Photomultiplier tube gain

The flow cytometer is set up to read linear-scale fluorescence intensities. For each sample, analyze 5000-10,000 cells. Analyze the resting, unstimulated control samples first to determine the appropriate gain/ voltage setting for the photomultiplier tube then analyze the stimulated samples. 

With the development of the photomultiplier tube the measurement of very low light intensities has become relatively simple and the photoelectric recording of fluorescence emission spectra can now compete in terms of sensitivity with the less convenient photographic method. During the last decade the development of the experimental technique has gained considerable impetus as a result of the requirements of analytical chemists for methods of extreme sensitivity. A variety of spectro-fluorimeters have been described in the literature and commercial instruments of high sensitivity are also available. Recent reviews1-2 deal with the principles and analytical applications of fluorescence spectrometry and a textbook of biochemical applications has been published.2... 

The photomultiplier tube a very sensitive device that has a linear response over seven decades - has for a long time been the most widely used detector in spectrophotometers. Its efficiency depends on the yield of the photocathode, which varies with wavelength (e.g. 0.1 e /photon at 750 nm). and with the signal gain provided by the dynode cascade (e.g. gain of 6 x 105). With such values, the impact of 10000 photons per second produces a current of 0.1 nA. 

For detection of the small number of scattered photons, modern photomultiplier tubes having low internal noise and high gain are used. The amplification method employed is generally direct-current amplification. 

The densitometer uses a two beam comparator technique of measurement. A single lamp illuminates, through a variable neutral density filter, the emitter of an eleven stage photomultiplier tube. Because the frequency of measurement is five times the AC line frequency and the amplifiers are extremely high gain, the lamp must be operated from a regulated DC source, otherwise, 120 Hz brightness ripple of the lamp is recorded in the data. 

Fig. 3.9. The effect of changes in the photomultiplier tube voltage and amplifier gain on the appearance of six signals with intensities in the relationship of 1 2 10 20 100 200 to each other. A Linear amplification. B Logarithmic amplification, (continued on next page)...

Typical connections for a photomultiplier tube. Secondary emission of electrons from 5 to 15 dynodes provides gains of up to 10. The dynode resistors are usually all the same and are about 50 kfl. The capacitors on the last few dynodes store charge for improved operation with high current pulses. 

The second gain control, which is exerted at each individual discriminator, produces a similar effect to that at the level of the photomultiplier tubes. In this case, however, only individual discriminator circuits are affected (Figure 3-12A and B). If the gain in channel B is decreased... 

Raman spectra of monolayers were first obtained on roughened silver surfaces that exhibit strong field enhancement, since the scanning/photomultiplier tube spectrometers of the time needed the gain in sensitivity of a factor of to provide useful spectra (3,4). Multichannel spectrometers permitted spectrum acquisition without field enhancement about 8 years later (12), for the reasons discussed in Section 13.2. Surface Raman without field enhancement is conceptually simpler, so it will be discussed first. Section 13.5 describes the requirements and additional benefits when field enhancement occurs. 

They are more sensitive than a vacuum phototube but considerably less sensitive than a photomultiplier tube. Diode arrays can also be obtained commercially with front-end devices called image intensifiers to provide gain and allow the detection... 

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