Radiation detectors enable scientists to observe some of the universe’s smallest physical phenomena and refine theories of subatomic physics and galactic dynamics alike. To better distinguish minuscule differences in incoming signals, some of these instruments operate at very low temperatures, often just hundredths of a degree above absolute zero. Low-temperature environments minimize background noise caused by thermal fluctuations, clearing the way for detectors to measure and interpret the energy signatures of single particles. Similar to adjusting a tuning knob to hear a radio station in full clarity, scientists seek to optimize the ratio of information carriers to noise from other sources, referred to as the system’s energy resolution.
No one detector system is best for studying all types of radiation. Yet, for a selected energy range, researchers can improve resolution and differentiate signals with greater accuracy to resolve subtle features that carry strong analytical significance. Three examples illustrate the range of applications of low-temperature detectors: a superconducting tunnel junction (STJ) to detect sterile neutrinos, a magnetic microcalorimeter to distinguish between isotopes of radioactive materials, and another microcalorimeter that has enabled the study of massive, highly dynamic objects in the universe.
Superconducting Tunnel Vision
Interacting primarily through the weak nuclear force, neutrinos are elusive subatomic particles that physicists observe and study in detail. “Neutrinos are a hot topic because they are so peculiar, with fundamental properties that we don’t understand,” says Livermore physicist Stephan Friedrich. “Roughly 100 trillion neutrinos pass through each person’s body every second. Yet, no one feels a thing because the particle hardly interacts with anything, including the detectors meant to find them.” Friedrich leads an experiment at the Laboratory to detect and measure the mass of sterile neutrinos, theorized heavier variants of the particles that only interact with gravity without interacting with the weak nuclear force, as neutrinos typically do.
The Beryllium-7 Electron capture in Superconducting Tunnel junctions, or “BeEST”, experiment is measuring neutrino properties through their effect on other particles. In BeEST, radioactive beryllium-7 nuclei are implanted in an STJ using a particle accelerator. When a beryllium-7 nucleus decays to lithium-7, it emits a neutrino, which makes the lithium-7 nucleus recoil to conserve momentum. The STJ is sensitive enough to measure the energy of the recoil, allowing researchers to infer the mass of the neutrino. Regarding the signal BeEST is looking for, Friedrich explains, “If, once in a blue moon, a heavy, sterile neutrino is emitted in this decay instead of a standard neutrino, then we should measure the resulting lithium-7 nucleus to have lower recoil energy than usual.”
BeEST must therefore be able to detect incredibly slight deviations from the expected lithium-7 recoil energy spectrum. The detection system incorporates superconducting materials, which provide for much greater energy sensitivity than detectors using silicon- or germanium-based semiconductors. To generate signals from a detector, incoming particles must deliver sufficient energy to promote electrons from a material’s valence band to its conduction band. The energy difference between these two states is known as bandgap (see S&TR, December 2023, Ultrawide Bandgap Materials in the Spotlight). The bandgap value for a more common semiconductor-based detector is typically around 1 electronvolt (eV), whereas a superconductor’s bandgap is 1,000 times smaller. Therefore, an energetic particle impacting a superconducting detector (such as BeEST) can generate 1,000 times more conduction-band electrons than in semiconductor systems, giving a roughly 30-fold increase in energy resolution. “The more precisely we can measure a particle’s energy, the better we can enable scientific discovery. This tiny detector enables us to do competitive searches for unknown particles in our usual laboratory without using a billion-dollar accelerator,” says Friedrich.
Detector experiments such as BeEST are vital to advance a fundamental understanding of phenomena not yet explained by the Standard Model of particle physics. Sterile neutrinos are considered a leading candidate component for dark matter. The team’s findings could validate existing theoretical models or suggest improvements beyond the Standard Model necessary to account for new observations.
Magnetic Microcalorimeter
Livermore physicist Geon-Bo Kim leads a research team that has developed a magnetic microcalorimeter (MMC), a device so sensitive it can measure the temperature increase from a single radioactive decay event. At its core, an MMC consists of three major components: an absorber heated by radioactive decay, a sensor material that changes its magnetic properties when heated, and a quantum magnetometer device that measures the change. Kim’s MMC device is different from many calorimeter designs because it embeds nuclear material—plutonium, uranium, americium, or other radioactive elements—directly into the absorber. This configuration achieves a 5,000 times finer resolution than the magnitude of decay energy, enabling it to distinguish the decay signatures of plutonium-239 and plutonium-240, which differ by only 0.2 percent. Without the high-energy resolution, the detector would not be able to distinguish the specific signatures. In addition to this application, MMCs offer scientists and regulatory agencies a valuable capability for assessing the composition of nuclear material samples.
When radioactive material releases energy via nuclear decay, the event causes a slight temperature spike within the MMC’s gold absorber, which surrounds the sample. This temperature signal is transferred through a thermal link (typically gold wirebonds) to a sensor composed of silver and erbium. This sensor material is a paramagnet whose magnetic strength varies with temperature. The sensor achieves peak magnetism near absolute zero and loses magnetism at a predictable rate as temperature increases. To measure this minuscule change in magnetism, the MMC incorporates a superconducting quantum interference device (SQUID). SQUIDs excel at measuring weak magnetic fields and ultimately convert the changing magnetic flux into a voltage signal.
Tallying the voltage readings from decay events over the course of hours or days reveals the energy spectrum of the material. When analyzing the data using decay energy spectroscopy, scientists seek features indicative of the energy released associated with specific isotopes, for instance, energies associated with decays of plutonium-239 versus that of plutonium-240. Detection of certain isotopes and concentrations within the sample informs researchers of a material’s history regarding age, source, and chemical processes in which it may have taken part. “This method can play a significant role in the nuclear and energy security landscape. Being complementary to the current approach of mass spectrometry, the two technologies act as a cross-check to increase confidence in our analysis of samples,” says Kim.
The Resolve Quantum Microcalorimeter
Analyzing energetic photons also offers insight into the universe’s largest structures. Operating above Earth’s atmosphere aboard the X-ray Imaging and Spectroscopy Mission (XRISM) satellite observatory led by the Japanese Space Agency and NASA’s Goddard Space Flight Center, XRISM’s premier instrument is Resolve, a combination of an x-ray mirror and an x-ray calorimeter spectrometer. Resolve is used to study massive, highly dynamic objects such as galaxy clusters, active galaxies, accreting black holes, and supernova remnants. By taking advantage of Resolve’s ability to measure objects’ composition and dynamics in never-before-seen detail, the XRISM mission provides, for the first time, a more detailed and complete picture of some of the most immense, energetic objects in the universe.
Resolve was designed to provide high-resolution spectroscopy in a region of the x-ray spectrum out of reach to many of its predecessors. Large, energetic objects contain regions of hot gas that are best analyzed in the soft x-ray band of 0.3 to 10 kiloelectronvolts (keV). Important spectral lines from astrophysically abundant elements from carbon to nickel fall into this waveband. “The Resolve calorimeter provides diagnostic capabilities and high-precision measurements that were not possible until now,” says Livermore physicist Greg Brown. “With it, we can see x-ray emissions from every highly charged, astrophysically relevant element.”
Benefiting from the many of Lawrence Livermore’s contributions that took place back on Earth, Resolve now makes astrophysical observations from orbit. Livermore scientists played a major role in the ground calibration efforts for the Resolve detector system and led the calibration of Resolve’s optical blocking filters and mirror thermal shield. Initial calibration of prototype blocking filters enabled the Resolve team to correct issues with the fabrication prior to flight. During these calibration tests, Livermore researchers used radiation-generating equipment to bombard the thin filters and the detector array with x-rays, ensuring the degree of x-ray transmission was well known and that the calorimeter achieved the desired energy resolution. In addition, using an engineering spare calorimeter spectrometer coupled with the Laboratory’s electron beam ion trap (EBIT), Livermore researchers perform laboratory astrophysics experiments. These laboratory experiments simulate the processes taking place in celestial sources and measure transition energies to accuracies unachievable by calculations. The results reduce the uncertainties of, for example, the elemental abundances and motion of a source, and they are often required for building a physically consistent picture of the source. “Livermore’s contributions to the instrument testing, calibration, and laboratory astrophysics are fundamental to Resolve’s ability to precisely measure spectral features and draw scientific conclusions from those measurements,” says physicist Megan Eckart, director of Livermore’s Space Science Institute.
Consisting of a 6-by-6 array of pixels that independently function as quantum calorimeters, each Resolve pixel can precisely measure the heat signature from being struck by a single x-ray photon. To ensure such stellar sensitivity, the detector system is cooled to 0.05 Kelvin, providing the low-temperature environment essential for accurately discerning the signature of each photon. “In detector design, many ‘dials’ exist that can be turned depending on desired performance such as the energy band and spectral resolution,” says Eckart. “Resolve is optimized to deliver the highest possible signal-to-noise ratio given the spectrometer’s energy band and area of each pixel, the latter of which is driven by the characteristics of Resolve’s x-ray mirror.”
When an incoming x-ray photon races toward the Resolve detector, the photon is absorbed in a mercury telluride (HgTe) tile whose thickness is tuned to the targeted energy band of study. For Resolve, the absorber thickness is approximately 10 micrometers, enabling high stopping efficiency and good spectral resolution of photons with energies below approximately 10 keV. The HgTe absorber is attached to a micromachined sensor made of doped silicon. The sensor, a resistive silicon thermistor, is an electrical component whose resistance changes with temperature. An incident photon heats up both the absorber and sensor, and the energy the photon delivers is measured as a change in electrical resistance and then read out as a voltage pulse. The height of the pulse is proportional to the energy of the incident photon. This setup provides Resolve an energy resolution of approximately 4.5 eV for 6-keV x-ray signals.
Leveraging Resolve’s impeccable energy resolution, scientists can distinguish closely spaced spectral lines and gain insight into the materials and mechanisms associated with the evolution of cosmic objects. As part of the XRISM science team, Livermore researchers play a role in analyzing incoming data and publishing new scientific findings on cosmic objects. “These spectra improve our understanding of some of the largest objects in our universe,” says Livermore’s Natalie Hell. “Obtaining just one spectrum, we can determine an object’s composition and dynamics, making it possible to unravel the physics and evolutionary history of a variety of celestial sources, many for the first time. Nearly every source that has been analyzed so far has revealed new insights.”
While Resolve focuses on energetic sources on a cosmic scale, Livermore scientists are also adapting NASA-built calorimeter detectors for fusion energy purposes. One such detector is currently deployed on the Madison Symmetric Torus, a magnetic fusion research device at the University of Wisconsin–Madison. Researchers seek to apply the x-ray microcalorimeters’ high spectral resolution and broadband coverage to monitor the plasma state and to detect the presence of “impurity ions.” Caused by interaction between the plasma and the reactor wall, these impurities can altogether jeopardize a sustained fusion reaction. As this new detector application gains traction, the Livermore team is investigating multiple avenues for deploying x-ray microcalorimeters to tokamaks. They are also incorporating next-generation calorimeters with thousands of pixels that operate on the edge of superconductivity to improve spectral resolution. From space satellites to fusion devices, the breadth of these detectors’ applications underscores the far-reaching capabilities of low-temperature systems.
—Elliot Jaffe
For further information contact Stephan Friedrich (925) 423-1527 (friedrich1 [at] llnl.gov (friedrich1[at]llnl[dot]gov)), Geon-Bo Kim (925) 422-4232 (kim90 [at] llnl.gov (kim90[at]llnl[dot]gov)), Megan Eckart (925) 422-7889 (eckart2 [at] llnl.gov (eckart2[at]llnl[dot]gov)), or Greg Brown (925) 422-6879 (brown86 [at] llnl.gov (brown86[at]llnl[dot]gov)).
