X-ray Microcalorimeter Research Articles

Design and Performance of a TES X-ray Microcalorimeter Array for Energy Dispersive Spectroscopy on Scanning Transmission Electron Microscope

We discuss the design and performance of a transition edge sensor (TES) X-ray microcalorimeter array for scanning transmission electron microscope (STEM)–energy dispersive X-ray spectroscopy (EDS). The TES X-ray microcalorimeter has better energy resolution compared to conventional silicon drift detector and STEM–EDS utilizing a TES detector makes it possible to map the distribution of elements on a specimen in addition to analyze the composition. The requirement for a TES detector is a high counting rate (\(>\)20 kcps), wide energy band (0.5–15 keV) and good energy resolution (\(<\)10 eV) full width at half maximum. The major improvement of this development is to increase the maximum counting rate. In order to accommodate the high counting rate, we adopted an \(8 \times 8\) format, 64-pixel array and common biasing scheme for the readout method. We did all design and fabrication of the device in house. With the device we have fabricated most recently, the pulse decay time is 40 \(\upmu \)s which is expected to achieve 50 kcps. For a single pixel, the measured energy resolution was 7.8 eV at 5.9 keV. This device satisfies the requirements of counting rate and energy resolution, although several issues remain where the performance must be confirmed.

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

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.

Flight model measurements of the porous plug and film flow suppression system for the ASTRO-H Soft X-ray Spectrometer dewar

Flight model measurements of a porous plug phase separator and a film flow suppression system for the ASTRO-H Soft X-ray Spectrometer dewar are described. ASTRO-H is the sixth Japanese astronomy satellite and will be launched in 2016. It carries the Soft X-ray Spectrometer consisting of an X-ray optic and an X-ray microcalorimeter system operated at 50mK. Superfluid liquid He is employed as a part of the cooling system. A wide range of He flows from 28μg/s to 3.2mg/s in various operation cases must be safely vented under zero gravity. At the same time, superfluid He film flow through the vent line must be suppressed to <2μg/s in a nominal case to avoid extra loss of the liquid He. For this purpose, a porous plug phase separator together with a film flow suppression system is installed. To verify its performance, the mass flow rates and the film flow rate of the flight model system were measured at component level. The mass flow rates at various He tank temperatures (1.15, 1.30, 1.50, and 2.00K) were obtained and also the film flow rate was measured at 1.15K. Then, the mass flow rates were measured after installing the whole system into a flight model dewar at the He tank temperature of 1.16, 1.30, 1.50, and 2.00K. The dewar was tilted so that the porous plug located at the top of the dewar is immersed in the liquid He and the porous plug separates the liquid and vapor He by the thermomechanical effect as in orbit. The obtained mass flow rates and the film flow rate in these tests were confirmed to meet the requirements and to be consistent with each other. No abnormal event such as large mass flow rates was observed. All these experimental results strongly suggest that this flight model of the porous plug and the film flow suppression system will work properly in space.

The Practice of Pulse Processing.

The analysis of data from x-ray microcalorimeters requires great care; their excellent intrinsic energy resolution cannot usually be achieved in practice without a statistically near-optimal pulse analysis and corrections for important systematic errors. We describe the essential parts of a pulse-analysis pipeline for data from x-ray microcalorimeters, including steps taken to reduce systematic gain variation and the unwelcome dependence of filtered pulse heights on the exact pulse-arrival time. We find these steps collectively to be essential tools for getting the best results from a microcalorimeter-based x-ray spectrometer.

Developments in Time-Division Multiplexing of X-ray Transition-Edge Sensors.

Time-division multiplexing (TDM) is a mature scheme for the readout of arrays of transition-edge sensors (TESs). TDM is based on superconducting-quantum-interference-device (SQUID) current amplifiers. Multiple spectrometers based on gamma-ray and X-ray microcalorimeters have been operated with TDM readout, each at the scale of 200 sensors per spectrometer, as have several astronomical cameras with thousands of sub-mm or microwave bolometers. Here we present the details of two different versions of our TDM system designed to read out X-ray TESs. The first has been field-deployed in two 160-sensor (8 columns × 20 rows) spectrometers and four 240-sensor (8 columns × 30 rows) spectrometers. It has a three-SQUID-stage architecture, switches rows every 320 ns, and has total readout noise of 0.41 μΦ0/√Hz. The second, which is presently under development, has a two-SQUID-stage architecture, switches rows every 160 ns, and has total readout noise of 0.19 μΦ0/√Hz. Both quoted noise values are non-multiplexed and referred to the first-stage SQUID. In a demonstration of this new architecture, a multiplexed 1-column × 32-row array of NIST TESs achieved average energy resolution of 2.55±0.01 eV at 6 keV.

Fine pitch transition-edge sensor X-ray microcalorimeters with sub-eV energy resolution at 1.5 keV

We are developing arrays of X-ray microcalorimeters on a 50-µm pitch that utilize transition-edge sensors as the sensor to measure the temperature rise when X-rays are absorbed. An array of this type of pixel has great potential for the study of point sources in future X-ray observatory missions. The pixels have gold absorbers with dimensions 45 × 45 × 4.2 µm3. We measured an energy resolution of 0.72 ± 0.03 eV full width at half maximum for the Al Kα complex in a subset of pixels within the array, which is the best resolution to date using a non-dispersive detector at this energy. We describe our characterization of this device including measurements of the heat capacity, thermal conductance to the heat bath, and the temperature and current sensitivity of the detector, and discuss its potential for improved performance.

Operating modes and cooling capabilities of the 3-stage ADR developed for the Soft-X-ray Spectrometer instrument on Astro-H

A 3-stage adiabatic demagnetization refrigerator (ADR) (Shirron et al., 2012) is used on the Soft X-ray Spectrometer instrument (Mitsuda et al., 2010) on Astro-H (Takahashi et al., 2010) [3] to cool a 6×6 array of X-ray microcalorimeters to 50mK. The ADR is supported by a cryogenic system (Fujimoto et al., 2010) consisting of a superfluid helium tank, a 4.5K Joule–Thomson (JT) cryocooler, and additional 2-stage Stirling cryocoolers that pre-cool the JT cooler and cool radiation shields within the cryostat. The ADR is configured so that it can use either the liquid helium or the JT cryocooler as its heat sink, giving the instrument an unusual degree of tolerance for component failures or degradation in the cryogenic system. The flight detector assembly, ADR and dewar were integrated into the flight dewar in early 2014, and have since been extensively characterized and calibrated. This paper summarizes the operation and performance of the ADR in all of its operating modes.

Checking the potassium origin of the new emission line at 3.5 keV using the K xix line complex at 3.7 keV

Whether the new line at ~3.5 keV, recently detected in different samples of galaxy clusters, Andromeda galaxy and central part of our Galaxy, is due to Potassium emission lines, is now unclear. By using the latest astrophysical atomic emission line database AtomDB v. 3.0.2, we show that the most prospective method to directly check its Potassium origin will be the study of K XIX emission line complex at ~3.7 keV with future X-ray imaging spectrometers such as Soft X-ray spectometer on-board Astro-H mission or microcalorimeter on-board Micro-X sounding rocket experiment. To further reduce the remaining (factor ~3-5) uncertainty of the 3.7/3.5 keV ratio one should perform more precise modeling including removal of significant spatial inhomogeneities, detailed treatment of background components, and further extension of the modeled energy range.

Developments of Frequency-Domain Multiplexing of TES Arrays for a Future X-Ray Satellite Mission

Arrays of transition-edge sensors (TESs) X-ray microcalorimeters can provide high energy resolution and a large area necessary for the future X-ray mission Athena (2028~). An array with 4000 TESs will be employed on the X-ray satellite for the first time. The detector can achieve energy resolution of 2.5-3.0 eV and a high quantum efficiency in the energy range of 0.3-12 keV. Multiplexing the readout is necessary to minimize the number of readout amplifiers, the amount of wiring in the cryostat, and the cooling power required at the base-temperature. We are developing frequency-domain multiplexing (FDM) readout of TES microcalorimeters. In the FDM configuration, the TES is ac voltage biased at well-defined frequencies (between 2 and 6 MHz). For the development of the readout of the Athena instrument, we are using a nearly quantum-limited high-dynamic-range two-stage superconducting quantum interference device amplifier from VTT and high-Q lithographical LC resonators. In this paper, we will present the current status of the development of FDM readout of TES microcalorimeters. We will report on the results obtained using arrays fabricated at the NASA/Goddard Space Flight Center group.

Nearly Quantum Limited Two-Stage SQUID Amplifiers for the Frequency Domain Multiplexing of TES Based X-ray and Infrared Detectors

The frequency domain multiplexing (FDM) readout of arrays of transition edges sensors for the X-IFU/Athena and SAFARI/SPICA instruments requires SQUID amplifiers with low input inductance, low flux noise, high-dynamic range and low power consumption. We report the performance of two-stage SQUID amplifiers developed at VTT coupled to multiple high-Q LC resonators of the SRON FDM readout operating in the frequency range from 2 to 5 MHz. The SQUIDs have been tested in the following configurations: 1) the two SQUIDs at base temperature with open input; 2) the amplifier SQUID at 1K temperature and the front-end SQUID at base temperature in the FDM readout of low-G TES bolometers; and 3) and in the FDM readout of X-ray microcalorimeter arrays with both SQUIDS at base temperature. We measured an input flux noise equal to 0.18 μΦ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> /Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> at 20 mK, equivalent to an energy resolution of 9 b. The measured current noise at the SQUID input is ~1 pA/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> at 20 mK, which corresponds to a coupled energy resolution of about 15 b.

Radiance line ratios Ly-β/Ly-α, Ly-γ/Ly-α, Ly-δ/Ly-α, and Ly-ε/Ly-α for soft X-ray emissions following charge exchange between C6+ and Kr

The radiance line ratios Ly-β/Ly-α, Ly-γ/Ly-α, Ly-δ/Ly-α, and Ly-ε/Ly-α for soft X-ray emission following charge exchange (CX) between C6+ and Kr are reported for collision energies between approximately 320 and 46,000eV/u. The corresponding collision velocities (250–3000km/s) are characteristic of the solar wind. X-ray spectra were obtained at the Oak Ridge National Laboratory Multicharged Ion Research Facility using a microcalorimeter X-ray detector with a resolution on the order of 10eV FWHM. The measured Ly-ε/Ly-α is zero for all considered energies and suggests that very little, if any, capture to 6p occurs. The measured Ly-β/Ly-α and Ly-γ/Ly-α ratios intersect and form a well resolved node around (950±50)km/s, which could be used as an astrophysical velocity indicative tool. The results reported here are compared to calculations for C6+ +H since no published theory for C6+ +Kr is known to exist. Double-electron-capture (DEC) and other multi-electron processes are possible. True double capture is estimated to be only 10% of the single-electron-capture (SEC).

Three-Dimensionally Assembled TES X-ray Microcalorimeter Arrays for a TEM EDS System

Three-dimensionally assembled TES X-ray microcalorimeter arrays may be utilized for three purposes: (1) to obtain wide X-ray energy coverage of TES microcalorimeters, (2) to distinguish charged particle events from X-ray events, (3) to reconstruct Compton-scattering geometry for hard X-ray Compton cameras. We have designed and fabricated three-dimensionally assembled array of the minimum format i.e. 2 × 2 × 2 array in order to obtain a good energy resolution in a wide energy range of 0.5–20 keV and a high maximum counting rate of 2000 cps for energy dispersive X-ray spectrometer (EDS) system for a transmission electron microscope (TEM). Although we could not obtain required energy resolution because of a problem in the refrigerator system, we confirmed the operation of the three-dimensional array.

In-orbit background of X-ray microcalorimeters and its effects on observations

Methods.There are no experimental data about the background experienced by microcalorimeters in the L2 orbit, and thus the particle background levels were calculated by means of Monte Carlo simulations: we considered the original design configuration and an improved configuration aimed to reduce the unrejected background, and tested them in the L2 orbit and in the low Earth orbit, comparing the results with experimental data reported by other X-ray instruments.To show the results obtainable with the improved configuration we simulated the observation of a faint, high-redshift, point source (F[0.5-10 keV]~6.4E-16 erg cm-2 s-1, z=3.7), and of a hot galaxy cluster at R200 (Sb[0.5-2 keV]=8.61E-16 erg cm-2 s-1 arcmin-2,T=6.6 keV). Results.First we confirm that implementing an active cryogenic anticoincidence reduces the particle background by an order of magnitude and brings it close to the required level.The implementation and test of several design solutions can reduce the particle background level by a further factor of 6 with respect to the original configuration.The best background level achievable in the L2 orbit with the implementation of ad-hoc passive shielding for secondary particles is similar to that measured in the more favorable LEO environment without the passive shielding, allowing us to exploit the advantages of the L2 orbit.We define a reference model for the diffuse background and collect all the available information on its variation with epoch and pointing direction.With this background level the ATHENA mission with the X-IFU instrument is able to detect ~4100 new obscured AGNs with F>6.4E-16 erg cm-2 s-1 during three years, to characterize cluster of galaxies with Sb(0.5-2 keV)>9.4E-16 erg cm-2 s-1 sr-1 on timescales of 50 ks (500 ks) with errors <40% (<12%) on metallicity,<16% (4.8%) on temperature,2.6% (0.72%) on the gas density, and several single-element abundances.

Development of Compact Dry 3He-4He Dilution Refrigerator for Transition Edge Sensor Microcalorimeter X-ray Detector Operation on Electron Microscopes

A compact dry 3He- 4He dilution refrigerator (DR) was developed for operation of a superconducting transition edge sensor (TES) microcalorimeter x-ray detector mounted on a transmission electron microscope. A Gifford-McMahon (GM)-type mechanical cryocooler was used as a DR pre-cooler to operate the TES microcalorimeter without consuming liquid helium. To reduce the noise and vibrations generated by the GM cryocooler, the DR was separated from the cryocooler in a split system that allows a remote helium cooling loop to pre-cool the DR. In the DR unit, the dilution components of the still, the heat exchangers, and the cascade pair of mixing chambers were assembled into a compact cylinder. A snout made from an oxygen-free copper rod was attached to the DR mixing chamber for placement of the TES microcalorimeter x-ray detector. The lowest temperature and the cooling power in the mixing chamber were measured to be 87 mK and 20 μW at 100 mK, respectively. The DR unit was shown to maintain the temperature on top of the snout such that it was below the temperature required for multi-pixel-array TES microcalorimeter x-ray detector operation.

Interdisciplinary X-Ray Microanalysis: From Planets and Comets to Artifacts and Fine Art

A cryogenic microcalorimeter x-ray spectrometer is coupled to an FEI XL30 environmental scanning electron microscope (ESEM). The microcalorimeter is designed to obtain high resolution broadband xray spectra in the energy range of 150 eV to 10 keV. An x-ray optic enhances the solid angle for x-ray energies between 150 eV and 2300 eV. This enables precise microanalysis with relatively low energy (< 5 keV) electrons. The detected x-ray photons are time-tagged and synchronized with the electron beam scan to generate high resolution x-ray maps.

Development status of the mechanical cryocoolers for the Soft X-ray Spectrometer on board Astro-H

Astro-H is the Japanese X-ray astronomy satellite to be launched in 2015. The Soft X-ray Spectrometer (SXS) on board Astro-H is a high energy resolution spectrometer utilizing an X-ray micro-calorimeter array, which is operated at 50mK by the ADR with the 30liter superfluid liquid helium. The mechanical cryocoolers, 4K-class Joule Thomson (JT) cooler and 20K-class double-staged Stirling (2ST) cooler, are key components of the SXS cooling system to extend the lifetime of LHe cryogen beyond 3years as required. Higher reliability was therefore investigated with higher cooling capability based on the heritage of existing cryocoolers. As the task of assessing further reliability dealt with the pipe-choking phenomena by contaminant solidification of the on-orbit SMILES JT cryocooler, outgassing from materials and component parts used in the cryocoolers was measured quantitatively to verify the suppression of carbon dioxide gas by their storage process and predict the total accumulated carbon dioxide for long-term operation. A continuous running test to verify lifetime using the engineering model (EM) of the 4K-JT cooler is underway, having operated for a total of 720days as of June 2013 and showing no remarkable change in cooling performance. During the current development phase, prototype models (PM) of the cryocoolers were installed to the test SXS dewar (EM) to verify the overall cooling performance from room temperature to 50mK. During the EM dewar test, the requirement to reduce the transmitted vibration from the 2ST cooler compressor was recognized as mitigating the thermal instability of the SXS microcalorimeter at 50mK.

X-ray-emission measurements following charge exchange between C6+and H2

Lyman x-ray spectra following charge exchange between C$^{6+}$ and H$_2$ are presented for collision velocities between 400 and 2300 km/s (1--30 keV/amu). Spectra were measured by a microcalorimeter x-ray detector capable of fully resolving the C VI Lyman series emission lines though Lyman-$\delta$. The ratios of the measured emission lines are sensitive to the angular momentum $l$-states populated during charge exchange and are used to gauge the effectiveness of different $l$-distribution models in predicting Lyman emission due to charge exchange. At low velocities, we observe that both single electron capture and double capture autoionization contribute to Lyman emission and that a statistical $l$-distribution best describes the measured line ratios. At higher velocities single electron capture dominates with the $l$-distribution peaked at the maximum $l$.

He flow rate measurements on the engineering model for the Astro-H Soft X-ray Spectrometer dewar

The sixth X-ray Japanese astronomy satellite, namely Astro-H, will be launched in 2015. The Soft X-ray Spectrometer onboard the Astro-H is a 6×6 X-ray microcalorimeter array and provides us with both a high energy resolution of <7eV at 0.5–10keV and a 3′×3′ modest imaging capability for the first time. To cool the detector down to the operation temperature of 50mK, five cryocoolers, a 30-L superfluid helium cryostat, and a 3-stage adiabatic demagnetization refrigerator are utilized. A very small heat load up to ∼0.9mW on the helium tank is allowable to realize the helium lifetime of >3years, which consequently requires that the vapor flow rate out of the helium tank should be very small <42μg/s. We adopted a porous plug phase separator in combination with a film flow suppression system composed of an orifice, a heat exchanger and knife edge devices to retain the liquid helium under zero gravity and safely vent the small amount of the helium vapor. We measured helium mass flow rates from the helium tank equipped in the engineering model dewar. We tilted the dewar at an angle of 75° so that one side of the porous plug located at the top of the helium tank attaches the liquid helium and the porous plug separates the liquid and vapor helium by thermomechanical effect. Helium mass flow rates were measured at helium tank temperatures of 1.3, 1.5 and 1.9K. We confirmed that resultant mass flow rates are in good agreement within the systematic error or low compared to component test results and achieve all the requirements. The film flow suppression also worked normally. Therefore, we concluded that the SXS helium vent system satisfactorily performs integrated into the dewar.

Performance of TES X-ray Microcalorimeters with AC Bias Read-Out at MHz Frequencies

At SRON we are developing Frequency Domain Multiplexing for the read-out of superconducting transition edge sensor microcalorimeters for future X-ray astrophysical missions. We will report on the performance of Goddard Space Flight Center pixels under AC bias in the MHz frequency range. Superconducting flux transformers are used to improve the impedance matching between the low ohmic TESs and the SQUID. We connected 5 pixels to the LC filters with resonant frequencies ranging between 1 and 5 MHz. For X-ray photons of 6 keV we measured a best X-ray energy resolution of 3.6 eV at 1.4 MHz, consistent with the integrated Noise Equivalent Power. In addition, we improved the electrical circuit by optimizing the coupling ratio of the impedance matching transformer. In addition, we improved electrical circuit for impedance matching; modified transformer coupling ratio. As a result, we got the integrated noise equivalent power resolution of 2.7 eV at 2.5 MHz. A characterization of the detector response as a function of the AC bias voltage, bias frequency and the applied magnetic field is presented.

Progress on the FDM Development at SRON: Toward 160 Pixels

SRON is developing the electronic read-out for arrays of transition edge sensors using frequency domain multiplexing in combination with base-band feedback. The astronomical applications of this system are the read-out of soft X-ray micro-calorimeters in a potential instrument on the European X-ray mission-under-study Athena+ and far-IR bolometers for the Safari instrument on the Japanese mission SPICA. In this paper we demonstrate the simultaneous read-out of 38 bolometer pixels at a 1 $$-$$ 2 aW/ $$\surd $$ Hz dark NEP level. The stability of the read-out is assessed over 400 s. time spans. Although some 1/f noise is present, there are several bolometers for which 1/f-free read-out can be demonstrated.