Enectali Figueroa-Feliciano

First Astronomical Application of a Cryogenic Transition Edge Sensor Spectrophotometer

We report on the first astronomical observations with a photon-counting pixel detector that provides arrival time (δt = 100 ns) and energy (δEγ ≤ 0.15 eV) resolved measurements from the near-IR through the near UV. Our test observations were performed by coupling this transition edge sensor device to a 0.6 m telescope; we have obtained the first simultaneous optical near-IR phase-resolved spectra of the Crab pulsar. A varying infrared turnover gives evidence of self-absorption in the pulsar plasma. The potential of such detectors in imaging arrays from a space platform is briefly described.

Arraying compact pixels of transition-edge microcalorimeters for imaging x-ray spectroscopy

We are developing superconducting transition-edge sensor (TES) microcalorimeters for astronomical x-ray spectroscopy. We have obtained very high energy resolution (2.4 eV at 1.5 keV and 3.7 eV at 3.3 keV) in large, isolated TES pixels using Mo/Au proximity-effect bilayers on silicon-nitride membranes several mm wide. In order to be truly suitable for use behind an x-ray telescope, however, such devices need to be arrayed with a pixel size and focal-plane coverage matched to the telescope focal length and spatial resolution. For the Constellation-X mission, this requires fitting the TES, its thermal link, and contact wiring into a 0.25 mm square, a far more compact geometry than has previously been investigated. We have demonstrated that the weak thermal link can be restricted to a narrow (∼10 micron) perimeter of membrane around the TES and still provide a thermal conductance in the acceptable range. Varying the size and placement of slits in that nitride perimeter, we can tune that value.

Progress in the Development of Mo-Au Transition-Edge Sensors for X-Ray Spectroscopy

X-ray microcalorimeters using transition-edge sensors (TES) show great promise for use in astronomical x-ray spectroscopy. We have obtained very high energy resolution (2.8 eV at 1.5 keV and 3.7 eV at 3.3 keV) in a large, isolated TES pixel using a Mo/Au proximity-effect bilayer on a silicon nitride membrane. We will discuss the performance and our characterization of that device. In order to be truly suitable for use behind an x-ray telescope, however, such devices need to be arrayed with a pixel size and focal-plane coverage commensurate with the telescope focal length and spatial resolution. Since this requires fitting the TES and its thermal link, a critical component of each calorimeter pixel, into a far more compact geometry than has previously been investigated, we must study the fundamental scaling laws in pixel optimization. We have designed a photolithography mask that will allow us to probe the range in thermal conductance that can be obtained by perforating the nitride membrane in a narrow perimeter around the sensor. This mask will also show the effects of reducing the TES area. Though we have not yet tested devices of the compact designs, we will present our progress in several of the key processing steps and discuss the parameter space of our intended investigations.

Optimal filter analysis of energy-dependent pulse shapes and its application to TES detectors

Abstract We present a method for applying optimal filtering to data sets containing energy-dependent pulse shapes. This occurs frequently in transition edge sensors (TES) when dealing with signal energies that are close to the saturation point of the detector. Different filter templates are created which span the dynamic range desired for the TES. These filters are then used as templates to filter the data, using interpolation to bridge the gap between templates. The method has been demonstrated on our tungsten (W) TES. We present the latest results from heat-pulse data on a (125 μm ) 2 TES with resolutions of 3 eV FWHM at 42 eV and 4 eV FWHM at 1.42 keV with the same sensor demonstrating the wide band operation possible with this technique.

Development of wide-band, time and energy resolving, optical photon detectors with application to imaging astronomy

Abstract Superconducting transition edge sensors (TESs) are showing promise for the wide-band spectroscopy of individual photons from the mid-infrared (IR), through the optical, and into the near ultraviolet (UV). Our TES sensors are ∼20 μm square, 40 nm thick tungsten (W) films with a transition temperature of about 80 mK. We typically attain an energy resolution of 0.15 eV FWHM over the optical range with relative timing resolution of 100 ns. Single photon events with sub-microsecond risetimes and few microsecond falltimes have been achieved allowing count rates in excess of 30 kHz per pixel. Additionally, tungsten is approximately 50% absorptive in the optical (dropping to 10% in the IR) giving these devices an intrinsically high quantum efficiency. These combined traits make our detectors attractive for fast spectrophotometers and photon-starved applications such as wide-band, time and energy resolved astronomical observations. We present recent results from our work toward the fabrication and testing of the first TES optical photon imaging arrays.

Fabrication of Mo/Au transition-edge sensors for X-ray spectrometry

We present fabrication details of our Mo/Au X-ray microcalorimeters, which are being developed as one of the candidate high resolution spectrometers for the Constellation-X mission. We have reproducibly fabricated Mo/Au transition-edge sensors with Tcs of /spl sim/100 mK on etched silicon nitride membranes and connected via superconducting Nb leads. Our single pixel devices have, so far, attained resolution of 3.7 eV at 3.3 keV. We also discuss our plans for fabrication and testing of fully functional multi-pixel array of X-ray microcalorimeters.

Toward a 2-eV microcalorimeter x-ray spectrometer for Constellation-X

COnstellation-X is a cluster of identical observatories that together constitute a promising concept for a next- generation, high-throughput, high-resolution, astrophysical x-ray spectroscopy mission. The heart of the Constellation-X mission concept is a high-quantum-efficiency imaging spectrometer with 2 eV resolution at 6 keV. Collectively across the cluster, this imaging spectrometer will have twenty times the collecting efficiency of XRS on Astro-E and better than 0.25 arc minute imaging resolution. The spectrometer on each satellite will be able to handle count rates of up to 1000 counts per second per imaging pixel for a point source and 30 counts per second per pixel for an extended source filling the array. Focal plane coverage of at least 2.5 arc minutes X arc minutes, comparable to XRS but with a factor of thirty more pixels, is required. This paper will present the technologies that have the potential to meet al these requirements. It will identify the ones chosen for development for Constellation-X and explain why those were considered closer to realization, and it will summarize the results of the development work thus far.

Transition edge sensors as single photon detectors

We have recently demonstrated wideband detection of individual photons from the mid infrared (IR), through the optical, and into the near ultraviolet (UV). We use thin film tungsten transition edge sensors about 20 /spl mu/m on a side to detect single photon events above a noise threshold of 0.3 eV (4 /spl mu/m wavelength), with an energy resolution of 0.12 eV FWHM (full width at half maximum). The observed events have a risetime (falltime) of 0.5 /spl mu/s (30 /spl mu/s). In this paper we present a summary of recent calibration data and resolution measurements as well as two proof-of-principle experiments to show the ability of TES detectors to extract both time and energy information from photons arriving at the detector during astronomical observations.

Position-sensing transition-edge sensors for large-field high-energy-resolution x-ray imaging spectroscopy

In the X-ray astrophysics community, the desire for wide- field, high-resolution, X-ray imaging spectrometers has been growing for some time. We present a concept for such a detector called a Position-Sensing Transition-edge sensor (PoST). A PoST is a calorimeter consisting of two Transition- Edge Sensors (TESs) on the ends of a long absorber to do one dimensional imaging spectroscopy. Comparing the rise time and energy estimates obtained from each TES for a given event, the position of that event in the PoST is determined. Energy is inferred from the sum of the two signals on the TESs. We have designed 7, 15, and 32 pixel PoSTs using our Mo/Au TESs and bismuth absorbers. We discuss the theory, modeling, operation and readout of PoSTs and the latest results from our development.

Wideband observation of the Crab pulsar using a superconducting transition-edge sensor

Our detectors are superconducting transition edge sensors (TESs) optimized for the wide band detection of individual photons from the mid infrared, through the optical, and into the near ultraviolet. We typically achieve an energy resolution of 0.15 eV FWHM over this range with timing resolution of 100 ns. We have measured photon events with sub- microsecond rise times and 3 microsecond fall times allowing count rates as high as 30 kHz without significant degradation in energy resolution. Such characteristics along with the predicted high quantum efficiency (10% in IR to 50% in optical-UV) make our TES detectors very appealing for low-flux applications which have energy and timing requirements, such as fast spectrophotometry for observational astronomy. We present results from our recent observation of the Crab Pulsar (PSR BO531 + 21) which demonstrate the ability of our sensors to extract wide band phase-resolved spectroscopic information of the pulsar using the student-class 24 inch telescope on the campus of Stanford University. We present a description of the optical system and an analysis of the single pixel energy response.