X-ray emitters

X-ray spectrometry is generally carried out with Si(Li) detectors. The set-up is similar to that applied to y-ray spectrometry with i-Ge or Ge(Li) detectors cooling of the detector in a cryostat, operation in combination with a preamplifier, an amplifier and a multichannel analyser. The energy resolution is very good, as already mentioned in section 7.6, and makes it possible to distinguish the characteristic X rays of neighbouring elements. Some X-ray emitters that may be used for calibration purposes are listed in Table 7.6. 

The range of alpha, beta and X-ray emitters determined was based on the nuclides that had been positively detected in previous Bradwell FED analysis programmes. The dominant nuclides present in the pre-abated dissolution liquors were the beta emitters Ni, and... 

There are several sources of X-rays such as a Coolidge tube, vacuum sparks, hot-dense fusion plasmas, synchrotron, pinch devices, muonic atoms, beam-foil interaction, stellar X-ray emitters, solar flares, etc. The X-rays originating from all these sources can be broadly categorized into main types (1) atomic inner shell transitions, (2) emission by free electrons, (3) X-rays from few electron systems. The basic spectroscopic aspects of the various types of X-rays are discussed in this article. 

These include sources such as laser produced plasmas, tokamak plasmas, pinch plasmas, solar flares, stellar X-ray emitters, etc. In such plasmas, the electron temperature (corresponding to a Maxwellian velocity distribution) can be a few hundreds of eV to several keV. On collision with plasma ions these energetic electrons undergo acceleration/deceleration and thereby emit Bremsstrahlung radiation. Electron-electron collisions do not emit any net radiation as the two colliding electrons undergo exactly equal and opposite accelerations. The radiation emitted by the two electrons is therefore equal in magnitude and opposite in phase. Hence, there is no net radiation emitted. 

And a rotation of the emitter-receiver transducer around the "object" (or a rotation of the object) gives a annulus of center O and radii [Km, Km] [2]. The situation is identical to that of X-ray tomography (slice-by-slice spectral coverage), but with a band-pass spectral filter instead of a low-pass spectral filter. ... 

Fluorescence and phosphorescence are types of luminescence, ie, emission attributed to selective excitation by previously absorbed radiation, chemical reaction, etc, rather than to the temperature of the emitter. Laser-iaduced and x-ray fluorescence are important analytical techniques (see... 

Eigure 4.30 is an example of X-ray mapping of an (In,Ga)As quantum wire structure using a TEM/STEM Philips GM20 equipped with a thermally-assisted field-emitter and a Ge EDXS detector (Tracor Northern) [4.124]. The cross-section STEM bright-... 

Celsius. The energy distribution of the radiation emitted by this surface is fairly close to that of a classical black body (i.e., a perfect emitter of radiation) at a temperature of 5,500°C, with much of the energy radiated in the visible portion of the electromagnetic spectrum. Energy is also emitted in the infrared, ultraviolet and x-ray portions of the spectrum (Figure 1). 

Fig. 1-7. Schematic diagram of Barkla s experiment. Barkla proved that the component of the x-ray beam reaching the ionization chamber is independent of angle and characteristic of the element used as secondary emitter.

Positron emission tomography (PET) makes use of a short-lived positron emitter such as fluorine-18 to image human tissue with a degree of detail not possible with x-rays. It has been used extensively to study brain function (see illustration) and in medical diagnosis. For example, when the hormone estrogen is labelled with fluorine-18 and injected into a cancer patient, the fluorine-bearing compound is preferentially absorbed by the tumor. The positrons given off by the fluorine atoms are quickly annihilated when they meet... 

Scintillation counters, which constitute an extremely important group, depend upon the absorption of radiation by a scintillator to produce UV light scintillations, which are detected and converted into amplified voltage pulses by a photomultiplier (Figure 10.10). Solid scintillators are used extensively for the detection and analysis ofy-rays and X-rays, while liquid scintillators find widespread employment in the measurement of pure negatron emitters, especially where the particle energy is low (< 1 MeV). 

For X-ray generation, other radionucleides such as 241 Am (r = 430 years) can be used. This nucleus emits a radiation accompanied by b photons of 60 keV energy. Finally, it is possible to mix a / -emitter and a second element, which is used as a target and plays the role of the anticathode in an X-ray tube. For example, the source 147Pm/Al (r — 2.6 years) emits a decelerating radiation between 10 and 200 keV. 

Each of the types of radiation has a characteristic way of interacting with matter and transferring its energy. Alpha radiation has the least penetrating power and its effects are limited to the surface layers of a material, so it only needs to be considered when a surface is contaminated by an alpha emitter. Beta radiation has a range of up to a centimetre or two whilst X-ray, gamma... 

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