Performance of an X-Ray Microcalorimeter with a 240 Micron Absorber and a 50 Micron TES Bilayer

We have been developing superconducting transition-edge sensor (TES) microcalorimeters for a variety of potential astrophysics missions, including Athena. The X-ray Integral Field Unit (X-IFU) instrument on this mission requires close-packed pixels on a 0.25 mm pitch, and high quantum efficiency between 0.2 and 12 keV. The traditional approach within our group has been to use square TES bilayers on molybdenum and gold that are between 100 and 140 microns in size, deposited on silicon nitride membranes to provide a weak thermal conductance to a 50 mK heat bath temperature. It has been shown that normal metal stripes on top of the bilayer are needed to keep the unexplained noise at a level consistent with the expected based upon estimates for the non-equilibrium non-linear Johnson noise.In this work we describe a new approach in which we use a square TES bilayer that is 50 microns in size. While the weak link effect is much stronger in this size of TES, we have found that excellent spectral performance can be achieved without the need for any normal metal strips on top of the TES. A spectral performance of 1.58 eV at 6 KeV has been achieved, the best resolution seen in any of our devices with this pixel size. The absence of normal metal stripes has led to more uniform transition shapes, and more reliable excellent spectral performance. The smaller TES size has meant that that the thermal conductance to the heat bath, determined by the perimeter length of the TES and the membrane thickness, is lower than on previous devices, and thus has a lower count rate capability. This is an advantage for low count-rate applications where the slower speed enables easier multiplexing in the read-out, thus potential higher multiplexing factors. In order to recover the higher count rate capabilities, a potential path exits using thicker silicon nitride membranes to increase the thermal conductance to the heat bath

Fabrication of a Hybrid Transition Edge Sensor Array for Lynx

Lynx is a proposed NASA X-Ray telescope flight mission aimed at achieving state-of-the-art angular and energy resolution with a 100 kilopixel array to probe the hot energetic young universe in unprecedented detail. To achieve these goals, our team plans on leveraging our current work in development of the focal plane for the Athena X-Ray Integral Field Unit (X-IFU) while advancing the state-of-the-art in transition edge sensor (TES) X-ray detector technology. The TES is an optimal technology for achieving both high energy and fine angular resolution at the same time because pixel features can be made extremely small and the absorber which dominates the heat capacity can be tuned to meet resolution requirements. Specifically, the proposed mission concept calls for a hybrid detector of three different arrays fabricated in the same planar process in one focal plane and optimized for different science goals. The main arrays consist of 5x5 hydras, 25 pixels of 4 micron thick Au absorbers each with a different thermal link to one common TES. The outer array has absorbers on a 50-micron pitch for most of the 5 arc-minute field-of-view, and the inner array has 25-micron absorbers for the central 1 arc-minute region. A high resolution array consisting of single pixel 1 micron thick Au absorbers on 50-micron pitch will lie off to the side. Reading out an array of this magnitude will likely require improvements in indium bump bonding to superconducting flexible wiring. Fabrication of absorbers of two different sizes requires electroplating through a photoresist mold by careful tuning of the current density to achieve uniform flat absorbers on a fine pitch scale, followed by ion milling to yield narrow streets separating the pixels while preserving high quantum efficiency. We report on progress made at fabricating the hybrid array with different absorber sizes and thicknesses. Further, we also report on ongoing work to adequately heat sink the pixels with backside wire bonding and copper coating. We also report on work to improve detector pixel yield and top side indium bump bonding to flexible wiring

Development of Kilo-Pixel Arrays of Transition-Edge Sensors for X-Ray Spectroscopy

We are developing kilo-pixel arrays of transition-edge sensor (TES) microcalorimeters for future X-ray astronomy observatories or for use in laboratory astrophysics applications. For example, Athena/XMS (currently under study by the european space agency) would require a close-packed 32x32 pixel array on a 250-micron pitch with < 3.0 eV full-width-half-maximum energy resolution at 6 keV and at count-rates of up to 50 counts/pixel/second. We present characterization of 32x32 arrays. These detectors will be readout using state of the art SQUID based time-domain multiplexing (TDM). We will also present the latest results in integrating these detectors and the TDM readout technology into a 16 row x N column field-able instrument

Large Format Transition Edge Sensor Microcalorimeter Arrays

We have produced a variety of superconducting transition edge sensor array designs for microcalorimetric detection of x-rays. Designs include kilopixel scale arrays of relatively small sensors (approximately 75 micron pitch) atop a thick metal heat sinking layer as well as arrays of membrane-isolated devices on 250 micron and up to 600 micron pitch. We discuss fabrication and performance of microstripline wiring at the small scales achieved to date. We also address fabrication issues with reduction of absorber contact area in small devices

Performance of Large Format Transition Edge Sensor Microcalorimeter Arrays

We have produced a variety of superconducting transition edge sensor array designs for microcalorimetric detection of x-rays. Arrays are characterized with a time division SQUID multiplexer such that greater than 10 devices from an array can be measured in the same cooldown. Designs include kilo pixel scale arrays of relatively small sensors (-75 micron pitch) atop a thick metal heatsinking layer as well as arrays of membrane-isolated devices on 250 micron and up to 600 micron pitch. We discuss fabrication and performance of microstripline wiring at the small scales achieved to date. We also address fabrication issues with reduction of absorber contact area in small devices

Fabrication and Performance of Large Format Transition Edge Sensor Microcalorimeter Arrays

We have produced a variety of superconducting transition edge sensor array designs for microcalorimetric detection of x-rays. Designs include kilopixel scale arrays of relatively small sensors (~75 micron pitch) atop a thick metal heatsinking layer as well as arrays of membrane-isolated devices on 250 micron pitch and smaller arrays of devices up to 600 micron pitch. We discuss the fabrication techniques used for each type of array focusing on unique aspects where processes vary to achieve the particular designs and required device parameters. For example, we evaluate various material combinations in the production of the thick metal heatsinking, including superconducting and normal metal adhesion layers. We also evaluate the impact of added heatsinking on the membrane isolated devices as it relates to basic device parameters. Arrays can be characterized with a time division SQUID multiplexer such that greater than 10 devices from an array can be measured in the same cooldown. Device parameters can be measured simultaneously so that environmental events such as thermal drifts or changes in magnetic fields can be controlled. For some designs, we will evaluate the uniformity of parameters impacting the intrinsic performance of the microcalorimeters under bias in these arrays and assess the level of thermal crosstalk

Multi-Absorber Transition-Edge Sensors for X-Ray Astronomy Applications

We are developing multi-absorber Transition-Edge Sensors (TESs) for applications in x-ray astronomy. These position-sensitive devices consist of multiple x-ray absorbers each with a different thermal coupling to a single readout TES. Heat diffusion between the absorbers and the TES gives rise to a characteristic pulse shape corresponding to each absorber element and enables position discrimination. The development of these detectors is motivated by a desire to maximize focal plane arrays with the fewest number of readout channels. In this contribution we report on the first results from devices consisting of nine) 65 X 65 sq. microns Au x-ray absorbers) 5 microns thick. These are coupled to a single 35 X 35 sq. microns Mo/Au bilayer TES. These devices have demonstrated full-width-half-maximum (FWHM) energy resolution of 2.1 eV at 1.5 keV) 2.5 eV at 5.9 keV and 3.3 eV at 8 keV. This is coupled with position discrimination from pulse shape over the same energy range. We use a finite-element model to reproduce the measured pulse shapes and investigate the detector non-linearity with energy) which impacts on the devices position sensitivity and energy resolution

Advances in Small Pixel TES-Based X-Ray Microcalorimeter Arrays for Solar Physics and Astrophysics

We are developing small-pixel transition-edge-sensor (TES) for solar physics and astrophysics applications. These large format close-packed arrays are fabricated on solid silicon substrates and are designed to accommodate count-rates of up to a few hundred counts/pixel/second at a FWHM energy resolution approximately 2 eV at 6 keV. We have fabricated versions that utilize narrow-line planar and stripline wiring. We present measurements of the performance and uniformity of kilo-pixel arrays, incorporating TESs with single 65-micron absorbers on a 7s-micron pitch, as well as versions with more than one absorber attached to the TES, 4-absorber and 9-absorber "Hydras". We have also fabricated a version of this detector optimized for lower energies and lower count-rate applications. These devices have a lower superconducting transition temperature and are operated just above the 40mK heat sink temperature. This results in a lower heat capacity and low thermal conductance to the heat sink. With individual single pixels of this type we have achieved a FWHM energy resolution of 0.9 eV with 1.5 keV Al K x-rays, to our knowledge the first x-ray microcalorimeter with sub-eV energy resolution. The 4-absorber and 9-absorber versions of this type achieved FWHM energy resolutions of 1.4 eV and 2.1 eV at 1.5 keV respectively. We will discuss the application of these devices for new astrophysics mission concepts

Portraying the nature of corruption: Using an explorative case-study design

What is the nature of corruption in Western democracies? To answer this research question, the authors study 10 Dutch corruption cases in depth, looking at confidential criminal files. The cases allow them to sketch a general profile of a corruption case. The authors offer nine propositions to portray the nature of corruption. They conclude that corruption usually takes place within enduring relationships, that the process of becoming corrupt can be characterized as a slippery slope, and that important motives for corruption, aside from material gain, include friendship or love, status, and the desire to impress others. The explorative multiple case study methodology helps to expand our understanding of the way in which officials become corrupt. © 2008 The American Society for Public Administration

Gendering the careers of young professionals: some early findings from a longitudinal study. in Organizing/theorizing: developments in organization theory and practice

Wonders whether companies actually have employees best interests at heart across physical, mental and spiritual spheres. Posits that most organizations ignore their workforce – not even, in many cases, describing workers as assets! Describes many studies to back up this claim in theis work based on the 2002 Employment Research Unit Annual Conference, in Cardiff, Wales