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Chapter 12 Atomic X-Ray Spectrometry

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Chapter 12 Atomic X-Ray Spectrometry 授課教師: Professor 吳逸謨 教授 Warning: Copyrighted by textbook publisher. Do not use outside class. Principles of Instrumental Analysis Chapter 12 Atomic X-ray Spectrometry 1 X-ray wavelengths are shorter than those of UV rays and typically longer than those of gamma rays. 2 Ｘ光機器被選為史上最佳的科學發明 • Ｘ光機器被選為史上最佳的科學發明，1928年問世的抗生素藥物盤尼西林（Penicillin） • 排名第二。 • 慶祝建館100週年，位在倫敦的科學博物館邀請5萬名民眾票選對過去、現在及未來最偉大 • 的科學發明，項目涵蓋醫學、科技、科學及工程等，票選結果今天公布。 • 由德國物理學家倫特根（William Roentgen）114年前發明的Ｘ光機器，獲得近1萬 張選票，高居榜首，倫特根本人並因為這項發明在1901年獲得諾貝爾物理獎殊榮。 • 10項最偉大科學發明依序是：Ｘ光機器、盤尼西林抗生素、1953年發現的DNA雙螺旋（DNA double helix）、1969建造的阿波羅10號太空船（Apollo 10Capsule）、1944年建造的火箭引 擎（V2 RocketEngine）。1829年史蒂芬生（George Stephenson）建造的世界第一部蒸汽火 車，排名第6；1950年由英國數學家杜林（Alan Turing）發明電子計算機（Pilot ACEComputer），開啟電腦新紀元；1712年首次建造的蒸汽機排名第8；1908年福特公司設計 的第一款T型汽車（Model T Ford），開啟人類運輸史新頁。 • 排名第10的是1837年發明的電報機（ElectricTelegraph），它縮短人類通訊的時間，掀起傳播 革命。 981104 X-ray: X-radiation to signify an “unknown type of radiation”. X-ray is also called: Röntgen radiation, after Wilhelm Röntgen (a German physicist, who discovered this radiation. Atomic X-ray Spectrometry Atomic X-ray Spectrum Methods include: X-ray Emission method (Ｘ光發射分析) X-ray Absorption method (Ｘ光吸收分析) X-ray Fluorescence method (Ｘ光螢光發射分析) X-ray Diffraction method (Ｘ光繞射分析) [skipped] 12A. FUNDAMENTAL PRINCIPLE X-rays are short-wavelength (high-energy) electromagnetic radiation produced by the deceleration of high-energy electrons or by electronic transitions of electrons in the inner orbitals of atoms. The wavelength range of X-rays is from about 10-5 Å to 100 Å; conventional X-ray spectroscopy is, however, largely confined to the region of about 0.1 Å to 25 Å (1 Å = 0.1 nm = -10 10 m). 4 12A-1 Emission of X-rays • For analytical purposes, X-rays are generated in four ways: • (1) by bombardment of a metal target with a beam of high-energy electrons, [This is the most common source.] • (2) by exposure of a substance to a primary beam of X-rays to generate a secondary beam of X-ray fluorescence, • (3) by use of a radioactive source whose decay process results in X-ray emission, and • (4) from a synchrotron radiation source. • Only a few laboratories in the United States (in the world) have facilities to produce X-rays from synchrotron radiation. For this reason, we will consider only the first three sources. - (Note: This is not true anymore. Many labs outside US now have synchrotron facilities, including one in Hsinchu, Taiwan. National Synchrotron Center) Note: When the energy source is a synchrotron, the X-ray beam can be very small (microns) and very intense. 5 Note: Snychrotron For X-ray sources Cyclotron The electromagnetic radiation emitted when charged particles 6 are accelerated radially ( ) is called synchrotron radiation. X-ray sources produce only line and continuum emissions, but no bands. X-ray sources, like ultraviolet and visible emitters, often produce both continuum and line spectra; both types are of importance in analysis. Continuum radiation is also called white radiation or bremsstrahlung. Bremsstrahlung means radiation that arises from retardation of particles; such radiation is generally a spectral continuum. Lines – correspond to quantum energies of inner electrons. Continuum –radiation from retardation by particles. Note: Characteristic X-rays were discovered by Charles Glover Barkla in 1909,[1] who later won the Nobel Prize in Physics for his discovery in 1917. Continuum Spectra from Electron Beam Sources In an X-ray tube, electrons produced at a heated cathode are accelerated toward a metal anode (the target) by a potential difference as great as 100 kV; when electrons collide with the anode, part of the energy of the beam is converted to X-rays. Continuum emission -radiation from retardation by particles – Continuous and not quantum. Under some conditions, only a continuum spectrum [i.e., no lines] such as that shown in results; Figure 12-1 FIGURE 12-1 Distribution of continuum under other conditions, a line radiation from an X-ray tube spectrum is superimposed on the with a tungsten target. The numbers above the curves indicate the continuum (see Figure 12-2). accelerating voltages. 8 Bremsstrahlung – brake radiation Continuum X-ray Bremsstrahlung (German pronunciation) from bremsen "to brake" and Strahlung "radiation", i.e. "braking radiation" or "deceleration radiation") is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus. The moving particle loses kinetic energy, which is converted into a photon, thus satisfying the law of conservation of energy. The term is also used to refer to the process of producing the radiation. Bremsstrahlung has a continuous spectrum, which becomes more intense and whose peak intensity shifts toward higher frequencies as the change of the 9 energy of the accelerated particles increases. Continuum X-ray Strictly speaking, braking radiation is any radiation due to the acceleration of a charged particle, which includes synchrotron radiation, cyclotron radiation, and the emission of electrons and positrons during beta decay. However, the term is frequently used in the more narrow sense of radiation from electrons (from whatever source) slowing in matter. Bremsstrahlung emitted from plasma is sometimes referred to as free-free radiation. This refers to the fact that the radiation in this case is created by charged particles that are free both before and after the deflection (acceleration) that caused the emission. 10 – The continuum X-ray spectrum shown in the two figures is characterized by a well-defined, short-length limit (λ0), which depends on the accelerating voltage V but is independent of the target materials. [Left (Fig. 12-1): tungsten, Z=74; Right (Fig. 12-2): molybdenum, Z=42] – Thus, λ0 (0.35 Å) for the spectrum produced with a molybdenum target at 35 kV (Figure 12-2) is identical to λ0 for a tungsten target at the same voltage (Figure 12-1). 11 X-ray emission X-ray notation (K, L, M) of line emissions vs. atomic notation: “1s, 2s, 3p,”, etc., will be discussed later. 12 Cont’d – Continuum in X-ray emission • The continuum radiation from an electron beam source results from collisions between the electrons of the beam and the atoms of the target material. At each collision, the electron is decelerated and a photon of X-ray energy is produced. • The energy of the photon is equal to the difference in kinetic energies of the electron before and after the collision. • Generally, the electrons in a beam are decelerated in a series of collisions; the resulting loss of kinetic energy differs from collision to collision. Thus, the energies of the emitted X-ray photons vary continuously over a considerable range (continuum). [Fig. 12-1, next page, again] 13 The high velocity electrons collide with a metal target, the anode, creating the X-rays.[17] -In medical X-ray tubes the target is usually tungsten (W, Z=74) or a more crack-resistant alloy of rhenium (5%) and tungsten (95%), but sometimes molybdenum (Mo, Z=42) for more specialized applications, such as when softer X-rays are needed as in mammography [Note: low-energy X-rays (usually around 30 kV) to examine the human breast]. -In crystallography, a copper target is most common [Kα =1.54A is used.] Characteristic X-ray emission lines for some common anode materials.[15][16] Photon energy Wavelength Anode Atomic [keV] [nm] material number Kα1 Kβ1 Kα1 Kβ1 W 74 59.3 67.2 0.0209 0.0184 Mo 42 17.5 19.6 0.0709 0.0632 Cu 29 8.05 8.91 0.157 0.139 Ag 47 22.2 24.9 0.0559 0.0497 Ga 31 9.25 10.26 0.134 0.121 14 In 49 24.2 27.3 0.0512 0.455 Conversion[edit] X-ray notation is a method of labeling atomic orbitals that grew out of X-ray science. It is still traditionally used with most X-ray spectroscopy techniques including AES and XPS. In X-ray notation, every principal quantum number is given a letter [K, L, M, etc.] associated with it. Therefore, the X-ray notation (K, L, M) is different from atomic notation: “1s, 2s, 3p,”, etc., used in the atomic spectroscopy discussed earlier. Conversion[1][2] Quantum Numbers Atomic Notation X-ray Notation (n l s j) 1 0 ±1/2 1/2 1s(1/2) K1 2 0 ±1/2 1/2 2s(1/2) L1 2 1 -1/2 1/2 2p(1/2) L2 2 1 +1/2 3/2 2p(3/2) L3 3 0 ±1/2 1/2 3s M1 3 1 -1/2 1/2 3p1/2 M2 3 1 +1/2 3/2 3p3/2 M3 3 2 -1/2 3/2 3d M 3/2 4 15 3 2 +1/2 5/2 3d5/2 M5 Duane-Hunt law – energy transfer - electron to photon The maximum photon energy generated corresponds to the instantaneous deceleration of the electron to zero kinetic energy in a single collision. For such an event, we can write: E=hνo =hc/λo = V e (12-1) where V e, the product of the accelerating voltage and the charge on the electron, is the kinetic energy of all of the electrons in the beam; h is Planck's constant; and c is the velocity of light. The quantity ν0 is the maximum frequency of radiation that can be produced at voltage V, and λ0 is the low wavelength limit for the radiation. This relationship is known as the Duane-Hunt law. (Note that Equation 12-1 provides a direct means for the highly accurate determination of Planck’s constant.) 16 Cont’d - Duane-Hunt law When we substitute numerical values for the constants and rearrange, Equation 12-1 becomes: λo=12,398/V (12-2) where λo and V have units of angstroms and volts, respectively. [λ0, as discussed, is the low wavelength limit for the radiation.
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