The Photon-Assisted Cascaded Electron Multiplier Operation in CF$_{4}$ for Ion Backflow Suppression

The operation of the recently introduced Photon Assisted Cascaded Electron Multiplier (PACEM) in CF$_{4}$ is investigated. The PACEM uses the VUV scintillation produced in the electron avalanches of the first multiplier of the cascade to transfer the signal to the subsequent ones. The VUV scintillation induces the emission of a large number of photoelectrons from a CsI photocathode placed on the top-surface of the second multiplier. The photoelectrons are further multiplied in the subsequent stages of the cascade, resulting in efficient signal amplification. A mesh, set at a fixed voltage, is placed between the first and the second multipliers to block the charge transfer between them, thus suppressing all the ion backflow (IBF) heading to the first cascade element. The PACEM electrically isolates the first multiplier of the cascade and only the ions produced in the electron avalanches of the first element may flow back into the drift region. Operating in CF$_{4}$, absolute IBFs as low as ${\sim} 1$ ion per primary electron are achieved for drift fields of 0.1 kV/cm, while for fields of 0.5 kV/cm the absolute IBF is ${\sim} 10$ ions/primary electron. This corresponds to an IBF of 10$^{-4}$ at gains of 10$^{4}$, for Time Projection Chambers (TPC) operating conditions, and IBFs of ${\sim} 10^{-5}$ at gains of 10$^{6}$ for Gaseous Photomultipliers (GPM) operating conditions.