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As promised, more on work. The time slot started Friday morning and was set to end at 8 Tuesday morning. This is a pretty substantial number of shifts to get on short notice...one of the advantages of having the synchrotron on site is that when things like that open up we don't have to fly halfway across the country to use the time.

Anyway, I'm helping out with a project on characterizing and optimizing materials for x-ray imaging detectors: specifically, scintillators (materials that emit visible light when struck by x-rays) and storage phosphors (materials that will store x-ray energy until stimulated with low-energy light, when they release it as visible light of a shorter wavelength). The materials are based on so-called ZBLAN glass, a mixture of metal fluorides (Z=zirconium, B=barium, L=lanthanum, A=aluminum, and N=sodium) that's been doped with europium(II) and possibly other halogens--chlorine and/or bromine. The fluoride mixture melts around 800°C. It's solidified and rapidly cooled to below 200°C, at which point it's completely amorphous. If it's then heat-treated at 235°C for several hours, barium fluoride begins to crystallize out of the glass, and the Eu(II) partitions into it. Depending on the doping level and the heat-treating time, the barium fluoride crystals can be very small--a few tens of nanometers across. These crystallites are the actual scintillating phase. However, if the glass is heat-treated at higher temperatures for shorter times, the crystallites tend to be larger, and have a different structure. After this treatment the crystallites act as storage phosphors.

What I am trying to find out is how the high-temp heat-treating process works as far as crystallite growth and phase transformation kinetics. That's why I used x-ray diffraction of storage phosphor samples that've been heat-treated for different lengths of time--it lets me identify the crystal structure, the average crystallite size, and the strain state of the crystallite. In simpler systems this can be done with a laboratory x-ray source, but to get good response with these materials takes a high-intensity single-energy source. This is where synchrotrons excel.

So I had eleven samples (seven storage phosphors, four scintillators) to look at over four days. Each scan took a little over eight hours. Fortunately, there's a sample changer at the beamline, so I could load up five samples and go away for a day or so. I got them all in, despite a problem with the ring that ate the intensity for the last two scans. Now all I have to do is process the data, figure out what it means, and write the papers. Nothing to it!


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July 2017

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