Secondary x-rays

Fig. 1-6. Roentgen s production of secondary x-rays. The secondary x-rays from zinc are most efficiently absorbed by the emulsion and give the maximum darkening.

The secondary x-rays emitted by certain substances are remarkably homogeneous in character, though the primary radiation producing them is very heterogeneous. 34... 

Fig. 6—1. Diagram of Beeghly s experiment. Note the attenuation of both the primary and secondary x-rays by the tin plate. The position of the detector is so chosen as to avoid diffraction peaks resulting from the interaction of the primary beam and metal crystals.

Roentgen, definition of the, 248 Roentgen, W. C., discovery and investigation of x-rays by, 2, 9-12, 43 Nobel Prize awarded to, 2 production of secondary x-rays by, 12 x-ray absorption studies by, 11 x-ray papers of, 2... 

When primary X-rays are directed on to a secondary target, i.e. the sample, a proportion of the incident rays will be absorbed. The absorption process involves the ejection of inner (K or L) electrons from the atoms of the sample. Subsequently the excited atoms relax to the ground state, and in doing so many will lose their excess energy in the form of secondary X-ray photons as electrons from the higher orbitals drop into the hole in the K or L shell. Typical transitions are summarized in Figures 8.35 and 8.36. The reemission of X-rays in this way is known as X-ray fluorescence and the associated analytical method as X-ray fluorescence spectrometry. The relation between the two principal techniques of X-ray emission spectrometry is summarized in Figure 8.37. 

Thus, on passage through matter, both the primary and the secondary X-ray beams will be attenuated as a result of these processes. Absorption follows Beer s law (Section 12.4), in which the intensity of the beam, 1(A), after traveling a distance x through a solid, is given by ... 

In EDXRF the secondary X-ray emitted by the excited atom is considered to be a particle (an X-ray photon) whose energy is characteristic of the atom whence it came. The major development which has facilitated this technique is the solid state semiconductor diode detector. An EDXRF system consists of a solid state device which provides an electronic output that is... 

All Mars rovers to date have carried alpha-particle X-ray spectrometer (APXS) instruments for chemical analyses of rocks and soils (see Fig. 13.16). The source consists of radioactive curium, which decays with a short half-life to produce a-particles, which then irradiate the sample. Secondary X-rays characteristic of specific elements are then released and measured by a silicon drift detector. The Mars Pathfinder APXS also measured the backscattered a-particles, for detection of light elements, but the Mars Exploration Rovers measured only the X-rays. 

XRF analysis involves directing a beam of X-rays at a small area ( 2 x 5mm) on an artifact or sample and measuring the wavelength and intensity of the secondary X-rays that are fluoresced by the area hit by the primary X-rays. The wavelengths correspond to the elements present, and their intensity is directly related to concentration. 

X-ray fluorescence spectrometry (XRF) is a non-destructive method of elemental analysis. XRF is based on the principle that each element emits its own characteristic X-ray line spectrum. When an X-ray beam impinges on a target element, orbital electrons are ejected. The resulting vacancies or holes in the inner shells are filled by outer shell electrons. During this process, energy is released in the form of secondary X-rays known as fluorescence. The energy of the emitted X-ray photon is dependent upon the distribution of electrons in the excited atom. Since every element has a unique electron distribution, every element produces... 

A spectrometer for the measurement of characteristic secondary X-rays was perhaps the first peripheral analytical device to be attached to a conventional Transmission Electron Microscope (TEM)... 

Figure 1. Schematic representation of secondary x-ray production for (a) bulk sample and (b) thin-film sample.

These thin-film techniques, which usually entail a small, focussed electron beam ( 20-50nm diameter) and an EDS detector for the measurement of secondary X-rays, have been applied to many mineralogical problems with considerable success. Many areas of geology and mineralogy, both terrestrial and extraterrestrial, can benefit from the analytical power of the modern-day AEM because the fundamental relationships between microchemistry and bulk physical properties can be effectively explored. 

In particle probe analysis systems, x-rays are generated from the elements due to an excitation caused by the impinging particles, whether they are electrons or protons, and these secondary x-rays are emitted in all directions. However, the detector can only cover a small part of the sphere of secondary radiation (Figure 5.3), even if the geometry of the experimental setup allows the detector to come very close to the object, which will increase the spatial angle from which the detector sees the volume of analysis. Here we see a factor which influences markedly the sensitivity of the analysis method. 

The heavy particles in the probe used in PIXE analysis are not as easily retarded as electrons by biological materials, and this results in a negligible background production allowing even weak secondary x-rays to be detected, hence the high sensitivity of 1 ppm. 

FIGURE 5.5 The secondary x-ray information will emerge from cellular structures in the depth of the section within volume of the proton beam volume of excitation. In addition, a probe diameter of 5 /im will result in lateral overlap of cellular compartments. Hence, the spatial resolution of the proton probe is restricted to strata rather than single cells. The resolution can be improved by diminishing the probe diameter (<2 //m) and the section thickness (<6 //m) at the cost of a substantial increase in acquisition time. 

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