Photomultiplier tube, high-gain

The detector in some inexpensive filter fluorometers is a phototube, but in most of the better filter fluorometers and in all spectrofluorometers a high-gain photomultiplier tube is used. The photomultiplier is far more sensitive to low radiation levels and is therefore recommended for trace analysis. 

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... 

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. 

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. 

On the fluorescence detection side, because individual photons must be detected, there is hardly a replacement for a photomultiplier-type detector with its high gain (of the order of >10 ). For a time resolution down to several tens of picoseconds, relatively simple photomultipliers have been used as detectors. One requirement is that the PM should have a narrow and well-defined electric response to a single photon and a low dark current. Traditionally, relatively fast front-end PM tubes have been... 

Because electrochemical experiments involve a direct conversion of chemical information to electricity, the instrumentation can be relatively simple. There is, for example, no need for high-quality power supplies to drive a light source or operate a photomultiplier tube. On the other hand, because the process often measures nanoamperes (or less) of current, electrochemical detectors are particulariy subject to dectrical interferences, and proper grounding can be crucial to successful experiments at high current-to-voltage gains. 

High voltage to the photomultiplier tubes is decreased while increasing the amplifier gain, to maintain counting efficiency of 70 percent. 

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