Download 2 - DORAS

background signal present. This problem can be minimised by detector cooling and also
by the suitable choice o f photocathode material.
Over the last 15 years or so array detectors have been developed to act as alternatives
to photographic and single-channel PMT type detectors. They offer the advantages of
electronic read-out of a PMT along with the spatial resolution and multiplexing
characteristics of a photographic plate.
These detectors are solid state devices,
consisting of a large number of light sensitive elements (usually 512 or 1024) closely
arranged in a row.
Each individual element represents one channel of an optical
multichannel analyser (OMA),
in which a count proportional to the intensity of the
incident radiation on the individual element is stored. An OMA is a electro-optical
signal-processing/readout system which when combined with an array detector allows
real-time detection along with image processing capabilities. Array detectors are placed
in the focal plane of the spectrometer and with geometry of 1024 photodiodes, 25 mm
diode separation and 2.5 mm height form effectively a strip of electronic 'photographic'
plate.
To improve the light sensitivity of these devices the array detector is placed
behind an image intensifier device such as a microchannel plate (MCP, see Chapter 2).
This device produces bunches of electrons that are spatially arranged corresponding to
the original radiation spectrum. These electron bunches impinge on a phosphorescent
material. The light thus produced is then transferred to the diode array with the spatial
information retained via fibre-optic coupling device. A more complete description of
such a detector assembly is given in Chapter 2. The above type of diode array detector
was initially developed for plasma impurity analysis in tokamak fusion devices [Fonck et
al 1982, Hodge et al 1984, Schwob et al 1983] but has become widely used in
laboratory spectroscopy. The main benefit of these detectors is their capability to record
simultaneously and digitally acquire whole spectra.
The speed and ease of data
acquisition and processing with these detectors has meant they are attractive alternatives
to photographic and PMT detection systems.
1.4 S PE C T R O SC O PIC IM A G E EN H A N C EM EN T
Spectroscopic imaging systems use either photographic (film, plates) or
photoelectric (PMT’s, Diode Arrays) detection for image sampling and recording and
thus set a limit on the highest spatial frequency which can be recorded.
Further the
optical elements used to produce the images (lenses, mirrors, stops, gratings, etc.)
degrade the quality of the image formed.
The images are therefore instrumentally
smeared with a resultant loss of spatial resolution. It is possible, with a knowledge of
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