Photocathodes conventional

A schematic cross-section of one type of photomultiplier tube is shown in Figure 26. The photomultiplier is a vacuum tube with a glass envelope containing a photocathode and a series of electrodes called dynodes. Light from a scintillation phosphor liberates electrons from the photocathode by the photoelectric effect. These electrons are not of sufficient number or energy to be detected reliably by conventional electronics. However, in the photomultiplier tube, they are attracted by a voltage drop of about 50 volts to the nearest dynode. 

The same types of photocathode compositions used in conventional photomultipliers also apply to MCP-PMs and similar considerations apply. Typical performance of MCP-PMs for the UV/visible is demonstrated in several chapters in an earlier volume in this series.(4)... 

The 5 ns pulses of about 10 electrons released at the anode by a photon absorbed by the photocathode of a PM tube can be used to count photons. In such instruments the intensity of light is displayed as a count per second which varies between about 15 (dark count) and 105. A photon-counting detector system is of course much more complex than the simple PM/ampli-fier used in conventional spectrofluorimeters. Figure 7.27(a) is a block diagram of such a photon counter (b) gives a simple illustration of the important process of pulse selection through a discriminator. The output of... 

Photomultiplier tubes are a variation of the conventional phototube. Such tubes have several intermediate electrodes, known as dynodes, in addition to the primary photocathode and anode. Electrons emitted from the cathode strike the first of these dynodes, whereupon by secondary emis-... 

Because tenfold-lower emittance beams can be generated from photocathodes compared with those from conventional thermionic guns, photocathode systems have significant advantages in efficient transportation of the charge from the gun to the radiolysis target and in magnetic pulse compression elfectiveness. 

Image dissector tubes (Fig. 26) make use of an entrance aperture behind the photocathode, by which the photoelectrons stemming from different locations of the photocathode can be scanned and measured after amplification in the dynode train, as in a conventional photomultiplier. Although used in combination with an echelle spectrometer with crossed dispersion for flexible rapid sequential analyses [59], these systems have not had any commercial breakthrough. This might be due to the limited cathode dimensions but also to stability problems. 

Conventional scintillation counters such as the Microbeta (Wallac/Perkin Elmer, Turku, Finland) or the TopCount (Packard, Meriden, USA) use photomultiplier detection systems that count 8 or 12 wells at a time, resulting in a readout time of 40 minutes per 384-well microplate. Bialkali photocathodes (Sb-Rb-Cs or Sb-K-Cs) used in standard photomultipliers have a maximum spectral response at about 420 nm, with a quantum efficiency for detection of up to 30%. Thus, the aforementioned instruments are ideally suited for filtration assays and SPA assays with the blue-emitting YSi and PVT beads. 

The most frequently used detectors for low-level detection of light are photomultiplier tubes. A conventional photomultiplier tube (PMT) is a vacuum device that eontains a photocathode, a number of dynodes (amplifying stages) and an anode that delivers the output signal (Fig. 6.1). 

The transit time between the absorption of a photon at the photocathode and the output pulse from the anode of a PMT varies from photon to photon. The effeet is called transit time spread", or TTS. There are three major TTS components in conventional PMTs and MCP PMTs - the emission at the photoeathode, the transfer of the photoelectron to the multiplieation system, and the multiplication process in the dynode system or mieroehannel plate. The total transit time jitter in a TCSPC system also contains jitter indueed by amplifier noise and amplitude jitter of the SER. 

In photomultiplier-based detectors, the incoming ion beam is first converted to a photon beam when ions strike a scintillation material. The emitted photons are amplified and detected by a conventional photomultiplier. The construction of a photomultiplier is similar to that of an EM, except that the conversion dynode, called the photocathode, is coated with a photoemissive material that emits electrons when struck by photons. Photomultipliers are usually employed in combination with postacceleration devices, which are discussed next. 

Conventional Photocathodes. The photocathode controls the spectral properties of the phototube. Photocathodes can be divided into two categories, conventional ones and negative electron affinity ones. The energy band diagrams of... 

Detection of Cerenkov photons is done with a new hybrid or vacuum APD (HAPD) with single-photoelectron capability from Advanced Photonix Inc. The HAPD combines an avalanche photodiode APD with a conventional photocathode. It is insensitive to magnetic fields at least up to 1.5 T [17] [18]. 

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