Detector photomultipliers

The variation of Bq causes all ions to pass sequentially in front of the exit slit behind which is positioned the photomultiplier detector. The pressure in the apparatus is held at 10 torr in order to achieve mean free paths of ions sufficiently high that all ions emitted from the source are collected. 

Fluorometry and Phosphorimetry. Modem spectrofluorometers can record both fluorescence and excitation spectra. Excitation is furnished by a broad-band xenon arc lamp foUowed by a grating monochromator. The selected excitation frequency, is focused on the sample the emission is coUected at usuaUy 90° from the probe beam and passed through a second monochromator to a photomultiplier detector. Scan control of both monochromators yields either the fluorescence spectmm, ie, emission intensity as a function of wavelength X for a fixed X, or the excitation spectmm, ie, emission intensity at a fixed X as a function of X. Fluorescence and phosphorescence can be distinguished from the temporal decay of the emission. 

Radiation from a xenon or deuterium source is focussed on the flow cell. An interchangeable filter allows different excitation wavelengths to be used. The fluorescent radiation is emitted by the sample in all directions, but is usually measured at 90° to the incident beam. In some types, to increase sensitivity, the fluorescent radiation is reflected and focussed by a parabolic mirror. The second filter isolates a suitable wavelength from the fluorescence spectrum and prevents any scattered light from the source from reaching the photomultiplier detector. The 90° optics allow monitoring of the incident beam as well, so that dual uv absorption and fluorescence... 

The ellipsometer used in this study is described elsewhere(3). It consists of a Xenon light source, a monochromator, a polarizer, a sample holder, a rotating analyzer and a photomultiplier detector (Figure 1). An electrochemical cell with two windows is mounted at the center. The windows, being 120° apart, provide a 60° angle of incidence for the ellipsometer. A copper substrate and a platinum electrode function as anode and cathode respectively. Both are connected to a DC power supply. The system is automated with a personal computer to collect all experimental data during the deposition. Data analysis is carried out by a Fortran program run on a personal computer. 

The basic instrumentation used for spectrometric measurements has already been described in the previous chapter (p. 277). Methods of excitation, monochromators and detectors used in atomic emission and absorption techniques are included in Table 8.1. Sources of radiation physically separated from the sample are required for atomic absorption, atomic fluorescence and X-ray fluorescence spectrometry (cf. molecular absorption spectrometry), whereas in flame photometry, arc/spark and plasma emission techniques, the sample is excited directly by thermal means. Diffraction gratings or prism monochromators are used for dispersion in all the techniques including X-ray fluorescence where a single crystal of appropriate lattice dimensions acts as a grating. Atomic fluorescence spectra are sufficiently simple to allow the use of an interference filter in many instances. Photomultiplier detectors are used in every technique except X-ray fluorescence where proportional counting or scintillation devices are employed. Photographic recording of a complete spectrum facilitates qualitative analysis by optical emission spectrometry, but is now rarely used. 

To make accurate measurements of the integrated absorption associated with such narrow lines requires that the linewidth of the radiation source be appreciably smaller than that of the absorption line. In practice, this could be achieved with a continuum source only if expensive instrumentation of extremely high resolving power were used, and it is doubtful whether conventional photomultiplier detectors would be sufficiently sensitive at the resulting low radiation intensities. An alternative arrangement is to... 

Constant instrumental error 1%. Curve A photovoltaic detector. Curve B photomultiplier detector. 

The design of a conventional atomic absorption spectrometer is relatively simple (Fig. 3.1), consisting of a lamp, a beam chopper, a burner, a grating monochromator, and a photomultiplier detector. The design of each of these is briefly considered. The figure shows both single and double beam operation, as explained below. 

Figure 3.1 Schematic diagram of an AAS spectrometer. A is the light source (hollow cathode lamp), B is the beam chopper (see Fig. 3.2), C is the burner, D the monochromator, E the photomultiplier detector, and F the computer for data analysis. In the single beam instrument, the beam from the lamp is modulated by the beam chopper (to reduce noise) and passes directly through the flame (solid light path). In a double beam instrument the beam chopper is angled and the rear surface reflective, so that part of the beam is passed along the reference beam path (dashed line), and is then recombined with the sample beam by a half-silvered mirror.

Essentially the same spectrometer as is used in atomic absorption spectroscopy can also be used to record atomic emission data, simply by omitting the hollow cathode lamp as the source of the radiation. The excited atoms in the flame will then radiate, rather than absorb, and the intensity of the emission is measured via the monochromator and the photomultiplier detector. At the temperature achieved in the flame, however, very few of the atoms are in the excited state ( 10% for Cs, 0.1% for Ca), so the sample atoms are not normally sufficiently excited to give adequate emission intensity, except for the alkali metals (which are often equally well determined by emission as by absorption). Nevertheless, it can be useful in cases where elements are required for which no lamp is available, although some elements exhibit virtually no emission characteristics at these temperatures. 

However, it is worthwhile to have a comparison of the merits and demerits of photographic and photomultiplier detectors side-by-side as follows ... 

Describe the two common detectors invariably used in emission spectroscopy. Differentiate the plus and negative aspects encountered in (a) Photographic Detector and (b) Photomultiplier Detector, briefly. 

Radiation from a Xenon-radiation or a Deuterium-source is focussed on the flow cell through a filter. The fluorescent radiation emitted by the sample is usually measured at 90° to the incident beam. The second filter picks up a suitable wavelength and avoids all scattered light to reach ultimately the photomultiplier detector. 

In TL, the light emission induced in the mineral or ceramics sample heated up to 500° C is measured by means of a photomultiplier detector. In addition to laboratory instrumentation, portable gamma spectrometers have been used when circumstances make sampling impractical. 

Zarowin (68) has made use of a multiple-sampling technique in the measurement of decay times. This method uses a periodically pulsed- or chopped-excitation source and a continuously operating photomultiplier detector. The fluorescent signal is displayed on an oscilloscope. The response of the photomultiplier tube must be fast enough to resolve individual photoelectron pulses, and the time density of pulses is then proportional to the light intensity. 

Color Plate 23 Polychromator for Inductively Coupled Plasma Atomic Emission Spectrometer with One Detector for Each Element (Section 21-4) Light emitted by a sample in the plasma enters the polychromator at the right and is dispersed into its component wavelengths by grating at the bottom of the diagram. Each different emission wavelength (shown schematically by colored lines) is diffracted at a different angle and directed to a different photomultiplier detector on the focal curve. Each detector sees only one preselected element, and all elements are measured simultaneously. [Courtesy TJA Solutions, Franklin, MA.J... 

In the photolysis of ozone, only emission from 02(1A9) can be detected,70 and the absence of 02(1S,+) is now understood in terms of the considerable reactivity of 02(123+) with ozone61 (see Sect. V-B-l). has been detected in the vacuum UV photolysis products of 0277 although absolute measurements of the excitation efficiency have not, so far, proved practicable. The limits of sensitivity of the photomultiplier detector to 1.27 [x emission suggest that reaction (19) could proceed perhaps 20 times more rapidly than reaction (20) work is at present in progress in an attempt to measure k12 directly. 

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