Conventional Photomultiplier Tubes

The most frequently used detectors for low-level detection of light are photomultiplier tubes. A conventional photomultiplier tube (PMT) is a vacuum device that eontains a photocathode, a number of dynodes (amplifying stages) and an anode that delivers the output signal (Fig. 6.1). 

A wide variety of photomultipliers are used, with different shapes, different cathode geometries and diameters, and different dynode geometries [219, 297, 348]. Some tube designs have been successfully used for more than 50 years. Typieal design principles are shown in Fig. 6.2. 

Of special interest for time-correlated single photon counting are the linear fo-cused dynodes, which give fast single electron response and low transit-time jitter, and the fine mesh and metal channel types, which offer position sensitivity when used with an array of anodes. Moreover, PMTs with fine-mesh and metal channel dynodes can be made extremely small, which results in low transit time, low transit-time jitter, and a fast single-electron response. 

The gain systems used in photomultipliers can also be used to detect electrons or ions. The detectors are then called Electron Multipliers. The particles to be detected are fed directly into the dynode system. 

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There can be little doubt there is still much progress to be made in the application of solid-state detectors to the study of near-IR fluorescence. However, solid-state photomultipliers capable of replacing the conventional photomultiplier tube in mainstream fluorescence applications would still seem to be a long way away. 

Time-Correlated Single-Photon Counting. For the application of TCSPC in the picosecond time domain, lasers with pulses whose half-widths are 20 ps or less are used. For better time resolution, the combination of a microchan-nel plate photomultiplier tube (MCP-PMT) and a fast constant fraction discriminator (CFD) are used instead of a conventional photomultiplier tube (PMT). A TCSPC system with a time response as short as 40 ps has at its core a Nd YLF (neodymium yttrium lithium fluoride) laser generating 70-ps, 1053-nm pulses at... 

The interfacing of GC with MI fluorescence spectrometry also has been accomplished for fluorometric examination of GC effluents, the conventional photomultiplier tube detector is replaced by a SIT vidicon to facilitate rapid acquisition of MI fluorescence spectra of individual GC fractions (30). 

Fluorescence droplet detection is conducted by illuminating the droplet of interest and detecting the emission of photons, e.g., a fluorescence microscope coupled with a sensitive CCD. This is the most commonly used methods for biological applications. However, the real-time and sensitive individual droplet detection is limited by the slow response of conventional CCDs. To address this issue, expensive high-speed cameras, conventional photomultiplier tubes, or avalanche photodiode detectors are often selected to capture... 

A schematic cross-section of one type of photomultiplier tube is shown in Figure 26. The photomultiplier is a vacuum tube with a glass envelope containing a photocathode and a series of electrodes called dynodes. Light from a scintillation phosphor liberates electrons from the photocathode by the photoelectric effect. These electrons are not of sufficient number or energy to be detected reliably by conventional electronics. However, in the photomultiplier tube, they are attracted by a voltage drop of about 50 volts to the nearest dynode. 

In conventional chip experiments, fluorescence scanners are used for chip read-out. In the case of laser scanners, HeNe lasers are used as excitation sources and photomultiplier tubes as detectors, whereas CCD-based scanners use white light sources. The optical system can be confocal or non-confocal. Standard biochip experiments are performed using two fluorescent labels as... 

In this final section, we summarize the operation and characteristics of the principal vacuum tube and solid state detectors that are available for red/near-IR fluorescence studies. These include conventional photomultipliers, microchannel plate versions, streak cameras, and various types of photodiodes. Detector applicability to both steady-state and time-resolved studies will be considered. However, emphasis will be placed on photon counting capabilities as this provides the ultimate sensitivity in steady-state fluorescence measurements as well as permitting lifetime studies. 

Photomultiplier tubes are a variation of the conventional phototube. Such tubes have several intermediate electrodes, known as dynodes, in addition to the primary photocathode and anode. Electrons emitted from the cathode strike the first of these dynodes, whereupon by secondary emis-... 

Image dissector tubes (Fig. 26) make use of an entrance aperture behind the photocathode, by which the photoelectrons stemming from different locations of the photocathode can be scanned and measured after amplification in the dynode train, as in a conventional photomultiplier. Although used in combination with an echelle spectrometer with crossed dispersion for flexible rapid sequential analyses [59], these systems have not had any commercial breakthrough. This might be due to the limited cathode dimensions but also to stability problems. 

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