Microchannel Plates
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The microchannel plate is an open multiplier of specific type intended for registration of particles and radiations. MCPs are used as multiplying systems in different types of detectors, including photomultiplier tubes /PMT/ and secondary-electron multipliers /SEM/ providing better time characteristics and magnetic-field resistance than those of the devices having a discrete structure. |
MCP represent 0.4-2.0 mm thick plates of round or rectangular shape. They have a honeycomb structure and contain in one square centimeter up to one million of separate channels of 10-15 microns diameter. In addition to design simplicity, small dimensions and absence of external voltage divider, MCP feature high time and spatial resolution capability.
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Honeycomb structure of Microchannel Plates. For additional images of Microchannel Plate surface at different spatial resolution click here. |
Construction and Operation
A MICROCHANNEL PLATE begins as a glass tube fitted with a solid, acid-etchable core and drawn via fiberoptic techniques to form single fibers. A number of these fibers are then stacked in a hexagonal array; the entire assembly is drawn again to form multi-fibers. The multi-fibers are then stacked to form a boule or billet, which is fused together at high temperature.
The fused billet is sliced on a wafer saw to the required bias angle, edged to size, and then ground and polished to an optical finish. The individual slices are chemically processed to remove the solid core material, leaving a "honeycomb" structure of millions of tiny holes. Each hole is capable of functioning as a single channel electron multiplier which is relatively independent of the surrounding channels.
Through subsequent processing, this glass wafer is given its conductive and secondary emissive properties. Finally, a thin metal electrode (usually inconel or nichrome) is vacuum-deposited on both input and output surfaces of the wafer to electrically connect all the channels in parallel.
In this manner, microchannel plate arrays may be fabricated in a wide variety of formats. The arrays may range in size from 6mm to 80mm or larger, and they may be circular, rectangular, or virtually any other shape as required by the application or instrument geometry. In addition, a cylindrical or spherical radius of curvature may be provided to conform to the focal plane of an instrument.
MCP resolution is determined by channel diameter and center-to-center (c-c) spacing. Channel diameters ranging from 10mm (12mm c-c) to 15mm (18mm c-c) are standard.
For normal operation, a bias of up to 1000 volts is applied across the microchannel plate, with the output at its most positive potential. The bias current flowing through the plate resistance is what supplies the electrons necessary to continue the secondary emission process.
When a photon or charged particle is incident at the input of a microchannel, secondary electrons are generated. Due to the bias voltage applied these electrons are accelerated down the channel toward the output end. When they strike the channel walls en route, additional secondary electrons are generated. This process is continuously repeated down the channel until, at the output, a pulse of up to 10.000 electrons may be realized. If two or more MCPs are operated in series, a single input event will generate a pulse of 107 or more electrons at the output.
The output signals are typically collected in any of several ways, including metal or multimetal anodes, resistive anode (one- or two- dimensional), wedge and strip anode, Delay-Line Readout or on a phosphor screen deposited on a fiberoptic or other substrate.
Dynamic and Noise
Rise time The rise time of MCP assemblies is very small, usually it's less than 1 ns. (Rise time of the electron avalanche in MCP channels is about 100 ps). The width of the single ion peak determined mainly by temporal characteristics of electronics used to detect electron avalanche.
Dead time To discuss the dead time we should first determine the meaning of this time. Usually in our own research we use different characteristic -maximum average current at the output face of the MCP at which MCP has linear response (Imax). When the output current is larger than Imax then charge on the output face appears leading to smaller gain (nonlinear response). Practical criteria for Imax which we use is: Imax=Imcp/10 (*) here Imcp=Vmcp/Rmcp. Typical value of Imcp= 10mA, so Imax=1mA. This criteria for constant ion current can be expressed in terms of maximum number of ions per channel per second (Nmax). For two-stage MCP amplifier with gain 107 for MCP-34 (number of channels about 5*106 this corresponds to Nmax=0.1 ions/channel/s. From the relation (*) we can estimate the "dead time" of the MCP in pulsed experiments (like TOF MS - Time-of-flight Mass Spectrometer). This is the time between ion pulses corresponding to the average current Imax. Let n be the number of ions per pulse, k - gain, t - time between ion pulses. Then I=n*k*e/t. The dead time Tdead = n*k*e/Imax =10*n*k*e*Rmcp/Vmcp. For n=103, k=107, R=108, V=103 Tdead= 1.6*10-3 s. For n=104, k=108 (maximum values) Tdead is about 0.1s.
Noise characteristic Noise characterized by the dark current. For MCP-34 the area of the plate is about 9cm2. The dark current is about 3*10-12A. From this figure one can estimate noise level at given experimental conditions.
Detection Efficiency
The detection efficiency of an Microchannel Plate (MCP), sometimes termed the Quantum Detection Efficiency (DQE), is defined as the probability of an incident source of radiation (a photon, electron or ion) leading to an amplified output electron pulse. When incident radiation strikes the surface of a channel, there is a probability that an electron will be ejected from the wall and in turn amplified. In the case of charged particles this probability is relatively high (-60%) for moderately energetic electrons and ions (0.3 - 3.0 keV), but is strongly dependent on incident energy. The DQE is comparatively low for X-ray and ultraviolet radiation's (5-10%) generally increasing with photon wavelength from 0.01 nm until a sharp drop-off at 120 nm. However, the sensitivity to ultra-violet radiation is commonly increased with various photon-cathode coating applied to the input face of the MCP. Under most normal operating conditions the DQE of an MCP is limited to the open-area-ratio (OAR), since generally electrons resulting from radiation strikers in the interchannel web area are not collected. Typically the OAR for MCP is approximately 60% and is practically limited to 80%.Bias angle, the angle of the channel with respect to the surface normal can have a dramatic effect on Quantum Detection Efficiency (DQE). The incident angle of radiation to the channel wall will have a similar effect. There is typically a sharp drop off DQE below a ~5° bias angle resulting from an increased reflectivity of the channel wall for low grazing angles of radiation. Although the peak detection for electrons falls about ~5° , the peak in DQE is at ~10° for x-ray ultraviolet (XUV 10-50 nm), and at ~15° for extreme-ultraviolet (EUV 50-100 nm). At progressively higher than ~15° bias angles the incoming radiation penetrates more deeply into the channel wall, thus lower the probability that a secondary electron created by incident strike can escape the surface and resulting in low DQE.
Pore size has no correlation with DQE Instead it is typical that smaller pore size MCPs have higher open-area-ratios resulting in an increase in the limit to DQE imposed by this geometric constraint.
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