Main beam attenuation

The El source precedes the FD source and is used as a collision cell. The gas has a major role in collisions collision gas pressure required to attenuate the main beam by 2/3 varies as a function of the products, their molecular weights and the type of gas chosen. 

X-ray and neutron imaging are complementary techniques for materials research. X-rays interact mainly with the electronic shell of atoms whereas neutrons as charge-neutral particles interact with the nuclei (Figure 18.1a,b). The different interaction mechanisms yield different beam attenuation properties. Figure 18.1c shows the values for the attenuation coefficients of X-rays and neutrons for different element numbers. In the case of X-rays, the attenuation increases with the number of electrons in the atom and, therefore, with the element number. In case of neutrons, no clear dependence on the amount of nuclei within the atomic core can be found. In contrast to X-rays, some light elements such as H and Li have a very... 

Practical X-ray energies do not exceed 100 keV. The primary beam is mainly attenuated by the photoelectric effect. Scattering, both elastic (Rayleigh) and inelastic (Compton), represents a minor contribution to attenuation at energies below 100 keV. 

If the crystal is a semiconductor such as Si, it can be used as the working electrode itself and this was the means employed in the early experiments. However, the limited number of suitable electrode materials available was a severe restriction and attempts to use metal-coated crystals suffered severely from the low sensitivity caused by the attenuation of the IR beam by the coating. In addition, lock-in detection is mainly limited to those electrochemical systems capable of responding sufficiently rapidly to the imposed potential modulation. 

The experimental simplicity of turbidity spectra is the main reason why the technique has received so much attention in the past years (1-5). Turbidity gives a measure of the attenuation of a beam of light traversing a dispersion of non-absorbing spheres ... 

Fluctuations in the dielectric properties near the interface lead to scattering of the EW as well as changes in the intensity of the internally reflected wave. Changes in optical absorption can be detected in the internally reflected beam and lead to the well-known technique of attenuated total reflectance spectroscopy (ATR). Changes in the real part of the dielectric function lead to scattering, which is the main topic of this review. Polarization of the incident beam is important. For s polarization (electric field vector perpendicular to the plane defined by the incident and reflected beams or parallel to the interface), there is no electric held component normal to the interface, and the electric field is continuous across the interface. For p polarization (electric field vector parallel to the plane defined by the incident and reflected beams), there is a finite electric field component normal to the interface. In macroscopic electrodynamics this normal component is discontinuous across the interface, and the discontinuity is related to the induced surface charge at the interface. Such discontinuity is unphysical on the molecular scale [4], and the macroscopic formalism may have to be re-examined if it is applied to molecules within a few A of the interface. 

The main problem in IR measurements in situ is a strong IR absorption by the solvent water. Therefore the measurements in the classical transmission mode are practically excluded (the path length must not exceed 15 tan). Short path lengths are provided by the attenuated total reflection (ATR) mode. The probing light beam undergoes multiple total internal reflection in the ATR element (e.g. a cylindrical rod made of ZnSe, with two cone-sharpened ends) and each reflection... 

ATR (Attenuated Total Reflection) spectroscopy [27] was invented and applied mainly for investigations of surfaces.This method entails an experimental set-up which reflects the IR beam via a mirror system to a crystal with a high refraction index. Normally, germanium (refraction index n = 4), silver chloride (n = 2) or diamond (n = 2,4) crystals are used. The crystal is standing in direct contact with the surface of the sample (Fig. 16.3). 

Under poor mixing conditions (dominance of component A), there are sharp concentration discontinuities instead of a true gradient function along the flowing sample. A myriad of transient liquid mirrors is established within the poorly mixed fluid elements with different concentrations, and hence refractive indices. The incident beam then undergoes attenuation, mainly by reflection, leading to a decrease in the power of transmitted radiation and therefore to an increase in, e.g., absorbance. 

Instead of measuring the attenuation of a beam, one may also count the ions produced with very high efficiency by the use of channelplates or a hot-wire detector [387], an approach which has mainly been applied in laser spectroscopy, where high sensitivity can be achieved by space charge amplification. The principle of the thermionic diode is that the atomic vapour under study is formed within the detector, and a current limited by the space charge is obtained by appropriately biasing a diode, consisting of an external anode (often the outer wall of the vacuum system, formed by a metal tube) and a heated cathode made of a suitable material to emit many electrons (thoriated W is suitable in many cases). A sketch of... 

The most widely available technique for identifying mainly polymer, but also additives in plastics, is Fourier Transform Infrared (FTIR) spectroscopy. Samples are exposed to infrared light (4000-400 wavelengths per centimetre or cm ) causing chemical bonds to vibrate at specific frequencies, corresponding to particular energies. In the last 5 years, an accessory for FTIR has been developed, which enables non-destructive examination of surfaces and so is ideal for analysis of plastics in museum collections. Attenuated Total Reflection-FTIR (ATR-FTIR) requires samples to be placed on a diamond crystal with a diameter of 2 mm through which the infrared beam is reflected... 

This is conceptually the most straightforward technique. In view of the small relative change in laser intensity, it is mainly used in cases where two different laser beams are crossed in the sample. Often one of the lasers is a high-power, fixed-frequency device attenuation is measured on the second, weaker laser appropriately termed the probe, or monitor, laser. The method was used in early days of two-photon absorption spectroscopy and applied mostly to high-density and liquid samples. Its obvious drawback is poor sensitivity, as the desired quantity is obtained as a small difference between two large numbers. 

Fig. 5-8 Scattering of K -I- CH3I. (a) Intensity of scattered signal as measured by Pt ( ) and W (O) surface-ionization detectors. Pt measurements are normalized to W measurement in main K beam. Main K beam, shown at laboratory angle = 0, is attenuated by CH3I crossed beam, (b) Detected KI distribution for two experiments. (Adapted from Herschbach [22].)...

The main measure of the intensity of neutron radiation is the neutron flux J. It is a product of the concentration of free neutrons in a pile and their mean speed, [7] = n/m xs. When a beam passes through an absorber material, it is attenuated. The degree of attenuation depends on the absorption process according to Lambert s law ... 

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