Photocathode spectral sensitivity

Figure 12.21. Photocathode spectral sensitivity curves in imlliampcres of cathode current per watt ofradiam power falling on the photocathode, q is the photocathode quantum efficiency, i.e, the probability of photoelectron emission (Adapted from Ref 76.)...

The absolute and spectral sensitivities can often vary by up to 100% within a few millimeters on the surface of the photocathode [49]. Figure 19 illustrates this effect for a sideways and vertical adjustment of a photomultiplier, in addition slight maladjustment of the light entrance can lead to zero hne runaway as a result of thermal effects. 

The photoelectrochemical activity inherent in thin films of aggregated cyanine dyes permits them to act as the spectral sensitizers of wide bandgap semiconductors [69]. It is seen from Fig. 4.14 that the photoelectrochemical behaviour of semiconductor/dye film heterojunctions fabricated by deposition of 200 nm-thick films of cyanine dyes on the surface of TiC>2 and WO3 electrodes, bears close similarity to that of semiconductor electrodes sensitized by the adsorption of dye aggregates. Thus, both anodic and cathodic photocurrents can be generated under actinic illumination, the efficiency of the photoanodic and photocathodic processes and the potential at which photocurrent changes its direction being dependent on dye and semiconductor substrate [69]. 

The operation of photocells and photomultipliers is based on the external photoelectric effect. Photons impinging on the surface of a photosensitive cathode (photocathode) knock out electrons which are then accelerated in the electrical field between the cathode and the anode and give rise to electric current in the outer circuit. The spectral sensitivity of a photocell depends on the material of the photocathode. The photocathode usually consists of three layers a conductive layer (made, e.g., of silver), a semiconductive layer (bimetallic or oxide layer) and a thin absorptive surface layer (a metal from the alkali metal group, usually Cs). A photocathode of the composition, Ag, Cs-Sb alloy, Cs (blue photocell), is photosensitive in the wavelength range above 650 nm for longer wavelengths the red photocell with Ag, Cs-O-Cs, Cs is used. The response time of the photocell (the time constant) is of the order of 10" s. 

A very important parameter of every photomultiplier tube is the spectral sensitivity of its photocathode. For best results, the spectrum of the scintillator should match the sensitivity of the photocathode. The Cs-Sb surface has a maximum sensitivity at 440 nm, which agrees well with the spectral response of... 

Several different definitions are used to speeify the effieieney of a PMT eathode. Often the sensitivity of a PMT is speeified in units of eathode luminous sensitivity". This is the cathode eurrent per lumen incident light from a tungsten lamp operated at a temperature of 2,856 K. Beeause the intensity maximum of the lamp is at about 1,000 nm, the luminous sensitivity may not represent the efficiency at a given wavelength. Photocathodes of different spectral sensitivity are therefore not directly comparable. Moreover, the cathode luminous sensitivity does not include the effieieney of the eleetron transfer from the cathode into the dynode system and the possible loss of photon pulses due to incomplete resolution of the pulse height distribution (see Fig. 6.27, page 241). 

The spectral response of a photomultiplier tube varies with the coating materials used on the photocathode. Spectral responses of various photomultiplier tubes are given in Table 6-3. Chapter 6 also includes a general discussion of photomultiplier phototubes. The 1P28 tube (S-5 response) is sensitive from 2000 to 6500 A and is frequently used for atomic absorption spectroscopy. The Hamamatsu R106 also has an S-5 response but uses a silica window to lower the usable short wavelength response to about 1700 A. The S-20 response of the RCA 4459 permits measurements to 8500 A and is very useful for most of the alkali metals. 

Figure 4.96 shows the spectral sensitivity S(X) of some typical photocathodes, scaled in milliamperes of photocurrent per watt incident radiation. For comparison, the quantum efficiency curves for = 0.001, 0.01 and 0.1 are... 

The material used for manufacture of photocathode governs both the sensitivity and the spectral range of the PMT. The spectral characteristics of PMTs are presented in Figure 5. 

The major characteristics of the photomultiplier with which the user is generally most concerned include (1) sensitivity, spectral response, and thermal emission of photocathodes (2) amplification factor and (3) noise characteristics and the signal-to-noise ratio. 

MicroChannel plates are becoming widely used in spectroscopy and two dimensionaj imaging at EUV (100 - 1000 A) and soft X-ray wavelengths (10 - 100 A) for astronomy and microscopy. Although bare microchan-nel plates have low sensitivity (5% - 10% quantum detection efficiency) in this spectral region, use of photocathodes can increase substantially (to 30%-40%) microchannel plate performance. This, combined with the high spatial resolution (<50/rm), fast time response (<300 ps), and large effective area (up to 100 mm diameter) achievable with microchannel plates make the latter a very attractive and versatile tool. We discuss the properties of microchannel plates, photocathode materials, and various microchannel plate detector readout schemes that have been used for soft X-ray and EUV detection. 

Thin composite layers of CS2O and Cs on Ag play an important role in the IR sensitive SI photocathodes which have been manufactured for approximately sixty years. [245, 246] The characteristic spectral response of such cathodes can be explained by allowing for the presence of alkali metal suboxides, CsnOj or CS3O , since these have the appropriate electronic properties. Thus, these materials have a low work function of approximately 1 eV (compared to 2 eV for elemental Cs) and low energies of the surface plasmons (1.5 eV for (IIS11O3). The enhancement of the photoelectric yield is due to surface plasmon decay. [247]... 

The quantum efficiency r] of CCD arrays depends on the material used for the substrate, it reaches peak values over 90%. The efficiency rj(X) is generally larger than 20% over the whole spectral range from 350—900 nm (Fig. 4.93c) and exceeds the efficiencies of most photocathodes (Sect. 4.5.4). The spectral range of special CCDs ranges from 0.1 —1000 nm. They can therefore be used in the VUV and X-ray regions, too. The highest sensitivity up to 90% efficiency is achieved with backward-illuminated devices (Fig. 4.94). Table 4.2 compiles some relevant data of commercial CCD devices. 

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