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Superconducting Materials and Devices

Above left: BICEP2 South Pole station. Photo courtesy Keith Vanderline.

TES and Cosmic Microwave Background Detection

Transition edge sensor (TES) bolometers sense small temperature changes that occur when photons are absorbed and converted to heat. The use of TESs enables arrays with a much larger number of pixels than is practical with spider-web bolometers. Sustaining its leading role in superconducting TES array technology, MDL developed and continues to improve a process to create arrays of thousands of TESs with high yield (>90 percent). These arrays are being employed on three major astro physics projects, all with the same goal: generating detailed maps of the polarization of the cosmic microwave background (CMB).

In 2009, MDL delivered TES focal plane arrays for BICEP2 (Background Imaging of Cosmological Extragalactic Polarization 2), a ground-based South Pole CMB polarimeter telescope. Building on that success, MDL completed the final two of five focal planes for the Keck array 2012 observation season — a CMB polarimeter experiment that is five times larger than BICEP2.

A view of all five Keck telescopes installed at the South Pole (March 2012).

In 2012, MDL increased the optical efficiency of our science-grade arrays by ~1.5 times. These more efficient detector arrays were hybridized into the Keck experiment and placed into two of the five science-grade focal planes that are now deployed and observing the CMB.

Each focal plane unit (FPU) consists of four 150-GHz focal plane arrays that are fabricated at MDL. A focal plane array contains 64 dual-polarization pixels for a total of 512 pixels in each FPU.

Each pixel absorbs radiation from the CMB using a phased array of slot antennas, then transfers the absorbed radiation via a niobium superconducting strip-line to a thermally isolated termination resistive element. On the thermally isolated membrane, a superconductive transition-edge sensor (TES) is employed to measure the change in temperature of the isolated membrane. The TES elements are measured using a multiplexed superconducting quantum interference device (SQUID) ammeter from NIST.

The Keck focal plane consists of four 64-pixel dual polarization arrays.

The final of the three missions that will host MDL’s CMB focal plane arrays is Spider, a high-altitude, balloon-borne CMB instrument providing a higher sensitivity to the CMB signal by eliminating much of the atmospheric interaction. Two of the six focal planes for Spider’s initial launch were completed in 2011, with four more to be completed prior to its deployment in late 2012.

TES arrays are also being developed for imaging, spectroscopy, and polarimetry for future missions such as JPL’s proposed BLISS instrument for the Japanese SPICA mission and SAFIR (Single Aperture Far-Infrared observatory).


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