Geology 777 Lab 3. September 28, 2009 Name____________________ Introduction to X-ray analys is: Ene rgy Dispe rsive Spe ctrometry (EDS) Purpose : The purpose of this lab is to introduce you to some basic concepts of X-ray analysis, via an introduction to EDS. These concepts include the generation of multiple lines for one element, overvoltage, X-rays as both characteristic peaks and continuum, deadtime, EDS spectral resolution, and EDS spectral artifacts. 1. Physical layout - Pointing out the features on an old EDS detector, and some features of the SX50's EDS, including the aperture switch (limiting the amount of X-rays that can hit the detector). Liquid nitrogen, rules for dry dewar. 2. Spectrum Acquisition: Setup EDS software for 60 sec acquisitions (Acquire Attributes). Set the pulse process time set to 51 microsec (under Acquire->Setup pulse processor), set accelerating voltage (E0) to a “normal” value of 20 kV on the SX51. Adjust beam current to produce ~35-45% deadtime, which will probably be around 2500 cps. The EDS aperture (on SX51 column) should be in middle position (see appendix). Record your beam current (on SX51). Current = _____nA at ______ kV with ______ deadtime. The instructor will position an unidentified sample under beam if not done so already. Three major peaks should be visible (disregard any “zero” peak that is at the extreme far left; this is electric noise which should be ignored (should be below 0.5 keV). Make sure you count for at least 60 seconds (need good statistics!) Can say immediately how many elements are present in the sample?___ Why or why not? ____________________________________________________ Determine the peak position (=energy) of each of these peaks by both expanding each peak (position red marker over it and the expanding with the black <> areas, and fine tuning the cursor position; read the x-ray peak position at top. The software should also aid by telling you the peak identity (9th icon from left: page with sma ll ? –NOT the big black ?, which is Help file. Low energy Peak _____ keV peak counts _____ ->?? Element___ Line ___ Mid energy Peak _____ keV peak counts _____ ->?? Element___ Line ___ High energy Peak _____ keV peak counts _____ ->?? Element___ Line ___ Now repeat for 15 kev and10 keV (trying to maintain Deadtime around same value you used before, 30-40% by changing beam current—DO NOT change the column aperture) and record the results. We are going to ignore the highest energy (highest keV—furthest to the right) peak. Record Faraday cup current for each measurement.15 keV curr=___nA 10 keV curr=___nA deadtime _____ deadtime ______ low energy element __ line ___ pk counts _____ pk counts _____ mid energy element __ line ___ pk counts _____ pk counts _____ Calculate La/Ka count ratios: 10 keV ______ 15 keV=______ 20 keV=_______ (The Cu La/Ka ratio at some commonly used accelerating voltage [E0] can be an important number to monitor regularly, for indications that there is no degradation of the EDS system, particularly the low energy end. What happens to the La/Ka ratio with increasing accelerating voltage?___________ Let’s try to understand why this trend occurs. First, realize that we are looking at a ratio: Numerator = low energy peak; Denominator = high energy peak. Second, in order to produce x-rays you should have your accelerating voltage at least ____ times the particular edge energy – this is known as the OVERVOLTAGE. An overvoltage of 1.1 would not yield many x-rays, while an overvoltage varying from 10 to 20 is not going to produce many additional x-rays (the cross section is slowly decreasing). Third, there is another factor that works AGAINST detecting x-rays….ABSORPTION. And low energy x-rays in many cases suffer high absorption, and the deeper the electrons penetrate, the greater the amount of absorption. So at 20 keV, Cu La x-rays will have maybe twice? the distance to travel to exit the sample (Monte Carlo simulation will give you the answer), and be absorbed that many times more. Look up the mass absorption coefficient (MAC) for Cu Ka by Cu, Table 14.3 p 747 in Goldstein et al (1992) or on the CD for the new edition. The x-ray is “emitter” and the material it is absorbed by is the absorber (here Cu, i.e. “self-absorption”) It is _________. Now look up the MAC for Cu La by Cu, Table 14.4, p.752. It is _________. Overvoltage is the ratio of the accelerating voltage over the minimum excitation (or absorption edge) energy. Calculate the overvoltages (U) for each line of the element used above (divide the E0 by the critical excitation energy, found in Goldstein et al (1992)Tables 14.6 and 14.7. K edge energy_________ L edge energy________ Ka overvoltage La overvoltage 10 kV 15 kV 20 kVIn your final writeup, you will discuss these fa ct ors that contribute to the Cu La /Ka vs kV trend. 3. Continuum: Set the E0 back to 15 kV. Blow up the x-axis of the spectrum between 15 and 20 keV. Extrapolate a straight line thru the x-axis. Where does it intersect? ___ Why? (this is called the Duane-Hunt limit and is a useful check on an instrument that you might find yourself using some day, to verify that it is operating properly) 4. Dead time, pulse processing time, and FWHM: On Cu metal, at 15 keV. TC of 51.2 microsec [“Real time”=clock time; “Live time”=time detector is accessible to new counts] You will vary the current, to produce higher and higher count rates, that progressively yield higher and higher dead times. Go from low to high dead time. Collect data in the format of the following table (DisplayAttributes for “Real Time” and ProcessPeak Stats for other info). Set the live time to 20 sec (in Acquire). Read off Dead Time and “thruput” (cps) off the bottom of the screen display. Beam current Real Time Dead Time (%) Total count Rate (cps) Cu Ka peak counts FWHM 4 nA 20 nA 40 nA 80 nA Note that the x-ray intensity (“Rate” in cps, counts/second) displayed is the TOTAL x-rays being detected (continuum plus characteristic). With the conditions still set to 80 nA, change the pulse processing time constant to 1.6 usec (Acquire => Setup Pulse Processor). Now repeat the last measurement and record your results below. Beam current Real Time Dead Time (%) Total count Rate (cps) Cu Ka peak counts FWHM 80 nA What is
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