T. C. Chen

Design and fabrication of superconducting transition edge X-ray calorimeters

Abstract We report on progress made so far at NASA Goddard Space Flight Center towards the development of arrays of X-ray microcalorimeters as candidates for the high-resolution X-ray spectrometer on the Constellation-X mission. In the design concept presently under consideration, the microcalorimeter consists of (i) a Bi/Cu multilayer absorber for stopping and thermalizing the incident X-rays, (ii) an e-beam evaporated Mo/Au proximity bilayer with sputtered Nb leads for sensing the resultant temperature rise, and (iii) a silicon nitride membrane to provide a weak thermal link to the sink temperature so that the calorimeter can return to its equilibrium temperature. Fabrication details and preliminary results are reported.

Fabrication of superconducting bilayer transition edge thermometers and their application for spaceborne X-ray microcalorimetry

The transition between normal conduction and superconductivity in superconducting materials can be exploited as a highly sensitive thermometer. Transition temperatures can be tailored through the selection of materials, their component cases of more than one material. Two bilayer configurations, Ag/Al and Au/Mo, are examined, including details of preparation, testing, and encountered difficulties. Proposed designs for spaceflight detector applications are discussed.

MUSTANG: 90 GHz science with the Green Bank Telescope

MUSTANG is a 90 GHz bolometer camera built for use as a facility instrument on the 100 m Robert C. Byrd Green Bank radio telescope (GBT). MUSTANG has an 8 by 8 focal plane array of transition edge sensor bolometers read out using time-domain multiplexed SQUID electronics. As a continuum instrument on a large single dish MUSTANG has a combination of high resolution (8) and good sensitivity to extended emission which make it very competitive for a wide range of galactic and extragalactic science. Commissioning finished in January 2008 and some of the first science data have been collected.

Optical performance of frequency-selective bolometers

Frequency-selective bolometers (FSBs) are a new type of detector for millimeter and submillimeter wavelengths that are transparent to all but a narrow range of frequencies as set by characteristics of the absorber itself. Therefore stacks of FSBs tuned to different frequencies provide a low-loss compact method for utilizing a large fraction of the light collected by a telescope. Tests of prototype FSBs indicate that the absorption spectra are well predicted by models, that peak absolute absorption efficiencies of the order of 50% are attainable, and that their out-of-band transmission is high.

Molybdenum-gold proximity bilayers as transition edge sensors for microcalorimeters and bolometers

Mo/Au proximity bilayers as transition edge sensors (TESs) are promising candidates for low-temperature thermometry. The transition temperature of the bilayers can be easily tuned between 50 and 600 mK, yielding sensors which can be used in a variety of calorimetric and bolometric applications. With phase transition widths of less than 1 mK, Mo/Au TESs show very high temperature sensitivity (d(logR)/d(logT )~2500). Also, Mo/Au TESs show improved thermal and chemical stability compared to most other bilayer configurations. Fabrication issues and detector performance of Mo/Au TESs on Si3N4 membranes are discussed.

THE TOPHAT EXPERIMENT: A BALLOON-BORNE INSTRUMENT FOR MAPPING MILLIMETER AND SUBMILLIMETER EMISSION

The TopHat experiment was designed to measure the anisotropy in the cosmic microwave background radiation on angular scales from 03 to 30° and the thermal emission from both Galactic and extragalactic dust. The balloon-borne instrument had five spectral bands spanning frequencies from 175 to 630 GHz. The telescope was a compact, 1 m, on-axis Cassegrain telescope designed to scan the sky at a fixed elevation of 78°. The radiometer used cryogenic bolometers coupled to a single feed horn via a dichroic filter system. The observing strategy was intended to efficiently cover a region 48° in diameter centered on the south polar cap with a highly cross-linked and redundant pattern with nearly uniform sky coverage. The Long Duration Balloon flight over Antarctica in 2001 January surveyed about 6% of the sky. Here we describe the design of the instrument and the achieved in-flight performance and provide a brief discussion of the data analysis.

The spectral energy distribution camera for the LMT

Advances in bolometer device and readout technologies make it possible to build photon-noise limited bolometric cameras for ground-based observations at mm-wave frequencies. However, todays bolometer cameras are limited not by photon-noise of the telescope and atmosphere but by fluctuations in the atmosphere signal. To realize the full potential of bolometer cameras on large aperture ground-based telescopes, one must find a way to defeat this foreground. The SPEctral Energy Distribution Camera - or SPEED - is a four pixel, four frequency camera planned for eventual use on the Large Millimeter Telescope (LMT). A prototype version of this camera is currently being built for initial operation on the Heinrich Hertz Telescope (HHT). SPEED incorporates Frequency Selective Bolometers to sample the sky with a frequency-independent beam simultaneously at four frequencies (from 150 to 375 GHz) in each pixel. SPEEDs ability to separate the temporally varying atmospheric signal from the true sky signal will potentially result in a per-detector sensitivity between 2 and 5 times greater than that achieved with contemporary bolometer cameras. We describe the basic design and motivation for SPEED, the expected sensitivity of the camera on the LMT, and give examples of some of the science programs we will undertake.

First results from Mo/Au transition-edge sensor X-ray calorimeters

Abstract Superconducting bilayers made of thin films of molybdenum and gold show promise as robust transition-edge sensor (TES) thermometers for calorimeters. We present our first X-ray results from experiments with Mo/Au TES calorimeters on silicon-nitride membranes. These results include analysis of the signal pulse shape and noise as functions of the bias point, which is varied through changing the bias voltage for operation at different places within the superconducting transition and changing the heat sink temperature relative to the transition temperature. Ultimately, we determined that the performance of our devices is limited by the slew rate of the SQUID amplifier used to measure the change in current, which restricts the choice of bias. The amplifier must be replaced before further device characterization and optimization can proceed.

Development of frequency selective bolometers for ground-based MM-wave astronomy

The Frequency Selective Bolometer (FSB) is a bolometer with a patterned frequency selective absorber, coupled with a band-reflecting backshort. The resulting unit absorbs in-band radiation, and passes out-of-band radiation. Thus a series of FSBs tuned to different bands packed in series in a light pipe forms a compact multi-band photometer. The compact form factor makes it an attractive detector for a mm-wave array camera. We have built and characterized prototypes that demonstrate this technology. We are now developing a set of FSBs for SPEED (the SPEctral Energy Distribution camera), an FSB array camera which will observe 4 pixels in 4 mm-wave spectral bands, to be used on the Heinrich Hertz Telescope and the Large Millimeter Telescope. These FSBs are fabricated on a free-standing SiN film with TES thermometers. We will discuss the design and performance of these detectors.

Design and fabrication of novel photodetector arrays

We are developing novel photodetector arrays based on superconducting transition-edge sensor (TES) and pop-up detector (PUD) technologies. The TES has the potential for a new generation of high sensitivity photodetectors from the IR to the x-ray. This is directly due to the sharpness of the resistance change with temperature at the superconducting transition. The TESs are deposited on the PUD arrays and serve as the sensing elements. The PUDs are close-packed, folded membrane arrays that provide the TES substrate and the thermal isolation required by the bolometers and microcalorimeters. This paper presents the processing-related characterization result of preliminary TES and PUD designs. The gaol of this work is to fabricate a new generation of x-ray calorimeters and IR bolometers for space flight projects.