Optimization of signal extraction and front-end design in a fast, multigap ionization chamber

This paper discusses the criteria that have been adopted to optimize the signal processing in a shower detector to be employed as LHC beam luminosity monitor. The original aspect of this instrument is its ability to operate on a bunch-by-bunch basis. This means that it must perform accurate charge measurements at a repetition rate of 40 MHz. The detector must withstand an integrated dose of 100 Grad, that is, two to three orders of magnitude beyond those expected in the experiments. To meet the above requirements, an ionization chamber consisting of several gaps of thickness 0.5 mm, filled with a gas that is expected to be radiation resistant, has been designed. Crucial in the development of the system is the signal processing, as the electronics noise may set the dominant limitation to the accuracy of the measurement. This is related to two aspects. One is the short time available for the charge measurement. The second one is the presence of a few meter cable between the detector and the preamplifier, as this must be located out of the region of highest radiation field. Therefore the optimization of the signal-to-noise ratio requires that the best configuration of the chamber gaps be determined under the constraint of the presence of a cable of non negligible length between detector and preamplifier. The remote placement of the amplifying electronics will require that the front-end electronics be radiation hard although to a lesser extent than the detector.