Joel N. Ullom

Scuba-2: The 10 000 pixel bolometer camera on the james clerk maxwell telescope

SCUBA-2 is an innovative 10000 pixel bolometer camera operating at submillimetre wavelengths on the James Clerk Maxwell Telescope (JCMT). The camera has the capability to carry out wide-field surveys to unprecedented depths, addressing key questions relating to the origins of galaxies, stars and planets. With two imaging arrays working simultaneously in the atmospheric windows at 450 and 850µm, the vast increase in pixel count means that SCUBA-2 maps the sky 100–150 times faster than the previous SCUBA instrument. In this paper we present an overview of the instrument, discuss the physical characteristics of the superconducting detector arrays, outline the observing modes and data acquisition, and present the early performance figures on the telescope. We also showcase the capabilities of the instrument via some early examples of the science SCUBA-2 has already undertaken. In February 2012, SCUBA-2 began a series of unique legacy surveys for the JCMT community. These surveys will take 2.5years and the results are already providing complementary data to the shorter wavelength, shallower, larger-area surveys from Herschel. The SCUBA-2 surveys will also provide a wealth of information for further study with new facilities such as ALMA, and future telescopes such as CCAT and SPICA.

Characterization and reduction of unexplained noise in superconducting transition-edge sensors

The noise in superconducting transition-edge sensors (TESs) commonly exceeds simple theoretical predictions. The reason for this discrepancy is presently unexplained. We have measured the amplitude and frequency dependence of the noise in TES sensors with eight different geometries. In addition, we have measured the dependence of the noise on operating resistance, perpendicular magnetic field, and bath temperature. We find that the unexplained noise contribution is inversely correlated with the temperature width of the superconducting-to-normal transition and is reduced by a perpendicular field and in certain geometries. These results suggest paths to improved sensor performance.

Prototype system for superconducting quantum interference device multiplexing of large-format transition-edge sensor arrays

We discuss the implementation of a time-division superconducting quantum interference device (SQUID) multiplexing system for the instrumentation of large-format transition-edge sensor arrays. We cover the design and integration of cryogenic SQUID multiplexers and amplifiers, signal management and wiring, analog interface electronics, a digital feedback system, serial-data streaming and management, and system configuration and control. We present data verifying performance of the digital-feedback system. System noise and bandwidth measurements demonstrate the feasibility of adapting this technology for a broad base of applications, including x-ray materials analysis and imaging arrays for future astronomy missions such as Constellation-X (x-ray) and the SCUBA-2 instrument (submillimeter) for the James Clerk Maxwell Telescope.

14-pixel, multiplexed array of gamma-ray microcalorimeters with 47eV energy resolution at 103keV

The authors present a prototype for a high-energy-resolution, high-count-rate, gamma-ray spectrometer intended for nuclear forensics and international nuclear safeguards. The prototype spectrometer is an array of 14 transition-edge-sensor microcalorimeters with an average energy resolution of 47eV (full width at half maximum) at 103keV. The resolution of the best pixel is 25eV. A cryogenic, time-division multiplexer reads out the array. Several important topics related to microcalorimeter arrays are discussed, including cross-talk, the uniformity of detector bias conditions, fabrication of the arrays, and the multiplexed readout. The measurements and calculations demonstrate that a kilopixel array of high-resolution microcalorimeters is feasible.

Cooling of bulk material by electron-tunneling refrigerators

Improved refrigeration techniques have lead to scientific discoveries such as superconductivity and Bose-Einstein condensation. Improved refrigeration techniques also enhance our quality of life. Semiconductor processing equipment and magnetic-resonance imaging machines incorporate mechanical coolers operating below 10 K. There is a pressing need for refrigeration techniques to reach even lower temperatures because many next-generation analytical and astronomical instruments will rely on sensors cooled to temperatures near 100 mK. Here we demonstrate a solid-state, on-chip refrigerator capable of reaching 100 mK based on the quantum-mechanical tunneling of electrons through normal metal-insulator-superconductor junctions. The cooling power and temperature reduction of our refrigerator are sufficient for practical applications and we have used it to cool bulk material that has no electrical connection to the refrigerating elements.

Review of superconducting transition-edge sensors for x-ray and gamma-ray spectroscopy*

We present a review of emerging x-ray and gamma-ray spectrometers based on arrays of superconducting transition-edge sensors (TESs). Special attention will be given to recent progress in TES applications and in understanding TES physics.

Optimized transition-edge x-ray microcalorimeter with 2.4eV energy resolution at 5.9keV

We present measurements from a series of transition-edge x-ray microcalorimeters designed for optimal energy resolution. We used the geometry of the sensors to control their heat capacity and employed additional normal metal features and a perpendicular magnetic field to control the sharpness of the superconducting-to-normal transition. These degrees of control allow an optimal selection of sensor saturation energy and noise. Successive design changes improved the measured energy resolution of the sensors from 4.5eV full width at half maximum at 5.9keVto2.4eV at 5.9keV. Sensors with this energy resolution are well matched to applications in x-ray astrophysics and terrestrial materials analysis.

A high resolution gamma-ray spectrometer based on superconducting microcalorimeters

Improvements in superconductor device fabrication, detector hybridization techniques, and superconducting quantum interference device readout have made square-centimeter-sized arrays of gamma-ray microcalorimeters, based on transition-edge sensors (TESs), possible. At these collecting areas, gamma microcalorimeters can utilize their unprecedented energy resolution to perform spectroscopy in a number of applications that are limited by closely-spaced spectral peaks, for example, the nondestructive analysis of nuclear materials. We have built a 256 pixel spectrometer with an average full-width-at-half-maximum energy resolution of 53 eV at 97 keV, a useable dynamic range above 400 keV, and a collecting area of 5 cm(2). We have demonstrated multiplexed readout of the full 256 pixel array with 236 of the pixels (91%) giving spectroscopic data. This is the largest multiplexed array of TES microcalorimeters to date. This paper will review the spectrometer, highlighting the instrument design, detector fabrication, readout, operation of the instrument, and data processing. Further, we describe the characterization and performance of the newest 256 pixel array.

Practical electron-tunneling refrigerator

We demonstrate a thin-film, solid-state refrigerator based on the removal of hot electrons from a metal by quantum-mechanical tunneling. We have reduced the electronic temperature in a metal film from 260 to ∼130 mK. The base temperature of the device is predicted to increase to near 140 mK under a power load of 10 pW. Both the cooling power and temperature reduction of the refrigerator are well matched to practical applications. This refrigerator will make high-performance cryogenic photon sensors more accessible to the astronomical and analytical communities.

Array-compatible transition-edge sensor microcalorimeter γ-ray detector with 42eV energy resolution at 103keV

The authors describe a microcalorimeter γ-ray detector with measured energy resolution of 42eV full width at half maximum for 103keV photons. This detector consists of a thermally isolated superconducting transition-edge thermometer and a superconducting bulk tin photon absorber. The absorber is attached with a technique compatible with producing arrays of high-resolution γ-ray detectors. The results of a detailed characterization of the detector, which includes measurements of the complex impedance, detector noise, and time-domain pulse response, suggest that a deeper understanding and optimization of the thermal transport between the absorber and thermometer could significantly improve the energy resolution of future detectors.