Photomultiplier spectral sensitivity

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. 

Range of application and spectral sensitivity The photomultipliers most frequently employed in scanners possess antimony-caesium cathodes. These alloy cathodes are primarily sensitive to the short-wavelength part of visible light (Tab. 4). 

Phosphorus-containing pesticides la 254 Phosphorus insecticides lb 83 Phosphorus pesticides lb 32 Photochemical activation lb 13 Photochemical reactions lb 15,17 Photodiodes la 24,29 Photo effect, external la 24 -, internal la 24, 29 Photo element la 24,29 Photography, exposure times la 137 -, instmmentation la 137 Photomultiplier la 25ff -, disadvantages la 27 -, energy distribution la 26 -, head on la 27 -, maximum sensitivity la 28 -, side on la 27 -, spectral sensitivity la 28 -, window material la 28 Photocells la 25 Phloxime lb 116... 

Emission from dimols of singlet oxygen may be detected by photomultipliers used for measurement of chemiluminescence from hydrocarbon polymers with a maximum spectral sensitivity at 460 nm. The above scheme, however, requires the presence of at least one molecule of hydrogen peroxide in close vicinity to the two recombining peroxyl radicals and assumes a large heterogeneity of the oxidation process. 

The simplest method of determining the function S, in the visible region of the spectrum is to take photomultiplier readings when the entrance slit of the monochromator is illuminated by a tungsten lamp giving light of known spectral distribution. If RSL represents the values so obtained, the spectral sensitivity is then calculated from... 

The method just described is not usually applicable in the ultraviolet because ultraviolet lamps of known spectral distribution are not readily available at present. The spectral sensitivity caii be calculated directly if the values of B L and P, are known. The first of these is obtained from the dispersion curve of the monochromator the second is somewhat difficult to measure—for prism instruments over restricted wavelength regions above 250 m t it is often reasonably constant. The photomultiplier sensitivity, P can be determined by comparison with a thermopile or with the ferrioxalate actinometer.11 12 Direct calculation of S, is subject to inaccuracies due to the accumulation of errors in the measurement of the three separate quantities B L and P,. A more convenient... 

Fig 5. Spectral sensitivity curves for quartz prism monochromator, (a) With EMI 6256 photomultiplier tube (6) with EMI 9558 Q photomultiplier tube. 

Figure 7.25 Examples of spectral sensitivities of a photodiode (a) and of two types of photomultipliers (b). The window transmissions are shown as q (silica) and g (optical glass). Horizontal axes, A in nm/100 vertical axes,

For the measurement only two materials were selected rhodamine B, which shows a beautiful orange luminescence, and luminol—i.e., 3-aminophthalhydrazide or 2,3-phthalazdione. Luminol was preferred because the yield seemed to be higher and its emission spectrum corresponded better to the spectral sensitivity curve of the photomultiplier. The rhodamine type of compound—i.e., the compound without any substitution—prepared in the authors laboratory, was not found suitable. 

Photomultipliers used within a range of 200 to 650 nm serve as detectors. Only recently broadband photomultipliers with a spectral sensitivity of 185-850 nm have appeared on the market1). In the range above 650 nm the photocell operating up to 1100 nm or the photoresistance responding up to 2500 nm are used. Occasionally attempts have been made to apply flame ionisation detectors to thin-layer chromatography 8 10). In this case, however, it is necessary to carry out the chromatography on rods, or strips or tubes coated with adsorbents. 

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... 

The spectra were not corrected for the spectral variation in the excitation source and photomultiplier tube sensitivity. Background scattered light was zeroed electronically before the acquisition of the fluorescence of 4-methylcoumaro-[222]cryptand. Absorption spectra were obtained using Perkin Elmer Lambda 7 UV/VIS Spectrophotometer. 

FIGURE 6-12. Spectral sensitivities of the R106 and R166 photomultiplier tubes. [From A. Walsh, Physical Aspects of Atomic Absorption, ASTM-STP 433 (1968). Used by permission of the American Society for Testing Materials.]... 

Rgura IX) The spectral sensitivity curve of a Hamamatsu R955 photomultiplier tube. 

Great progress has also been achieved in the field of lov -level signal detection. Apart from new photomultipliers with an extended spectral sensitivity range and large quantum efficiencies, new detection instruments have been developed such as image intensifiers, infrared detectors, or optical multichannel analyzers, which could escape from classified military research into the open market. For many spectroscopic applications they prove to be extremely useful. 

Photomultipliers are generally used to convert the spectral radiation to an electrical current and often phase-sensitive lock-in amplifiers are used to amplify the resulting current. AES and AFS require similar read-out systems because both methods are measuring small signals. The difficulty associated with both these methods is the separation of the signal for the atomic transition of interest from the background radiation emitted by excited molecular species produced in the atom reservoir. AFS phase locks the amplifier detection circuit to the modulation frequency of the spectral source. Modulation of the source is also used in AAS. 

In the chemiluminescent detection of nitrogen oxides, a constant source of ozone reacts with a metered air sample containing nitric oxide. Fontijn et al. suggested that this method could also be used for ozone detection by using a constant nitric oxide source for reaction with ozone in the air sample. The ozone-nitric oxide reaction is carried out at reduced pressure, to avoid quenching the chemiluminescent reaction. Detection of the emission in the spectral r on involved (600-3,000 nm) requires using a near-infrared-sensitive photomultiplier tube. The noise of such a photomultiplier tube is reduced by cooling it to about - 20 C. ... 

The activation [672] of Lil with Eu2+ and the use of an activated Lil phosphor as a scintillation detector for slow neutron detection [673] has been investigated. Blue, fluorescent Lil (0.03 mole % Eu) phosphor was found to be the most useful [673] phosphor because of its ease to growth, relatively high light output, chemical stability and good match with spectral characteristics of the 6260 type photomultiplier. Lil (Eu), however, does have an interfering y radiation sensitivity. Fast neutron scintillation spectra of Li6(w, a)H3 in Eu doped Lil crystals has also been investigated [674]. 

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