Photomultipliers microchannel plate

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. 

MicroChannel plate photomultipliers are preferred to standard photomultipliers, but they are much more expensive. They exhibit faster time responses (10- to 20-fold faster) and do not show a significant color effect (see below). 

With mode-locked lasers and microchannel plate photomultipliers, the instrument response in terms of pulse width is 30-40 ps so that decay times as short as 10-20 ps can be measured. 

For high-frequency measurements, normal photomultipliers are too slow, and microchannel plate photomultipliers are required. However, internal crosscorrelation is not possible with the latter and an external mixing circuit must be used. 

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). 

Streak cameras and multianode microchannel plate photomultipliers (MCP-PMs) interfaced to a polychromator also permit multiwavelength fluorescence decay measurements, the spectral response of both being determined by the photocathode composition. 

H. Kume, T. Taguchi, K. Nakatsugawa, K. Ozawa, S. Suzuki, R. Samuel, Y. Nishimura and I. Yamazaki, Compact ultrafast microchannel plate photomultiplier tubes, in Time-Resolved Laser Spectroscopy in Biochemistry III (I. R. Lakowicz, ed.), Proc. SPIE 1640 440-447 (1992). 

H. Kume, K. Koyama, K. Nakatsugawa, S. Suzuki and D. Fatlowitz, Ultrafast microchannel plate photomultipliers, Appl. Opt. 27, 1170-1178 (1988). 

Fig. 2. Experimental setup for the kinetic energy experiment. The PSC solenoid provides an axial magnetic field of 5 Tesla. PPAC Parallel plate avalanche chamber. MCP MicroChannel plate. The photomultipliers and light-guides coupled to the Csl and plastic scintillators are not shown...

A time-to-amplitude converter (TAC) system was also employed to measure fluorescence decays without the microscope. Then, the fluorescence decay and the fluorescence lifetime were obtained precisely with the microchannel-plate photomultiplier (MCP-PM) as detection. The time resolution of the lifetime was determined, using a convolution method, to be 10ps. 

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. 

It should be noted that internal cross-correlation is not possible with microchannel plate photomultipliers, but an external mixing circuit can be used. 

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