Appendix H Glossary 223
MCP Gating: Available for PI-MAX
MG
cameras. Applies the primary gating pulse to the
MCP portion of the tube and, if chosen by the user, applies the bracket pulse to the
photocathode.
MCP Resolution: The MCP is a slightly conductive glass substrate with millions of
parallel traversing channels containing a secondary electron emitter on their inner
walls. The smaller the diameter and more tightly grouped the channels, the higher the
resolution.
Phosphor Type: A phosphor is a chemical substance that fluoresces when excited by x-
rays, an electron beam, or ultraviolet radiation. Phosphors usually emit green light
with decay times ranging from hundreds of nanoseconds to a few milliseconds. P43
offers high resolution (3 ms decay) while P46 offers fast decay for high-repetition
rate spectroscopy (3 µs decay). The shutter compensation time (see below), inserted
between the end of the exposure time and the beginning of the array readout, allows
for the decay time.
Photocathode Coatings: Coatings on the photocathode convert a portion of the incident
photons into electrons. Any photons that are not captured by the photocathode are
lost from the final signal produced by the intensifier. Therefore, the kind of coating
and the resulting QE of the photocathode is very important. The choice of coating
determines the most effective spectral range for the intensifier. For example GaAs
(gallium arsenide) has high QE in the VIS and NIR regions. Multi-alkali coatings
have fair photoconversion in the visible (VIS) and ultraviolet (UV) but have
relatively limited response in the near IR (NIR).
Pulse Ensemble: PTG term. Consists of a Gate Start pulse, a Gate Stop pulse, and an
Auxiliary pulse. At the end of the ensemble, the photocathode is gated off, shutter
compensation time elapses to allow for phosphor decay, and then the CCD array is
readout.
QE: Quantum Efficiency. The percentage of incident photons converted to electronic
charge. The throughput of the input windows, the spectral sensitivity of the
photocathode and the CCD array, the illuminated surface (front or back) of the CCD
array, the intensifier on/off ratio, the MCP resolution, the MCP gain, and the
intensifier-CCD coupling all contribute to the total system QE.
RAM: Random Access Memory used to store data such as experiment parameters.
Region of Interest (ROI): A square or rectangular set of contiguous pixels on the CCD
array that is usually smaller than the full frame. Using an ROI to acquire data results
in a faster readout of the array since data from pixels outside of that ROI is discarded.
Safe Mode: PI-MAX intensifier mode in which the photocathode is continuously biased
off.
Saturation: Caused when a pixel well is completely filled with charge. Once a pixel is
saturated, additional charge will spillover (bloom) into adjacent pixels. Ways to deal
with saturation include lowering the array temperature (to reduce the dark charge
component), shortening the exposure time (to reduce the signal component), and
decreasing the gain (also to reduce the signal component).
Scan or Scanning: The process of reading out the contents of a CCD array.
Shutter Compensation Time: This is the amount time inserted between the end of the
exposure time and the beginning of the array readout. For a PI-MAX, the appropriate
To Next Page
Loading...
