【正文】 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。 Fig Goldstein Xrays are absorbed by Si, with photoelectrons ejected. This photoelectron then creates electronhole pairs as it scatters inelastically. The Si atom is unstable and will either emit a characteristic Auger electron or Si ka Xray. If Auger, it scatters inelastically and produces electronhole pairs. If Si Ka Xray, it can be reabsorbed, in a similar process, or it can be scattered inelastically. In either case, the energy will end up as electronhole pairs. The result, in sum, is the conversion of all the Xray’s energy into electronhole pairs with 2 exceptions. UW Madison Geology 777 Artifacts: Siescape peak。 the grid (., Si or Ni) takes up about 15% of the area, but the window material is thin enough that low energy Xrays pass through. “Windowless”: Here there is no film, and there is a turret that allows swapping with a Be window. Difficult to use as oil or ice can coat the detector surface. Not used much. Goldstein Fig. , p. 318 This plots shows the transmittance of Xrays thru different types of window material. (Quantum [BN] um, diamond um). The higher the transmission number, the better UW Madison Geology 777 EDS Windows Goldstein Fig. , p. 318 This plots shows the transmittance of Xrays thru different types of window material. (Quantum [BN] um, diamond um). The higher the transmission number, the better UW Madison Geology 777 How it works: energy gap Xray hits the SiLi crystal, producing a specific number of electronhole pairs proportional to Xray energy。 . one pair for every * eV, so for incident Fe Ka, 6404 eV, 1685 ehole pairs are produced. With a bias** applied across the crystal, the holes are swept to one side, the electrons to the other, producing a weak charge. Boron is important acceptor impurity in Si and degrades it (permits thermal excitation: bad)。 Si internal fluorescence peak Fig Goldstein et al There are 2 exceptions to the neat explanation of how