UPDATE (1/9/2013): The SNAP Instrument team also did a synopsis of UT’s week working on the CCR! Read about it on their blog.
This last week I was up at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory (ORNL) working at the Spallation Neutrons and Pressure Diffractometer (SNAP)… that’s a lot of acronyms. Basically, the goal of the week was to test a new piece of equipment for doing low temperature, high pressure neutron research. This is the beamline:
The neutrons come in from the lower left, hit the sample, and then the ones that diffract at ~90˚ (±5˚) hit the detectors (the big square things to the left and right), and the rest keep going through the sample to be stopped in that big beam-stop/collimator in the back.
So why are we working on a new instrument? Why hasn’t high pressure neutron diffraction been done at low temperatures before?
Well, when we say “high pressure” what we’re trying to do is get to 15+ GPa range. Which basically means the pressures of the Earth’s mantle. Currently there are cells available for neutron diffraction at cold temperatures that can do up to about ~10 GPa, and it is possible to use a Diamond Anvil Cell (DAC) to do the pressures/temperatures in the range we’re interested in using synchrotron x-rays. But using the DAC system with neutrons is tricky: it’s a lot harder to “see” only your sample as opposed to the diamond.
In the middle there (in the shadow) are two diamonds. Basically is works like this: imagine the diamond on an engagement ring… now flip it upside down so the pointy end is up, now polish the tip down so you get a little flat platform that’s ~200-1200µm in diameter. Now still a sample on that tip, and cover with another diamond you did the same thing to. Now press them both together using a lot of force. That’s how a DAC works.
Now, with x-rays it’s easy to shoot the photons through the diamond, hitting the sample, and then coming out through the diamond on the other side without too much interference. It’s also easier to make teeny-tiny x-ray beams so that you can aim the beam at only what you want to see. With neutrons, the beam can’t get as small as easily and it “sees” more of the DAC, causing a lot of interference.
Another problem is you have to cool the whole thing down. And the more material you’re cooling, the harder it is to control and the longer it takes to cool. In order to press on the DAC, we have to stick it in this:
That’s a lot of steal to cool down.
When it’s all in place, the sample and DAC are connected to a chilling element that goes down to 4˚K (liquid Helium temperature), but the rest of the contraption is connected to a chilling element that only goes down to 77˚K (liquid Nitrogen temperature). And it looks like this:
Installing the device into the vacuum pressure can (we have to cool it down in high vacuum or the water in the air will freeze onto all the electronic equipment and make things go haywire):
There are many improvements to be made (the temperatures for this first try were higher than we would have liked, and the pressures lower than we would have liked), but this is a great first stab at opening up this area of science. It helps to have a great team:
From left-to-right that’s me, Malcolm Guthrie of Carnegie Institute of Washington, Junjie Wu from Geophysics at the University of Texas (UT). Junjie’s supervisor, Jung-Fu “Afu” Lin, was also there:
Afu recently received tenure here at UT, so congrats to him!
We also worked with the SNAP beamline team who were amazing and incredibly hospitable. So many thanks to Chris Tulk, Jaimie Molaison and Antonio Moreira de Santo!