The L3 siliconmicrovertexdetector Dedicatedtothememory ofVanko Krastevandhisfamily.

AbstractThedesign andconstruction ofthesilicon strip microvertex detector(SMD)oftheL3experiment at LEPare described.We present the sensors, readout electronics, data acquisition system, mechanical assembly and support, displacementmonitoring systems and radiation monitoring system of the recently installed double-sided, double-layered SMD. Thisdetector utilizes novelandsophisticatedtechniques for its readout.At thebeginning of 1991, the radius of the LEPbeampipe at its four interaction pointswasreducedfrom8.0 cmto 5.5 cm. L3 decided to take advantage of this newlyacquired space by installing a silicon microstrip detector(SMD) to upgrade its central tracking capability. Theintention was to design, build and install the SMDin aperiod ofjust twoyearsso that this newdetector wouldbetaking data by 1993. Consequently, our general designphilosophy was to use existing technology as much aspossible to guideourchoices for such key itemsas sensordesign, readout electronics, data acquisition system, cool-ingtechniques anddisplacement monitoring systems.The advantages of placing a tracking system with acoordinate resolution of -10 lam close to thebeam pipeare well known [1]. For L3, the SMDwill significantlyimprovetransverse momentumand impact parameterreso-lution for studying W-pair production physics at LEP200(rs -200 GeV), and to provide enhanced b-quark tag-ging capability to aid a potential Higgs detection in this - energy regime. Installation during LEP100 (~s_ mz)facilitates detector debugging due to the large reactioncross section at theZresonance.29° SMD two-layer coverage'~220 SMDone-layercoverageFig. 1. Thetop view showsthe SMDpositioned inside the