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Microchannel plate detector
A micro-channel plate (MCP) is a planar component used for detection of particles (electrons or ions) and impinging radiation (ultraviolet radiation and X-rays). It is closely related to an electron multiplier, as both intensify single particles or photons by the multiplication of electrons via secondary emission.
Additional recommended knowledge
A micro-channel plate is a slab made from highly resistive material of typically 2 mm thickness riddled with tiny tubes or slots (microchannels) leading from one face to the opposite, densely distributed over the whole surface. The microchannels are typically approx. 10 micrometer in diameter and have an approx. 15 micrometer spacing between each other, are parallel to each other and often enter the plate at a small angle to the surface (~8° from normal).
Each microchannel is a continuous-dynode electron multiplier, in which the multiplication takes place under the presence of a strong electric field. A particle or photon 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, which amplifies the original signal by several orders of magnitude depending on the electric field strength and the geometry of the micro-channel plate. After the cascade, the microchannel takes time to recover (or recharge) before it can detect another signal.
The electrons exit the channels 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.
Most modern MCP detectors consist of two microchannel plates with angled channels rotated 180° from each other producing a chevron (v-like) shape. In a chevron MCP the electrons that exit the first plate start the cascade in the next plate. The advantage of the chevron MCP over the straight channel MCP is significantly more gain at a given voltage. The two MCPs can either be pressed together or have a small gap between them to spread the charge across multiple channels.
An external voltage divider is used to apply 100 volts to the acceleration optics (for electron detection), each MCP, the gap between the MCPs, and the backside of the last MCP and the collector (anode). The last voltage dictates the time of flight of the electrons and in this way the pulse-width. The anode is a 0.4 mm thick plate with an edge of 0.2 mm radius to avoid high field strengths. It is just large enough to cover the active area of the MCP, because the backside of the last MCP and the anode act as a capacitor with 2 mm separation and large capacitance slows down the signal. The positive charge in the MCP influences positive charge in the backside metalization. A hollow torus conducts this around the edge of the anode plate. A torus is the optimum compromise between low capacitance and short path and for similar reasons usually no dielectric (Markor) is placed into this region. After a 90° turn of the torus it is possible to attach a large coaxial waveguide. A taper allows to minimize the radius so that an SMA connector can be used. To save space and make the impedance match less critical, the taper is often reduced to a small 45°Cone on the backside of the anode plate.
The typical 500 volts between the backside of the last MCP and the anode cannot be fed into the preamplifier. Therefor the inner or the outer conductor needs a DC-block, that is a capacitor. Often it is chosen to only have 10-fold capacitance compared to the MCP-anode capacitance and is implemented as a plate capacitor. Rounded, electro-polished metal plates and the ultra high vacuum allow very high field strengths and high capacitance without a dielectric. The bias for the center conductor is applied via resistors hanging trough the waveguide (see bias tee). If the DC block is used in the outer conductor, it is in a parallel circuit with the larger capacitor in the power-supply. Assuming good screening the only noise is due to current noise from the linear power regulator. Because the current is low in this application and space for large capacitors is available, and because the DC-block capacitor is fast, it is possible to have very low voltage noise, so that even weak MCP signals can be detected. Sometimes the preamplifier is on a potential and gets it power through a low power isolation transformer and outputs its signal optically.
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. The jitter can be further reduced by means of a constant fraction discriminator. That means that MCP and the preamplifier are used in the linear region (space charge negligible) and the pulse shape is assumed to be due to an impulse response with variable height but fixed shape from a single particle.
Because MCPs have a fixed charge, that they can amplify in their life, especially 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. (see: Secondary emission#Special amplifying tubes, , , . ).
Delay line detector
The electrons are accelerated to 500 eV between the back of the last MCP and a grid. Then they fly for 5 mm and are dispersed over an area of 2 mm. A grid follows. Each element has a diameter of 1 mm and consists of an electrostatic lenses focusing arriving electrons through a 30 µm hole of a grounded sheet of aluminum. Behind that a cylinder of the same size follows. The electron cloud induces a 300 ps negative pulse when entering the cylinder and a positive when leaving. After that another sheet, a second cylinder follows, and a last sheet follow. The cylinders are fused into the center-conductor of a stripline. These striplines meander across the anode to connect all cylinders, to offer each cylinder 50 ohm impedance, and to generate a position dependent delay. The sheets minimize cross talk between the layers and adjacent lines in the same layer, which would lead to signal dispersion (optics) and ringing, as do the 180° turns. So the number of turns is limited and for high resolution multiple meanders are needed (you get what you pay). At both ends the meanders are connected to the electronic. The first layer generates the X-coordinate the second layer the Y-coordinates. Sometimes a hexagonal grid and 3 coordinates are used. This redundancy reduces the dead space-time.
After extracting 1 coulomb from 1 cm² gain is half. For chevron these are 1013 detected particles. With a megacountrate these takes 107 seconds = 4 month. Most detectors have a larger area.
Single MCPs produce a negative exponential PHD . Channels space charge saturation is achieved near the channel output of the second plate in a chevron configuration. Nothing is said on the effect of saturation on timing. Saturation is necessary to prevent the ringing of the electronics to outshine following pulses. Saturation not only leads to a faster than exponential decrease of high charge pulses, but for non dark-current events also to a decrease in low the amount of charge pulses.
Reducing channel diameter for chevron from 25 µm to 10 µm increases pulse rise time from 640 ps to 283 ps.
For most assemblies the anode has to be on ground
for some a version with opto-isolator is available, which still delivers 2 ns pulse width, which is typical for plastic scintillators, which are not bakeable for ultra high vacuum applications]
MCP Assembly technical information by Hamatsu:
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Microchannel_plate_detector". A list of authors is available in Wikipedia.|