MCP:Ion Feedback

ION feedback, also known as ion feedback effect, occurs in microchannel plates (MCPs). When an MCP is operated at high gain, residual gases within the channels can become ionized by multiplied electrons. These ions are then accelerated towards the input side of the MCP by the electric field. Upon reaching the input side, these ions can initiate additional electron avalanches, generating false signals that degrade the signal-to-noise ratio. This phenomenon can lead to unstable behaviour in the MCP. Ion bombardment can also damage the photocathode, reducing quantum efficiency (QE) and causing a drop in overall gain.

Factors Influencing Ion Feedback:

  • MCP Gain: Operating an MCP at higher gain increases the likelihood of residual gas ionization and, consequently, ion feedback.
  • Vacuum Quality: Poorer vacuum conditions, meaning a higher presence of residual gases, increase the probability of ion feedback.

Impact of Ion Feedback:

  • False Signals: Ion feedback generates spurious pulses, creating unwanted signals that can be mistaken for real events.
  • Pulse Height Distribution (PHD) Degradation: The spurious pulses caused by ion feedback negatively impact the PHD, making it broader and less distinct.
  • MCP Lifetime Reduction: The ion bombardment resulting from ion feedback can damage the channel walls, ultimately shortening the operational lifespan of the MCP.

Ion Feedback Mitigation

  • Improved Vacuum Quality: Enhancing vacuum quality is a key strategy for reducing ion feedback. This minimizes the presence of residual gas molecules within the MCP channels that can be ionized by multiplied electrons. The ionized gas ions travel back towards the input side of the MCP, leading to false signals and potential discharge events. Using an oil-free vacuum system is recommended, and if an oil diffusion pump is unavoidable, an oil trap is essential.
  • MCP Stacking and Bias Angle: In multi-stage MCP configurations like Chevron or Z-stack, arranging two or three MCPs with opposing bias angles effectively suppresses ion feedback. This arrangement traps ions generated from residual gases at the junctions between each MCP.