Beam Switching

In beam switching, the telescope acquires total power data at signal and reference positions by repositioning one of the optical components of the telescope system, usually the secondary mirror (see Fig. 3). Since the optical components are usually relatively small, they can be switched and positioned accurately at a rapid rate, often several Hz. This allows for rapid switching between source and reference positions, which often allows for better subtraction of the fluctuating emission from the earth s atmosphere, but is limited by the relatively small reference position offsets attainable with most telescope optics. 

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Measurements were made with a cathode-ray beam switched on and off with a flat-topped pulse having 20 nsec rise and fall times. For the europium-doped sample, the rise time to 90 per cent of full intensity was 10 4 sec the decay to 10 per cent luminescence was also 10 4 sec. 

In conclusion thre first half-life measurements of light neutron rich nuclei using the MSU Reaction Product Mass Separator has resulted in the measurement of eight half-lives,two of which represent first time measurements and three of which are second measurements.The RPMS coupled with fast beam switching has proven to provide a very clean environment in which to study the decays of neutron rich nuclei. 

In all methods of background correction, two measurements are needed. The background correction requires that the spectrometer rapidly alternates between the total absorbance measurement and the background absorbance measurement, especially in electrothermal AAS where the absorption signal lasts only for a few seconds. A beam switching frequency of at least 150 Hz is required. 

TOP analysers also provide the highest practical mass range of all MS analysers, but require pulsed ionisation method or ion beam switching. For most MS/MS experiments the ion selectivity is hmited. 

Figure 4.18. Sketch of Ketr Cell illustrating application as a light beam switch.

Apart from transistors, several other solid state devices have been discussed [78], like junctions, photon and electron beam switches and various kinds of sensors. One property of diamond which has stimulated considerable interest in the recent years is the negative electron affinity (NEA) of suitably prepared surfaces [78,80]. The electron affinity, of a material is defined as the difference between the energy of a free electron in vacuum and the bottom of the conduction band Fyac - E. In Fig. 8 the electronic bands of p-doped clean and H-terminated (111) diamond surfaces near the surface are depicted, based on the results of UV-photoemission measurements. For the H-terminated surface, the electron affinity becomes negative once an electron is injected into the conduction band from a suitable contact or by UV excitation, it will easily leave the crystal and be emitted into vacuum. This effect, which is also observed on monohydride terminated (100) surfaces, is not unique to diamond but was also observed in a few other semieonductors with high band gaps [80]. Apart from a scientific interest, the NEA of diamond makes it an attractive eandidate for the replacement of thermionic emitters as electron beam sourees and as a miniature electron emitter for field emission displays. 

As shown in Fig. 5.108 a beam switch for the high energy electrons allows the operation of two FELs. 

A binary shaft encoder, which produces 100 pulses per revolution of its input shaft, is geared to the lead screw by means of a timing belt. This shaft encoder supplies its output to a beam switching counter which stores the pulses and presents the total accumulated number of pulses to the digital printer on command. With a 50-ml syringe one pulse or count is approximately... 

Position Switching Beam Switching Frequency Switching... 

Although the descriptions of hght emission still abound with subjective statements such as bright , weak and easily visible , there are very many ways in which a quantitative result can be obtained. Absolute intensity measurements are, however, seldom performed. Most quantum yields are reported relative to one or other of the available standards [3, 4]. Chemiluminescent spectra are routinely produced by fluorescence spectrometers with the excitation beam switched off or obscured. For very weak spectra, filter systems rather than true dispersive (e. g. grating) spectrometers are often employed [3]. It has been estimated in a recent review [5] that some 14% of the full papers in Analytical Chemistry include the techniques of phosphorescence, fluorescence or chemiluminescence. 

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