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Hanoi 4- 2013 Class: Materials Science Engineering Doctor : Phạm Mai Khánh Presenters : Hoàng Văn Tiến Nguyễn Đình Trung

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  • 1. Hanoi 4-2013Class: Materials Science EngineeringDoctor : Ph m Mai KhnhPresenters : Hong Vn Ti nNguy n nh TrungPh m c Th nh

2. contents- Introduction- Functions and properties of EDS- The working of EDS- Resolution of detector- Summary 3. functions EDS can be used to find the chemicalcomposition of materials down to a spot sizeof a few microns, and to create elementcomposition maps over a much broaderraster area. 4. X-ray detectors 5. 1. Collimator assembly The collimator provides a limiting aperturethrough which X-rays must pass to reach thedetector. This ensures that only X-rays fromthe area being excited by the electron beamare detected. 6. 2. Electron trap Electrons that penetrate the detector causebackground artifacts. The electron trap is apair of permanent magnets that stronglydeflect any passing electrons that could causebackground artifacts 7. 3. Window The window provides a barrier to maintainvacuum within the detector whilst being astransparent as possible to low energy X-rays.CharFac has a polymer-based thin windowthat can transmit X-rays from elementsheavier than Beryllium. 8. 4. Crystal The material used for the crystal is silicon (Si), into which isdrifted lithium (Li) to compensate for small levels of impurity. When an incident X-ray strikes the detector crystal its energyis absorbed by a series of ionizations within the semiconductorto create a number of electron-hole pairs. An electron-hole pair is created for every 3.76 eV of incomingX radiation. Thus, for example, a Ni K X-ray photon (7,471eV) will produce a current of 1,966 electrons. The electrons are raised into the conduction band of thesemiconductor and are free to move within the crystal lattice. When an electron is raised into the conduction band it leavesbehind a hole, which behaves like a free positive charge withinthe crystal. A high bias voltage, applied between electrical contacts on thefront face and ba ck of the crystal, then sweeps the electronsand holes to these opposite electrodes, producing a chargesignal, the size of which is directly proportional to the energyof the incident X-ray. 9. 5. FET (Field Effect Transistor ) The Field Effect Transistor is the firststage of the amplification process thatmeasures the charge liberated in thecrystal by an incident X-ray and convertsit to a voltage output. During operation, charge is built up on thefeedback capacitor. The sharp steps on the voltage buildup aredue to the charge created by each X-rayevent. The voltage step size is proportional tothe incident X-ray energy. This accumulating charge has to beperiodically restored to prevent saturationof the preamplifier. 10. 6. Cryostat- The charge signals generated by the detector are small and can onlybe separated from the electronic noise of the detector if the noise isreduced by cooling the crystal and FET.- The noise determines the resolution of a detect or particularly at lowenergies.- The natural width (FWHM) of an X-ray peak is on the order of 2-10eV .- The actual FWHM is at least an order of magnitude greater thanthis due to noise in the system.- The EDS detector is cooled using a reservoir of liquid nitrogen heldin a dewar .- The vacuum is maintained at a low enough level to prevent thecondensation of molecules on the crystal. 11. Pulse Processor The signal (voltage step) from thepreamplifier is transformed into avoltage pulse that is suitable for themulti channel analyzer. The time over which the waveform isaveraged is called the process time (Tp). Tp is under control of the operator. Thelonger the Tp, the lower the noise butmore time is spent measuring each X-ray, and the fewer events that can bemeasured. If noise is minimized, the resolution ofthe peak displayed in the spectrum isimproved, and it becomes easier toseparate or resolve, from another peak 12. Deadtime Deadtime = (1 Outputrate/Input rate) x 100. Deadtimes of 30-60% willtend to maximize output. The operator can and shouldmaximize output rates for agiven sample and processtime by controlling probecurrent/spot size. 13. Multi Channel Analyzer(MCA) The MCA takes the data from the pulse processor anddisplays it as a histogram of intensity (number of counts)vs voltage. The voltage rang e (for ex., 20 keV) displayed on the x-axis is divided into a number (1024, 2048, etc) ofchannels each corresponding to a given energy range (forexample, 5,280 eV 5,300 eV). The MCA takes the peak height of each voltage pulse,converts it into a digital value, and puts it into theappropriate channel. Thus a count is registered at that energy level. 14. Computer controls Control whether the detector is on or off (swithed offwhen an X-ray signal is detected) Control the processing electronics , setting the timerequired to analyze the X-ray signal and assigningthe signal to the correct channel in the MCA (MicroChannel Architecture) Computer software governs the calibration of thespectrum readout on the MCA screen and all thealpha-numerics (conditions under which we acquiredthe spectrum Any data processing is also carried out using thecomputer 15. Geometry of the Detector : The geometry of the beam--specimendetector is such that maximum signal is obtainedfrom the specimen at 10 mm working distance. Accelerating voltage: One must exceed the critical ionization energy of theelement(s) of interest by a factor of 1.5 to 3 to efficiently excitethe X-ray line(s) with an electron beam. Exploratory analysisis often conducted with an accelerating voltage in the 1520keV range since a broad array of elements will be detected. An addition spectrum collected at 5-10 keV could help to avoidmissing low atomic number elements at low concentrations.The accelerating voltage can subsequently be tailored to theelements and shell levels of your specimen. Keep in mind thataccelerating voltage and atomic number are two factors thatdetermine the spatial resolution of and depth of signal fromyour specimen. 16. Process time: There are a rangeof available process times toselect from on the software. Therange is: high resolution/lowthroughput (65 us time constant)--high throughput/low resolution(4 us time constant). Duration of signal acquisition: set a live time limit. Th e system willautomatically compensate for thedeadtime value; set a maximum peak count.Acquisition will terminate when thespectral vertical scale reaches thisvalue; select of region of interest (energyrange) and terminate acquisition whena set number of count s have beenaccumulated within that region. 17. Quantitative X-Ray Analysis Measured intensities from the specimen needto be corrected for a host of matrix effects(ZAF): Z: Atomic Number Effect : A: X-Ray Absorption Effect : F: X-Ray Fluorescence Effect: 18. The working of EDS The detector generates a charge pulseproportional to the X-ray energy This pulse is first converted to a voltage Then the signal is amplified through a fieldeffect transistor (FET) ,isolated from otherpulses , further amplified ,then identifiedelectronically as resulting from an X-ray ofspecific energy Finally, a digitized signal is stored in a channelassigned to that energy in the MCA 19. Principal of EDS- The incident beam may excite an electronin an inner shell, ejecting it from the shellwhile creating an electron hole where theelectron was.- An electron from an outer, higher-energyshell then fills the hole, and the differencein energy between the higher-energy shelland the lower energy shell may bereleased in the form of an X-ray.- The number and energy of the X-raysemitted from a specimen can be measuredby an energy-dispersive spectrometer.the energy of the X-rays are characteristicof the difference in energy between the twoshells, and of the atomic structure of theelement from which they were emitted, thisallows the elemental composition of thespecimen to be measured. 20. Mosley principal X-rays emitted frequency is characterized for each atom insolids. 21. EDS system in TEM 22. Resolution of detector P: a measure of the quality of the associated electronics(definedas the full width at half maximum (FWHM) of a randomizedelectronic pulse generator X : the FWHM equivalent attributable to detector leakagecurrent and incomplete charge collection I : the intrinsic line width of the detector which is controlled byfluctuations in the numbers of electron hole pairs creat by agiven X-ray 23. Accuracy of EDS Accuracy of EDS spectrum can be affected by various factors : the nature of the sample. The likelihood of an X-ray escaping the specimen, and thusbeing available to detect and measure, depends on the energyof the X-ray and the amount and density of material it has topass through..EDS spectrum of the mineral crust of Rimicaris exoculata 24. summary The EDS is used to examine and analyze thechemical composition of materials down to a spotsize of a few microns, and to create elementcomposition maps over a much broader raster area. Energy Dispersive Spectrometer (EDS) micro-analysis is performed by measuring the energy andintensity distribution of X-ray signals generated by afocused electron beam on the specimen (EDS). Withthe attachment of the energy dispersivespectrometer , the elemental composition ofmaterials can be obtained. 25. references http://www.met-tech.com/sems_eds.html http://en.wikipedia.org/wiki/Energy-dispersive_X-ray_spectroscopy Joseph Goldstein et al. Scanning El ectronMicroscopy and X- Ray Microanalysis. http://www.microscopy.ethz.ch/aed.htm John J. Friel. X-ray and Image Analysis in ElectronMicroscopy, Princeton Gamma-Tech.