A radiation monitor for the ZEUS detector at HERA

A radiation monitoring and automatic beam-dump system has been installed in the interaction region of the ZEUS experiment during the last upgrade of the electron/positron-proton collider HERA. It is used to prevent the ZEUS Silicon Microvertex Detector (MVD) from beam-background induced radiation damage during HERA operation. The dose rate measurement is based on the readout of radiation induced signal currents from silicon PIN diodes. A precise temperature measurement allows to correct for temperature induced changes in the offset leakage current of the diodes. The online dose rate measurement is complemented by a system of radiation field effect transistors (RadFETs), which are sensitive to accumulated surface damage from ionizing radiation. The integrated dose is obtained from the change in the threshold voltage of the RadFET devices. In addition, thermo-luminescense dosimeters (TLDs) measuring the integrated dose of photon and neutron irradiation are used to control and calibrate the online dose measurements. The radiation monitoring system has been designed to measure the dose on a time scale from milliseconds up to the lifetime of the experiment of several years. It provides online information on the actual level of background radiation to both the HERA and ZEUS shift crews and triggers an automatic dump of the lepton beam in case of excessively high instantaneous dose rates.