LIMITATIONS OF THE PULSE-SHAPE TECHNIQUE FOR PARTICLE DISCRIMINATION IN PLANAR SI DETECTORS

Limitations of the pulse-shape discrimination (PSD) technique - a prom ising method to identify the charged particles stopped in planar Si-de tectors - have been investigated. The particle resolution turned out t o be basically determined by resistivity fluctuations in the bulk sili con which cause the charge-collection time to depend on the point of i mpact. Detector maps showing these fluctuations have been measured and are discussed. Furthermore we present a simple method to test the per formance of detectors with respect to PSD. Another limitation of the P SD technique is the finite energy threshold for particle identificatio n. This threshold is caused by an unexpected decrease of the total cha rge-collection time for ions with a short range, in spite of the fact that the particle tracks are located in a region of very low electric field.