We’ve analyzed five of our interstellar candidates on beamline ID22 at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. Our colleagues there are Alexandre Simionovici (University of Grenoble), Laurence Lemelle (University of Lyon), Pierre Bleuet (ESRF), and Romain Basset (University of Lyon). This is a highly sophisticated synchrotron beamline called the x-ray nanoprobe. It uses Kirkpatrick-Baez mirrors to focus the intense x-ray photons from the synchrotron to a spot only 150nm by 90nm in size. (1 nm — a nanometer — is a billionth of a meter). This beamline is very well-suited for the analysis of the expected interstellar dust particles.
This analysis was challenging technically. We used two different kinds of sample mounting techniques, each of which has its advantages and disadvantages.
The first is a silicon nitride window sandwich (picture below). The picokeystone is trapped between the two silicon nitride windows. The silicon window frames are each 200 microns thick, so the total sandwich thickness is 400 microns. The window frames are 5mm x 5mm and the windows are 1500 microns wide and 70nm (!) thick. This is a really nice mounting technique for numerous reasons — one of which is critical: the keystone cannot be easily lost because it is trapped in the sandwich. The windows are very clean and have undetectable levels of trace elements. We can place the picokeystone essentially anywhere in the window to help with any geometrical issues. The advantage of this method is that it is very easily to see the sample and to navigate, and the silicon nitride is quite robust. The disadvantage is that this precludes fluorescence-tomography.
The second is an ultralene bubble — the picokeystone is sandwiched between two 4-micron ultralene plastic sheets, then sealed in with a soldering iron with a Nb wire tip. The advantage is that one has no geometrical issues with seeing the sample through the side. The disadvantage is that we found that is very difficult to navigate — the plastic is hard to see through, and there are interesting optical effects that offset the optical image with respect to the actual position. We never could converge on a suitable fiducial. Fine (10-micron) chromel wire produced too much Ni background in the fluorescence detector. For scale, the Al holder in the image is the same size as the one in the image with the silicon nitride window sandwich.
Experience has shown that it is not wise to speculate about what we have until we’ve really digested the data. We expected from the beginning that at least some of our candidates would not turn out to be real — comparing the number of candidates and the number of expected events in the same area, we anticipate that at most 40% of our candidates would be real. The candidates that we looked are: 9471219V1, 5637295V1, 404198V1, 3602277V1, and 9267050V1. You can look at these on the alpha list.
This was a successful synchrotron run at one of the most sophisticated beamlines at one of the three most powerful synchrotrons in the world. It will tell us a lot about how to recognize real tracks as we continue our search. Stay tuned!