Detector microchannel-plate photomultiplier

The time resolution of a phase fluorometer using the harmonic content of a pulsed laser and a microchannel plate photomultiplier is comparable to that of a single-photon counting instrument using the same kind of laser and detector. 

A much better time resolution, together with space resolution, can be obtained by new imaging detectors consisting of a microchannel plate photomultiplier (MCP) in which the disk anode is replaced by a coded anode (Kemnitz, 2001). Using a Ti-sapphire laser as excitation source and the single-photon timing method of detection, the time resolution is <10 ps. The space resolution is 100 pm (250 x 250 channels). 

A laser system that delivers pulses in the picosecond range with a repetition rate of a few MHz can be considered as an intrinsically modulated source. The harmonic content of the pulse train - which depends on the width of the pulses - extends to several gigahertz. The limitation is due to the detector. For high frequency measurements, it is absolutely necessary to use microchannel plate photomultipliers (that have a much faster response than usual photomultipliers). The highest available frequencies are then about 2 GHz. As for pulse fluorometry, Ti sapphire lasers are most suitable for phase fluorometry, and decay times as short as 10-20 ps can be measured. 

Other, faster detectors that can be used in place of a conventional PMT include microchannel plate photomultiplier tubes (MCP-PMTs) (31) and streak cameras (37). Because of their expense, the use of these devices is usually confined to home built fluorimeters foimd in dedicated fluorescence laboratories, and is therefore not discussed here. 

The time resolution of a photomultiplier is limited mainly by the variations in the paths that electrons take in reaching the anode. Because of the spread in transit times, the anode pulse resulting from the absorptitm of a single photon typically has a width on the order of 10 -10 s. The spread of transit times is smaller in microchannel plate photomultipliers, which work on the same principles as ordinary photomultipliers except that the electronic amplification steps occur along the walls of small capillaries. The anode pulse width in a microchannel plate detector can be as short as 2 x 10 s. 

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. 

A variant of the EM is the Daly detector (Daly, 1960) in which ions are accelerated by 30 KV (this is called post-acceleration because it occurs after mass analysis) into a conversion dynode, which generates a significant number ( 10) of secondary electrons. These electrons are accelerated into a scintillator and converted into light, which is detected with a photomultiplier. The Daly detector offers high gain, low noise, and excellent stability. Other variants of the post-acceleration detector exist a simple configuration uses a metal plate to convert the ions into electrons for detection with an EM. Another variant of the EM is the microchannel plate (Coplan et al 1984 Odom et al 1990 Wiza, 1979). MicroChannel plate EM detectors have excellent sensitivity but poor gain stability. When operated in... 

Hybrid detectors are different from dual MCPs in that they possess a single microchannel plate. The second amplification stage is often achieved using a scintillator which surmounts the entrance window of a photomultiplier. Sometimes a venetian-blind steel dynode is found in the initial part of a hybrid detector in order to perform ion-to-electron conversion. 

A sensitive light detector that can rapidly and repeatedly be activated and inactivated is an ideal detector for time-resolved measurement of luminescence intensity. Photomultiplier tubes (PMTs) in its variants and microchannel plates (MCPs) are detectors that fill these requirements to a large extent - and in practice the only... 

The stop detector, usually a high gain (>10 ) photomultiplier (nowadays also a microchannel plate), displays an electric signal (a pulse) when hit by the first (and lonely) photon this pulse stops the time cycle initiated by the start signal then the detector quits until a subsequent start signal begins a new excitation-emission cycle. 

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