Measurement photomultiplier tube

Figure 2.26 Photoemissive tubes. Light enters a simple phototube (a) and causes the release of electrons from the photoemissive alloy of the cathode. Owing to the potential difference between the anode and the cathode, the electrons are captured by the anode and the resulting current can be amplified and measured. Photomultiplier tubes (b) are a development of simple phototubes and result in internal amplification of the current initially developed at the photocathode.

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. 

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... 

The P-emission of P is energetic enough in its passage through water to emit light (Cherenkov effect). This emission can be measured by a photomultiplier tube with a typical efficiency of ca 40%. 

Air passing through the NO pathway enters the reaction chamber, where the NO present reacts with the ozone. The light produced is measured by the photomultiplier tube and converted to an NO concentration. The NO2 in the air stream in this pathway is unchanged. In the NO pathway, the NO- and N02-laden air enters the converter, where the NO2 is reduced to form NO all of the NO exits the converter as NO and enters the reaction chamber. The NO reacts with O3 and the output signal is the total NO concentration. The NO2 concentration in the original air stream is the difference between NO and NO. Calibration techniques use gas-phase titration of an NO standard with O3 or an NOj permeation device. 

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. 

Direct-reading polychromators (Figure 3b) have a number of exit slits and photomultiplier tube detectors, which allows one to view emission from many lines simultaneously. More than 40 elements can be determined in less than one minute. The choice of emission lines in the polychromator must be made before the instrument is purchased. The polychromator can be used to monitor transient signals (if the appropriate electronics and software are available) because unlike slew-scan systems it can be set stably to the peak emission wavelength. Background emission cannot be measured simultaneously at a wavelength close to the line for each element of interest. For maximum speed and flexibility both a direct-reading polychromator and a slew-scan monochromator can be used to view emission from the plasma simultaneously. 

Optieal partiele eounters provide information on the partieles present in different size ranges. A beam of light is eollimated and foeused onto a measurement eelL Light impinging on a partiele is seattered and reaehes a photomultiplier tube and eonverted to an output proportional to partiele size. Partiele size distributions are eomputed by appropriate software. 

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. 

The sensitivity of a photo-emissive cell (phototube) may be considerably increased by means of the so-called photomultiplier tube. The latter consists of an electrode covered with a photo-emissive material and a series of positively charged plates, each charged at a successively higher potential. The plates are covered with a material which emits several (2-5) electrons for each electron collected on its surface. When the electrons hit the first plate, secondary electrons are emitted in greater number than initially struck the plate, with the net result of a large amplification (up to 106) in the current output of the cell. The output of a photomultiplier tube is limited to several milliamperes, and for this reason only low incident radiant energy intensities can be employed. It can measure intensities about 200 times weaker than those measurable with an ordinary photoelectric cell and amplifier. 

Figure 2. Schematic of the SLM 8000 fluorometer. Excitation occurs through the excitation monochromator, and light emitted from the sample is observed in as many as four different positions. Photomultiplier tubes (PMTs) A, B, and C can be used to monitor fluorescence or right-angle light scatter through the monochromator (PMT A) or through filters (PUT B and C), and position D measures transmittance. Three channels can be monitored simultaneously with measurements being acquired at intervals of 1 s or less. The data are stored by the computer for subsequent manipulation.

The branching ratios for the fine structure states of the 0(3Pj) were measured by placing a 2-inch diameter active area photomultiplier tube... 

The experiment is performed with a spectrofluorometer similar to the ones used for linear fluorescence and quantum yield measurements (Sect. 2.1). The excitation, instead of a regular lamp, is done using femtosecond pulses, and the detector (usually a photomultiplier tube or an avalanche photodiode) must either have a very low dark current (usually true for UV-VIS detectors but not for the NIR), or to be gated at the laser repetition rate. Figure 11 shows a simplified schematic for the 2PF technique. 

---