X-rays interaction with matter

All analytical methods that use some part of the electromagnetic spectrum have evolved into many highly specialized ways of extracting information. The interaction of X-rays with matter represents an excellent example of this diversity. In addition to straightforward X-ray absorption, diffraction, and fluorescence, there is a whole host of other techniques that are either directly X-ray-related or come about as a secondary result of X-ray interaction with matter, such as X-ray photoemission spectroscopy (XPS), surface-extended X-ray absorption fine structure (SEXAFS) spectroscopy, Auger electron spectroscopy (AES), and time-resolved X-ray diffraction techniques, to name only a few [1,2]. 

X-rays interact with matter because their electromagnetic oscillations are affected by the electrons of the material. Neutrons take no notice whatsoever of electrons when they pass through matter. They interact with the nuclei. Neutron diffraction is sensitive to the atomic number and atomic weight of the atoms constituting the substance. For example, it can distinguish easily between Fe and Co in alloys and between isotopes such as and Cl. 

They are much more penetrating it takes about 5 cm of lead to stop typical y radiation. Because of their high ionizing power y rays destroy tissue and inflict serious burns quite rapidly. Both y and X rays interact with matter in three different ways ... 

X-Ray Interactions With Matter (2003,2013), http //henke.lbl.gov/optical constants/... 

Fig. 1. Comparison of the four different physical processes which can be observed during the interaction of X-ray photons with matter 2 1. The two phenomena scetched below, namely photoelectron emission and Auger electron emission, can be detected and measured in a photoelectron spectrometer by determining the kinetic energy of the ejected free electrons...

When X-rays interact with any kind of materials, absorption and phase shifts effects occur. Conventional X-ray radiography relies on the absorption properties of the sample. The image contrast is produced by a variation of density, a change in composition or thickness of the sample, and is based exclusively on the detection of an amplitude variation of X-rays transmitted through the sample itself. Information about the phase of X-rays is not considered. The main limitation of this technique is the poor intrinsic contrast in samples with low atomic number (i.e., the case of soft matter ) or in materials with low variation of absorption from point to point. 

X-rays interact with electrons in matter. When a beam of X-rays impinges on a material it is scattered in various directions by the electron clouds of the atoms. If the wavelength of the X-rays is comparable to the separation between the atoms, then interference can occur. For an ordered array of scattering centres (such as atoms or ions in a crystalline solid), this can give rise to interference maxima and minima. The wavelengths of X-rays used in X-ray diffraction experiments therefore typically lie between 0.6 and 1.9 A. 

Much like X-rays, the interactions of neutrons with matter are atomic in nature. The difference is that neutrons are sensitive to nuclei directly, whereas X-rays interact with electrons. Hence, while X-rays are unsuitable to detect light elements because of the low atomic electron count, neutron scattering factors depend on the properties of the nucleus [206]. The most relevant consequence in the context of this discussion is that neutron-based tools are better suited for the detection of H and Li than X-rays, as H and Li are among the most highly neutron-absorbing atoms, and that they offer isotope resolution capability. In principle, they are also nondestructive. 

A narrow, monochromatic beam of X-rays of energy E and intensity Iq, which is transmitted through a foil of thickness d is attenuated to a value I d). The attenuation is the result of the interaction of the X-ray photons with matter and varies with the foil thickness according to... 

FIG U RE 7.13 Schematic representation of the main processes involved in the interaction of x-ray radiation with matter there are also other processes (i.e.. Auger electrons and electron-positron pair formation) that are usually minor and not represented here. 

A monograph centered on polymer morphology should undoubtedly include a chapter on these techniques. This chapter is therefore focused on the possibilities offered by X-ray and neutron scattering/diffraction for determining the structure of polymer systems. These techniques are often complementary as X-ray photons and neutrons do not see matter the same way. The difference of neutrons versus X-rays lies in the way they interact with atoms X-rays interact with the electron cloud, while neutrons interact with the nucleus. Neutrons have an unquestionable advantage over X-rays when polymers are at stake thanks to the difference in scattering amplitude between hydrogen and deuterium, as is detailed below. 

First of all, these different types of radiation differ in the manner they interact with matter the photons of visible light are scattered by atoms while X-rays interact with electrons and neutrons with atomic nuclei. 

X-rays are detected by observing an effect of their interaction with matter. The name x-ray detector came into use when such observations were predominantly qualitative. Nowadays, the emphasis is on high precision and efficiency so that most modern observations are measurements either of intensity or of dosage (x-ray quanta absorbed during exposure time). X-ray detector as a name has survived this change in emphasis although it does not describe the quantitative function of these devices. 

The first theoretical attempts in the field of time-resolved X-ray diffraction were entirely empirical. More precise theoretical work appeared only in the late 1990s and is due to Wilson et al. [13-16]. However, this theoretical work still remained preliminary. A really satisfactory approach must be statistical. In fact, macroscopic transport coefficients like diffusion constant or chemical rate constant break down at ultrashort time scales. Even the notion of a molecule becomes ambiguous at which interatomic distance can the atoms A and B of a molecule A-B be considered to be free Another element of consideration is that the electric field of the laser pump is strong, and that its interaction with matter is nonlinear. What is needed is thus a statistical theory reminiscent of those from time-resolved optical spectroscopy. A theory of this sort was elaborated by Bratos and co-workers and was published over the last few years [17-19]. 

X-rays are electromagnetic radiation with short wavelengths of about 0.01 to 10 nm. X 0.15 nm is the typical wavelength for the study of soft condensed matter. Whenever X-rays are interacting with matter, their main partners are the electrons in the studied sample. Thus X-ray scattering is probing the distribution of electron density, p (r), inside the material. 

X-rays, often used in radiation chemistry, differ from y-rays only operationally namely, X-rays are produced in machines, whereas y-rays originate in nuclear transitions. In their interaction with matter, they behave similarly—that is, as a photon of appropriate energy. Other radiations used in radiation-chemical studies include protons, deuterons, various accelerated stripped nuclei, fission fragments, and radioactive radiations (a, /, or y). 

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