Development of the silicon multiplicity detector for the NA50 experiment at CERN

We have designed a silicon microstrip detector to measure angular distribution and multiplicity of charged particles produced in high-energy Pb-Pb interactions for the NA50 experiment. NA50 will investigate the production of resonances decaying to dimuons at the CERN SpS. Given the low cross-section for these reactions, the experiment will have to function at very high rate, while the need to limit the background due to decays of π and K mesons imposes an absorber very close to the target, resulting in a very short space available for the multiplicity detector. The multiplicity detector will therefore have to be very fast (dead time below 50 ns), radiation resistant (up to the Mrad level as dose and up to more than 10¹³ particles/cm² as non-ionizing damage), compact (less than 12 cm along the beam) and of high granularity (in order to achieve a good resolution in central collisions with up to 1000 particles in the acceptance). The conditions on noise, speed and radiation hardness are comparable to the ones foreseen at the future Large Hadron Collider at CERN. We present the detector design and the first results on the components of the system which have been produced and tested up to now. In particular, we report on the performance achieved by the first complete detector modules (including two full-custom VLSI ASICs) and by the transputer-based readout system.