FIU CHM 4130 - CHAPTER 12_Xiao_X_Ray_Spectrometry_2018 (82 pages)

Atomic X Ray Spectroscopy Chapter 12 X ray range 10 5 to 100 Used 0 1 to 25 Atomic X Ray Spectrometry Atomic X Ray Spectrometry Emission absorption scattering fluorescence and diffraction Emission absorption scattering fluorescence and diffraction Fundamentals Instruments Fundamentals X ray Fluorescence Instruments X rayFluorescence Absorption X ray X rayAbsorption diffraction X ray X ray diffraction 8 1 8 1 Making X rays 1 By bombardment of a metal target with a beam of high energy electrons 2 By expose of a substance to a primary beam of X rays to generate a secondary beam of Xray fluorescence 3 By use of a radioactive source whose decay process results in X ray emission 4 From a synchroton radiation source Formation of X Rays emission Bombardment of a metal target with a beam of high energy electrons Formation of X Rays fluorescence Exposure of a substance to x ray radiation absorption and then secondary fluorescence Formation of X Rays decay synchroton Radioactive decay X ray emission common in medicine Synchrotron source radiation accelerated particles very few of these available X Rays are just like any kind of electromagnetic radiation Two different atomic processes to produce X ray photons Bremsstrahlung K shell emission Bremsstrahlung braking radiation X rays are generated by interactions encountered by free electrons Tungsten is the best element a high melting point and a good heat conducting The same bremsstrahlung pattern for most of heavy elements A range of photons emitted X ray A simplified diagram of a water cooled X ray tube X ray tube emission Continuum Spectra Results from Collisions between the electrons and the atoms of target materials Ee E e h At o E e 0 h 0 hc o Ve V accelerating voltage e charge on electron 0 12 398 V Duane Hunt Law Independent of material Related to acceleration voltage E 0 K shell emission X rays are generated by electrons changing energy levels within an atom K shell knock out on Innermost electron K shell spectrum depends on the target element Line spectra is possible From electron transitions involving inner shells Atomic number 23 0 L 2 line series K and L E K EL Atomic number 23 K only Line Spectrum of a Molybdenum target A minimum acceleration voltage required for each element increases with atomic number Line spectra 0 Bohr s atomic model shell model X ray line labeling Electron Transitions X Rays 1 Transitions that involve the innermost atomic orbitals 2 Energy difference between the L and K levels that between the M and L levels 3 Energy difference between the transitions labeled 1 and 2 as well as those between 1 and 2 are so small single lines 4 Energy difference between the levels increase regularly with atomic number 5 Energy of characteristic X ray lines are independent of the physical and chemical state of the element Relationship between X ray emission frequency and atomic number for K 1 and L 1 Line spectra from fluorescent sources Line or continuum spectra from radioactive sources Fe 26 55 Mn 25 55 h Interac3on of X Ray with Ma er 1 X ray absorption Transitions resulting from absorption of X ray X ray absorption spectra Ln P0 P X 1 Absorption edges is the linear absorption coefficient is characteristic of the Element and of atoms in the path of the beam X is sample thickness Ln P0 P M X 2 is density of the sample M is mass absorption coefficient 2 X Ray Fluorescence From X ray spectroscopy Transferring electrons to water Anders Nilsson Nature Chemistry 2 800 802 2010 The Basic Process X Ray Fluorescence Intensity Mul3ple Transi3ons 3 X Ray Diffraction What is X ray Di rac3on Bragg s Law of Diffraction light scattering by lattice of atoms AP PC n AP PC d sin n 2d sin n sin 2d Constructive interference only at angles proportional to and d If is known and can be measured then you can calculate d If d is known and can be measured then you can calculate Instrumenta3on of Atomic X ray Spectrometry Instrument components 1 Sources Tubes Radioisotopes Secondary fluorescence source X Ray Tube electron beam sources Determining the energy of the X Ray 100KV Controlling the intensity of the emitted X Ray Radioisotopes line spectra or continuum Secondary Fluorescent source line spectra As a source for absorption or fluorescence studies Instrumenta3on of Atomic X ray Spectrometry Instrument components 1 Monochromators Filters Monochromators 1 Filters Target filter combination 2 X ray Monochromators Flat Crystal Design sin n 2d How a collimator filters a stream of rays Simplicity Decreased radiation Increased scattering Flat crystal with Soller Collimators Rowland circle Rowland circle Curved crystal with slits Bent Crystal Design Higher intensities Higher resolution Lower background At least two interchangeable crystals d n d 2d cos Double Crystal Design Instrumenta3on of Atomic X ray Spectrometry Transducers photon counters Gas filled counters Ionization due to photo interaction with gas Three types Ionization chambers Proportional counters Geiger counters Scintillation counters Semiconductor transducers Lithium drifted silicon detectors Lithium drifted germanum detectors Gas Filled Transducers Ar ee Ar Gas ampli ca3on for various types of gas lled detectors Geiger counter Scheme of scintillation counters Si Li crystal Crystal of pure silicon with lithium diffused in to compensate for any residual carriers Much greater depletion depths about 3mm thick and 3 6 mm diameter Electrodes plated on front and back Front electrode is thin to allow X rays to enter Biased by a voltage of 3 500V Cooled to Liq N2 Semiconductors Si Li semiconductor Si Li semiconductor Energy of an x ray generates electron hole pairs These are swept from the crystal by the bias voltage and are detected in the external circuitry as a pulse of charge Since the average energy required to create an electron hole pair is constant and predictable about 3 8eV the external charge is proportional to the x ray energy XRF Spectroscopy 1 Source of X Rays used to irradiate the sample 2 Sample 3 Detection of the emitted fluorescence X Rays Why is using XRF Non destructive Minimal preparation Fast Easy to use Energy Dispersive XRF Better Efficiency Wavelength Dispersive XRF better resolution Micro EDXRF Micro WDXRF Comparison between EDXRF and WDXRF WDXRF vs EDXRF Resolution 5ev 20eV for WDX and 150eV 600eV for EDX Spectral Overlaps no need for WDX and requirement for EDX Background the amount of continuum radiation in WDX and at least 10 fold reduced the background