MCP:Bias Angle

Impact of Bias Angle on MCP Performance
The bias angle in an MCP (Microchannel Plate) significantly influences its performance characteristics, impacting factors like detection efficiency, ion feedback, and spatial resolution.

• Detection Efficiency: The bias angle affects the depth at which incident particles or photons penetrate the MCP channels. Aligning the incident beam with the bias angle leads to deep penetration into the channels. However, this can reduce the multiplication of electrons and result in a lower gain. The optimum bias angle ensures efficient detection by balancing penetration depth with electron multiplication.
• Ion Feedback: Ion feedback occurs when residual gases inside the MCP channels are ionized by multiplied electrons. These ions travel back towards the input side, generating noise and potentially causing discharges. Utilising a two or three-stage MCP with opposing bias angles helps mitigate this by trapping ions at the junctions between the MCPs.
• Spatial Resolution: When stacking multiple MCPs to achieve higher gain, the bias angle plays a crucial role in determining spatial resolution. The relative tilt between the channels in consecutive MCPs, determined by the bias angle, influences the spread of the electron cloud as it travels through the stages. This, in turn, affects the accuracy of determining the spatial location of the detected event.
• The optimum bias angle for an MCP typically falls between 5° and 15°. This range represents a compromise between optimising the aforementioned performance factors. The specific choice within this range depends on the intended application and the type of particle or radiation being detected. For instance, while a bias angle of 5° is suitable for photons, 8° is preferred for electrons, and 12° for ions.