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Microcalorimeter Spectrometers for X-ray and Gamma-ray Spectroscopy


Superconducting devices at very low temperatures can be used to measure very small amounts of energy. Using this effect, the Quantum Sensors Group is building single photon detectors for large regions of the electromagnetic spectrum.


The Quantum Calorimeters Project in the Quantum Sensors Group develops and applies sensors that detect the energy of single photons or particles. For example, Transition-Edge Sensors (TESs) are able to measure the energy of single x-ray and gamma-ray photons with a precision better than one part per thousand. When assembled into arrays, TESs combine good spectral resolving power and good collecting efficiency in a way that is not possible with other detector technologies. Application areas include materials analysis, astrophysics, microelectronics defect analysis and supply chain verification, and nuclear security.

Major Accomplishments

Top: A side view of TOMCAT, showing the horizontal electron column (SEM) and the NIST TES detector, integrated into an UHV chamber containing the sample and target.  The system currently utilizes a 1,000 pixel TES microcalorimeter detector package, and will be upgraded to a 3,000 pixel array in 2023. Bottom Left: A single slice from a tomographic reconstruction of an integrated circuit (IC) sample measured in TOMCAT. This slice represents one layer of metal wiring in the IC. Bottom Right: The design file IC wiring for the layer shown on the left, representing the ground truth for the reconstruction. The minimum feature size in the IC is 160 nm. A comparison between the lower left and lower right images indicates that TOMCAT can resolve all features present in nanoscale IC samples.

Nanoscale X-Ray Tomography of Integrated Circuits: X-ray nanotomography is a powerful tool for the characterization of nanoscale materials and structures. Many nanoscale x-ray imaging measurements are limited to x-ray synchrotron facilities, where high x-ray flux can be maintained in a small x-ray spot size. The Quantum Sensors Group has developed a laboratory-scale x-ray nanotomography instrument, deemed TOMCAT (TOMographic Circuit Analysis Tool), capable of imaging nanoscale features in integrated circuits (ICs). The structural complexity and nanoscale feature sizes in an IC make them a critical use case for nanotomography, as characterization of subsurface layers is integral to IC diagnostics such as defect detection or failure analysis. TOMCAT combines the electron beam of a scanning electron microscope (SEM) with a NIST-developed superconducting transition-edge sensor (TES) spectrometer. The electron beam generates a highly focused x-ray spot in a metal target and the precise, broadband x-ray detection of the TES spectrometer allows x-rays from the nanoscale spot in the target to be isolated from x-rays generated in other materials in the instrument. TOMCAT was demonstrated on an IC with 160 nm feature sizes, with the first results published in Nakamura et. al, arXiv:2212.10591 (2022) and Levine et. al., Microsystems & Nanoengineering, submitted (2023).

Future iterations of TOMCAT aim to improve spectral imaging capabilities, the achievable spatial resolution, and imaging speed. These improvements can be made in part through the development of the next generation of TES spectrometers, containing up to 12.5x more TES detector pixels than current instruments. Scaling up both the detection efficiency and number of TES pixels in a given spectrometer allows for higher total x-ray count rates, which improves the imaging speed. A 1,000 pixel TES microcalorimeter spectrometer has been demonstrated, with a 3,000 pixel instrument under development for use in TOMCAT. Results on these spectrometers can be found in Szypryt et. al., arXiv:2212.12073 (2022) and Szypryt et. al., IEEE Transactions on Applied Superconductivity 31, 1-5 (2021).

X-ray Spectroscopy for Exotic Atoms: The sub-atomic particles in an atom can be replaced by other particles of the same charge, forming an exotic atom. For example, a muon can peel off electrons of a normal atom and form a muonic atom with the nucleus. High-resolution x-ray spectroscopy of exotic atoms can be used to study quantum electrodynamics (QED) and strong force physics under extremely strong electric fields. At J-PARC, researchers are measuring exotic atom x-ray decays with transition-edge sensor (TES) arrays developed by the QSG, utilizing their superior energy resolution and high collection efficiency.  Recent results include measuring the deexcitation dynamics of muonic atoms as shown in T. Okumura, et al., Physical Review Letters, 127(5), 053001 (2021) and a measurement of the strong interaction’s effect on Kaonic Helium isotopes reported in T. Hashimoto et al. Physical Review Letters 128, 112503, (2022). In 2024, the QSG will delivered an upgraded spectrometer to J-PARC that will increase the efficiency and maximum energy range to probe higher-order transitions across multiple elements.  In 2025, the team will deliver a spectrometer to the ELENA beamline at CERN for similar studies using anti-protons.

photo of the team from NIST, the University of Colorado, and RIKEN installing the TES
A photo of the team from NIST, the University of Colorado, and RIKEN installing the TES ahead of the muonic x-ray spectroscopy experiment at J-PARC.
Credit: Shinji Okada

The NIST’s Electron Beam Ion Trap (EBIT) in Gaithersburg also serves as a source of exotic ion beams. It has been used to take measurements across a variety of fields, including fundamental atomic physics, spectroscopy of highly charged ions for fusion and laboratory astrophysics, and nuclear physics. The QSG has successfully commissioned and took initial measurements with a TES x-ray spectrometer at the NIST EBIT facility. Compared to the old EBIT germanium microcalorimeters, the TES has achieved more than 30 times increase in the active area and a modest improvement in energy resolution. A detailed description of this work is published in P. Szypryt, et al., Review of Scientific Instruments, 90 (12), 123107 (2019). And the most recent results are published in G. C. O’Neil, et al., Physical Review A, 102 (3), 032803, (2020). This project is also reported in a PML highlight.  The team is also developing a miniaturized EBIT for the NIST Boulder campus.  The more-compact system provides more stable operation and access to lower energy transitions. 

magazine cover
Left:  Cover of the January 2022 issue of The Journal of Physical Chemistry Letters showing a stylized representation of NIST's superconducting x-ray sensors collecting x-rays from a sample. The cover accompanied the publication “Metastable Brominated Nanodiamond Surface Enables Room Temperature and Catalysis-Free Amine Chemistry”. Right: NIST x-ray spectrometer installed at beamline U71 of the National Synchrotron Light Source, Brookhaven National Laboratory. The spectrometer contains 240 x-ray sensors and demonstrates NIST's ability to develop and deliver complete measurement systems.

Development of Superconducting X-ray Sensor Technology and Its Dissemination to U.S. Light Sources: Synchrotrons and XFELs are large facilities that provide x-ray beams of unmatched brightness and quality. As a result, they are heavily used by industrial and academic researchers to understand and develop advanced materials such as next-generation battery components. The Quantum Sensors Group has developed superconducting x-ray sensors that can provide a unique combination of spectral resolution, broad-band response, and collecting efficiency. In order to disseminate these sensors to users at U.S. light sources, the QSG has also developed the cryogenics, readout electronics, and control software needed to construct complete superconducting x-ray spectrometers.

NIST deployed the first of these spectrometers to the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory in 2010. Since then, permanent general-user systems have been deployed to beamlines 10-1 and 13-3 at Stanford Synchrotron Radiation Lightsource (SSRL) at SLAC National Laboratory. The success and scientific impact of these early instruments has spurred further interest from the x-ray science community. In 2022 the QSG in collaboration with other groups at NIST was awarded and NIST Innovations in Measurement Sciences grant to develop the most powerful soft x-ray spectrometer for Carbon chemistry ever conceived. Our goal is to improve the collection efficiency by more than 40x and the energy resolution by 4x compared to our existing spectrometers. This instrument will be able to directly measure the electron state of carbon bonded to catalysts and measurements such as these should enable more rapid catalyst development.

Overviews of this work have been published in K. Morgan, Physics Today, 71 (2018) 28-34 and J. Ullom et al., Synchrotron Radiation News 27 (2014) 24 and a detailed technical description, including overviews of the breadth of science they enable, is published in R. Doriese et. al., Review of Scientific Instruments 88, 053108 (2017). This work was also described in a recent PML highlight and was awarded a DOC Bronze medal in 2015.

Gamma-Ray Spectroscopy for Nuclear Safeguards: Arrays of cryogenic microcalorimeters are an emerging tool for performing high resolution gamma-ray spectroscopy on materials relevant to the nuclear fuel cycle. As seen below (left), microcalorimeter spectrometers consist of pixelated arrays where each pixel is an independent energy-dispersive spectrometer. As shown below (right), the co-added spectra from the pixels in an array provide 5x to 10x better spectral resolution than widely-used High-Purity Germanium (HPGe) detectors, while still detecting 35 % of 100 keV photons incident on the detector. This makes microcalorimeters the only path to gamma-ray sensors with both better energy resolution than HPGe and acceptable detection efficiency. The excellent energy resolution reduces peak overlaps and improves peak-to-background ratios, allowing isotopic analysis using gamma-ray microcalorimeters to achieve 1 % uncertainties, while being less susceptible to systematic error than HPGe detectors [Hoover-2014]. These features make gamma-ray spectroscopy highly appealing for Nuclear Material Accountancy and Control.

96-pixel gamma-ray microcalorimeter array
Photograph of a 96-pixel gamma-ray microcalorimeter array on a “microsnout” sub-module. The Sn absorbers used by the detectors are the grid at the top. The sides of the microsnout contain chips used to read out and bias the detectors.
Plot from [Hoover-2014] showing energy resolution advantage of cryogenic microcalorimeters (solid black) compared to HPGe detectors (upper black line) for actinide measurements.

The Quantum Sensors Group conducted the foundational technology development for microcalorimeter gamma-ray spectrometers. Over a 10+ year collaboration with the University of Colorado and Los Alamos National Laboratory, we have evolved gamma-ray microcalorimeters from single-pixel prototypes to deployed analytical instruments at operating nuclear facilities. We have achieved energy resolutions as good as 42 eV at 103 keV [Zink-2006]; developed the first microcalorimeter gamma-ray spectrometer to demonstrate isotopic analysis [Bennet-2012, Hoover-2013]; deployed SOFIA (Spectrometer Optimized for Facility Integrated Applications), a small-footprint spectrometer optimized for use in working nuclear facilities, and used it to make the first spectroscopic observation of the 103 keV and 159 keV lines of Pu-242 in an unprepared sample [mercer-2022]; demonstrated improvements in accuracy of isotopic analysis using microcalorimeters compared to competing technology; pioneered data analysis techniques required to make these precise measurements [Hoover-2013, Becker-2019]; deployed a 384-pixel instrument to Idaho National Laboratory in 2022; and are currently assembling a 672-pixel instrument for deployment at Pacific Northwest National Laboratory in 2023. In 2022, our SOFIA instrument won a prestigious R&D 100 award.

microcalorimeters graph

Theoretical modeling of the superconducting phase transition in TES microcalorimeters: The resistance of a superconductor drops abruptly to zero at its critical temperature Tc. This phenomenon can be exploited to measure small temperature changes. A current through the superconductor keeps it in the transition region between normal and superconducting states (an equilibrium point indicated in the figure by the red dot). Then the superconductor is heated by an amount ∆T, its resistance increases proportionally to the amount of input energy, as indicated by the red arrow. The resultant change in current can be measured to determine the amount of energy. This device is called a transition edge sensor (TES). By optimizing the materials, temperature, and size of the TES we can measure the energy of single photons from optical to gamma ray energies. Although an individual TES device is typically small – linear dimensions are typically on the order of a few hundred microns – arrays of hundreds or even thousands of TESs can be multiplexed with SQUID electronics, making them a powerful tool for high efficiency, high energy resolution spectroscopy.

surface plot
A ‘RIT’ surface plot showing a simulation of TES resistance as a function of current and temperature. The red line shows the equilibrium values across the transition. The other solid color lines are the trajectories of a 15 keV photon, assuming the photon is thermalized on times scales much faster than the detector response, for different values of the bias circuit inductance. The blue dashed line represents the initial response to the photon being rapidly thermalized on time scales faster than the TES current can respond.

A longstanding obstacle to advancing the design of TES detectors is that detailed understanding of the superconducting transition is limited. The microscopic physics of the superconducting state was successfully explained by Bardeen-Cooper-Schrieffer theory in 1957, but BCS theory doesn’t tell us about how a superconductor’s resistivity changes in the transition. Our group’s goal is to find predictive models for TES behavior based on the fundamental physics of superconductors. We are developing a new model based on solving the time-dependent Ginzburg-Landau equations in two-dimensions. The two-dimensional nature of this model enables exploration of different geometries and additional features like the normal banks on the side of the TES or normal metal stripes. With the full geometry of the device taken into account, this model has the potential to work across the entire resistive transition, including the small-scale structure in the RIT surface that has historically eluded TES models. For this work, Doug Bennett received a NIST Bronze medal in 2020.  Further work has focused on explaining the “excess” noise in TESs as the result of Johnson noise mixed down to low frequencies by Josephson oscillations that occur in devices with nonlinear current-voltage relationship, as described in Wessels et. al., Applied Physics Letters 118, 202601 (2021). These theoretical efforts support our TES development program’s goals to improve the energy resolution performance of our microcalorimeter sensors from the soft x-ray regime to MeV particle detection.

scanning electron microscope
A scanning electron microscope (center-left) with a commercial superconducting x-ray spectrometer (left) developed at STAR Cryoelectronics. The energy resolution of the spectrometer enables elemental and, in some cases, chemical speciation.
Credit: Robin Cantor

Support of Commercialization of Superconducting X-ray Sensors: These high energy resolution x-ray sensors were originally pioneered at NIST, and have considerable potential for materials analysis if they can be developed into spectrometers compatible with the ubiquitous scanning electron microscopes used throughout materials intensive industries. By measuring x-rays produced by the exciting electron beam, these sensors can provide improved elemental and chemical information, as well as information on thinner and smaller structures than is presently possible. The QSG has worked to disseminate this sensor technology so that it can be developed commercially. In particular, the QSG has supported STAR Cryoelectronics' efforts to develop a spectrometer that can be mounted on an electron microscope. STAR Cryoelectronics is a small company located in Santa Fe, New Mexico. STAR now provides a complete commercial instrument based on an array of superconducting x-ray sensors and has already sold several spectrometers to private customers. STAR also provides commercial analysis services using a superconducting x-ray spectrometer located at STAR's Santa Fe facility.

Improving the X-Ray Reference Database: The Quantum Sensors Group initiated a project in 2015 to improve x-ray reference data in order to support emerging needs in industrial and scientific materials analysis. A central goal of this work is an improved tabulation of x-ray line energies and shape profiles. The project is a collaboration between the QSG and three other Divisions at NIST. The QSG will use the high resolving power, broadband response, and high collecting efficiency of its microcalorimeter x-ray sensors to perform absolute energy metrology on a wide range of x-ray lines, including lines that were too weak for exact measurement using previous techniques. Proof-of-principle measurements of x-rays from four lanthanide-series elements were highly successful, establishing the viability of this new technique. These results were published as J. W. Fowler, et al. “A reassessment of absolute energies of the X-ray L lines of lanthanide metals,” Metrologia 54, 494, (2017). Further measurements made in 2018 improved the systematic uncertainties in the calibration of absolute energies by use of more linear sensors, more calibration materials, and an automated, rotating sample holder to separate calibration from lanthanide spectra. It also uses an improved method to characterize spectra by a transferrable smooth model of the line profiles. The full spectra and selected details are shown at right. New results and favorable comparisons against existing reference data have recently been published in J.W. Fowler, et al., “Absolute energies and emission line shapes of the x-ray lines of lanthanide metals,” Metrologia 58, 1, (2021). This work received a DOC Silver medal in 2020.

Fluorescence emission for four lanthanide metals
Fluorescence emission for four lanthanide metals excited by a broad-band x-ray source in the L line region of each spectrum. Spectra were measured by an array of 192 NIST superconducting microcalorimeters. Two dozen distinct emission features are visible for each element. The Nd and Ho spectra are multiplied by 1000 for clearer separation of details.
L3M emission graphs
The L3M emission features for four lanthanide metals. Microcalorimeter spectra are shown as colored dots, backgrounds in gold, and the spectral model as black. The several Voigt components that make up the model are shown in gray. This spectral model serves as a transferrable energy (or wavelength) standard for x-ray emissions.

Laboratory-Scale Picosecond X-ray Material Probe: X-rays are among the most common methods to probe materials and chemical reactions. Using picosecond x-ray pulses available at synchrotron and x-ray free electron laser (XFEL) facilities, it is possible to monitor the evolution of chemical changes in materials on the picosecond timescale. This capability is being used to study the pathways of photoreactions in environmentally and technologically relevant molecular systems, such as new solar cell materials. However, beam time at these facilities is very limited, making it possible to explore only a few materials systems with this technique. The QSG has developed a laboratory-scale picosecond x-ray probe system to compliment the more powerful (but much harder to access) XFEL and synchrotron lightsources. This system relies upon an x-ray source that produces picosecond duration pulses from a laser-generated plasma in a liquid jet, and the high resolving power of the NIST-developed superconducting x-ray spectrometer. High impact scientific results from this system include identification of the long-lived spin-crossover state in optically excited iron-tris-bypyridine and elucidation of the process of photo-disassociation in ferrioxalate. Soon the measurement speed of the system will be increased by a factor of 10 or more when the water jet used to generate x-rays via laser induced plasma is replaced with a liquid metal jet. The first dynamical x-ray results were published in O’Neil, G. C. et. al., The Journal of Physical Chemistry Letters 8 (5), 1099 (2017) and Miaja-Avila, L. et. al., Physical Review X 6 (3), 031047 (2016).  The work received a DOC Gold Medal in 2017 and a U.S Patent in 2021.

x-ray probe schematic
Top: schematic of the NIST table-top x-ray probe.  A pulsed laser induces photochemical reactions in the material under study.  The same laser is focused onto a water jet, producing an x-ray generating plasma.  The x-rays are used to interrogate the sample at controlled time delays after photoexcitation.  Bottom: (left) measured time-resolved Kβ emission spectrum of iron-tris-bipyridine probed at a 3 ps time delay.  The spectral shape changes in the excited (pumped) state due to a spin crossover in the system.  (right) resultant time evolution of the high-spin fraction deduced from emission measurements at several time delays.  Our data (see inset) show a fast rise of the high-spin fraction at time-zero followed by an exponential decay (see main figure) of the high-spin-state fraction with a time constant of 566 ± 100 ps.
Examples of devices developed by the Quantum Sensors Group to measure the total energy of single radioactive decays. These devices are used by Los Alamos National Laboratory to determine the elemental and isotopic composition of trace radioactive samples. In both devices, a solid silicon platform is suspended by flexible silicon beams. Radioactive samples can be attached to the dark brown squares. The device at left also has an embedded thermometer. A U.S. dime in the background provides scale.
Credit: Daniel Schmidt

Development of Sensors to Measure the Total Energy of Single Radioactive Decay Events:  The energy released by a single radioactive decay can be used to identify the specific element involved. However, measuring such a small energy is extremely difficult. The superconducting sensors developed at NIST have the energy resolution to perform this measurement, provided they can be engineered into a system compatible with total energy measurements. We have developed new sensors are fabricated on micromachined silicon (rather than silicon nitride) to simultaneously achieve good thermal isolation while preserving mechanical robustness. Robustness is needed to allow the easy attachment of metal foils containing embedded radioactive material, including particulates. The energy of single radioactive decays is sufficient to produce a measureable temperature rise in the sensors. Performed in collaboration with Los Alamos National Laboratory, the goal of this work is to develop new capabilities for nuclear forensics, treaty verification, and environmental monitoring. These sensors have shown their suitability for a wide range of analytical measurements. The attractions of the technique are spectral simplicity, the ability to simultaneously measure multiple isotopes including both alpha- and beta-emitters, and greatly reduced sample preparation. The ability of this technique to accurately measure the mass ratio of the isotopes 240Pu and 239Pu from soil samples was published in D. Mercer et al, “Gamma and Decay Energy Spectroscopy Measurements of Trinitite,” Nuclear Technology, 207:sup1, S309-S320, (2021). This work was also described in a recent PML highlight.

Expanding applications of radioactivity in medicine, energy, and national security demand quantification of complex radionuclide mixtures at uncertainty levels that are currently unachievable (sometimes by a factor of 10). Starting in 2021, NIST began a multidisciplinary project to develop a new capability for primary standardization of radionuclides. This “TrueBq” project focuses on developing Decay Energy Spectrometery (DES) of quantitatively-prepared sources using ultra-sensitive, cryogenic, Transition Edge Sensors (TESs).

Our team draws from 5 Divisions and 3 Operating Units at NIST with expertise in milligram mass and inkjet metrology, radioactive source preparation, detector electronics and modeling, algorithms and optimization with our group providing expertise in TES design and fabrication.  More details on this effort can be found on in R. Fitzgerald et al., “Toward a New Primary Standardization of Radionuclide Massic Activity Using Microcalorimetry and Quantitative Milligram-Scale Samples,” Journal of Research (NIST JRES), 126048, (2022), and on the TrueBq program page.

NRC Fellowship Opportunities

Highlights and Awards:

Created March 13, 2018, Updated March 1, 2024