Radiation detector photomultiplier

Motored engine cool flames are discernible with the naked eye only with great difficulty, hence the reliance on detection apparatus for quantitative investigations. The essential features of radiation detection apparatus, such as shown in Figure 1, include a quartz window mounted in the combustion chamber, a photomultiplier radiation detector, and an amplifier-oscilloscope arrangement (107). 

Two radiation detector stations were used along the gun with each station consisting of two units, one on each side of the barrel. A unit radiation detector was composed of two photomultiplier tubes immersed in a liq scintillator consisting of xylene saturated with terphenyl. The container for the soln was a 10 x 10 x 13cm aluminum-lined brass box fitted with appropriate electrical connections for the signals. These detectors provided the desired high count... 

Some radiation detectors, i.e., photoemissive detectors (vacuum phototubes or photomultipliers) or semiconductor detectors (photodiodes or phototransistors) directly produce an electrical signal by quantum effects. Their output is strongly dependent on the wavelength of the detected radiation. Thermal detectors, i.e., thermocouples and thermopiles, bolometers, pyroelectric detectors, or pneumatic and photoacoustic detectors record a temperature increase through radiation and convert this into an electrical signal. This is proportional to the flux of the absorbed radiant power, independent of the wavelength. 

A reverse-biased silicon diode can serve as a radiation detector because ultraviolet and visible photons are sufficiently energetic to create additional electrons and holes when they strike the depletion layer of a pn junction. The resulting increase in conductivity is easily measured and is directly proportional to radiant power. A silicon-diode detector is more sensitive than a simple vacuum phototube but less sensitive than a photomultiplier tube. 

ICP-AES is a technique of measurement used for the detection and determination of elements with the aid of atomic emission. The solution for measurement is atomized and the aerosol is transported into an inductively coupled plasma (ICP) with the aid of a carrier gas. There, the elements are excited such that they emit radiation. This is spectrally dispersed in a spectrometer and the intensities of the emitted element lines are measured by means of detectors (photomultipliers). A quantitative statement is possible by means of calibration with reference solutions, there being a linear relationship between the intensities of the emission lines and the concentrations of the elements over a broad range (usually several powers of ten). The elements may be determined either simultaneously or consecutively. 

The photomultiplier tube is a radiation detector using the photoelectric principle. The radiation falling on a photocathode causes the emission of primary electrons that are released into the surrounding vacuum. As a result of the applied voltage between the electrodes (dynodes), the primary electron is accelerated so that when it strikes a dynode several secondary electrons are emitted, leading to a cascade effect. 

The emission of the excited analytes in the plasma is polychromatic (several different wavelengths per analyte). The spectrophotometers used to detect and quantify the emitted radiation contain a monochromator (polychromator for simultaneous measurements at several different wavelengths) and a detector (photomultiplier tube) to quantify the amount of radiation for each specific wavelength. 

Ultraviolet and visible radiation detectors There are three basic kinds of detectors in this region. Photocells, phototubes, and photomultiplier tubes. 

When illuminated with monochromatic radiation a single monochromator usually shows continuous stray light of the order of 10" of the intensity of the monochromatic radiation. Therefore, 2 or 3 monochromators in series combined with additive dispersion reduce the stray radiation by about 10 or 10 , respectively. However, the intensity of the Raman lines is also reduced when passing a monochromator Since every monochromator has a transmittance only of about 30%, this means that a double monochromator has only a transmittance of 9%, a triple monochromator of 2.7%. Such monochromators are usually very voluminous and expensive. However, they are widely used for recording of Raman spectra with single detectors (photomultipliers). 

The detector for conventional Raman instrumentation involves photon counting by photomultiplier tubes (PMTs). A phototube consists of an evacuated glass envelope containing a surface coated with an active metal. Incident radiation causes emission of electrons from this surface by the photoelectric effect a positively charged plate termed dynodes collects these electrons. The plate current is proportional to the intensity of the radiation. The photomultiplier effect arises when the electrons emitted from the cathode of a phototube are accelerated by a large potential and then allowed... 

Physical detection methods are based on inclusion of substance-specific properties. The most commonly employed are the absorption or emission of electromagnetic radiation, which is detected by suitable detectors (the eye, photomultiplier). The / -radiation of radioactively labelled substances can also be detected directly. These nondestructive detection methods allow subsequent micropreparative manipulation of the substances concerned. They can also be followed by microchemical and/or biological-physiological detection methods. 

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). 

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... 

Any device that can monitor radiation can be used as detector. In most cases, end-on and side-on photomultiplier tubes (PMT) are used. The end-on PMT has the benefit of circular photocathode, which views more effectively the radiation emitted from the circular flow cells. Nonetheless, the side-on PMTs are more commonly used due to their lower cost. The signal from the PMT is current and, hence, in most applications a current-to-voltage converter is required to convert the current to voltage, which is then monitored. 

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... 

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