Hyperspectral x-ray imaging mapping capabilities for nuclear forensics

Our recent development of transition-edge sensor (TES) microcalorimeters for future X-ray astronomical missions such as DIOS is reported. In-house micromaching processes has been established aiming at prompt fabrication of TES devices. With a single-pixel TES microcalorimeter and an Au absorber, the energy resolution of 4.8 eV at 5.9 keV is achieved. 16×16 pixel arrays of TES microcalorimeters are successfully fabricated by using deep dry etching technique. The energy resolution is 11 eV and 26 eV with and without an Au absorber, respectively. The worse energy resolution than a single-pixel TES is due to large decrease of TES sensitivity and increase of transition temperature after etching. The reason for these phenomena is under investigation. In parallel, mushroom-type Au absorber structures are being tested. Furthermore, to precisely measure TES sensitivities and heat capacity, an experimental setup for impedance measurements is established.

A phase transition edge sensor (TES) microcalorimeter was developed for the energy-dispersive measurement of LX-ray photons emitted by transuranium elements. The phase transition temperature of the TES was designed to be 200mK using a bilayer structure of Au of 120 nm thickness and Ti of 50 nm thickness. A Au layer of 5.0 mm thickness was deposited on the Au/Ti bilayer to achieve an absorption efficiency of 50% and counting rate of 100 counts per second in the detection of LX-ray photons with energy from 10 to 20 keV. The TES microcalorimeter was operated for the detection of LX-ray photons emitted by 241Am, 238Pu, and 239Pu sources. A decay time constant of 180 ms for the detection signal pulses allowed the TES microcalorimeter to operate with a counting rate higher than 100 counts per second. The achieved energy resolution was 50 eV for the full width at half maximum of a peak corresponding to a 237Np Lβ1 X-ray of 17.75 keV.

Microcalorimeter detectors with embedded radioactive material offer many possibilities for new types of measurements and applications. We will discuss the designs and methods that we are developing for precise deposition of radioactive material and its encapsulation in the absorber of transition-edge sensor (TES) microcalorimeter detectors for two specific applications. The first application is total nuclear reaction energy (Q) spectroscopy for nuclear forensics measurements of trace actinide samples, where the goal is determination of ratios of isotopes with Q values in the range of 5–7 MeV. Simplified, rapid sample preparation and detector assembly is necessary for practical measurements, while maintaining good energy resolution. The second application is electron capture spectroscopy of isotopes with low Q values, such as \(^{163}\)Ho, for measurement of neutrino mass. Detectors for electron capture spectroscopy are designed for measuring energies up to approximately 6 keV. Their smaller heat capacity and physical size present unique challenges. Both applications require precise deposition of radioactive material and encapsulation in an absorber with optimized thermal properties and coupling to the TES. We have made detectors for both applications with a variety of designs and assembly methods, and will present their development.

Pulses of narrow line-width optical photons can be used to calibrate and test sub-2 eV full-width at halfmaximum (FWHM) energy resolution transition-edge sensor (TES) microcalorimeters at low energies (< 1 keV), where it is very challenging to obtain X-ray calibration lines comparable to (or narrower than) the detector resolution. This scheme depends on the ability to resolve the number of 3 eV photons in each pulse, which we have recently demonstrated up to photon numbers of about 300. At LTD-18 we showed preliminary results obtained with this technique on a 0.25 eV baseline resolution TES microcalorimeter designed for the ultra-high-resolution subarray of the Lynx mission. The line-shape was well described by a simple Gaussian. However, the difficulty of delivering photons to the small 46 μm square absorbers resulted in a large thermal crosstalk signal, whose random nature is expected to rapidly degrade the observed energy resolution towards higher photon numbers/energies. We have since improved the coupling between the optical fiber and the TES absorber and report here our current results.

A four-pixel-array superconducting transition-edge sensor (TES) microcalorimeter with a mushroom-shaped absorber was developed for energy dispersive spectroscopy performed on a transmission electron microscope. The TES consists of a bilayer of Au/Ti with either a 120- or 50-nm thickness. The absorber is made from a Au layer and its stem is deposited in the center of the TES surface. A Ta\({_{2}}\)O\({_{5}}\) insulating layer of 100-nm thickness is inserted between the overhang region of the absorber and the TES surface. Two types of microcalorimeter were fabricated with differing absorber thicknesses of 0.5 and 5.0 \({\upmu }\)m. An energy resolution of 15 eV FWHM with 5.9-keV X-rays was obtained using the 0.5-\({\upmu }\)m-type microcalorimeter.

Line-scan-based hyperspectral imaging techniques have often served as a research tool to develop rapid multispectral methods based on only a few spectral bands for rapid online applications. With continuing technological advances and greater accessibility to and availability of optoelectronic imaging sensors and spectral imaging spectrographs, the range of implementation for hyperspectral imaging has been broadening across quality and safety inspection needs in the food and agricultural industries. We have developed a series of food inspection imaging systems based on hyperspectral line-scan imaging with the use of a low-light sensitive, electron-multiplying charge-coupled device (EMCCD). In this methodology article, the spectral and spatial system performance of the latest generation of the ARS hyperspectral imaging system, which is capable of reflectance and fluorescence measurements in the visible and near-infrared (NIR) spectral regions from 400 to 1000 nm, is evaluated. Results show that the spectral resolution of the system is 4.4 nm at full-width at half-maximum (FWHM) and 6 nm FWHM at our typical operation mode (6-pixel spectral binning). We enhanced the system throughput responses by using spectral weighting filters to better utilize the dynamic range of the analog-to-digital converter. With this system throughput adjustment, noise-equivalent reflectance measurements were significantly reduced by approximately 50% in the NIR region for a range of standard diffuse reflectance targets. The responsivity of the system from 450 to 950 nm was determined to be linear.

We are developing high energy resolution X-ray microcalorimeters based on iridium and gold (Ir/Au) phase transition thermometers. Here we analyze the signal behavior of a 200×200 µm2 Ir/Au transition edge sensor (TES) with iridium and gold having the thicknesses of 100 and 25 nm, respectively. The energy resolution of this device was 15.4 eV full width at half maximum (FWHM) at 5.9 keV X-ray energy. For operation at bias resistances lower than 140 mΩ, we observed two distinct decay components, a slow component followed by a very fast component, which is thought to be due to the variation of current distribution inside the TES. However, taking into consideration the nonlinearity associated with the slow component, a very fast signal response makes the operation of this device promising at low-resistance bias points.

Superconducting transition-edge sensor (TES) microcalorimeters have great utility in x-ray applications owing to their high energy resolution, good collecting efficiency and the feasibility of being multiplexed into large arrays. In this work, we develop hard x-ray TESs to measure the absolute energies of muonic-argon ($\mu$-Ar) transition lines around 44 keV and 20 keV. TESs with sidecar absorbers of different heat capacities were fabricated and characterized for their energy resolution and calibration uncertainty. We achieved ~ 1 eV absolute energy measurement accuracy at 44 keV, and < 12 eV energy resolution at 17.5 keV.

Transition-edge sensor microcalorimeters with recovery times near 1 μs have become highly desirable in quantum science applications as near-infrared single-photon detectors with photon-number resolving capability. Previously, the recovery times of these devices could be decreased only by changing device material or modifying the superconducting-to-normal transition. We demonstrate a method for improving this speed that uses a normal-metal heat-sink. This demonstration with tungsten devices realizes a factor of 4 decrease in recovery time without significantly affecting energy resolution. Our approach may enable the creation of high-efficiency transition-edge sensors with decay times short enough to operate with 80 MHz pulsed single-photon sources.

Position-sensitive transition-edge sensor (TES) microcalorimeters (PoSTs) were developed to detect gamma rays up to a few MeV. Each gamma-ray PoST comprised a 0.5 mm × 0.5 mm × 19 mm lead absorber with a TES on each end and worked as a 1D imaging spectrometer. Only one TES worked at low temperature and its results are reported herein. We irradiated the PoST device with gamma rays from a Cs-137 source, and the resultant pulses showed strong correlations between the pulse height and the rise time. To determine effective pixels, we separately divided the photopeak and lead-K escape peak pulses into 19 groups after sorting them by their rise times. From the pulse shape distributions, we concluded that the TES was sensitive to the gamma-ray interaction position as long as it was not located too far away from the TES, and that it was probably possible to determine the interaction positions of the entire absorber, if both TESs were working. The PoST device was then modeled and the average pulses of the 19 effective pixels were compared to those of numerical simulations, where the rise time distributions of the actual pulses and simulated pulses were in good agreement.

We are developing a superconducting transition-edge sensor (TES) microcalorimeter array for the Diffuse Intergalactic Oxygen Surveyor (DIOS) mission. DIOS is a relatively small Japanese X-ray mission which will study large-scale distribution of the warm-hot intergalactic medium (WHIM) using O_VII and O_VIII emission lines. The satellite weighs about 400 kg equipped with a four-reflection X-ray telescope (FXT) and a TES microcalorimeter array (XSA). The design goal of the observing system is an effective area larger than 100 cm² at the oxygen line energy, a field of view about 50 arcmin square, and an energy resolution about 2 eV in the energy range of 0.3-1 keV. The TES microcalorimeter array provides the large field of view and good energy resolution at the same time. We plan to install an array comprising 16 x 16 pixels with an overall size of 1 cm square, which is cooled with a cryogen-free cooler. Pixels are readout by multiplexing signals using a multi-input SQUID amplifier, with each input connected to a TES microcalorimeter which is AC biased with a different frequency. We report the design and present status of the XSA system development.

The x-ray spectroscopy of the muonic atom has attracted atomic, nuclear, and particle physicists since its discovery. The properties of a muonic atom, such as its binding energy or atomic radius, are different from an ordinary atom because of the difference in the mass between the muon and electron. Our collaboration has employed superconductor transition-edge sensor (TES) microcalorimeters for the x-ray spectroscopy of the muonic atom. Thanks to the recent detector development, the 44-keV lines from muonic Ar, which is important for the precision test of bound-state quantum electrodynamics, and the 76-keV lines from muonic Si, which is of interest from the viewpoint of the measurement of nuclear radii, have been reached by the dynamic range of the state-of-art TES microcalorimeters. An accelerator facility that can produce a high-intensity muon beam is necessary for such spectroscopic experiments. We performed a commissioning experiment of the hard x-ray and gamma-ray TES microcalorimeter at the J-PARC MLF MUSE muon beam line. The energy resolution, gain stability, and performance of timing selection of the pulses were evaluated in the environment of a large-scale accelerator facility.

In recent years, cryogenic microcalorimeters using their superconducting transition edge have been under development for possible application to the research for astronomical X-ray observations. To improve the energy resolution of superconducting transition edge sensors (TES), several correction methods have been developed. Among them, a clustering method based on digital signal processing has recently been proposed. In this paper, we applied the clustering method to Ir/Au bilayer TES. This method resulted in almost a 10% improvement in the energy resolution. Conversely, from the point of view of imaging X-ray spectroscopy, we applied the clustering method to pixellated Ir/Au-TES devices. We will thus show how a clustering method which sorts signals by their shapes is also useful for position identification

We present finite element simulations of the transition-edge sensor (TES) microcalorimeter behavior using COMSOL Multiphysics. The simulated detector has a large absorber with a size of 990×990μm2. The simulation calculates the TES response after x-ray impact for a single pixel with a known, realistic, current-dependent resistive transition R(T,I). The simulation includes the full electrothermal feedback effects and heat conduction through the multiple contact points of the absorber to the TES thermometer and supporting membrane. The presented model, especially the 2D model, has been optimized for high accuracy, making it suitable for simulating the detector response when incident photons hit different positions on the absorber. We study the effects of position dependence using Principal Component Analysis with the aim of extracting correct photon energies from the pulses. The simulations show that the degradation, i.e., the FWHM broadening, due to the pulse variations will be less than 0.7 eV at below 2 keV, while it can be significant (&amp;gt;1 eV) at a higher energy band. The simulation result for position dependence can guide the design of large-absorber detectors for future x-ray missions, such as the Hot Universe Baryon Surveyor.

In many superconducting transition-edge sensor (TES) microcalorimeters, the measured electrical noise exceeds theoretical estimates based on a thermal model of a single body thermally connected to a heat bath. Here, we report on noise and complex impedance measurements of a range of designs of TESs made with a Mo/Au bilayer. We have fitted the measured data using a two-body model, where the x-ray absorber and the TES are connected by an internal thermal conductance Gae. We find that the so-called excess noise measured in these devices is consistent with the noise generated from the internal thermal fluctuations between the x-ray absorber and the TES. Our fitted parameters are consistent with the origin of Gae being from the finite thermal conductance of the TES itself. These results suggest that even in these relatively low resistance Mo/Au TESs, the internal thermal conductance of the TES may add significant additional noise and could account for all the measured excess noise. Furthermore, we find that around regions of the superconducting transition with rapidly changing derivative of resistance with respect to temperature, an additional noise mechanism may dominate. These observations may lead to a greater understanding of TES devices and allow the design of TES microcalorimeters with improved performance.