Selective chopper

Starting with the selective chopper radiometers (SCR) on Nimbus 4 and 5, the Department of Physics at the University of Oxford developed a series of instruments observing infrared emission from the atmosphere. A pressure modulated instrument flew on Nimbus 6 and the Stratospheric and Mesospheric Sounder (SAMS) flew on Nimbus 7 (Table 1). SAMS measured in limb viewing geometry the gases C02, H20, CO, N20, CR, and NO in the stratosphere and mesosphere. An improved version of ISAMS flew aboard the UARS and added channels for N02, N205, HN03, 03, and H20 (e.g. Barnet et al., 1992) (Table 1). 

To view the Chopper window, from the View menu, select Chopper or press Alt+2. The Chopper window displays the entire media file of the selected track. Click on a different track to load a different media file into the Chopper. To trim and insert events into the timeline using the Chopper ... 

Another way to measure the radiation absorbed by the gas in the cell is to construct two identical cells, one with and one without a gas inside, expose both cells to radiation from the object of interest, and measure the difference between the transmitted signals. This difference is just the radiation removed by absorption in the gas contained in the first cell. Indeed, the Selective Chopper Radiometer flown on Nimbus 4 is based precisely on this principle (Houghton Smith, 1970). However, balancing radiation from both cells is difficult. 

Light transmitted by the spectrometer is detected by an EMI 9558 photomultiplier. This is combined with a selective chopper and phase-sensitive detector to generate a sig-... 

Neutrons with a wavelength of 5 to 10 A are often used. A narrow range of wavelengths is usually selected to give a quasi-monochromatic beam. This can be achieved by a crystal monochromator or, more usually, by a velocity-chopper system of rotating discs which select neutrons of a given velocity. 

The nondispersive (filter-based) PAS detector consists of very similar components to the original setup used by Alexander Bell an IR light source, a chopper wheel and a measurement cell. In addition, optical filters have been added to improve selectivity, as has a pump to introduce the sample into the measurement cell. 

Figure 2 Schematic view of the apparatus used in studies of the steric effects in gas-surface scattering. A detail of the crystal mount with die orientation rod at 1 cm in front of the surface is shown in die right hand corner. A detailed drawing of the hexapole state selector is given below the main figure. The voltage is applied to die six small rods indicated by an arrow. Key Q quadrupole mass spectrometer R Rempi detector M, crystal manipulator SI, beam source for state selected molecules H electric hexapole state selector C mechanical beam chopper V pulsed gas source S2, continuous molecular beam source. From Tenner et al. [34].

When Walsh started to think about using AAS for analytical purposes back in 1952, one of his key conclusions was that, in order to carry out absorption measurements on luminous atomic vapors, it would be necessary to employ a modulated light source and a synchronously tuned detection system, so that any radiation emitted by the sample would produce no signal at the output of the detection system [2]. This modulation principle, using either an AC-operated radiation source or a chopper in the radiation beam, and a selective amplifier tuned to the same modulation frequency, has ever since been applied in all commercially available atomic absorption spectrometers. It has been considered one of the major advantages of... 

This means that HR-CS AAS, due to its special features, does not need any modulation of the source or any selective amplifier. This also means that a potential source of noise has been eliminated, as both AC operation of hollow cathode lamps and the mechanical choppers are contributing to noise in LS AAS. In addition, other problems that are associated with strong emission of the atomizer source in LS AAS - such as the emission noise caused by the nitrous oxide -acetylene flame in the determination of Ba and Ca due to the CN band emission [3] - are equally absent in HR-CS AAS for the same reasons, that is, the higher intensity of the primary radiation source, and the high resolution. 

We have developed a novel ultrasensitive detection method, thermal lens microscopy (TLM), for nonfluorescent species [13]. TLM is photothermal spectroscopy under an optical microscope. Our thermal lens microscope (TLM) has a dual-beam configuration excitation and probe beams [13]. The wavelength of the excitation beam is selected to coincide with an absorption band of the target molecule and that of the probe beam is chosen to be where the sample solution (both solvent and solute) has no absorption. For example, in determination of methyl red dye in water, cyclohexane, and n-octanol, a 514-nm emission line of an argon-ion laser and a 633-nm emission line of a helium-neon laser were used as excitation and probe beams, respectively [21], Figure 4 shows the configuration and principle of TLM [13]. The excitation beam was modulated at 1 kHz by an optical chopper. After the beam diameters were expanded, the excitation and probe beams were made coaxial by a dichroic mirror just before they were introduced into an objective lens whose magnification and numerical aper-... 

The pulsed primary beam is passed through a skimmer into the main chamber a chopper wheel located after the skimmer and prior to the collision center selects a slice of species with well-defined velocity that reach the interaction region. This section of the beam then intersects a pulsed reactant beam released by a second pulsed valve under well-defined collision energies. It is important to stress that the incorporation of pulsed beams allows that reactions with often expensive (partially) deuterated chemicals be carried out to extract additional information on the reaction dynamics, such as the position of the hydrogen and/or deuterium loss if multiple reaction pathways are involved. In addition, pulsed sources allow that the pumping speed and hence costs can be reduced drastically. 

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