An important performance
feature of a Microchannel Plate (MCP) detector is the dynamic
range. The dynamic range is the range of detectable signal level
over which the MCP linearly amplifies a signal.
The dynamic range of a microchannel
plate or any electron multiplier when detecting extremely low
signal levels is limited by the dark current or the background
noise inherent in the multiplier. There are a number of ways
to minimize the effects of the dark current or dark count, including
the use of some recently developed specialized low noise glass
formulations.
The background count rate of a
microchannel plate is limited ultimately by the cosmic ray background
found here on earth. The background count rate of the glass
itself is limited by the amount of radioactive decay of trace
impurities found in the glass. The glasses used to manufacture
BURLE electron multipliers have been specially formulated to
produce low background noise.
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The upper limit of dynamic range
is ultimately limited by charge saturation effects within the
multiplier itself.
There are a number of ways to raise
the high end limit of the dynamic range of a microchannel plate
based detector. The most effective way is to simply lower the
resistance of the individual channel. Each channel can be considered
an effective resistor capacitor network -- by simply lowering
the resistance of the channel, the "dead time" of
the channel is diminished proportionally to the change in the
RC time constant.
The BURLE Long-Life™
glass composition is specifically tailored to be able to produce
very low resistance channels. Low resistance microchannel plates
are called Extended Dynamic Range MCPs or EDR plates.
Another effective means of increasing
the upper count rate of a microchannel plate is to increase
the channel redundancy per unit area. Microchannel plates used
around the world are manufactured with pores as large as 100
microns and now as small as two microns.
By effectively increasing the number
of channels covering the same area of a microchannel plate,
the number of missed events caused by a second event entering
the specific channel within the dead time of the channel is
greatly diminished.
Figure 1 is an example of going
from a 25 micron pore channel down to as small as a five micron
pore channel. If an electron were to enter the 25 micron channel,
and a second event came during the time frame it took for that
channel to regenerate, it would be totally lost. If, however,
the larger diameter (25µm) were replaced by a number of smaller
pores (5µm), only the individual 5 micron channel would be dead
for that time period, and a second event entering another channel
displaced by as little as five microns would be detected and
amplified. BURLE now manufactures microchannel plates with pore
sizes as small as two microns.
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