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A Micro-channel plate (MCP) detector is a type of detector used in mass spectrometry to detect ions and in physics to detect X-rays, electrons and ultraviolet radiation. It is closely related to an electron multiplier. Microchannel plates consist of an array of miniature electron multiplier channels (~10micrometer diameter, ~15 micrometer spacing between channels). These channels are parallel to each other and often enter the plate at a small angle to the surface (~8° from normal). Most modern MCPs consist of two plates with angled channels rotated 180° from eachother producing a chevron (v-like) shape. An ion that enters one of the channels through a small orifice is guaranteed to hit the wall of the channel due to the channel being at an angle to the plate and thus the angle of impact. The impact starts a cascade of electrons that propagates through the channel. In a chevron MCP the electrons that exit the first plate start the cascade in the next plate. The electrons exit the chevron on the opposite side where they are themselves detected by additional means, often simply a single metal anode measuring total current. In some applications each channel is monitored independently to produce an image. Phosphors in combination with photomultiplier tubes have also been used.
The advantage of the chevron MCP over the straight channel MCP is significantly more gain at a given voltage. One disadvantage of MCPs is that each channel takes time to recover (or recharge) before it can detect another ion. For example, if all channels are saturated with a large signal at one moment, the MCP will not detect a small signal that follows directly after.
The gain of a MCP is very noisy, especially for single particles. With two thick MCPs (>1 mm) and small channels (< 10 µm), saturation occurs, especially at the ends of the channels after many electron multiplications have taken place. The last stages of the following semiconductor amplifier chain also go into saturation. A pulse of varying length, but stable height and a low jitter leading edge is sent to the Time to digital converter.
Because MCPs have a fixed charge, that they can amplify in their life, expecially the second MCP has a lifetime problem. It is important to use thin MCPs, low voltage and instead more sensitive and fast semiconductor amplifiers after the anode.
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