Toward a 256-Pixel Array of Gamma-Ray Microcalorimeters for Nuclear-Materials Analysis

We are developing a gamma-ray spectrometer for the analysis of nuclear materials based on an array of superconducting transition-edge-sensor microcalorimeters. The instrument will include eight columns of time-division-SQUID multiplexing circuitry capable of reading out 256 sensors. Our most recent sensors are bulk (1.5 mm square×0.25 mm thick) superconducting Sn absorbers glued to Mo/Cu bilayer thermometers. When fully populated, the active area of the spectrometer will be 5.76 cm², and the maximum count rate of the array will approach 20 kHz. Thus, our spectrometer will be comparable to the state-of-the-art 100 keV high-purity-Ge detector in count rate and collecting area, but with an order of magnitude better energy resolution. Half the detectors will be optimized for operation up to 100 keV, and the other half for operation up to 200 keV. A version of the spectrometer with a partially populated detector array was delivered to Los Alamos National Laboratory in June, 2007. We describe the present status of that instrument. In addition, we review results from a prototype array of 14 detectors that achieved 47 eV average energy resolution (full width at half maximum at 103 keV) and 25 eV resolution in the best detector. An important application of this technology is determining the total Pu content in spent reactor fuel without detailed knowledge of the reactor’s operating history. This work was supported in part by DOE-NNSA, NIST-EEEL Director’s Reserve, and the Intelligence Community Postdoctoral Fellowship Program.     

Development of Low Temperature SQUID Gradiometer Array for Metallic Magnetic Microcalorimeters

We report on the performance of two types of SQUID gradiometers developed for the readout of magnetic calorimeters. Our previously developed low dissipation SQUID gradiometer optimized for low temperature operation has demonstrated the flux noise level of under a magnetic field of 2.5 mT and 150 mK. With a cylindrical Au:Er paramagnetic sensor mounted inside the octagonal pickup washer of the SQUID gradiometer, we succeeded in detecting X-ray signals. However, our achieved energy resolution was 47.2±2.1 eV at 5.9 keV limited by the high operating temperature of 150 mK and by a magnetic field, small for that temperature, due to the limited critical current of the field coils. Based on these results, we fabricated new arrays of SQUID gradiometer by tuning the line width and the number of turns of the field coils and shunt resistance to realize a lower noise level and a larger magnetic field. Furthermore, arrays of SQUID gradiometer with meander patterned pickup washer was fabricated which provides a stronger coupling between the paramagnetic sensor and the pickup washer, and a larger magnetic field at the sensor.      

Effect of On-Chip Magnetic Shielding for TES Microcalorimeters

We investigated the magnet field dependence of the X-ray pulse height and the critical current of a Ti/Au bilayer TES micro-calorimeter. The pulse height was strongly affected by the magnetic field intensity applied perpendicularly to the TES surface. We found that the critical current at zero temperature, I _c0, decreased by a factor of two by applying a magnet field of ∼10 μT. Our data are consistent with a TES sensitivity proportional to (I/I _c0)^−2/3, as predicted by the Ginzburg-Landau theory. This fact implies that the shape of the R–T curve of the TES is partly determined by the critical current of the superconductor. In order to make our TES microcalorimeters less sensitive to the external magnetic field, we fabricated devices equipped with on-chip magnetic shielding. One device has a turn-around style electrical lead made of Al, in which the return line is laid beneath the Ti/Au TES. Another device has a Nb layer deposited between a multi-layer membrane. We demonstrated that the devices were usable below 200 μT with small degradation of the pulse height and energy resolution.      

High-Resolution Kaonic-Atom X-ray Spectroscopy with Transition-Edge-Sensor Microcalorimeters

We are preparing for an ultra-high resolution X-ray spectroscopy of kaonic atoms using an X-ray spectrometer based on an array of superconducting transition-edge-sensor microcalorimeters developed by NIST. The instrument has excellent energy resolutions of 2–3 eV (FWHM) at 6 keV and a large collecting area of about 20 mm \(^2\) . This will open new door to investigate kaon-nucleus strong interaction and provide new accurate charged-kaon mass value.     

Development of High Count Rate TES Microcalorimeters

To realize new TES imaging for high energy X-ray or gamma-ray spectrometer with a large pixel array and high count rate, we will try to develop a new radiation absorber self-adjusting the operating temperature and adopting an original reading out method of large pixel array using parallel biasing scheme. Our first prototype TES pixel using Au/Ti/Au tri-layer which consists of the small heat sensors and the radiation absorption area, and these are biased in serial. In this device, the current pulse with the decay time constant of less than 20 microseconds of incident X-rays from ⁵⁵Fe source were observed.      

Study of Microcalorimeters for Astrophysics Applications

In the framework of the Italian Space Agency R&D project, which is focused on the development of microcalorimeters for applications on astrophysics, we are studying different methods for TES microcalorimeter production and developing simulations of various absorber performances. In this paper are presented preliminary results obtained with two different geometries: front back and planar on SiN membrane.      

Development of Superconducting Multilayer Wiring for Large Arrays of TES X-Ray Microcalorimeters

We are developing TES (Transition Edge Sensor) X-ray Microcalorimeters for future X-ray astronomy missions such as DIOS (Diffuse Intergalactic Oxygen Surveyor). Standard wiring for a large-format array requires narrow and closely-packed configuration. This can cause deleterious crosstalk especially in the Frequency Domain Multiplexing readout. Hence we have employed multilayer wiring technique. In this paper, we tested a deposition of a TES film on the multilayer wiring by sputtering. Our first trial showed that the TES pixels have large residual resistances >50?mΩ and small critical currents of <1?μA. To improve the coverage of the TES film on the wiring, we increased the thickness of TES film (Ti/Au thickness of 100/200?nm). Also to remove an oxidation layer on the Al wiring, we strengthened a reverse-sputtering (150 W, 3?min) before the sputtering of TES. After these treatments, the TES film showed a sharp transition with small residual resistance (～1?mΩ) and large critical current (>30?μA).     

Development of Transition Edge Sensor Gamma-Ray Detectors for Positron-Annihilation-Spectroscopy

We are beginning development of a Transition Edge Sensor (TES) based gamma-ray detector for use in Coincidence-Doppler-Broadening (CDB) Positron-Annihilation-Spectroscopy (PAS) studies. We are specifically motivated to asses and study damage to nuclear Reactor Pressure Vessels (RPVs) which results from neutron irradiation during reactor operation. In this paper we provide a brief introduction to CDB-PAS and discuss our detector design.      

Absorber Materials for Transition-Edge Sensor X-ray Microcalorimeters

Arrays of superconducting transition-edge sensors (TES) can provide high spatial and energy resolution necessary for X-ray astronomy. High quantum efficiency and uniformity of response can be achieved with a suitable absorber material, in which absorber X-ray stopping power, heat capacity, and thermal conductivity are relevant parameters. Here we compare these parameters for bismuth and gold. We have fabricated electroplated gold, electroplated gold/electroplated bismuth, and evaporated gold/evaporated bismuth 8×8 absorber arrays and find that a correlation exists between the residual resistance ratio (RRR) and thin film microstructure. This finding indicates that we can tailor absorber material conductivity via microstructure alteration, so as to permit absorber thermalization on timescales suitable for high energy resolution X-ray microcalorimetry. We show that by incorporating absorbers possessing large grain size, including electroplated gold and electroplated gold/electroplated bismuth, into our current Mo/Au TES, devices with tunable heat capacity and energy resolution of 2.4 eV (gold) and 2.1 eV (gold/bismuth) FWHM at 5.9 keV have been fabricated. A.-D. Brown’s and S. Smith’s research was supported in part by appointments to the NASA Postdoctoral Program at NASA Goddard Space Flight Center administered by Oak Ridge Associated Universities through a contract with NASA.     

Characterization of Iridium Thin Films for TES Microcalorimeters

We are developing transition edge sensors (TES) using single layer iridium (Ir) as the detector and deposited tin (Sn) as the absorber. We obtained good Ir devices with reproducible and uniform transition temperature around 120 mK, transition widths of 1–2 mK, residual resistivity ratio (RRR) between 1.8–3 and high sensitivity α. Our Ir films are obtained using radio frequency (RF) magnetron sputtering and photolithography. In this paper we present a full low temperature characterization of the Ir films.      

Narrow Line X-Ray Calibration Source for High Resolution Microcalorimeters

We are developing a narrow line calibration source for use with X-ray microcalorimeters. At energies below 300 eV fluorescent lines are intrinsically broad, making calibration of high resolution detectors difficult. This source consists of a 405 nm (3 eV) laser diode coupled to an optical fiber. The diode is pulsed to create approximately one hundred photons in a few microseconds. If the pulses are short compared to the rise time of the detector, they will be detected as single events with a total energy in the soft X-ray range. Poisson fluctuations in photon number per pulse create a comb of X-ray lines with 3 eV spacing, so detectors with energy resolution better than 2 eV are required to resolve the individual lines. Our currently unstabilized diode has a multimode width less than 1 nm, giving a 300 eV event a FWHM less than 0.1 eV. By varying the driving voltage, or pulse width, the source can produce a comb centered on a wide range of energies. The calibration events are produced at precisely known times. This allows continuous calibration of a flight mission without contaminating the observed spectrum and with minimal deadtime.     

Performance of Micro-fabricated Magnetic Calorimeters Arrays for X-Ray Spectroscopy

We have been developing array technology for fabricating magnetic calorimeters for X-ray astronomy. The magnetization change in each pixel of the paramagnetic sensor material due to the heat input of an absorbed X-ray is sensed by a meander shaped coil. With this geometry it is possible to obtain excellent energy sensitivity, low magnetic cross-talk and large format arrays fabricated on wafers that are separate from the SQUID read-out. A magnetic bias field for each pixel is generated by the use of a persistent current that is stored. We report on the results from our prototype arrays, which are coupled to low noise DC-SQUIDs. The first test results are presented and the sensitivity is compared with calculations.     

Predicted Energy Resolution of a Running-Sum Algorithm for Microcalorimeters

The energy resolution of a high-pulse-rate filtering algorithm recently introduced by Hui Tan et al., based on running sums of TES microcalorimeter output streams, is predicted from average pulse shape and noise autocovariance. We compare with empirical resolution, and with optimal filtering predicted and empirical resolution, for a ⁵⁵Fe source measured by multiplexed 2×4, 2×8, and 2×12 arrays of microcalorimeters.     

Transformers to Readout Arrays of Microcalorimeters

We investigated the possibility of using transformers to replace SQUIDs for the readout of microcalorimeters. This simple scheme has been used in the past for bolometers, however it was discarded for the use with TES microcalorimeters because of the inadequate performance. Our work shows that, with a few simple changes, the performance of transformers as current transducers, while still not comparable to that of SQUIDs, is sufficient to read out the signal from TES microcalorimeters without any degradation in speed or energy resolution. In contrast to SQUIDs, transformers do not dissipate any power and their working principle makes them natural candidates for frequency multiplexing. Their extension to several channels is therefore straightforward.      

Characterization and Performance of Magnetic Calorimeters for Applications in X-ray Spectroscopy

We have developed prototype arrays of metallic magnetic calorimeters for applications in X-ray astronomy. Each pixel consists of an all-gold X-ray absorber in good thermal contact to a gold-erbium paramagnetic thin film thermometer that is operated in the temperature range of 30–100 mK. The para-magnetic response is coupled to a SQUID amplifier. We have characterized pixels in an array and observed the expected temperature dependence of the magnetization and heat capacity. We have demonstrated a full width at half maximum energy resolution of 1.7  \(\pm \) 0.1 eV at 6 keV and have also read out these devices using time-division multiplexing.     

Optimization and Analysis of Code-Division Multiplexed TES Microcalorimeters

We are developing code-division multiplexing (CDM) systems for TES arrays with the goal of reaching multiplexing factors in the hundreds. We report on x-ray measurements made with a four-channel prototype CDM system that employs a flux-summing architecture, emphasizing data-analysis issues. We describe an empirical method to determine the demodulation matrix that minimizes cross-talk. This CDM system achieves energy resolutions of between 2.3?eV and 3.0?eV FWHM at 5.9?keV.