harada 8, T. Haruyama 17, K. Hasuko 1, K. Hayashi 17, M. Hazumi 17, E. M. Heenan 6, T. Higuchi 17, H. Hirai 8, N. Hitomi 17, A. Igarashi 18, Y. Igarashi 17, H. Ikeda 17, H. Ishino 15, K. Itoh 14, S. Iwaida 18, J. Kaneko 15, P. Kapusta 4, R. Karawatzki 19, K. Kasami 17, H. Kawai 14, T. Kawasaki 8, A. Kibayashi 15, S. Koike 17, S. Korpar 5, P. Krizan 5, H. Kurashiro 15, A. Kusaka 14, T. Lesiak 23, A. Limosani 6, W. C. Lin 12, D. Marlow 10, H. Matsumoto 8, Y. Mikami 13, H. Miyake 9, T. Mori 15, T. Nakadaira 14, Y. Nakano 18, Z. Natkaniec 4, S. Nozaki 13, R. Ohkubo 17, F. Ohno 15, S. Okuno 3, Y. Onuki 8, W. Ostrowicz 4, H. Ozaki 17, L. Peak 11, M. Pernicka 19, M. Rosen 2, M. Rozanska 4, N. Sato 17, S. Schmid 19, T. Shibata 8, R. Stamen 17, S. Stanic 18, H. Steininger 19, K. Sumisawa 9, J. Suzuki 17, H. Tajima 14, O. Tajima 13, K. Takahashi 15, F. Takasaki 17, N. Tamura 8, M. Tanaka 17, G. N. Taylor 6, H. Terazaki 18, T. Tomura 14, K. Trabelsi 2, W. Trischuk 22, T. Tsuboyama 17, K. Uchida 2, K. Ueno 12, K. Ueno 17, N. Uozaki 14, Y. Ushiroda 17, S. Vahsen 10, G. Varner 2, K. Varvell 11, Y. S. Velikzhanin 12, C. C. Wang 12, M. Z. Wang 12, M. Watanabe 8, Y. Watanabe 15, Y. Yamada 17, H. Yamamoto 13, Y. Yamashita 7, Y. Yamashita 14, M. Yamauchi 17, H. Yanai 8, R. Yang 10, Y. Yasu 17, M. Yokoyama 14, T. Ziegler 10, D. Zontar 5 The Belle SVD Group 1 Riken/Brookhaven Research Center, BNL, Physics510A, Upton, NY , USA 2 University of Hawaii, Department of Physics and Astronomy, 2505 Correa Road, Honolulu, HI 96822, USA 3 Kanagawa University, Faculty of engineering, Rokkakubashi, Kanagawa-ku, Yokohama, Kanagawa, , Japan 4 H. Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences, ul. Radzikowskiego 152, Krakow, Poland 5 Jozef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia 6 University of Melbourne, School of Physics, Victoria, 3010, Australia 7 Nihon Dental College, 1-8 Hamaura-cho, Niigata , Japan 8 Niigata Graduate School of Science and Technology, Niigata university, Niigata ,Japan 9 Osaka University, Department of Physics, Graduate School of Science, 1-1 Machikaneyama, Toyonaka, Osaka , Japan 10 Princeton University, Department of Physics, PO Box 708, Princeton, NJ 08544, USA 11 University of Sydney, School of Physics, Darlington 2006, NSW, Australia 12 National Taiwan University, Department of Physics HEP Lab, Taipei 10764, Taiwan, R.O.C. 13 Tohoku University, Department of Physics HEP Group, Aramaki, Aobaku, Sendai , Japan 15 Tokyo Institute of Technology, Department of Physics Group HP, , Oh-okayama, Megro-ku, Tokyo , Japan 14 University of Tokyo, Department of Physics, 7-3-1, Hongo, Bunkyo-ku, Tokyo , Japan 16 Toyama National College of Maritime Technology, Toyama, Japan 17 KEK, High Energy Accelerator research Organization, 1-1 Oho, Tsukubashi, Ibaraki-ken , Japan 18 University of Tsukuba, Institute of Applied Physics, Tenodai, Tsukuba-shi, Ibaraki-ken , Japan 19 IHEP Vienna, Nikolsdorfergasse 18, Vienna, Austria A Tohoku University, present address KEK 21 National Taiwan University, present address, IHEP Beijing 22 Princeton University, visitor from Toronto University 23 KEK, on leave from ifj Krakow Abstract The Belle collaboration, at the KEKB B factory, has constructed an upgraded Silicon Vertex Detector, SVD2, which has been installed into the Belle detector in September The design goals of SVD2 are aimed at overcoming shortcomings of the existing SVD detector at Belle, SVD1. To achieve these goals substantial changes have been made to the mechanical design of the SVD. These changes have led to a re-design of the detector ladder systems. A fine pitch (50 micron) flexible kapton circuit has been developed as an interconnect between the silicon detector signal strips and the readout electronics. The ladder design for SVD2 also introduces a step structure, and a fine pitch flexible kapton interconnect, between the silicon sensors and the readout electronics. Index Terms Belle, KEKB, silicon vertex detector, detector geometry I. INTRODUCTION The Belle experiment has been established at the KEK accelerator laboratory to study Charge Parity symmetry (CP) violation in the decay of B mesons [1]. An upgraded Silicon Vertex Detector (SVD) has been installed into the Belle detector [2] in September Since 1999, the previous detector design, SVD1 [3], has provided Belle with precision measurements for the extrapolation of charged particle trajectories to the interaction point. These measurements have been essential for the separation of B meson decay vertices in time dependent CP violation measurements. The upgraded detector, SVD2, has been installed into the Belle detector in September 2003, and commenced taking

2 collision data in October The design goals for the SVD upgrade include [4]: Improved radiation tolerance Higher resolution for vertex reconstruction Improved charged particle tracking in the low transverse momentum region Trigger capability Increased solid angle of the SVD Tolerance to pin-hole defect To achieve these goals substantial changes have been made to the design for the upgraded SVD. Some of the changes which are not related to the detector ladder geometry include: The inner radius of the beam pipe has been decreased from 20 mm to 15 mm to allow a reduced radius for the inner detector layer. The inner portion of the Belle Central Drift Chamber has been replaced by a small cell drift chamber, allowing the outer radius of the SVD to increase from 72 mm to 101 mm. The 0.8 µm CMOS VA1 preamplifier chips used in SVD1 have been replaced with VA1TA chips implemented in a 0.35 µm CMOS process. The reduced feature size increases the expected radiation tolerance from 1 Mrad to greater than 20 Mrad [5], [6]. The VA1TA includes a trigger circuit which can produce a fast trigger signal for use in the event trigger of the Belle detector [7]. The SVD trigger provides additional discrimination of the expected increase in beam background, coming both from expected increases in KEKB luminosity and the reduced beam pipe radius. A redesigned flash ADC based front-end digitisation system is used to digitise the shaped analogue signals the VA1TA chips. The online data acquisition system is performed by a farm of Linux PCs through a PCI-based readout card [8]. II. THE DETECTOR LAYOUT SVD1 and SVD2 are both constructed from detector ladders, arranged in concentric layers around the beam pipe, as shown in Figs. 1 and 2. Some important dimensions of the geometric layout of the detector ladders for SVD1 and SVD2 are shown in tables I and II. The inner radius of the beam pipe is also shown. The changes which have been made to the detector geometry include: The length of the detector ladders has been increased to extend the coverage of the SVD in the forward and backward directions to match the acceptance of the Belle Central Drift Chamber (CDC). The number of silicon detector layers has been increased from three to four. This provides improved track reconstruction efficiency for charged particles with low transverse momentum, and improved vertexing robustness against large beam background. SVD1 SVD2 Fig. 1. Axial cross-section view of the detector ladder geometry for SVD1 and SVD2. θ=150 Hybrids ο DSSDs Beam axis θ=17 Fig. 2. Longitudinal view of the SVD2 detector layout. The forward backward asymmetry matches the boosted centre of mass of the KEKB accelerator. Layer Radius Length Ladders DSSDs (mm) (mm) per layer per ladder Inner radius of beam pipe : 20.0 mm Size of DSSD : mm Polar Angle Coverage : 23 < θ < 139 TABLE I GEOMETRIC DESIGN PARAMETERS FOR SVD1. Layer Radius Length Ladders DSSDs (mm) (mm) per layer per ladder Inner radius of beam pipe : 15.0 mm Size of DSSD : Layer 1, 2, 3 : mm Layer 4 : mm Polar Angle Coverage : 17 < θ < 150 TABLE II GEOMETRIC DESIGN PARAMETERS FOR SVD2. ο

3 The radius of the inner layer has been decreased from 30 mm to 20 mm, improving the resolution for vertex extrapolation in Belle. Each detector ladder in SVD2 consists of between 2 and 6 Double Sided Strip Detectors (DSSDs) glued end to end onto mechanical supporting ribs, such that the long edge of the sensors is parallel to the beam axis when mounted onto the detector. The signal strips on the p side of the sensors run perpendicular to the strips on the n side, with the p side signal strips providing a measurement of the z coordinate and the n side strips the φ coordinate of particle trajectories. At each end of the detector ladder an electronic hybrid is glued to the supporting structure to provide signal preamplification and readout. For SVD1 the hybrids were glued directly onto the end of the DSSDs. For SVD2 it is difficult to accommodate the geometric bulk of the hybrids at the 20 mm radius of the inner layer. A flexible kapton interconnect has been used between the DSSDs and hybrids, allowing the hybrids to be placed at a larger radius to that of the DSSDs, as shown in Fig. 2. III. THE SVD2 DETECTOR LADDER A. The half ladder design A ladder consists of a pair of half ladders aligned to each other by a mechanical support structure. Half ladders consist of one, two or three DSSDs, a hybrid pair, and a pair of fine pitch (50 µm) flexible kapton interconnects. Figure 3 shows views of a 2 DSSD half ladder from the z strip side and the φ strip side. R φ Side connector connector Hybrid Z Side VA1TA chips Kapton Flex DSSDs Wire bonding pads Kapton Flex Fig. 3. SVD2 half ladder view from the φ strip side, and the z side. The heavy, dark lines represent the rows of wire-bonding pads on the flex, DSSD and VA1TA preamplifier chips. In the multi DSSD half ladders, the readout strips and detector bias from adjacent DSSDs are ganged together. On the φ side, this is achieved by wire bonding strips from adjacent DSSDs. For SVD1, a second metallisation layer was used on the z strip side to carry the signal from the strips to the bonding pads at the ends of the DSSD suitable for wire bonding to the readout hybrid and for ganging adjacent DSSDs together. For the SVD2 silicon sensors there is no second metallisation layer, and the z-side kapton flex extends over the entire length of the DSSDs providing readout traces for the transverse z strips, as shown in Fig. 3. The readout strips on the kapton circuit are wire bonded to the z strips using the wire-bonding pads along the edge of the DSSDs. The z-side kapton flex has a lower capacitance between the readout traces and strips than a double metal layer providing a reduced readout noise. The hybrid pair consists of two hybrids, with 0.5 mm aluminium nitride substrates, glued back-to-back with a 0.5 mm copper heater-spreader between. Each hybrid reads out strips from one side of the half ladder the z strip side or the φ strip side. The aluminium nitride substrates and the copper heat spreader provide a high thermal conductivity path between the VA1TA chips and the water cooled SVD ladder mounting structure. The kapton flex connects DSSD readout strips to the input channels of the preamplifier chips on the hybrid. Wire bonding is used to connect signal traces between the DSSD strips and the flex circuit, and between the flex and the VA1TA preamplifier chips on the hybrids. The requirements for the SVD2 kapton flex circuits are quite demanding. A 50 µm strip pitch must be maintained over circuit lengths up to 260 mm. The assembly procedure for the half ladders consists of gluing the DSSDs, kapton interconnects and hybrids together, then applying the wire bonds for signal readout and detector bias. To achieve the required production rate, the production was split across two sites, with the layer 4 ladders occurring at a different site from the layer 1, 2 and 3 assembly. It was necessary to not clamp the kapton circuits to the DSSDs and hybrids during glue curing to minimise the flow of glue out from the edges of the kapton onto the wire bonding pads. Surface tension and capillary attraction were sufficient to ensure the glue would flow to fill the contact area under the kapton and no further. Variation in the amount of glue dispensed resulted in a variation in the thickness of the glue gap, rather than in excess glue covering the wire bonding pads. The half ladders were tested after assembly by reading out noise and pedestal distributions and VA1TA channel gain, using the internal test function of the VA1TA chips. B. The full ladder assembly The full ladders are assembled by placing two half ladders onto vacuum holding jigs and gluing a mechanical support structure onto the half ladders. The mechanical support is provided by a box-rib, to which the DSSDs are glued, and a plastic bridge piece which is glued to the box-rib and the hybrid, as shown in Fig. 4. Fig. 4. AlN Hybrid VA1TA preamplifier chips Kapton Flex Circuit Detail of the ladder support structure of SVD2. Support Bridge DSSD Support Rib The box-rib consists of two zylon ribs, extending the length of the silicon sensors glued to a CFRP piece which provides the

4 roof for the box-rib. Finite element analysis has been used to calculate mechanical properties of the assembled ladders. Table III shows the calculated sag for ladders in each layer. The analysis show the CFRP roof is not necessary for the layer 1 ladders, and these ladders are fabricated without the CFRP roof to minimise material in the central region of the tracking volume. after alignment are shown in Fig. 6. From the distributions, the intrinsic resolution of SVD2 after alignment is: σ x = 12.0 ± 0.4 µm σ z = 22.3 ± 0.8 µm The strip pitch of the layer 1 to 3 sensors is 50 µm in x and 75 µm in z. Layer Calculated Sag 0.4 µm 1.3 µm 6.4 µm 16.4 µm TABLE III SAG OF THE SVD2 DETECTOR LADDERS CALCULATED FROM FINITE ELEMENT ANALYSIS. Assembled full ladders are shown in Fig. 5. Readout tests are again performed on the fully assembled ladders, before shipping to KEK for assembly onto the SVD2 mechanical support structure. Fig. 5. Assembled SVD2 detector ladders from layers 1, 2, 3 and 4. Fig. 6. Track residuals calculated before and after internal alignment. The upper plots show the residuals before alignment, and the lower plots after alignment. The plots on the left side show the x residuals, which provide a measure of the φ of the track. The plots on the right side are for the z coordinate measurement. IV. SVD2 PERFORMANCE The electrical and mechanical performance of the assembled SVD2 detector, complete with readout system and offline software, has been successfully tested at KEK. The detector has shown good performance, with a preliminary strip signal to noise ratio of 30. The average measured noise for the readout channels is shown in Tab. IV. The noise for the different type of half ladders are categorised by the number of DSSDs in the half ladder. Number of DSSDs (layer 4) Noise (electrons): p side n side TABLE IV AVERAGE NOISE FOR SVD2 READOUT CHANNELS, CATEGORISED BY NUMBER OF DSSDS. Precision alignment of the silicon sensors in the SVD detector is performed through an iterative residual minimisation procedure using a large data sample of cosmic ray tracks recorded with SVD2. The calculated track residuals before and V. CONCLUSIONS The upgrade from SVD1 to SVD2 is expected to bring substantial improvements in performance and radiation tolerance. The imperative to reduce the radius of the inner layer has mandated substantial changes to the design of the detector ladders for SVD2, including the use of a fine pitch flexible kapton circuit. SVD2 has been assembled and installed into the Belle detector in September 2003, and commenced taking data for the Belle experiment in October REFERENCES [1] Belle Techical Design Report, KEK, Preprint 95-1 (1995) [2] A. Abashian, et al., (Belle Collaboration), Nucl. Instr. and Meth. A 479 (2002) 117 [3] G. Alimonti et al., (Belle Collaboration), Nucl. Instr. and Meth. A 453 (2000) 71 [4] T. Kawasaki, Nucl. Instr. and Meth. A 494 (2002) 94 [5] M. Yokoyama et al., IEEE trans. Nucl. Sci. 48 (2001) 440 [6] J. Kaneko et al., IEEE trans. Nucl. Sci. 49 (2002) 1593 [7] T. Ziegler et al., The Belle Trigger System with the new Silicon Vertex Detector SVD 2.0, Contribution for the IEEE 2003 Conference in Portland, USA, Oct To be published in these proceedings.

5 [8] H. Ishino et al., The Data Acquisition System of the Belle Silicon Vertex Detector (SVD) Upgrade, Contribution for the IEEE 2003 Conference in Portland, USA, Oct To be published in these proceedings.