【正文】 one steady decreasing curve from Na down to Zn, following the red curve superimposed here. UW Madison Geology 777 Evolution of EDS spectrum: from the specimen to the monitor 1 Goldstein et al Fig (by R. Bolon) p. 330 The spectrum on our monitor (d) is a result of many things impacting the real spectrum generated within the specimen (a). At instant of generation within the specimen, there is only the Ka, Kb and continuum. An instant later (b), as the Xrays leave the specimen, two things can happen: some of the continuum Xrays above keV are absorbed, producing the drop in the continuum there. Simulation of element (say V) Xray generation and display UW Madison Geology 777 Evolution of EDS spectrum: from the specimen to the monitor 2 Goldstein et al Fig (by R. Bolon) p. 330 Also in (b) the lower energy continuum is absorbed, causing the dropoff in the spectrum there. When the Xrays hit the detector (c), Si fluorescence peaks can result. And after signal processing (d), the display will show peak broadening, sum peaks, Siescape peaks, further decrease of intensity and low energy noise. Simulation of element (say V) Xray generation and display UW Madison Geology 777 Comments about LN2 and EDS UW Madison Geology 777 That big tank of liquid nitrogen cools the SiLi crystal and the FET, so the very low charge generated by the electronsholes can be detected with minimal noise. But what about letting the thing warm up when you’re on vacation? There is a lot of misunderstanding about this… Modern systems “can” be allowed to warm up without damage to the crystal (if the bias on it is turned off) BUT that is not the only thing to be concerned about when it warms up. Another important ingredient is the vacuum within the snout that extends from the bottom of the dewar to the end where the detector sits there is a “getter” (zeolites or Al wool) inside that absorb yucky contaminants. But if the getter warms up, they are released inside the snout, creating a poor vacuum, which then means the LN usage increases significantly as the vacuum is poor. Bottom line: keep it cold all the time. EDSWDS parison UW Madison Geology 777 Recent Developments UW Madison Geology 777 Over the past 1520 years, 2 new “spins” off the ‘old school’ Si(Li) EDS detector have entered the microanalysis world: 1. The microcalorimeter 2. The Silicon Drift Detector Microcalorimeter UW Madison Geology 777 The principal behind the microcalorimeter is that an xray hitting a very sensitive thermal absorber will register a very small temperature increase. However, this requires a very cold absorber, with liquid helium cooling required. It would provide the best of both EDS and WDS, with simultaneous capture of all xray energies AND with very tight spectral resolution (like with WDS). However, there apparently have been major engineering stumbling blocks and none have made it to the market. Silicon Drift Detector UW Madison Geology 777 The SDD is similar to SiLi Detector in that electronhole pairs are generated, but the physical design is radically different. There is a lower capacitance, and also a lower leakage current (high leakage current in SiLi is what requires LN cooling). And because the SDD has the FET “built in”, created during the lithography of the Si crystal, wires are eliminated, reducing capacitance more. Resulting advantages: 1. LN not needed (use a simple Peltier cooler) 2. Can handle high count rates 100,000 up to ~106 cps 3. Spectral resolution at 100,000 cps still good (~140150 eV) Image from Bruker web page The SDD is created from a single Si crystal using microlithography. “The major distinguishing feature of an SDD is the transversal field generated by a series of ring electrodes that causes charge carriers to 39。drift39。 to a small collection electrode. The 39。drift39。 concept of the SDD (which was imported from particle physics) allows significantly higher count rates.” Wikipedia Silicon Drift Detector Simulation For the full simulation, go to url updated 2/3/14 Further EDS details UW Madison Geology 777 There are several modern EDS panies, with most producing very informative brochures that go into the technical details of EDS hardware (and software): For example: Oxford Instruments dispersivexraysystemsedsedx/edsforsem/sdd has a nice technical publication explaining EDS using the SDD as the detector. Well worth downloading and reading. url updated 2/4/14