Atomic X-Ray Spectrometry • Emission, absorption, scattering, fluorescence and diffraction • • • • • Fundamentals Instruments X-ray Fluorescence X-ray Absorption X-ray diffraction 8-1 Fundamentals • X-rays X-ray wavelengths from 1E-5 angstrom to 100 angstrom De-acceleration of high energy electrons Electron transitions from inner orbitals * Bombardment of metal with high energy electrons * Secondary x-ray fluorescence by primary x-rays * Radioactive sources * Synchrotron sources 8-2 Continuum sources • Cathode produced x-rays Short wavelength limit (lo) Dependent upon accelerating voltage, not material X-ray produced by electron de-acceleration Line spectra Requires minimum voltage Electronic transitions amongst inner atomic orbitals * K and L lines 8-3 X-ray generation 8-4 8-5 Composite X-ray 8-6 8-7 Absorption Spectra • Edge keV A • K 115.6061 0.1072 • L-I 21.7574 0.5698 • L-II 20.9476 0.5919 • L-III 17.1663 0.7223 • M1 5.5480 2.2348 • M2 5.1822 2.3925 • M3 4.3034 2.8811 • M4 3.7276 3.3261 • M5 3.5517 3.4908 • N1 1.4408 8.6052 • N2 1.2726 9.7426 • N3 1.0449 11.8657 U absorption edges and scattering coefficients 8-8 Absorption Ln (P0/P)=mx x is sample thickness Transmitted (P) and incident (P0) beam power m is linear absorption coefficient Linear combinations of elements 8-9 X-ray Fluorescence • • • Emission of x-ray from excited elements Absorption removes electron Fluorescence involves inter electron transfers x-ray can either be absorbed by the atom or scattered x-ray absorbed by the atom by transferring all of its energy to an innermost electron is photoelectric effect if primary x-ray has sufficient energy electrons are ejected from inner shells, creating vacancies electrons from outer shells are transferred to the inner shells resulting in characteristic x-ray energy is the difference between the two binding energies of the corresponding shells each element produces x-rays at a unique set of energies The process of emissions of characteristic x-rays is X-ray Fluorescence (XRF) innermost K and L shells are involved in XRF detection 8-10 X-ray Fluorescence 8-11 Diffraction • Interaction between x-rays and electrons in sample Constructive and destructive interference Distance between scattering points are on same order as radiation wavelengths diffraction 8-12 Diffraction • Bragg equation sin q defines observed angles All others destructive interference 8-13 XRD • • • • Fixed wavelength, vary angle Powder specimen Grains act as single crystal Plot I vs angle At Bragg angle produce angle 8-14 Data analysis Normalize data to 1st sin^2theta Clear fractions Speculate on hkl Know wavelength from source, solve for a 8-15 Si glass XRD 8-16 Transducers • Photon counting Gas filled counters Ionization due to photon interaction with gas * Ionization chambers * Geiger counters * Proportional counters Scintillation counters Solid state Ge NaI 8-17 Gas filled counters 8-18 Solid state detector 4000 3500 Charactaristic X-ray Counts 3000 2500 Photopeak 2000 Lead X-ray 1500 Backscattering 1000 Compton Edge 500 0 0 100 200 300 400 500 Channel Number 137-Cs 8-19