Detection of x-rays

X-ray detectors are transducers that count individual photons. In a photoelectric interaction, the entire incident energy of the interacting photon is stored up in the detector (while in Compton scattering, only a portion of the incident energy is deposited). The detector works with greater accuracy, as the photon flux is weaker. The two most current types are  

Such tubes have been made experimentally [1.9, l.IO] and commercially [1.11]. They are small, only about 4 to 8 in. (10 to 20 cm) in length, and operate typically at a voltage of about 50 kV and a tube current of the order of 1 mA, as compared to 10 mA or more in conventional tubes. 

The principal means used to detect x-ray beams are fluorescent screens, photographic film, and counters. 

Fluorescent screens are made of a thin layer of zinc sulfide, containing a trace of nickel, mounted on a cardboard backing. Under the action of x-rays, this compound fluoresces in the visible region, i.e., emits visible light, in this case yellow light. Although most diffracted beams are too weak to be detected by this method, fluorescent screens are widely used in diffraction work to locate the position of the primary beam when adjusting apparatus. 

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Detectors. Two general types of detectors are used in x-ray medical imaging scintillation and gas ionisation. Scintillation detectors are used for both conventional projection and computerized tomographic imaging. Ionization detectors have been used only in CT appHcations. All detectors used in detection of x-ray radiation must be linear and have a maximum efficiency at the wavelength of the x-ray photon to be detected. 

It is difficult to overestimate the importance of the multiplier phototube, first made available by the Radio Corporation of America, in the detection of x-rays, 7-rays, and nuclear particles. The device is sensitive to x-rays directly, but better results are obtained if the x-rays are first converted to visible light b r a phosphor. A picture of a Du Mont No. 6291 multiplier phototube is shown in Figure 2-5b. 

Analysis by the Detection of X-rays or y rays. EPMA is a fully qualitative and quantitative method of non-destructive analysis of micrometre-sized volumes at the surface of materials, with sensitivity at the level of ppm. All elements from Be to U can be analysed, either in the form of point analysis, from line scans and also as X-ray distribution maps. Current software allows the combination of elemental data in the latter, so that, for example, the digital data for those elements that corresponds to a selected phase will produce an X-ray map of the distribution of that phase in a given microstructure. 

Scanning electron microscopy (SEM) can produce images of surface and objects at high magnification. If the scanning of the electron beam is also coupled to detection of x-rays from the electron impact on the... 

Waller H, Friess E, Kiefer J (1981) On the immunological detection of X-ray induced DNA damage. Radiat Environ Biophys 19 259-264... 

The experimental equipment requires a standard fine structure X-ray generator operated usually with monochromatic K -radiation. The measurements of the refraction effect are taken by using a commercial small angle X-ray camera of the Kratky type in combination with two scintillation detectors for simultaneous detection of X-ray refraction intensity Ir and sample absorption U- A standard DOS-computer handles the scattering intensity data acquisition and the micromanipulator scanning-system. Figure 1 shows the experimental setup. 

Fluorescent screens are used only for the detection of x-ray beams, while photographic film and the various kinds of counters permit both detection and measurement of intensity. Photographic film has the advantage of being able to record a number of diffracted beams at one time and their relative positions in space, and the film can be used as a basis for intensity measurements if desired. Intensities can be measured much more rapidly with counters, and these instruments are more popular for quantitative work. However, most counters record only one diffracted beam at a time. 

Without exception all electronic counters were developed by nuclear physicists for studies of radioactivity. They can detect not only x- and y-radiation, but also charged particles such as electrons and a-particles, and the design of the counter and associated circuits depends to some extent on what is to be detected. Here we are concerned only with counters for the detection of x-rays of the wavelengths commonly encountered in diffraction. 

Gas-filled chambers operating in the proportional counter range are used in certain special applications for the detection of X-rays with energy less than 100... 

Soderholm L, Antonio MR, Williams C, Wasserman SR (1999) XANES spectroelectrochemistiy A new method for determining formal potentials. Anal Chem 71 4622-4628 Soderholm L, Liu GK, Antonio MR, Lytle FW (1998) X-ray excited optical luminescence (XEOL) detection of X-ray absorption fine structure (XAFS). J Chem Phys 109 6745-6752. 

Discuss methods for extending the limits of detectability of x-ray fluorescence spectroscopy down to the ppm-ppb range. Also include comments regarding possible sources of systematic errors. 

X-ray absorption spectra are usually recorded as fluorescence excitation spectra. The limited resolution (150 eV) of the X-ray detectors has precluded obtaining useful information from the fluorescence spectra per se. However, the resolution of the fluorescence spectrum can be improved to about 1 eV by using energy resolving optics. The K jS X-ray emission spectra (3p-l5) of Mn complexes have shown that the spectrum is sensitive to the oxidation state and spin state of the metal. The potential for site-and spin-selective detection of X-ray absorption spectra in bioinorganic chemistry is considerable. 

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