X-Ray Fluorescence Analytical Techniques - CNSTN : Centre ...

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IV.1.2 End-window Tubes The distinguishing feature of the end-window tubes is that the anode has a positive high voltage and the beryllium exit window is located on the front end of the housing (Figure I.16). Figure I.16: The principle of the end-window tube. The cathode is set around the anode in a ring (anular cathode) and is set at zero voltage. The electrons emanate from the heated cathode and are accelerated towards the electrical field lines on the anode. Due to the fact that there is a difference in potential between the positively charged anode the surrounding material, including the beryllium window, the back-scattering electrons are guided back to the anode and thus do not contribute to the rise in the exit window’s temperature. The beryllium window remains cold and can therefore be thinner than the side-window tube. Windows are used with a thickness of 125 mm and 75 mm. this provide a prerequisite for exciting light elements with the characteristic L radiation of the anode material (e.g. rhodium). Due to the high voltage applied, non-conductive, de-ionised water must be used for cooling. Instruments with end-window tubes are therefore equipped with a closed, internal circulation system containing de-ionised water that cools the tube head as well. End-window tubes have been implemented by all renowned manufacturers of wavelength dispersive X-ray fluorescence spectrometers since the early 80’s. IV.2 Radioisotope Sources Radioisotopes are commonly used because of their stability and small size when continuous and monochromatic sources are required. Safety regulations require that X-ray emission from these sources is limited to about 10 7 photons s -1 steradian -1 compared with 10 12 or 10 13 photons for X-ray tubes; the difference is only partly compensated for by the small size of the source, which allows very compact source-specimen-detector assemblies to be constructed that are very convenient due to their portability. On the other hand, the low intensities preclude crystal dispersion so that these sources are used almost exclusively in energy dispersion techniques. Separation of analytical lines is sometimes done with selective
filters but more often with pulse height analysers in combination with high resolution Si(Li) semiconductor detectors. Radioisotope XRF systems are often tailored to a specific but limited range of applications. They are simpler and often considerably less expensive than analysis systems based on X-ray tubes, but these attributes are often gained at the expense and flexibility. Radioisotope excitation is preferred to X-ray tubes when simplicity, ruggedness, reliability and cost of equipment are important; when a minimum size, weight and power consumption are necessary; when a very constant and predictable X-ray output is required; and when the use of high energy X-rays is advantageous. Radioisotope systems, especially those involving scintillation or proportional detectors, must be carefully matched to the specific application. The activity of radioisotopes is specified in terms of the rate of disintegration of the radioactive atoms, i.e. decays per second or Becquerels (Bq) (the Becquerel replaces the non- SI unit, the Curie (Ci), which equals 3.7 10 7 Becquerels). The activity decreases with the time from Ao to A(t) after an elapsed time t: ( t) = A exp( −0. 693t / T ) , A o 1/ 2 where T1/2 is the half-life of the radioisotope. The source decays to half of its original emission rate after the time equal to its half-life has passed. The radioisotope source has usually to be replaced after several half-lives. Several sources are listed in Table I.2. Table I.2: Typical radioisotope sources used for XRF. Isotope Fe-55 Cm-244 Cd-109 Am-241 Co-57 Energy (keV) 5.9 14.3 - 18.3 22.88 59.5 122 Elements (K-lines) Al - V Ti - Br Fe - Mo Ru - Er Ba - U Elements (L-lines) Br - I I - Pb Yb - Pu None None An important property of a given radioisotope is the type of its decay and the spectrum of the electromagnetic radiation accompanying the nuclear disintegration. The basic radioactive decays are: 1. α decay: when a radioactive nucleus emits a helium nucleus (α particle) consisting of two protons and two neutrons. The energy spectrum of alpha particles is linear because of the quantization of the energy levels of nuclei. 2. β + decay: when one of the protons is transformed into a neutron, emitting a positron (β + particle) and a neutrino. The energy spectrum of positrons is also continuous. 3. K capture: when a nucleus captures one of the K-shell electrons, the final result also being the proton-into-neutron transformations, as in the case of the β + decay. In addition to the above nuclear transformations (decays), resulting in transformations of original nuclei into nuclei of other elements, the following accompanying processes may also occur: 1. Emission of gamma radiation: occurring when the resulting nucleus is not in its ground state. The existing energy surplus can be either emitted in the form of electromagnetic radiation or transferred to the atomic shell electrons (internal conversion). Sometimes a nucleus reaches its ground state through subsequent intermediate (compound) states. In such cases, every decay may be accompanied by several photons and/or internal conversion electrons, with energies being equal to the energy differences between the individual
Page 1 and 2: X-Ray Fluorescence Analytical Techn
Page 3 and 4: II.4 Energy Resolution III. Spectru
Page 5 and 6: Module: Title: X-Ray Fluorescence A
Page 7 and 8: very high resolution and X-ray phot
Page 9 and 10: where: c = speed of light (2.99782
Page 11 and 12: Figure I.2: Bohr’s atomic model,
Page 13 and 14: decelerated as they penetrate the m
Page 15 and 16: The mass attenuation coefficient ha
Page 17 and 18: e neglected provided that momentum
Page 19 and 20: In higher atomic number materials,
Page 21: Figure I.14: A Bremsstrahlung (Cont
Page 25 and 26: SECTION II ENERGY DISPERSIVE X-RAY
Page 27 and 28: just above the absorption edges of
Page 29 and 30: energy. Electronic noise is minimiz
Page 31 and 32: E is the photon energy, ε is the a
Page 33 and 34: IV.2 Compton Edge Figure II.8: Esca
Page 35 and 36: The relative error resulting from a
Page 37 and 38: Figure II.11: Schematic diagram of
Page 39 and 40: factor of 2, because also the refle
Page 41 and 42: IV. Instrumentation The major compo
Page 43 and 44: One of the inherent advantages in T
Page 45 and 46: Figure III.6: Sample preparation me
Page 47 and 48: III.1 gives an overview of various
Page 49 and 50: crystal mounted on a 2θ goniometer
Page 51 and 52: Figure IV.4: Collimators with diffe
Page 53 and 54: II.3.2 Reflections of Higher Orders
Page 55 and 56: Ge InSb PET AdP TIAP OVO-55 OVO-N O
Page 57 and 58: II.4.2 Scintillation Counters The s
Page 59 and 60: When using other counting gases (Ne
Page 61 and 62: SECTION V SAMPLE PREPARATION XRF an
Page 63 and 64: eground and pressed or cast as a di
Page 65 and 66: SECTION VI QUANTITATIVE ANALYSIS Th
Page 67 and 68: Cr 1.2 16 Sn 4.8 (Lα) 8 Mn 2.6 12
Page 69 and 70: The mechanism of the enhancement ef
Page 71 and 72: Mineralogical effect were discussed
Page 73 and 74:
In general this equation is referre
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from sugar, but then this involved
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I. (The Photon Concept) EXERCICES A
Page 79 and 80:
I. Solution SOLUTIONS An FM radio s
Page 81 and 82:
charge = 15× 10 Cs second −3 -1
Page 83:
h ∆λ = ( 1 − cosθ ) = 2λ c =
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