The detection efficiency of a Microchannel
Plate (MCP) is an important performance parameter. In many applications,
the quantity of the events to be detected is quite small. And,
therefore, it is important that event be detected and added
to the signal statistics.
Some of the features that can affect
the detection efficiency of a microchannel plate are the:
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Open
Area Ratio (OAR) - The open area ratio is the
percentage of the entire area of the microchannel
plate which consists of open pores. Open area
ratios range from 50-70% for most MCPs. Increased
OAR often improves the collection efficiency and
optimizes detector sensitivity.
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Bias
angle - The bias angle of a microchannel plate
is the angle of the channels from the perpendicular
or normal to the MCP surface. MCPs are typically
manufactured with bias angles of 5-30º. MCPs with
channels running perpendicular to the MCP surface
(0º bias) are used for collimation and filtration
applications, or when the input events arrive
at oblique angles. The detection efficiency of
an MCP for various charged particles and electro-magnetic
radiation can be optimized by controlling the
angle of incidence of the input event.
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Metallization
or electroding configuration - Nichrome electroding
is applied to both surfaces of a microchannel
plate to provide electrical contact. The electroding
also penetrates into the channel. The penetration
depth is minimized on the input face of the MCP
(typically 0.3-0.7 channel diameters) to maximize
the first strike conversion efficiency of incoming
events into the channel. The electrode penetration
on the output face of the MCP is much deeper (typically
1.7-3.0 channel diameters) to provide a lensing
effect for optimized spatial resolution.
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Microchannel plates can be customized
in a number of ways to enhance the detection efficiency or,
in some cases, to lower the detection efficiency for a particular
desired or undesired species that might be present in the instrument
or the experiment. Various types of secondary electron emissive
coatings, such as cesium iodide, magnesium fluoride, magnesium
oxide, copper iodide, and gold, can be used to significantly
enhance the detection efficiency for various charged particles
and electromagnetic radiation.
In some detector applications,
the use of a suppression grid is also helpful in enhancing the
detection efficiency.
Figure 1 shows the detection efficiencies
typically achieved for various charged particles and electro-magnetic
radiation.
