Detection of ionization electrons with hybrid pixel detectors for non-destructive beam profile measurements

Abstract

The Beam Gas Ionization (BGI) profile monitor is an Ionization Profile Monitor used for continuous, non-destructive transverse beam profile measurements in synchrotrons at CERN. Electrons are produced when residual gas in the beam pipe is ionized by the proton or ion beams orbiting the particle accelerator. In the BGI profile monitor, these ionization electrons are accelerated onto Timepix3-based silicon hybrid pixel detectors using co-linear electric and magnetic fields. Individual electrons are detected and subsequently summed to reconstruct the beam’s transverse profile. The energy absorption of the $\sim$8.6 keV ionization electrons in the sensor layers of the detectors induces electrical signals close to or below the noise floors of the Timepix3’s digitizers. This results in hyper-sensitivity of the detection efficiency to small changes in the detectors’ global thresholds, as demonstrated in simulations, and creates challenges for the equalization of detection efficiency across all four detectors in each BGI instrument. This equalization is necessary to utilize the full field-of-view of the instrument. Emittance measurements from an equalized BGI profile monitor are compared to measurements from Beam Wire Scanners and show good agreement. This constitutes the first validation of the primary use case of the BGI profile monitor.