Dynode structures

The operating principle of an MCP-PM is based on electron multiplication using a continuous dynode structure of ca. 10 um diameter holes, giving a more compact and hence faster time response when compared with conventional photomultipliers. Rise-times of 150 psec and transit-time jitter (i.e., impulse response) of ca. 25 psec FWHM at 200 counts/sec noise at room temperature have been recorded with the 6 fun channel Hamamatsu R3809 MCP-PM.(87)... 

Figure 7-12 Discrete dynode electron multiplier showing dynode structure and generation of electron cascade.

Figure 12.20 shows the structure of the side-window circular cage type and linear focused head-on type of photomultiplier which are both preeminent in fluorescence studies. The lower cost of side-window tubes tends to favor their use for steady-state studies, whereas the ultimate performance for lifetime studies is probably at present provided by linear focused devices. In both types internal current amplification is achieved by virtue of secondary electron emission from discrete dynode stages, usually constructed of copper-beryllium (CuBe) alloy, though gallium-phosphide (GaP) first dynodes have been used to obtain higher gains. 

The multiplier structures may be divided into two main types (1) dynamic and (2) static. The dynamic multiplier in its simplest form consists of two parallel dynode surfaces with an alternating electric field applied between them. Elections leaving one suiface at the piopei phase, of the applied field are accelerated to the other surface where they knock out secondary electrons. These electrons, in turn, are accelerated back to the first plate when the field reverses, creating still more secondary electrons. Eventually, the secondary electrons are collected by an anode placed in... 

Dark noise in photomultipliers is caused by (1) leakage current across insulating supports (2) field emission from electrodes (3) thermal emission from the photocathode and dynodes (4) positive ion feedback to the photocathode and (5) fluorescence from dynodes and insulator supports. Careful design can eliminate all but item (3). Associated with the photocurrent from the photocathode is shot noise. There is also shot noise from secondary emission in the multiplier structure. 

The pulse height distribution at high detector gain often shows a structure, probably due to the discrete numbers of secondary electrons emitted at the first dynode. An example for an H7422P-40 module is shown in Fig. 6.14. 

A PMT can develop instability or permanently increased dark count rate after being heavily overloaded for a period of several seconds or longer. The reason is probably heating of the anode and the last dynodes which releases gas from these structures. 

In the previous chapter we presented an overview of protein fluorescence. We described the spectral properties of the aromatic amino acids and how these properties depend on protein structure. We now extend this discussion to include time-resolved measurements of intrinsic protein flu( scence. Prior to 1983, most measurements of time-resolved fluorescence were performed using TCSPC. The instruments employed for these measurements typically used a flashlamp etcitation source and a standard dynode-chain-type PMT. Such instruments provided instrument response functions with a half-width near 2 ns, which is comparable to thedecay time of most proteins. The limited repetition rate of the flashlamps, near 20 kHz. resulted in data of modest statistical accuracy, unless the acquisition times were excessively long. Given the complexity of protein intensity and anisotropy decays, and the inherent difficulty of resolving multiexponential processes. ii was difficult to obUun definitive information on the decay kinetics of proteins. 

FIGURE 6 Photomultiplier structure. Photocathode, C, dynodes, 1-8, collecting anode, A... 

Optical absorption spectrophotometry is probably the most commonly used technique [4,a]. Reaction cells are similar to those used in flash work. Photomultipliers cover the uv-visible range the initial photoelectric signal is amplified internally, by an amoimt controlled by selection of the number of dynodes. Nanosecond equipment is commercially available. Picosecond time-resolution has been achieved [l,h]. For the infrared and Raman region, semiconductor photodiodes cover the range 400-3000 nm the vibrational spectra yield structural information about transient species much more detailed and precise than that from electronic spectra. Resonance enhancement of Raman spectra increases their intensity by a factor of 10, and makes them attractive for detection and monitoring [4,b]. They can be recorded with time-resolution down to sub-nanoseconds. Fluorescence detection is sensitive, and fast with single-photon counting or a streak camera (Section 4.2.4.2), it has been used for times down to 30 ps after an electron pulse. Conductivity also provides a fast and sensitive technique [4,c,d,l,m], especially in hydrocarbon solutions, where... 

While the principles of operation are similar, the structures of discrete-dynode multipliers and CEMs are quite different. Figure 3.1 illustrates each type schematically. 

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