# Europe’s first TES spectrometer can catch X-ray signals scientists used to miss

> A new study in Review of Scientific Instruments describes Europe's first synchrotron-based TES spectrometer, now operating at BESSY II in Berlin. The instrument gives scientists a far more sensitive way to capture faint X-ray signals from materials that were previously too thin,...

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Byline: Helmholtz-Zentrum Berlin
Published: 2026-07-10T00:10:17+00:00
Categories: News, Physics

![Superconducting TES Array X-ray Spectrometer at BESSY II](https://www.argo.net/wp-content/uploads/2026/06/Europes_first_TES_spectrometer_can_catch_X-ray_signals_scientists_used_to_miss.jpg)

A [new study](https://doi.org/10.1063/5.0332443) in Review of Scientific Instruments describes Europe's first synchrotron-based TES spectrometer, now operating at BESSY II in Berlin. The instrument gives scientists a far more sensitive way to capture faint X-ray signals from materials that were previously too thin, too dilute, or too delicate for many standard experiments.

Developed through a collaboration involving **Helmholtz-Zentrum Berlin**, the Max Planck Institute for Chemical Energy Conversion and NIST, the new system uses superconducting sensors to measure individual X-ray photons. Its detector array can collect signals 100 to 1,000 times more efficiently than conventional wavelength-dispersive X-ray emission spectrometers.

That leap matters because many of the most interesting materials in modern science are small by design. A single atomic layer, a trace impurity, or a dilute molecular sample can hold crucial information about chemistry, biology, quantum behavior and energy conversion. The new **Transition Edge Sensor** spectrometer gives researchers a way to study those faint signals directly.

## A new X-ray tool comes online at BESSY II

**BESSY II**, the synchrotron light source operated by Helmholtz-Zentrum Berlin, generates bright X-ray beams for probing matter. Scientists use these beams to examine the electronic and structural properties of materials, from catalysts and semiconductors to molecular systems and quantum materials.

The newly commissioned TES spectrometer is installed at the UE52-SGM beamline. This beamline provides full polarization control, which lets researchers tune the X-ray light for different kinds of measurements. The spectrometer is connected to a custom ultra-high vacuum sample chamber, giving scientists a controlled environment for delicate experiments.

According to the instrument paper, the system was designed for soft X-ray absorption and emission spectroscopy. It is also suited for resonant inelastic X-ray scattering, a method that can reveal how electrons behave inside a material after it absorbs X-ray energy.

The spectrometer is now entering service for the BESSY II user community. That means outside scientists can propose experiments that use the instrument's sensitivity for problems that were previously beyond reach at many facilities.

## Why faint samples were so hard to study

Many X-ray experiments work by exciting a sample and then measuring the photons it emits. Those emitted photons carry information about the sample's electronic structure. In practice, the signal can be extremely weak.

Traditional **X-ray emission spectroscopy** and **Resonant Inelastic X-ray Scattering** often need large numbers of emitted photons to build a useful spectrum. That requirement has favored bulk materials or samples with relatively high concentrations. Thin films, monolayers, impurities and dilute molecular solutions can produce too few photons for efficient measurements.

Low signal creates another problem. Longer exposure can damage sensitive samples. Some biological molecules, adsorbed molecules and nanoscale structures can change under intense X-ray illumination. A faster and more efficient detector can reduce the burden on fragile samples while still collecting meaningful data.

Régis Decker, the HZB scientist responsible for the new instrument, summarized the gain in simple terms. "The superconducting Transition Edge Sensor array photon detector that we have now put into operation at BESSY II is around 100 to 1000 times more efficient."

## Up to 1,000 times more photon detection

The central advantage of the TES spectrometer is collection efficiency. Conventional wavelength-dispersive spectrometers can achieve high energy resolution, but they capture only a small fraction of the photons emitted by a sample. The new detector array is built to catch far more of them.

This higher efficiency changes what counts as a measurable sample. Signals from an atomic monolayer or a very dilute molecule can become usable. In the paper, the team describes measurements on monolayer hexagonal boron nitride and a sub-millimolar molecular system as representative examples.

Efficiency also improves time. Some measurements that previously required hours may be completed in minutes, depending on the sample and experiment. That speed gives researchers more flexibility and can help limit radiation damage during studies of sensitive materials.

The result is a more practical route to studying low-dimensional and low-concentration systems. These are exactly the kinds of systems that appear in many frontiers of materials science, molecular chemistry, catalysis and quantum technology.

![The photo shows the detector array, composed of 248 sensors](https://www.argo.net/wp-content/uploads/2026/06/Europes_first_TES_spectrometer_can_catch_X-ray_signals_scientists_used_to_miss-1.jpg)

## A 248-sensor detector cooled near absolute zero

At the heart of the instrument is an array of **248 superconducting sensors**. Each sensor is designed to register the energy of an incoming soft X-ray photon. The array sits inside a compact detector assembly that is cooled to extremely low temperatures.

The system uses a **dilution refrigerator** based on helium-4 and helium-3 cooling. The refrigerator brings the bath temperature down to about 25 milli-Kelvin. That's only a tiny fraction of a degree above absolute zero, the temperature where thermal motion approaches its lowest possible limit.

At these temperatures, the sensors operate at the edge of superconductivity. When an X-ray photon hits a sensor, it deposits a small amount of energy. That energy slightly warms the sensor and changes its electrical resistance. The instrument measures that change and converts it into the photon's energy.

The readout relies on superconducting electronics called **SQUIDs**, short for Superconducting Quantum Interference Devices. These circuits can detect extremely small electrical changes. Together, the cryogenic sensors and SQUID-based readout allow the spectrometer to measure many photons with high precision.

The detector's cryogenic stability is important for long experiments. Stable temperatures help keep the energy calibration reliable over time. That is essential when researchers need to compare weak signals from samples that differ only subtly.

## New access to monolayers, nanostructures and dilute molecules

The instrument was built for samples that push X-ray spectroscopy into difficult territory. The paper lists target systems such as impurities in solids, mono- and few-layer van der Waals materials, nanostructures, molecules adsorbed on surfaces and dilute molecular systems.

**Atomically thin materials** are especially important because their properties can change dramatically when they are reduced to a single layer. Graphene, hexagonal boron nitride and other two-dimensional materials can behave differently from their bulk forms. X-ray spectroscopy can reveal how their electrons are arranged and how they respond to excitation.

Nanostructures and impurity-level systems raise a related challenge. The feature of interest may represent only a tiny part of the sample. A detector that collects far more emitted photons can help isolate the signature of that small component.

Decker said the new capability reaches into several scientific fields. "This can provide new insights into molecular chemistry or molecular biology." The same sensitivity can also support studies of quantum properties in reduced dimensions, including monolayers and nanoscale systems.

The spectrometer also complements methods such as ARPES, which maps electronic band structures. Together, these approaches can give researchers a richer picture of how electrons move, bind and respond inside advanced materials.

## Faster experiments for fragile materials

Speed can be just as valuable as sensitivity. When a detector captures more useful photons per unit time, researchers can shorten exposures. That can make a major difference for samples that degrade under X-rays.

Some molecular systems and biological samples are beam-sensitive. Prolonged irradiation can alter chemical bonds or change the state of the sample during measurement. A faster experiment can preserve more of the original structure while still producing a usable spectrum.

The BESSY II setup also includes a dedicated ultra-high vacuum sample chamber. The chamber allows transfer, preparation and analysis of samples under controlled conditions. A motorized cryostat can hold samples from 10 K to room temperature, which lets researchers study temperature-dependent behavior.

The sample environment supports multiple sample mounting and electric field-gated devices. That opens possibilities for experiments where researchers adjust a material's electronic state while measuring its X-ray response.

Future upgrades are expected to expand sample preparation capabilities and add magnetic-field experiments. Those enhancements would support X-ray Magnetic Circular Dichroism in absorption and emission, including RIXS-MCD studies.

## Europe's only synchrotron TES spectrometer

TES detectors have a history in astrophysics, where scientists needed to detect extremely weak photon signals from distant objects. The same basic strength now helps laboratory researchers capture faint X-ray signals from small and dilute samples.

Before the BESSY II installation, only five TES spectrometers were operating at X-ray sources worldwide. Four were in the United States and one was in Japan. With the new system, BESSY II hosts Europe's only synchrotron TES spectrometer.

That position gives European researchers local access to a technology that has been rare at large X-ray facilities. It also strengthens BESSY II's role as a user facility for materials science, chemistry, physics and related fields.

The collaboration behind the instrument reflects the technical difficulty of the project. The system required expertise in superconducting sensors, cryogenics, X-ray beamline science, vacuum engineering and photon detection electronics. Bringing all of those elements together is what turns a sensitive detector into a working scientific instrument.

## What scientists can propose next

The next step is use by the scientific community. HZB is inviting researchers to submit proposals for experiments that take advantage of the spectrometer's high collection efficiency and low-temperature sample environment.

Potential studies could examine how electrons behave in monolayer materials, how impurities shape quantum systems, or how dilute molecular complexes respond to X-ray excitation. The instrument is also well suited to questions in molecular chemistry and catalysis, where active sites can be scarce and signals can be weak.

The paper in **Review of Scientific Instruments** presents the spectrometer as a platform for soft X-ray absorption, emission and scattering experiments on low-dimensional and low-concentration systems. Its performance combines high collection efficiency, useful energy resolution, count-rate capability and long-term stability.

Decker expressed the facility's next goal plainly. "We are looking forward to receiving exciting research proposals from our user community."

For researchers working at the edge of detectable X-ray signals, the new spectrometer offers a practical shift. Samples that once demanded long exposures or unusually high concentrations can now be approached with a detector built for faint light.