Microchannel plate detector

A microchannel plate (commonly used is the English term micro -channel plate, abbreviated MCP ) is an areal, image resolution secondary electron multiplier.

It is used for low-noise amplification low currents of free electrons, ions, or high-energy photons. This strike the plate, where they generate secondary electrons which can then be detected.

Design and operation

Between the two sides of the plate is metallized at an acceleration voltage, the plate itself is made of lead glass. The inner walls of the channels are made of a semiconductor material. The board is similar to a sieve or perforated crossed by microscopic channels, which typically have a pitch of about 10 microns and a diameter of about 6-25 microns. The plate has a thickness of a few tenths of millimeters (up to around 1 mm) and the channels are tilted by about 10 ° relative to the plate axis, so that the incident electrons with safety repeatedly hit the duct wall. They are then accelerated by a between the plates along the channels applied electrical voltage and multiply at each wall collision, each channel behaves as a microscopic channel electron multiplier, or the gradual secondary electron multiplier in photomultipliers.

The number of electrons has increased by multiple collisions with the channel wall by approximately 1000-fold at the exit side. By Nachbeschleunigungsstrecke the reinforced ( = multiplied ) electrons on the actual detector directed, usually a fluorescent screen, but also for example a EBCCD the electron Bombarded CCD, a special form of CCD for detection of free electrons.

Application

Used microchannel plates for example in image intensifiers and electron spectroscopy as well as mass spectrometry. In the latter case, the inlet side to be coated with special materials to increase the sensitivity of the detected particle. This additional coating can be effectively converted into electrons than in uncoated MCPs for example ions. Since free electrons to be detected MCPs can only be used in a high vacuum. Another application of MCPs, the brightness increase of cathode ray tubes dar. MCPs are therefore in high-speed analog oscilloscope ( Tektronix 7104, 2467B ) used to make infrequent events visible.

Quality factors

Quality -determining parameters are in particular:

• Gain ( multiplication factor for the electron for a given accelerating voltage)
• Dynamics ( the ratio of the maximum to the minimum detected signal), the maximum signal is limited to about 10 % of the cross current. The cross- current flows due to the applied acceleration voltage by half conductive plate material itself - that is, regardless of the number of incident electrons ( the cross-flow, the multiplied electrons removed)
• Spatial resolution
• Sensitivity ( detection efficiency: the ratio of detected to incident electrons )
• The pulse height distribution (or pulse height distribution PHD abbreviated ) indicates how much the number of exiting electrons will vary if each individual electrons impinge on the input side. This variation should be as low as possible as the variation of the transit time of each electron showers during amplification. Both are strongly dependent on the acceleration voltage, which is typically between 400 volts and 1000 volts.

Finally, the homogeneity of all these parameters over the entire active area of the MCP is an essential feature of quality.

Production

The most critical factor of an MCP is the uniformity of microscopic channels. The preparation requires control techniques similar to an optical fiber: there is each one of the fused optical fibers of core glass and cladding glass are fused to each other in a first operation, such as single shrunk- on spaghetti macaroni. Subsequently an increasingly larger number of individual strands bundled and fused together is pulled or pressed in semi-liquid state, which can be obtained microscopically fine structures in extremely high regularity after several passes.

As the solid heterogeneous strand is then separated into thin slices from which the core glass is etched chemically in the case of MCP - so here is the remainder of the cladding glass of the active semiconductor device.