A distributed search by volunteers for interstellar dust in the Stardust Interstellar Dust Collector.
Anna wrote:To analyze the impacts we find, we extract a small volume of the aerogel that encapsulates the whole track and then we crank up the optical magnification to 50x or 100x and maybe see a particle in the track (resolved only to the refractory limit). Next we switch to soft x-rays and 25 nm spatial resolution to image both the track made in the aerogel and any particle residue. For this we use the Scanning Transmission X-ray Microscope at Beamline 11.0.2 of the Advanced Light Source synchrotron, Berkeley National Laboratory, thankfully referred to as STXM. http://beamline1102.als.lbl.gov/
There is no hard lower limit to the detection size and analysis of impacts. We have small particles (200 nm, say) which are "easy" to detect by STXM because they have a measurable density of an element in range e.g femtograms of Aluminum or Magnesium. At the other extreme, we have a bona fide hedgehog-shaped 20-µm impact track, imaged with high-resolution x-rays, but we haven't detected any recognizable impact residue above the background of the silica aerogel.
For some context, compared with electron microscopy, STXM spatial resolution is not that different from FEG-SEM (highly sample dependent, about 10 nm), but its energy resolution beats out even the best monochromated TEM-EELS (note: a list of acronyms follows). Sensitivity is on par with TEM techniques, but the sample prep for STXM is more forgiving and the radiation is less damaging. We have been able to analyze impact particles still in aerogel and though this is technically challenging in STXM, the thickness and electrically insulating aerogel make it impossible in TEM. When we figure out how to safely do the sample preparation, we will move on to TEM work for the full mineralogy work-up.
I hope that covers it!
STXM = Scanning Transmission X-ray Microscope (high resolution imaging, element spectroscopy and mapping)
TEM = Transmission Electron Microscopy (powerful, ultra-high resolution analytical suite, typically using sub-100 nm thin foils sample preparation. The best instruments are now capable of sub-Angstrom resolution)
EELS = Electron Energy Loss Spectroscopy (in the TEM gamut, EELS is the closest thing to STXM spectroscopy)
SEM = Scanning Electron Microscopy (workhorse elemental analysis and imaging instruments, used in many science and engineering disciplines)
FEG = Field Emission Gun (brighter, higher spatial resolution electron source than the standard thermionic emitters used in electron microscopy)
1 µm = 1,000 mm
1 nm = 1,000,000 mm
1 fg (femtogram) = 10E-15 grams. (1/1,000,000,000,000,000 grams)