Chopper device

The measured time distribution of the prebunched positrons at the entrance slit of the chopper is about 2 ns (FWHM). This width is mainly caused by the energy spread(A sa 0.4eVFWHM) of the moderated positrons [133,136]. The positrons, which travel in a longitudinal magnetic guiding field of about 7 mT, are deflected by 90° and enter the new chopper device with an energy of 200 eV (see Figure 4.39). This chopper is a double-plate system with an external resonator, positioned outside the vacuum system. 

NoU It is possible that at some loealioiis there is no a.e. source available, such as (or battery-operated lifts iirul motor vehicles,. Such applications may also call for a variable d.e. source. When it is so. it can be achieved with the use of a chopper circuit which uses the conventional semiconductor devices. The devices are switched at high repetitive frequencies to obtain the required variation in the output voltage as with the use of a phase-controlled lliyristor rectifier, A typical chopper circuit is shown in Ingure 6.2, i. using diodes and a controlled unidirectional semieonduetor switch, which can be a thyristor or tin IGBT. 

IR-emitting diodes (GaAs) Remote controls, optical switches, optoisolators, choppers, pattern recognition devices... 

The measurement of the growth and decay of fluorescence requires essentially two items (a) modulated excitation source and (b) a detector. The modulation of an excitation source may be accomplished in various ways. These range from simple mechanical choppers to highly sophisticated electronic pulsers. Detectors may be phototubes or semiconducting devices, or even the human eye. The detector itself, in some instances, may be modulated. Of course, the detector chosen must depend upon the spectral range to be studied and the response time desired. 

In FTIR, the incident radiation is automatically modulated in an acoustic frequency, so no special devices (such as choppers) are necessary. The modulation frequency of the interferogram is given by ... 

Separate meats from shells, grind twice at least 250 g in a food chopper equipped with 3 mm plate openings or other suitable devices that give a smooth homogeneous paste without loss of oil. Mix well and store in an airtight container. For butters and pastes, mix, warming semi-solid products. 

For example, various mechanical, electronic, or other technical devices created by man can be highly complex and totally unrelated to any natural objects. Yet these items share a common feature they were designed with a definite goal. Qualitatively speaking, their basic properties were also known in advance. If we design and build an airplane, we know it will never serendipi-tously turn out to be a tape recorder or a meat chopper it will be only a good airplane or a bad one. 

Modulation can be accomplished by interposing a motor-driven circular chopper between the source and the flame, as shown in Figure 28-18. Segments of the metal chopper have been removed so that radiation passes through the device half the time and is reflected the other half. Rotation of the chopper at a constant speed causes the beam reaching the flame to vary periodically from zero intensity to some maximum intensity and then back to zero. As an alternative, the power supply for the source can be designed to pulse the hollow-cathode lamps in an alternating manner. 

Chopper A mechanical device that alternately transmits and blocks radiation from a source. 

Hastie [131] coupled for the first time a quadrupole mass spectrometer with a Knudsen cell. One of the quadrupole mass spectrometer - Knudsen cell systems used at our laboratory is shown in Fig. 4. The system has been developed to study small alkali metal clusters under equilibrium conditions (see Sect. 3.2). Broad-band photoionization by a 1 kW Hg/Xe lamp is used for the first time in Knudsen effusion mass spectrometry to reduce fragmentation. Other quadrupole mass spectrometer - Knudsen cell systems have for example been developed by Hilpert [132, 133], Fraser and Rammensee [134], Plante [135], Ono et al. [136], Kematick et al. [137], and Edwards et al. [138]. Cryogenic pumping is used in the device by Hilpert to reduce mercury background ion intensities for the study of amalgams [132], The instruments described in Refs. 134,135 use a chopper to modulate the molecular beam from the Knudsen cell. Interfering background ion intensities can, thereby, be subtracted. The apparatus developed by the authors of Refs. 137, 138 renders possible the simultaneous application of Knudsen effusion mass spectrometry and the mass-... 

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