Modern Photon Detectors

Chapter 5 discusses Modern Photon Detectors in a general way. There are too many materials, devices, and variants to attempt a comprehensive review - and it would quickly become out of date. Much of the material in these chapters is new because there are many more detector types available now than there were in 1990. 

Modern UV/VIS spectrometers are generally hmited by shot noise in the photon detector as electrons cross a junction. In this case, the plot of relative uncertainty due to indeterminate instrument error looks very different from Fig. 2.15. For good quality, shot noise limited instruments, the relative error is high for very low values of A (high %T), but absorbance values from 0.2 to above 2.0 have approximately the same low (< 1 %) relative uncertainty. In other words, modern spectrometers are much more accurate and precise than older ones because of improvements in instrument components. 

Although most of these types of thermal detector have been in use for many years, they are all used in modern instrumentation. They have not been rendered obsolete by the more recent development of photon detectors because for many types of application use of the appropriate thermal detector is more suitable than a photon detector. 

The most widely used modern scintillation detector consists of a transparent crystal of sodium iodide that has been activated by the introduction of 0.2% thallium iodide. Often, the crystal is shaped as a cylinder that is 3 to 4 in. in each dimension one of the plane surfaces then faces the cathode of a photomultiplier tube. As the incoming radiation passes through the crystal, its energy is first lost to the scintillator, this energy is subsequently released in the form of photons of fluorescence radiation. Several thousand photons with a wavelength of about 400 nm are produced by each primary particle or photon over a period of about 0.25 ps, which is the dead time. The dead time of a scintillation counter is thus significantly smaller than the dead time of a gas-filled detector. 

Transducers. Most modern electron spectrometers are based on solid-state, channel electron multipliers, which consist of tubes of glass that have been doped with lead or vanadium. When a potential difference of several kilovolts is applied across these materials, a cascade or pulse of 10 to 10 electrons is produced for each incident electron. The pulses are then counted electronically (see Section 4C). Several manufacturers are now offering two-dimensional multichannel electron detectors that are analogous in construction and application to the multichannel photon detectors described in Section 7E-3. Here, all of the resolution elements of an electron spectrum are monitored simultaneously and the data stored in a computer for subsequent display. The advantages of such a system are similar to those realized with multichannel photon detectors. 

On the other hand, those characteristics of the laser, which are important for its applications in spectroscopy are treated in more detail. Examples are the frequency spectrum of different types of lasers, their linewidths, amplitude and frequency stability, tunability, and tuning ranges. The optical components such as mirrors, prisms, and gratings, and the experimental equipment of spectroscopy, for example monochromators, interferometers, photon detectors, etc., are discussed extensively because detailed knowledge of modern spectroscopic equipment may be crucial for the successful performance of an experiment. 

The first detector for optical spectroscopy was the human eye, which, of course, is limited both by its accuracy and its limited sensitivity to electromagnetic radiation. Modern detectors use a sensitive transducer to convert a signal consisting of photons into an easily measured electrical signal. Ideally the detector s signal, S, should be a linear function of the electromagnetic radiation s power, P,... 

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