NIST Repurposes SQUIDs to Sniff Nukes, Solve Big Bang

　　Superconducting quantum interference devices (SQUIDs), based on superconducting loops containing supercooled Josephson junctions, enable the most sensitive magnetometers in use today. Now researchers at the National Institute of Standards and Technology (NIST) and the University of Colorado, Boulder, are repurposing SQUIDs to yield microwave super-multiplexers capable of putting signals from up to 1,000 cryogenic microcalorimeters on a single coaxial cable.

　　The researchers are looking to solve diverse problems, ranging from keeping nuclear materials out of terrorists’ hands to clarifying details of the Big Bang and uncovering hidden aspects of matter. They describe their work in “Simultaneous readout of 128 X-ray and gamma-ray transition-edge microcalorimeters using microwave SQUID multiplexing,” published online this month in the American Institute of Physics’ (AIP’s) Applied Physics Letters.

　　To date, nuclear materials accounting, astrophysics and X-ray spectrometry applications have used time-division multiplexing to combine a maximum of 50 microcalorimeter outputs onto a single twisted pair. The biggest known array used 250 microcalorimeters but required 50 twisted-pair outputs. The new, microwave-multiplexing Spectrometer to Leverage Extensive Development of Gamma-ray transition-Edge sensors for Huge Arrays using Microwave Multiplexed Enabled Readout (Sledgehammer) is capable of multiplexing 1,000 microcalorimeters onto a single coax pair, according to the researchers, although so far they have demonstrated only 128 microcalorimeters on a single pair of coax cables.

　　The researchers used supercooled resonators regulated by radio-frequency SQUIDs to put a slightly different frequency for each microcalorimeter onto the coax pair. The team plans versions of the new instrument for measuring gamma-ray emissions from fissile materials, such as plutonium; high-energy X-rays; and background cosmic radiation. Current work aims at building a 1,000-microcalorimeter version of the 128-channel Sledgehammer.

　　Estimates are that terrorists would need to obtain only 8 to 10 kg of plutonium—an amount about the size of a softball—to construct a “suitcase” atomic bomb. Thus, the new array detectors are badly needed to improve the accuracy of accounting for inventories of plutonium at storage facilities.

　　In nondefense applications, astrophysicists could tune the array detectors to measure fluctuations in the polarization of cosmic background radiation more accurately in a bid to explain the inflationary epoch left over from the Big Bang. Likewise, the U.S. Department of Energy could more accurately measure high-energy X-ray sources, such as the Stanford Linear Accelerator Center’s (SLAC’s) free-electron laser, to reveal subtle properties of matter that remain masked today. The DOE’s Nuclear Energy University Program, NIST’s Innovations in Measurement Science, the NASA’s Astrophysics Research Program Agency, and the DOE's Basic Energy Sciences Advanced Detector Research program provided funding and other resources for the research.