Status of the Belle SVD detector

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Abstract

The Belle spectrometer was designed for studies of B meson decays at an asymmetric e+e collider operating at the ϒ(4S) resonance. One of its crucial components, a silicon vertex detector (SVD), is placed just outside a cylindrical beryllium beam-pipe. After a year of Belle operation an upgraded version of SVD was installed during the regular summer shut-down. The new SVD follows the same design, with a few important improvements. Rad-soft readout electronics was replaced by rad-tolerant one, allowing for longer lifetime of the detector. A new radiation and temperature monitoring system was developed and implemented. A saw-shaped inner surface was introduced in the beam-pipe to prevent potential synchrotron radiation damage. The upgraded detector started operating successfully in October 2000.

Introduction

A primary goal of the Belle experiment is to observe time-dependent CP asymmetries in the decays of B mesons [1]. To achieve it, a measurement of relative displacements of B meson decay vertices to a precision of ≈100μm is needed. This calls for the use of a silicon vertex detector (SVD) [2], [3] with its innermost layer as close to the beam-pipe as possible, light support structure and readout electronics placed outside the tracking volume.

Section snippets

General characteristics

The Belle SVD consists of 3 layers and covers 86% of the total solid angle. Each layer is constructed from independent ladders, which in turn comprise of either two, three or four double sided silicon strip detectors (DSSD) reinforced by support ribs and the DSSD readout electronics. A DSSD consists of 1280 sense strips with 25μm pitch on the φ-side and 1280 sense strips with 42μm pitch on the z-side. On the φ-side only every second strip is connected to a readout channel. On the z-side

Upgrades

The first SVD (version 1.0) was severely damaged by synchrotron light soon after installation in 1999. It was replaced by an equivalent detector, SVD 1.2. To reduce the low energy photon radiation background the beam-pipe was wrapped in 20μm thick Au foil and 300μm Au foil was placed in the SVD support structure. After a year of successful operation of SVD 1.2 the next SVD (version 1.4) with new radiation tolerant readout electronics was installed in Belle. Special care was taken to protect SVD

Performance

The S/N ratio of SVD 1.4 was found to be better than 19 (17 for SVD 1.2). The bad channel fractions are 3.6% in layer 1, 11.7% in layer 2 (5% due to a dead half-ladder) and 8.1% in layer 3, somewhat worse than SVD 1.2 (corresponding values are 1.2%, 3.7% and 6.5%). Bad channels are due to failures of the readout electronics and failed coupling capacitors on the DSSD. The helix parameter resolution was also studied for hits in the SVD detector only and for hits both in SVD and the Central Drift

References (3)

  • G Alimonti

    Nucl. Instr. and Meth. A

    (2000)
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