A fast low-noise readout channel for the Beam Monitor of the Large Hadron Collider

P.F. Manfredi, M. Manghisoni, L. Ratti, V. Re, V. Speziali, G. Traversi A segmented multi-gap ionization chamber will be used to measure the luminosity of the Large Hadron Collider, which is being built at CERN. The aim is to optimize the accelerator operation by monitoring the beam geometrical parameters and to collect data of interest for accelerator physics. The detector is expected to provide an accurate measurement of the charge associated to single collisions of proton bunches. The electronics will be located at about three meters from the interaction region, where very high levels of radiation are expected, and connected to the detector through a radiation-hard cable. The characteristic impedance of the cable is matched using the cold resistance termination technique, which takes advantage of the input impedance of the preamplifier, adding considerably lower noise than in the case of a traditional passive termination. Due to the high collision frequency which is expected at the LHC (40 MHz), a suitably fast readout system, capable of processing times shorter than 25 ns, is required. In order to comply with the system specifications, including noise considerations, silicon and silicon-germanium bipolar transistors with very high transition frequencies (fT>10 GHz) have been characterized in order to select the preamplifier input element. Laboratory tests have confirmed that the readout electronics can be operated at the required speed. A prototype detector, including the ionization chamber and the complete analog channel, has been tested on the beam of the Super Proton Syncrotron at CERN in September 2001. The system was able to detect the charge signal released by a single $pp$ interaction, which is the minimum expected value at the collider interaction points.

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