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IF Josephson junction brings to mind an intersection of two small back roads, it's time to change gears and think science. This term, along with quasi-particle and Cooper pair, is part of the large area of superconductors.
Simon Labov and his colleagues in Lawrence Livermore's Physics and Space Technology Directorate say these concepts and discoveries show great promise for applications in areas such as wireless communication, energy storage, and medical diagnostics. Labov and his fellow researchers are using superconductors to create a new generation of supersensitive detectors for nondestructive evaluation and astrophysics.
When ordinary metal conducts electricity, the electrons carrying the current collide with imperfections in the metal, thereby creating resistance. But when a superconducting material is cooled to its critical temperature, electrons pair off into Cooper pairs, named for Leon Cooper, one of the scientists who won a 1972 Nobel Prize in physics for explaining the now widely accepted theory. Any movement of one electron is matched by equal and opposite movement of the other. As a result, they don't hit the imperfections, no electrical resistance is generated, and electrons flow freely, without the addition of more energy.
But to put these theories to practical use in detectors requires a Josephson junction. Named for Brian Josephson, who described the theory when he was a graduate student at Cambridge University in 1962, a Josephson junction is two pieces of superconducting material linked by a weak insulating barrier. When an x ray hits a Josephson junction, the Cooper pairs break up, and quasi-particles are created. These quasi-particles, which are electronlike or holelike excitations in the superconductor, can tunnel through the weak insulating barrier of the Josephson junction, producing a pulse of electrical current. By measuring the number of Cooper pairs that are broken, scientists can determine the energy of the x ray up to ten times better than with conventional technology and can identify the material that emitted the x ray. These superconducting-tunnel-junction (STJ) detectors also work with optical, ultraviolet gamma-ray photons and large biomolecules. Labov and his team are working to use this new technology in applications for analyzing all of these particles.
Measuring Large, Slow Molecules
The Livermore group has, for example, teamed with scientists at Lawrence Berkeley National Laboratory and a commercial firm, Conductus Inc., of Sunnyvale, California, to measure massive, slow-moving macromolecules in DNA research. In a typical time-of-flight mass spectrometer using a microchannel-plate (MCP) ion detector, large ions move too slowly to be efficiently detected. Using an STJ detector, the team found that they could achieve close to 100% detection efficiency for all ions, including the slow, massive macromolecules. "A comparison of count rates obtained with both detectors indicated a hundred to a thousand times higher detection efficiency per unit area for the STJ detector at 66,000 atomic-mass units," Labov says. "For higher molecular masses, we expect an even higher relative efficiency for cryogenic detectors because MCPs show a rapid decline in detection efficiency as ion mass increases."
Even more exciting, STJ detectors can measure independently the mass and charge of the molecule. Current MCP detector technology cannot measure the charge of the molecule, and this inability often causes confusion in interpreting mass spectrometer data. According to Labov, if nonfragmenting ionization techniques can be perfected, cryogenic detectors could make possible the rapid analysis of large DNA molecules for the Human Genome Project and might be used to analyze intact microorganisms to identify viruses or biological weapons materials.
High Resolution for Soft X Rays
In another experiment using an STJ, Labov again teamed with Conductus and seven other Lawrence Livermore scientists to study energy resolution for soft x rays with energies between 70 and 700 electron volts. The results showed that STJ detectors can operate at count rates approaching those of semiconductor detectors while still providing significant improvement in energy resolution for soft x rays. "In this region, the STJ detector provides about ten times better resolution," Labov adds.
Astronomers also are looking to STJs as single-photon detectors of both x rays and visible wavelengths. In the visible band, silicon-based, charge-coupled devices cannot measure a photon's energy, but STJs can. One photon, depending on its energy, can generate thousands of quasi-particles. By measuring the photon's energy, STJ detectors will allow astronomers to study galaxies and stars that are barely bright enough to be seen with the largest telescopes. |