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.

Detecting Impurities as Semiconductors Shrink
As semiconductor devices continue to shrink, the industry needs to detect and identify small amounts of contamination on the devices. Microanalysis systems with conventional energy-dispersive spectrometers "excite" contamination on chips with fairly high (10-kiloelectron-volt) energy, which results in the surrounding material also being excited. When the surrounding material is excited, a flood of unwanted signals or noise is created, making it impossible to detect the contamination. But STJ detectors can operate with excitation energies of less than 2 kiloelectron volts, which produce signals from the contamination only, allowing the imperfections to be detected.

Helping to Enforce Nonproliferation
One of the Laboratory's important missions is to help guard against the proliferation of nuclear weapons. Labov and his team are conducting a research and development project that involves using a superconducting tantalum detector to improve gamma-ray resolution. This technology provides better diagnostic capability, particularly when there are large amounts of one isotope and small amounts of another. For example, when small quantities of nuclear materials are present, most of the gamma rays detected will be from background sources. Conventional detectors aren't sensitive enough to distinguish clearly between gamma radiation from the background source and from the nuclear material.
The team's high-resolution, superconducting spectrometer can detect special nuclear materials by isolating emissions from different radioisotopes. For example, if an inspector suspected that a heavily shielded barrel of spent plutonium from a reactor plant also contained weapons-grade plutonium, the superconducting spectrometer can measure the composition of the materials in the barrel much more accurately than a conventional detector. The technology also holds promise in environmental monitoring for the analysis of trace contaminants because it can detect levels that conventional detectors would miss.

Looking toward the Future
Traditional energy-dispersive and wavelength-dispersive spectrometers are fully developed technologies, leaving little room for significant performance improvements. Cryogenic detectors are still in a developmental stage, with significant progress having been made over the past few years. STJ detectors, although an "old" concept, are now better able to resolve low-energy x rays without sacrificing count-rate capability, and the x-ray collection efficiency of these detectors can be increased by orders of magnitude with focusing x-ray optics, which concentrate the x rays on the detector. These developments could greatly increase the use of these detectors in a wide range of applications.
--Sam Hunter

Key Words: atomic spectroscopy, Cooper pairs, detectors, Josephson junction, mass spectrometry, quasi-particles.

For further information contact Simon Labov (925) 423-3818 (labov1@llnl.gov).

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