Microchannel plates (MCP) use separate channels to detect particles and radiation. Electrons, ions, ultraviolet radiation, and x-rays are detected with spatial resolution by many channels of electron multipliers performing secondary emission to intensify detection signals. Applications for microchannel plates include mass spectrometry, high energy physics, remote detection, image intensification, space science, and surface science.

Operation

Microchannel plates have millions of independent channels of tiny glass capillaries fused together into a two-dimensional array, forming a honeycomb-like structure. This wafer has a thin metal electrode deposited on its input and output surfaces to electrically connect each channel in parallel.

A particle colliding with the microchannel plate enters a single channel and hits the channel wall, generating a secondary electron which is accelerated by an electric field produced by a voltage across both sides of the plate. The secondary electron spawns more electrons as it strikes the channel wall, ultimately resulting in thousands of electrons exiting the back of the plate.

The output of each individual channel is collected by anodes, phosphor screens on a fiber optic, or other methods in order to assemble the final spatial pattern image.

Coatings

Various types of coatings cover each surface of the microchannel plate to form an electrode. These coatings include nickel chromium (NiCr), cesium iodide (CsI), copper iodide (CuI), magnesium fluoride (MgF₂), potassium bromide (KBr), and gold (Au). CsI, CuI, and MgF₂ improve detection in the ultraviolet spectrum, while MgO enhances detection of electrons. KBr improves detection of soft x-rays in the 0.2 to 9 keV range, and Au electrodes allow for rapid charge replenishment.

Specifications

The following specifications are important when selecting a microchannel plate:

• Pore size (microns)—Pore size indicates the size of each individual channel on the plate. A smaller pore size results in higher spatial and temporal resolution of the image.
• Bias angle (degrees)—The angle between the channel and the surface normal. A bias angle is present so that incident particles do not pass directly through a channel without generating secondary electrons.  Values of 5° to 15° are typical.
• Aspect ratio (thickness : channel diameter)—Aspect ratio describes the ratio of the channel length to the channel diameter. Typical values range from 40:1 to 80:1. Values around 40:1 provide good performance for imaging applications, while higher ratios are preferred for detection applications. Higher ratios also result in thicker plates with more mechanical robustness.

References

Del Mar Photonics—Microchannel Plates 

Photonis—Microchannel Plates

Image credits:

Hamamatsu Photonics K.K. | PHOTONIS