Measurement techniques photomultiplier tubes

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... 

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. 

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. 

A photoelectric technique has been devised whereby the position of the deton front in an expl can be measured. The light from the deton front is transmitted by a small-diameter optical fiber to the cathode of a photomultiplier tube where the light pulse is converted to an electrical pulse. Additional pulses may be generated by other fibers located elsewhere in the expl. All of the pulses may be recorded on an oscilloscope. The usefulness of the technique has been demonstrated by the measurement of the transmission times of detonators, the propagation velocities-of expl columns, the timing of expl systems, the vels... 

Emission spectroscopy utilizes the characteristic line emission from atoms as their electrons drop from the excited to the ground state. The earliest version of emission spectroscopy as applied to chemistry was the flame test, where samples of elements placed in a Bunsen burner will change the flame to different colors (sodium turns the flame yellow calcium turns it red, copper turns it green). The modem version of emission spectroscopy for the chemistry laboratory is ICP-AES. In this technique rocks are dissolved in acid or vaporized with a laser, and the sample liquid or gas is mixed with argon gas and turned into a plasma (ionized gas) by a radio frequency generator. The excited atoms in the plasma emit characteristic energies that are measured either sequentially with a monochromator and photomultiplier tube, or simultaneously with a polychrometer. The technique can analyze 60 elements in minutes. 

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. 

With the development of the photomultiplier tube the measurement of very low light intensities has become relatively simple and the photoelectric recording of fluorescence emission spectra can now compete in terms of sensitivity with the less convenient photographic method. During the last decade the development of the experimental technique has gained considerable impetus as a result of the requirements of analytical chemists for methods of extreme sensitivity. A variety of spectro-fluorimeters have been described in the literature and commercial instruments of high sensitivity are also available. Recent reviews1-2 deal with the principles and analytical applications of fluorescence spectrometry and a textbook of biochemical applications has been published.2... 

Measurements of NO and N02 by Chemiluminescence. Most balloon-borne in situ measurements of NO and NO published to date have been made with the chemiluminescence technique (6, 63,64). Reagent ozone is added to the ambient flow in a reaction chamber, and the chemiluminescence observed from the reaction NO + 03 —> NO + 02 is detected by a photomultiplier tube that views the volume. The amount of light observed is proportional to the amount of NO in the ambient air. N02 is detected by exposing the ambient air going into the detection cell to a strong light source this exposure photolyzes N02 to NO, which can then be detected. 

The first quantitative studies were made by Johnston and Crosby,227 228 who used a stopped-flow technique to monitor the disappearance of ozone by absorption of 2537-A light. Intensity was measured with a photomultiplier tube, the output of which was displayed on an oscilloscope with sweep frequency 0.2-2 sec. Corrections were made for the absorption of N204 at 2537 A, because the absorption coefficient of that species is almost 5% that of ozone at that wavelength. The reaction was observed at 198 and 230°K, with initial NO and 03 concentrations of 0.17-1.85 x 10-5 M and 0.27-2.13 x 10 5 M, respectively. Nitrogen was present in pressures of 130-650 torr. The primary step was determined to be a bimolecular second-order reaction... 

The densitometer uses a two beam comparator technique of measurement. A single lamp illuminates, through a variable neutral density filter, the emitter of an eleven stage photomultiplier tube. Because the frequency of measurement is five times the AC line frequency and the amplifiers are extremely high gain, the lamp must be operated from a regulated DC source, otherwise, 120 Hz brightness ripple of the lamp is recorded in the data. 

Our primary measurement technique was to pull seawater through a 25-mL volume chamber with a pump. The organisms emit light when stimulated by the tur-bulently flowing seawater. This light is viewed by a photomultiplier tube (PMT). Two in situ measurement systems were used on surface ships. The on-board bioluminescence detector pulls seawater from below the ship s hull for continuous realtime measurements of surface bioluminescence a bathyphotometer was used on station to depths of 100 m. An additional laboratory system was used to measure bioluminescence flashes from individual plankters isolated from plankton tows and pumped collections. 

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