Vertical bunch size measurement at LEP using very fast x-ray detectors

Very fast X-ray detectors have been developed for beam diagnostics in the LEP machine (Large European Electron Positron Collider). Picosecond CdTe photoconductors allow the measurement of the vertical dimensions of electron and positron bunches (of length 50 picoseconds and vertical size 300 microns) during a single beam passage. Intense synchrotron X-rays emitted by the LEP beams are measured each turn by two sets of detectors. This article describes in detail the transverse profile detector and its associated electronics. A pulsed bias is used to gate the detector for time as short as 10 ns. The profile is then digitised in 10 µs and saved in memory. Typical results obtained in real time with LEP beams are used to illustrate the performance of the detector. The linear pulse bias provides a new way to make single shot X-ray measurements of the longitudinal and transverse bunch centre of gravity simultaneously with the transverse profile.