Monochromatic operation

The background of a diffraction pattern obtained with a diffractometer may be reduced by means of a single-channel pulse-height analyzer, as mentioned in Sec. 7-9. An even better method is to use a crystal monochromator in the diffracted beam. Balanced filters present still another option. 

Even though intensity is decreased during diffraction by a monochromator, a KP filter is not needed because the monochromator is set to diffract only Aa radiation. As a result, and because of the focusing action of the monochromator, the intensity of a diffraction line at the counter can actually be higher with a monochromator than without, particularly if the monochromating crystal is graphite. 

Placement of the monochromator in the diffracted beam has the advantage of suppressing background radiation originating in the specimen, such as fluorescent radiation and incoherent (Compton modified) scattered radiation. For example, if a steel specimen or any iron-rich material is examined with copper radiation in an ordinary diffractometer, the background due to fluorescent Fe K radiation will be unacceptably high. But if a monochromator is added and oriented to reflect only Cu Aa, the background is reduced practically to zero, because the fluoresced Fe Kol and Fe K(i do not enter the counter. A monochromator may therefore 

The diffractometer in Fig. 15-10(a) is equipped with a diffracted-beam monochromator. 

The isolation of Cu Kv. radiation may be taken as an example. Its wavelength is 1.542 A, which means that cobalt and nickel can be used as filter materials since their K absorption edges (1.608 and 1.488 A, respectively) effectively bracket the Cu Kti line. Their linear absorption coefficients p are plotted in Fig. 7-29(a), which shows that balancing can be obtained by making the nickel filter somewhat thinner than the cobalt one. When their thicknesses. v are adjusted to the correct ratio, then = l co co except in the pass band, and a plot of //.y versus /. has 

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Until the advent of lasers the most intense monochromatic sources available were atomic emission sources from which an intense, discrete line in the visible or near-ultraviolet region was isolated by optical filtering if necessary. The most often used source of this kind was the mercury discharge lamp operating at the vapour pressure of mercury. Three of the most intense lines are at 253.7 nm (near-ultraviolet), 404.7 nm and 435.7 nm (both in the visible region). Although the line width is typically small the narrowest has a width of about 0.2 cm, which places a limit on the resolution which can be achieved. 

Step 2. The computer opens a shutter, bathing the crystal in a monochromatic x-ray beam. The computer rotates the crystal for about one minute and the rotation diffraction image is stored on the detector and then read into the computer memory. When the operator examines the image and is confident that the sample is indeed a single crystal, the experiment can proceed. 

Photomultipliers are appreciably more sensitive sensors than the eye in their response to line or continuum sources. Monochromators are fitted to the light beam in order to be able to operate as substance-speciflcally as possible [5]. Additional filter combinations (monochromatic and cut-off filters) are needed for the measurement of fluorescence. Appropriate instruments are not only suitable for the qualitative detection of separated substances (scanning absorption or fluorescence along the chromatogram) but also for characterization of the substance (recording of spectra in addition to hR and for quantitative determinations. 

Technical Requirements. AXS requires an X-ray source with easily tunable, monochromatic photon wavelength. This means that a respective device can only be operated at a synchrotron. In general a 2D detector is used. 

The function of this subunit is to present so-called monochromatic radiation to the detector, i. e. to separate or disperse the radiation so that selected frequencies corresponding to particular energy transitions within the sample may be individually examined. For instruments designed to operate in the ultraviolet, visible and infrared regions of the spectrum, there are two approaches to this problem. 

X-ray Instrumentation. All experiments were performed at the Cornell High Energy Synchrotron Source (CHESS) operated at 5.8 GeV (Stations A-3, B and C-2). Monochromatic radiation was obtained with a Si (220) double crystal monochromator. In order to eliminate higher harmonics, 50% detunning was typically employed. ... 

All methods mentioned in Table 1 operate (typically) in the frequency domain a monochromatic optical wave is usually considered. Two basically different groups of modeling methods are currently used methods operating in the time domain, and those operating in the spectral domain. The transition between these two domains is generally mediated by the Fourier transform. The time-domain methods became very popular within last years because of their inherent simplicity and generality and due to vast increase in both the processor speed and the memory size of modem computers. The same computer code can be often used to solve many problems with rather... 

In the derivations above of the Stokes parameters we began with monochromatic light and then extended our results to the more general case of quasi-monochromatic light. However, the operational definition of the Stokes parameters in terms of a set of elementary experiments involving a detector and various polarizers, as opposed to the formal mathematical definitions (2.80) and (2.84), is independent of any assumed properties of the beam. Unless otherwise stated, we shall assume that all beams of interest are quasi-monochromatic, which includes as a special case monochromatic light. 

Chromatic aberrations do not arise in the acoustic microscope because in its usual mode of operation it may be considered essentially monochromatic. Even when it is necessary to take the spread of frequencies in the acoustic pulses into account, the media through which the waves pass are essentially non-dispersive in solids over the frequency range of interest the phonons are very near the centre of the first Brillouin zone where the dispersion relationship is linear, especially for sapphire. 

The second factor involves the theory that defines the natural width of the lines. Radiations emitted by atoms are not totally monochromatic. With plasmas in particular, where the collision frequency is high (this greatly reduces the lifetime of the excited states), Heisenberg s uncertainty principle is fully operational (see Fig. 15.4). Moreover, elevated temperatures increase the speed of the atoms, enlarging line widths by the Doppler effect. The natural width of spectral lines at 6000 K is in the order of several picometres. 

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