THIN DETECTORS FOR THE CHICSI DELTA-E-E TELESCOPE

A pilot series of 10 mu m to 15 mu m thin silicon detectors has been m ade for the Delta E-E telescopes in the CHICSi detector system. This s ystem will operate gt the CELSIUS heavy ion storage ring in Uppsala, S weden, Delta E-E telescopes provide isotope identification and energy determination of fragments from nuclear collisions. The thin detectors are made as p-i-n diodes in thin etched membranes in 280 mu m thick s ilicon wafers. The membranes are made with anisotropic etching using 2 5 w% tetramethylammonium hydroxide (TMAH) solution. The etch speed of this solution is very uniform across a wafer. As a result detectors wi th uniform thickness can be produced. The etch depth varies with less than +/- 0.3 mu m over a wafer and the surface microroughness is in th e range from 2 to 4 nm. Each detector has a 10.0 mm x 10.0 mm active a rea on a 10.2 mm x 10.2 mm membrane surrounded by a 1.1 mm wide suppor ting frame. The detectors have leakage currents in the active area of approximately 0.5 nA at 20 V. The breakdown voltage of the detectors i s above 100 V. Evaluation experiments with telescopes consisting of a thin detector in combination with a thick detector have shown excellen t isotope separation capabilities. Mass separation of Li-6 and Li-7 is clearly observable.