Thomas W. Cecil

Tungsten silicide films for microwave kinetic inductance detectors

Microwave kinetic inductance detectors are used to detect photons over a large range of wavelengths from submillimeter to X-ray. The common material requirements for this application are: high internal quality factor (Qi), high kinetic inductance fraction, long quasiparticle lifetime, and in the case of X-ray photons stopping power (i.e., dense, high atomic number materials). Superconducting tungsten silicide alloys have a tunable TC with silicon content, a high normal state resistivity, and a high density. In this work, we investigate the properties of thin films of tungsten silicide made of two different stoichiometry: WSi2 and W5Si3 with particular attention to their application to microwave kinetic inductance detectors. We present a study of the structural and transport properties of films deposited under different conditions for both stoichiometry. Quarter wavelength microwave coplanar waveguide resonators have been fabricated from films of both stoichiometry and we present measurements of the microwave properties of these films as well as quasiparticle lifetimes using X-ray photons.

X-ray photon detection using superconducting resonators in thermal quasi-equilibrium

Superconducting resonators have to date been used for photon detection in a non-equilibrium manner. In this paper, we demonstrate that such devices can also be used in a thermal quasi-equilibrium manner to detect x-ray photons. We have used a resonator to measure the temperature rise induced by an x-ray photon absorbed in normal metal and superconducting absorbers on continuous and perforated silicon nitride membranes. We observed two distinct pulses with vastly different decay times. We attribute the shorter pulses to non-equilibrium quasiparticle relaxation and the longer pulses to a thermal relaxation process. In addition, we have measured the temperature dependence of the x-ray induced temperature rise and decay times. Finally, we have measured the resonator sensitivity and energy resolution.

Development of Thick Electroplated Bismuth Absorbers for Large Collection Area Hard X-ray Transition Edge Sensors

Transition edge sensors (TES) offer some of the highest resolutions for solid-state X-ray spectrometers. We are developing TES detectors for use at hard X-ray synchrotron light sources for energy ranges up to 20 keV. Because TES resolving power scales inversely with the square root of heat capacity, it is important to have an absorber with both a small heat capacity and a large X-ray stopping power. We are developing electroplated bismuth (Bi) absorbers to meet these criteria. Although Bi has a smaller X-ray absorption at 20 keV than gold, the specific heat is up to two orders of magnitude smaller, allowing for much larger collection area (up to 1 mm

Processing of X-ray Microcalorimeter Data with Pulse Shape Variation using Principal Component Analysis

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Optimization of thermal kinetic inductance detectors for x-ray spectroscopy

) without significantly increasing the total device specific heat. However, due to its low thermal conductivity, Bi absorbers may have longer thermalization times. Also, some evaporated Bi absorbers may produce spectra with low-energy tails that will hinder X-ray line shape analysis and increase minimum detectability limits of trace metals for X-ray fluorescence microscopy. We examine the impact of plating current density, agitation, film thickness, and seed layer thickness on the grain size, residual resistance ratio, and uniformity of Bi absorbers. Additionally, we discuss processing considerations important for successful electroplating.

Tungsten Silicide Alloys for Microwave Kinetic Inductance Detectors

We present a method using principal component analysis (PCA) to process x-ray pulses with severe shape variation where traditional optimal filter methods fail. We demonstrate that PCA is able to noise-filter and extract energy information from x-ray pulses despite their different shapes. We apply this method to a dataset from an x-ray thermal kinetic inductance detector which has severe pulse shape variation arising from position-dependent absorption.

Characterization and performance of the second-year SPT-3G focal plane

Thermal kinetic inductance detectors (TKIDs) are promising new detectors for use in X-ray spectroscopy because of the relative ease with which they can be fabricated into large arrays. While initial results have hinted at their resolution capability, the ultimate resolution achievable with these detectors has not been described. Using a bolometer matrix formalism, we examine the parameter space of the detector design (i.e., film critical temperature, detector operating temperature, resonator coupling, etc.) to examine the detectors response to noise sources, including phonon, Johnson, bias, and amplifier noise. Finally, we present the design of TKID optimized for 6-keV X-ray spectroscopy.

Year two instrument status of the SPT-3G cosmic microwave background receiver

Microwave kinetic inductance detectors are used to detect photons over a large range of wavelengths from submillimeter to X-ray. The common material requirements for this application are: high internal quality factor (Qi), high kinetic inductance fraction, long quasiparticle lifetime, and in the case of X-ray photons stopping power (i.e., dense, high atomic number materials). Superconducting tungsten silicide alloys have a tunable TC with silicon content, a high normal state resistivity, and a high density. In this work, we investigate the properties of thin films of tungsten silicide made of two different stoichiometry: WSi2 and W5Si3 with particular attention to their application to microwave kinetic inductance detectors. We present a study of the structural and transport properties of films deposited under different conditions for both stoichiometry. Quarter wavelength microwave coplanar waveguide resonators have been fabricated from films of both stoichiometry and we present measurements of the microwave properties of these films as well as quasiparticle lifetimes using X-ray photons.

Development of ROACH Firmware for Microwave Multiplexed X-Ray TES Microcalorimeters

The third-generation instrument for the 10-meter South Pole Telescope, SPT-3G, was first installed in January 2017. In addition to completely new cryostats, secondary telescope optics, and readout electronics, the number of detectors in the focal plane has increased by an order of magnitude from previous instruments to ~16,000. The SPT-3G focal plane consists of ten detector modules, each with an array of 269 trichroic, polarization-sensitive pixels on a six-inch silicon wafer. Within each pixel is a broadband, dual-polarization sinuous antenna; the signal from each orthogonal linear polarization is divided into three frequency bands centered at 95, 150, and 220 GHz by in-line lumped element filters and transmitted via superconducting microstrip to Ti/Au transition-edge sensor (TES) bolometers. Properties of the TES film, microstrip filters, and bolometer island must be tightly controlled to achieve optimal performance. For the second year of SPT-3G operation, we have replaced all ten wafers in the focal plane with new detector arrays tuned to increase mapping speed and improve overall performance. Here we discuss the TES superconducting transition temperature and normal resistance, detector saturation power, bandpasses, optical efficiency, and full array yield for the 2018 focal plane.

Kinetic Inductance Detectors for X-Ray Spectroscopy

The South Pole Telescope (SPT) is a millimeter-wavelength telescope designed for high-precision measurements of the cosmic microwave background (CMB). The SPT measures both the temperature and polarization of the CMB with a large aperture, resulting in high resolution maps sensitive to signals across a wide range of angular scales on the sky. With these data, the SPT has the potential to make a broad range of cosmological measurements. These include constraining the effect of massive neutrinos on large-scale structure formation as well as cleaning galactic and cosmological foregrounds from CMB polarization data in future searches for inflationary gravitational waves. The SPT began observing in January 2017 with a new receiver (SPT-3G) containing ~16,000 polarization-sensitive transition-edge sensor bolometers. Several key technology developments have enabled this large-format focal plane, including advances in detectors, readout electronics, and large millimeter-wavelength optics. We discuss the implementation of these technologies in the SPT-3G receiver as well as the challenges they presented. In late 2017 the implementations of all three of these technologies were modified to optimize total performance. Here, we present the current instrument status of the SPT-3G receiver.