Irradiation Test of the ZEUS Vertex Detector Frontend Chips, the HELIX128-3.0

During the 2000/2001 HERA shutdown a silicon strip vertex detector (MVD) was installed in the ZEUS experiment. The frontend chip, Helix128-3.0, fabricated in the radiation tolerant 0.8 μm CMOS technology by AMS, will be exposed to an estimated dose of 20 krad/year. The chips have been irradiated up to an integrated dose of 500 krad using a ⁶⁰Co source. In a testbeam the effect of the radiation on the S/N and on the position resolution have been investigated. The tests show that the S/N-ratio drops from 22 to 12 after 500 krad. When the operation point of the chip is changed a S/N-ratio of 18 corresponding to a position resolution of 9.7 μm can still be achieved even after 500 krad.