Photomultiplier Tubes In optical emission spectrometry, photomultipliers are commonly used as detectors. They are photocell detectors. The incident photons coming from the exit slit liberate electrons from the photocathode and the electron flow is then amplified by a set of dynodes. The final anode current is proportional to the incident photon signal received by the photocathode.
The measurement dynamic range is very broad, i.e. 1015, and sensitivity is high, as the dark current is low. These detectors allow the detection of low intensities emitted by trace elements, as well as strong signals from major elements. They have very fast response times, typically 1-2 ns for a 10%-90% change in signal. The main inconvenience of photomultipliers is their cost.
There are several types of photomultipliers, which differ in the nature of the entrance window, either crystal or fluoride, and in the nature of the sensitive layer on the photocathode. Some are only sensitive in the far ultraviolet while others are more sensitive in the visible. The type of photomultiplier to be used is selected according to the wavelength of the line to be detected.
A fatigue lamp (a small incandescent light source) is often used with photomultipliers to keep the temperature of the tube and its associated electronics constant. The fatigue lamp is switched on when the emission source is off and switched off when the emission source is on.
Solid State Detectors
An example of an array solid state detector
Concept: Pass charge from one capacitor to another by changing applied voltage in a coordinated fashion. The photon strikes silicon and is converted to a charge that can be transported and measured by electronic structure built on monolithic Silicon chip.
Two types of optical solid state detectors
Charge Coupled Device (CCD) Charge Injection Device (CID) Advantages of Solid State Detectors Wide range of elements and wavelengths Global analysis over the range of the chip Retrospective analysis for 'extra' elements 'Simultaneous' analysis Simultaneous background correction Cheap Disadvantages (compared with photomultiplier tubes) Smaller signals, mainly because of the much smaller surface area of the light sensitive region Higher noise, chiefly counting noise Poorer signal to background ratios Worse detection limits Poorer spectral resolution - mathematical corrections required Resolution changes with wavelength in some designs Blooming at high intensities occurs in nearby pixels Slower response time Speed usually limited by the need to integrate to overcome counting noise Smaller dynamic range of intensities
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