The ATLAS beam conditions monitor

The ATLAS beam conditions monitor is being developed as a stand-alone device allowing to separate LHC collisions from background events induced either on beam gas or by beam accidents, for example scraping at the collimators upstream the spectrometer. This separation can be achieved by timing coincidences between two stations placed symmetric around the interaction point. The 25 ns repetition of collisions poses very stringent requirements on the timing resolution. The optimum separation between collision and background events is just 12.5 ns implying a distance of 3.8 m between the two stations. 3 ns wide pulses are required with 1 ns rise time and baseline restoration in 10 ns. Combined with the radiation field of 10/sup 15/ cm/sup -2/ in 10 years of LHC operation only diamond detectors are considered suitable for this task. pCVD diamond pad detectors of 1 cm/sup 2/ and around 500 /spl mu/m thickness were assembled with a two-stage RF current amplifier and tested in proton beam at MGH, Boston and SPS pion beam at CERN. To increase the S/N ratio two back-to-back diamonds were read out by a single amplifier and the detectors inclined to 45 degrees. Limiting the bandwidth at the readout to 200 MHz provided further improvement; S/N ratio of nearly 10:1 could be achieved with MIP's. Amplifiers of the two stages were irradiated with protons and neutrons to 10/sup 15/ cm/sup -2/. Evaluating the irradiated electronics with silicon pad detectors, 20% degradation in S/N ratio was observed. Ten detector modules are being assembled and tested at CERN for their final installation into the ATLAS pixel support structure in the beginning of 2006.