Fermilab-led microelectronics codesign team works to develop a state-of-the-art particle detector

Particle accelerators and detectors are the workhorses of researchers plumbing the depths of the quantum realm. Each update gives scientists more opportunities to study the building blocks of our universe.

Now, a unique collaboration of researchers, led by research engineer Davide Braga of the US Department of Energy’s Fermi National Accelerator Laboratory, is designing and fabricating three components of a new superconducting particle detector: chip, circuitry and sensor. Each piece of this project is cutting edge in itself, and together they create something completely new.

The detector will be able to operate in the ultracold, strong magnetic field, and high radiation environment found in particle accelerator facilities where others cannot. Once completed, scientists could use this detector for groundbreaking experiments in fields ranging from nuclear physics to the search for dark matter. The detector’s unique hybrid design allows for computation close to the sensor modeled on the human brain and will ultimately make the detectors scalable without sacrificing performance.

At a fundamental level, the way this sensor works is completely different from the common detectors we’ve been working with for 100 years in particle physics, said physicist Whitney Armstrong, an Argonne National Laboratory scientist collaborating on the project. The exciting part is thinking about all the new applications you can do with this new technology.

In 2021, DOE announced a $54 million call for laboratories to apply for a three-year fundamental research grant on microelectronics co-design. Fermilab was one of 10 institutions to receive the award for carrying out this innovative project.

DOE is making a conscious effort to get people with different domain expertise to work together so that they influence each other, said Braga, the principal investigator on the co-design team. That’s why we put together this collaboration, which is quite diverse, he explained.

The design team is made up of Fermilab scientists and researchers from seven other institutions, including the Massachusetts Institute of Technology and Argonne. In January 2023, the team met for the first time in person to discuss their progress.

The funded proposal aims to revolutionize cryogenic detectors capable of detecting single particles or photons. To this end, the team is developing two complementary classes of cryogenic detectors, one based on ultra-low noise semiconductor sensors and one based on superconducting nanowire single photon detectors, or SNSPDs, that operate below minus 268 degrees Celsius. Although the photon is in the name, Braga explained that these detectors can also detect charged particles.

We are now looking to incorporate this technology into particle detectors for accelerator and collider experiments, he said.

Building a better detector

There are advantages to using these superconducting detectors in the high magnetic field environment of a particle accelerator that stem from the way a signal is generated in the sensor. When a particle or other photon hits the superconducting nanowire, it heats the wire enough to break the superconducting state. The nanowire exhibits electrical resistance, and the resulting voltage spike is transmitted to a custom signal-processing microchip before being sent to an attached computer.

This process is markedly different from more traditional detectors that rely on ionization to generate a signal: When a charged particle hurtles through such a detector, it knocks electrons off the atoms it encounters. This causes an electrical signal to be detected, amplified, and finally sent to a computer.

Because the movement of electrons and ions is affected by the attraction of high magnetic fields, ionization detectors, or charge harvesting-based detectors, do not work well inside accelerator tunnels where high-power magnets shape beams of particles. Superconducting detectors, however, could fill that niche.

One thing I really want to do is push the idea of ​​integrating superconducting detectors into the cold mass of superconducting magnets, especially for the Electron-Ion Collider, Armstrong said. This future collider will break electrons into ions, such as the proton, to probe the 3D internal structure of particles.

There are no other detectors that can operate effectively in those high magnetic fields and low temperatures, he explained, and this new technology could help researchers tune their accelerators more efficiently and precisely. And, in general, these detectors work well. They have excellent position and temporal resolution and resistance to radiation, Armstrong said.

While generally considered disadvantageous, working at cryogenic temperatures can be a boon for new applications in nuclear and particle physics. It’s not necessarily very difficult for accelerators to reach liquid helium temperatures, he said.

The microelectronics co-design team’s device has three different components to develop and refine: a specialized microchip; an interface layer of superconducting electronics; and a superconducting nanowire sensor, all three operating at a few degrees above absolute zero. Each member is faced by a different group of the team.

Fantastic microchips

Braga is the leader of the Application Specific Integrated Circuit, or ASIC, development team that designs the chips for the end device. Ordinary chip integrated circuits, or microchips, operate at room temperature, Braga said, although they are sometimes heated to 100 degrees C or cooled to minus 40 degrees C for industrial applications. That’s a far cry from the researchers’ target operating temperature of minus 269 degrees C, the temperature at which helium becomes a liquid and the operating temperature of the rest of the device.

It’s not easy to design and operate a complex circuit at that temperature, Braga said, and there are no good models of how transistors behave at that temperature.

However, it is important for detector performance that the microchip operates as close as physically possible to the sensor. As the chip moves away from the sensor, the performance of the device decreases and it becomes more difficult to magnify the detector while retaining its desirable qualities.

The team at Bragas, Fermilab’s microelectronics group, specializes in the design of high-performance custom integrated circuits for extreme environments. He is currently developing a series of transistor designs needed to optimize the power and performance of this system.

Anything you do at minus 269 degrees C has to be ultra-low power, he explained. Having a good model helps minimize the amount of guesswork and testing needed to develop a reliable and optimized microchip. Making the chips is expensive, Braga said, so it’s important to have good models so you can be sure of the performance before starting production.

The model used by the team has paid off, the ASIC team is now testing several promising prototypes.

The challenge of cryogenic circuits

A few states away at MIT in Boston, electrical engineering professor Karl Berggren leads a team of graduate students in designing and building the circuit that will connect the Fermilab-designed cryoASIC chip to the Argonne lab’s sensor. But it’s not a simple plug-and-play; the MIT group will build the circuits from the same superconducting nanowire material used on the detector.

One of the challenges with this technology is that it’s very nascent, said Reed Foster, a graduate student working on system design. The researchers haven’t explained how exactly specific design factors affect the performance of the circuits we build with these nanowires, he said.

It means that every circuit they build must be carefully tested to ensure it works as intended.

Foster explained that they normally test circuits by building a smaller circuit nested into the overall design. The smallest circuit self-tests let the user know if the entire circuit is working properly. Right now, we’re not able to do that, Foster said. Instead, they must couple the superconducting cryogenic circuits to the ambient temperature circuits and use the ambient temperature circuits for self-testing.

These two devices are not easy to combine. While a huge advantage of superconducting circuits is the speed at which information travels through them, this process requires very fast room-temperature circuits, which are expensive and can usually only test a few circuits at a time. Their first prototype can test eight circuits simultaneously, Foster said, but he plans to scale it up to 16 or more. As they continue to work out how to integrate a self-test circuit into the final design, Fosters room temperature setting will continue to ensure the circuits are working properly.

Development of cryogenic sensors

Back in Illinois at Argonne National Laboratory, Armstrong leads the Argonne team in the development of the superconducting nanowire sensor. We just wrapped up some testing at Fermilab’s test beam facility, he said, we took the sensors that were manufactured here in Argonne, put them in a cryostat and essentially tested them with the 120 GeV proton beam to see we can get the signal.

The sensors fabricated by the Argonne researchers are silicon wafers inlaid with a maze of nanowires. Armstrong’s team tested the ability to detect nanowires of different sizes, ranging from 100 to 800 nanometers, nearly 1,000 times thinner than a strand of hair. While wider nanowires are easier to fabricate, they don’t necessarily have the same sensing capabilities as thinner nanowires.

The team is still analyzing the test results, but so far it looks promising, Armstrong said. We hope to optimize the sensor design for particle detectors at the Electron-Ion Collider, he said.

Now they can start to focus their efforts towards enhancing the sensors.

Eventually, these three components will be combined into a fully functional device capable of detecting particles at liquid helium temperatures.

I think there will be a large number of users of this technology in the future, Armstrong said.

Fermi National Accelerator Laboratory is supported by the US Department of Energy’s Office of Science. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information please visit science.energy.gov.