Signal detectors charge coupled devices

Great progress has also been achieved in the field of low-level signal detection. Apart from new photomultipliers with an extended spectral sensivity range and large quantum efficiencies, new detection instruments have been developed such as image intensifiers, infrared detectors, charge-coupled devices (CCDs) or optical multichannel analyzers, which could move from classified military research into the open market. For many spectroscopic applications they prove to be extremely useful. 

A photomultiplier tube is a sensitive detector of visible and ultraviolet radiation photons cause electrons to be ejected from a metallic cathode. The signal is amplified at each successive dynode on which the photoelectrons impinge. Photodiode arrays and charge coupled devices are solid-state detectors in which photons create electrons and holes in semiconductor materials. Coupled to a polychromator, these devices can record all wavelengths of a spectrum simultaneously, with resolution limited by the number and spacing of detector elements. Common infrared detectors include thermocouples, ferroelectric materials, and photoconductive and photovoltaic devices. 

The most recent advance in ROA instrumentation is the use of charge coupled device (CCD) detectors [86,87]. These detectors have extremely low background noise, large areas, and very high quantum efficiency (in the range of 25 to 80%). The use of these detectors in ROA measurements has given rise to a renaissance in this field. Applications that were previously impossible due to low signal quality have now become almost routine. 

The most common detector for AES is the photomultiplier tube (see p. 174). An alternative approach for the detection of multielement (multiwavelength) information is the charged-coupled device (CCD). A CCD is essentially an array of closely spaced metal-insulator emiconductor diodes formed on a wafer of semiconductor material. Incident light striking the CCD is converted into an electrical signal. 

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