Senior Astrophysicist, Smithsonian Astrophysical Observatory
Growing up during NASA’s quest to reach the Moon, the lure of space travel and discovery was always on Dr. Eric Silver’s mind. “My friends and I built rockets and small model airplanes and flew them, imagining ourselves at the helm, wondering what we would discover once we reached our destination.”
Today Dr. Silver is a senior scientist at the Smithsonian Astrophysical Obervatory, where he leads the research and development of a new instrument to measure X-ray spectra. This new instrument can measure the extremely small temperature change that occurs when X-rays interact with material in his detector that operates at a temperature near absolute zero. In the future these cryogenic microcalorimeters will be used on space missions.
Innovative ground-based applications
Although this new instrument was designed to study cosmic X-ray sources, Dr. Silver’s interdisciplinary approach and collaborative nature have led him to explore innovative ground-based applications. Dr. Silver’s teams include scholars from across the Smithsonian, the Museum Conservation Institute (MCI), the National Museum of Natural History (NMNH), the Smithsonian Institution Archives (SIA) as well as SAO and collaborators outside the Smithsonian.
With support from the Consortia, Dr. Silver and his teams have carried out important investigations in several areas including:
- Determining the composition of the early solar system using dust grains captured from the tail of a comet.
- Analyzing early photographs (daguerreotypes) and the first color photographs from the 1850’s to determine how they have aged and how to better conserve them.
- Measuring the chemical state of mineral specimens to determine if microscopic life played a role in their formation.
Analyzing dust from the tail of a comet
The Consortia has supported some of the projects carried out by Dr. Silver’s teams. One project was to analyze dust grains collected by the NASA STARDUST mission as the spacecraft flew through the tail of Comet 81P/Wild2. These tiny, pristine dust grains were captured in a special material called aerogel. “Imagine cotton candy that’s not as sticky and a little less dense,” explains Silver. “The grains struck the aerogel at speeds of up to 16 miles per second and the aerogel literally stopped the particles in their tracks.”
These tiny particles are only about the diameter of a strand of hair, so how do you analyze them? “My colleague, Milos Toth from the University of Technology, Sydney, Australia, and I came up with an idea to etch away the aerogel using the electron beam in a scanning electron microscope, revealing the particles without ever touching them” explained Dr. Silver.
Once the aerogel is removed and dust grains are exposed, electrons from a scanning electron microscope interact with the dust grains and emit X-rays at different energies, depending on what they are made of. “So, if you can measure the X-ray’s energy, you can say, for example, ‘Ah, there’s carbon present; There’s oxygen; There’s iron; There’s cobalt.’ We were able to determine what some of the earliest material in our Solar System is made of. We started this microanalysis with our first Consortia grant, which led to a NASA grant for about three years’ worth of work. The Consortia’s seed funding really served its purpose.”
Determining the microchemistry of early photographs
A second Consortia-sponsored effort was a partnership between Dr. Ed Vicenzi from the Smithsonian’s Museum Conservation Institute (MCI) and Dr. Silver. MCI is the center for specialized technical research and conservation for all Smithsonian museums and collections. “Ed Vicenzi had been studying an early form of photography known as a daguerreotype. Prior to his work, detailed descriptions of the microchemistry associated with the surfaces of these precious images did not exist. He confirmed that ninetieth century photographers were indeed nanotechnologists.”
“Dr. Vincenzi also realized that the assemblage of metals in a daguerreotype image makes it an ideal candidate for the microcalorimetry studies we do at SAO since many of the characteristic X-rays emitted from these metals fall in the same crowded region of the spectrum. He knew that my spectrometer would provide the detailed information that other spectrometers could not.”
Silver and Vicenzi analyzed a few daguerreotypes and detected elements like mercury and silver more precisely than ever before. This work led to the realization that the microcalorimeter data could determine the oxidation state of the different elements in a compound. This capability was somewhat speculative when they wrote last year’s Grand Challenge proposal. But, their studies of manganese and several of its oxidation states showed the expected variation in the line shapes and wavelengths of the X-ray emission, the tell-tale signs that the chemical bonds holding the atoms to their nearest neighbors perturb the atomic structure of the element.
“Up until now,” says Silver, “one has only been able to do this type of chemical state analysis at a facility with a $100 million synchrotron. Now we can do it in a small laboratory with a microcalorimeter spectrometer and a scanning electron microscope. This capability opens up a whole new area of chemistry, physics, and microanalysis. In particular, we will be able to produce chemical maps for a variety of museum specimens. The research will further position SI at the forefront in the field of ‘heritage science’.”
Uncovering a range of practical applications
The practical applications of this new technology are broad and numerous and the potential for touching the lives of many people worldwide are exciting. The computer and semiconductor industries were the first to approach Dr. Silver about adapting his space-based microcalorimeter to assist in developing smaller and faster microchips used in computers, cell phones and other microelectronics. “Imagine you need to put a trillion transistors on the head of a pin. Imagine wires, like those in a household fuse box, getting smaller and smaller and closer and closer together—so close that only a few microns separate them. Well it doesn’t take much to short one of these wires to another.”
On these very small electronic devices, in which the circuitry can be seen only under a microscope, even a tiny particle, only a few microns in size, between two wire traces can cause a short. By using a microcalorimeter to identifying the chemical composition of the particle, it should be possible to learn where the particle originated in the manufacturing process and modify the production methods to eliminate this type of contamination.
Dr. Silver also worked with the National Institutes of Health to apply X-ray spectroscopic imaging at the cellular level. One goal was to determine if the chemical signature of a healthy cell is different from a diseased cell. As Dr. Silver explains “It could be very helpful to know the chemical composition of a cancer cell, as well as mapping the locations of chemotherapy drugs in cells to better understand how the drug interacts with a cell. X-ray microanalysis using our spectrometer coupled to a scanning electron microscope may make it possible to see all of that.”
But the cellular studies pose more challenges than any other application because the density of a cell is relatively low compared to other materials. A cell doesn’t emit as many X-rays when irradiated by an electron beam as does a Stardust particle or a piece of photographic film. “High resolution spectroscopic maps require lots of X-rays,” says Silver. “So, we’ve been inventing new ways to collect more X-rays.”
From macro to micro, Dr. Silver’s innovative X-ray optics and spectrographic applications have exciting uses far beyond their original intention. “We originally developed X-ray telescopes and spectrometers to study cosmic X-ray sources; now we are using the same technologies to spin-off ground based X-ray optics and improved spectrometers to analyze very small particles or samples of cells in a scanning electron microscope.” With this wealth of creative options, we can only wonder what will be studied and discovered next.