Noise characterization of silicon strip detectors-comparison of sensors with and without integrated jfet source-follower.

Noise is often the main factor limiting the performance of detector systems. In this work a detailed study of the noise contributions in different types of silicon microstrip sensors is carried on. We investigate three sensors with double-sided readout fabricated by different suppliers for the ALICE experiment at the CERN LHC, in addition to detectors including an integrated JFET Source-Follower as a first signal conditioning stage. The latter have been designed as an attempt at improving the performance when very long strips, obtained by gangling together several sensors, are required. After a description of the strip sensors and of their operation, the “static” characterization measurements performed on them (current and capacitance versus voltage and/or frequency) are illustrated and interpreted. Numerical device simulation has been employed as an aid in interpreting some of the measurement results. The commonly used models for expressing the noise of the detector-amplifier system in terms of its relevant parameters are then presented. Two configurations of the first signal processing stage are considered and confronted: the usual charge-sensitive amplifier and the Source-Follower. Next, the noise measurements performed and their results are illustrated. Curves of the equivalent input noise charge versus shaping time of the filtering amplifier, for several values of the leakage current, have b een obtained. The leakage current has been varied by photogeneration, illuminating the sensor with a LED, or alternatively by injecting it into the strip from an external current source, through a high-value resistor. The noise measured with the strip sensors read-out by a charge- sensitive amplifier generally agrees well with the common model, but in some operating conditions unexpected contributions have been found. These have been interpreted by correlating them with some peculiar features of the capacitance-voltage measurements. The noise measured on the detectors with integrated JFET source-follower complies with the prediction of the model, using the measured values of the relevant parameters. Finally, the performances of the two different approaches are confronted.