Photomultiplier sensitivity

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... 

GaP/electrolyte interface. The electrolyte is 0.15M HN03 and the current density flowing through the interface is 20 mA/cm2. The low-energy limit of the spectrum is determined by the photomultiplier sensitivity, (b) Strongly cathodically biased p-GaP/electrolyte interface. Hot electrons are created by tunneling from valence to conduction bands. These may decay radioactively to fill empty states created by cation injection or drive other redox reactions. 

Fluorimetric measurements were carried out with a commercial Hitachi F-4500 spec-trofluorometer equipped with a 150 W xenon lamp and interfaced to a personal microcomputer for instrument operation and spectra processing. The slit-widths were adjusted at 2.5 nm both for excitation and emission and the photomultiplier (PMT R-3788) voltage set to operate at 950 V. The synchronous fluorescence spectra were corrected for variations with wavelength of the lamp intensity and photomultiplier sensitivity. 

Figure 25 Photomultiplier sensitivity curves for some cathode materials...

Luminescence Spectroscopy. Photoluminescence measurements were performed with the aid of a Fluorolog3 spectro-fluorometer Fl3—22 (Horibajobin Yvon) equipped with double Czerny—Turner monochromators, a 450 W xenon lamp and a R928P photomultiplier with a photon counting system. Cooling down to 10 K was achieved by a closed cycle He cryostat (Janis Research). All emission spectra were corrected for the photomultiplier sensitivity and all excitation spectra for the intensity of the excitation source. To avoid any contamination of water on the sample s surfaces, we carried out the measurements in silica ampules with extreme purity which show no luminescence of the ampules itself. Reflection spectra were recorded on a Cary 5000 UV—vis—NIR spectrophotometer (Varian), which were corrected for both the lamp intensity and the photomultiplier sensitivity. 

The intensity of the reflected light must also be measured. Historically, this was done using the eye. Since, in general, a null (a measurement of the point at which the light decreases to zero) is required, this can be relatively sensitive. However, nowadays, the light intensity is generally measured using a photomultiplier tube. 

Photomultipliers are used to measure the intensity of the scattered light. The output is compared to that of a second photocell located in the light trap which measures the intensity of the incident beam. In this way the ratio [J q is measured directly with built-in compensation for any variations in the source. When filters are used for measuring depolarization, their effect on the sensitivity of the photomultiplier and its output must also be considered. Instrument calibration can be accomplished using well-characterized polymer solutions, dispersions of colloidal silica, or opalescent glass as standards. 

Liquid scintillation counting is by far the most common method of detection and quantitation of -emission (12). This technique involves the conversion of the emitted P-radiation into light by a solution of a mixture of fluorescent materials or fluors, called the Hquid scintillation cocktail. The sensitive detection of this light is affected by a pair of matched photomultiplier tubes (see Photodetectors) in the dark chamber. This signal is amplified, measured, and recorded by the Hquid scintillation counter. Efficiencies of detection are typically 25—60% for tritium >90% for and P and... 

Cathodoluminescence, CL, involves emission in the UV and visible region and as such is not element specific, since the valence/conduction band electrons are involved in the process. It is therefore sensitive to electronic structure effects and is sensitive to defects, dopants, etc., in electronic materials. Its major use is to map out such regions spatially, using a photomultiplier to detect all emitted light without... 

An optical detector with appropriate electronics and readout. Photomultiplier tubes supply good sensitivity for wavelengths in the visible range, and Ge, Si, or other photodiodes can be used in the near infrared range. Multichannel detectors like CCD or photodiode arrays can reduce measurement times, and a streak camera or nonlinear optical techniques can be used to record ps or sub-ps transients. 

An instrument for measuring nitrogen oxides based on chemiluminescence is shown in Fig. 13.49. The ozone required for the reaction is produced in the ozone generator, which is part of rhe device. One of the reaction chamber walls is an optical filter through which a red-sensitive photomultiplier tube measures the chemiluminescence radiation intensity and converts it into a current signal. 

Colorless substances absorb at wavelengths shorter than those of the visible range (the UV range normally amenable to analysis X = 400...200 nm). Such compounds can be detected by the use of UV-sensitive detectors (photomultipliers. Sec. 2.2.3.1). Substances that absorb in the UV range and are stimulated to fluorescence or phosphorescence (luminescence) can be detected visually if they are irradiated with UV light. 

Photomultipliers are appreciably more sensitive sensors than the eye in their response to line or continuum sources. Monochromators are fitted to the light beam in order to be able to operate as substance-speciflcally as possible [5]. Additional filter combinations (monochromatic and cut-off filters) are needed for the measurement of fluorescence. Appropriate instruments are not only suitable for the qualitative detection of separated substances (scanning absorption or fluorescence along the chromatogram) but also for characterization of the substance (recording of spectra in addition to hR and for quantitative determinations. 

A role is also played by the temperature and frequency dependence of the photocurrent, the variable surface sensitivity at various parts of the cathode and the vector effect of polarized radiation [40]. All the detectors discussed below are electronic components whose electrical properties vary on irradiation. The effects depend on external (photocells, photomultipliers) or internal photo effects (photoelements, photodiodes). 

The requirements for successful operation are a stable operating voltage of between 400 and 3000 V. The sensitivity of the photomultipliers is also dependent on this if a special compensation is not incorporated. 

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. 

Head-on photomultipliers, on the other hand, possess a greater entry angle for the capturing photocathode (Fig. 20). A diffuse sereen in front of the photocathode also allows the capture of light falling at an angle. These conditions are realized in the Camag TLC/HPTLC scanner I. The sensitivity of such head-on photomultipliers is independent of frequency up to 10 Hz. 

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