Belle II SVD ladder assembly procedure and electrical qualification

Abstract

The Belle II experiment at the SuperKEKB asymmetric ${\mathrm{e}}^{+}{\mathrm{e}}^{-}$ collider in Japan will operate at a luminosity approximately 50 times larger than its predecessor (Belle). At its heart lies a six-layer vertex detector comprising two layers of pixelated silicon detectors (PXD) and four layers of double-sided silicon microstrip detectors (SVD). One of the key measurements for Belle II is time-dependent CP violation asymmetry, which hinges on a precise charged-track vertex determination. Towards this goal, a proper assembly of the SVD components with precise alignment ought to be performed and the geometrical tolerances should be checked to fall within the design limits. We present an overview of the assembly procedure that is being followed, which includes the precision gluing of the SVD module components, wire-bonding of the various electrical components, and precision three dimensional coordinate measurements of the jigs used in assembly as well as of the final SVD modules.

Introduction

The Belle II experiment will operate at an instantaneous luminosity of $8\times {10}^{35}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$, and will replace its predecessor (Belle) as the next-generation luminosity frontier experiment. The Belle II detector [1] has two inner layers of PXD [2] followed by four outer layers of SVD to perform a precise vertex measurement in this high luminosity environment. Each SVD layer is made up of overlapping ladder modules in a central cylindrical arrangement and a slanted conical region at one end.

The basic detecting elements are 6 in. double-sided silicon microstrip (DSSD) sensors provided by HPK and Micron. The cylindrical central region of the SVD consists of rectangular sensors and the slanted region will feature trapezoidal sensors. The origami flexible electrical boards are attached to the DSSDs in order to minimize the distance to the APV25 readout ASIC chips [3], thus minimizing capacitive noise. The total sensor area is $1.23{\mathrm{m}}^2$ with an overlap of 8–10% and a polar angle coverage between 17° and 150° with respect to the detector axis (the direction of the ${\mathrm{e}}^{-}$ beam is taken as the positive z direction).