The tiniest of kicks forms the subject of the latest Universities Research Association Early Career Award. The kick is the recoil of an electron from a collision with a very low-mass dark matter particle. The award recognizes Fermilab scientist Javier Tiffenberg, who drove the development of a new generation of charge-coupled devices, called CCDs, sensitive enough to detect those tiny electron kicks.
Silicon-based CCD chips have become commonplace in consumer devices such as digital cameras. The CCD chip is sectioned into tiny pixels, each of which records the light that hits it, analogous to the way film records the light that lands on it. Special scientific CCDs are employed as detectors in a variety of physics experiments and in telescopes to collect astronomical data.
Regular CCDs — even scientific CCDs — are not sensitive enough to detect electron recoils. A special kind of CCD that sensitive, called a “skipper” CCD, was proposed 30 years ago but its full potential for particle physics was not realized until Tiffenberg and his collaborators took on the challenge. He took advantage of a new a generation of sensors developed by Lawrence Berkeley National Laboratory-Microsystems Lab. These devices leverage improvements in CCD fabrication and technology developed over the past few decades.
“That is what allows this technique to look for much, much lighter dark matter candidates,” Tiffenberg said. “There is a whole new zoo of particles and theories that can be explored with this technology.”
The effort culminated in the first working skipper CCD instrument, called SENSEI, or Sub-Electron-Noise Skipper-CCD Experimental Instrument. A SENSEI prototype is currently deployed 350 feet underground at Fermilab and has achieved the best sensitivity in the world. Another instrument with far more target material is being constructed for deployment at the SNOLAB facility in Canada.
“There is a whole new zoo of particles and theories that can be explored with skipper CCD technology.”
The URA Early Career Award recognizes outstanding work by an early-career researcher at Fermilab or for work benefiting the Fermilab scientific program. It includes $5,000 from URA.
“Javier’s work is a prime example of the important role that early career scientists play in the critical research done at Fermilab and across the DOE national laboratory complex,” said URA Executive Director Marta Cehelsky. “His work also serves as an outstanding example of how fundamental breakthroughs in the tools of scientific experimentation push the frontiers of knowledge.”
Cosmologists are certain that dark matter exists because they can see its effects on ordinary matter. The mysterious substance reveals its gravitational presence, for example, in modifying the rotation of galaxies. Scientists have constructed many models of what it might be. But no one knows which model is correct, and no one has captured any likely candidate particles.
A number of technologies are being employed to detect the recoil of nuclei from a collision with a massive dark matter particle. Those dark matter particles, or weakly interacting massive particles, called WIMPs, form one class of the many candidates for what might constitute dark matter.
Skipper CCDs, by detecting electron recoils, allow Tiffenberg and his colleagues to explore the possibility that dark matter might be made up of particles that are lighter than what could be captured by the previous generation of detectors with their recoils from much heavier nuclei.
“There wasn’t one single thing that really was like the silver bullet,” said Tiffenberg. “It was many small things that needed to be improved in the technological implementation of the skipper CCD sensors, mostly related to being able to make multiple measurements nondestructively and independent from each other. And then the big challenge was to understand what was the way to operate these things. We need to understand the internal workings of these things in a detailed way.”
Detect, read, repeat
Skipper CCDs stand out for their pixels’ ability to be sampled more than once, which enables more precise measurements. In a typical scientific CCD, reading how much energy was deposited by a passing particle wipes away that energy.
“With a skipper CCD, we are able to take multiple readouts of this energy that was deposited in one pixel in a nondestructive way,” explained Tiffenberg. “So we can make as many nondestructive measurements as we want.”
The recoil of an electron in a skipper CCD translates to ionization, which is quantized, because you can only have an integer number of electrons. By making multiple, independent readouts of the energy deposited in a pixel of a skipper CCD, Tiffenberg’s team can reduce the noise in the readout to as close to zero as they like.
“When you look at data from these detectors, it’s really beautiful.”
“When you look at data from these detectors, it’s really beautiful,” because you can see the number of electrons without any ambiguities, said Tiffenberg. “One electron, two electrons, three, four, it’s perfectly quantized.”
Not only did Tiffenberg drive the development of a working skipper CCD instrument, but he developed a program for how these ultrasensitive CCDs can be employed in other fields of study as well.
“Javier didn’t just look at the cosmic frontier, he saw the skipper could be used in neutrino physics, nuclear reactor monitoring and quantum imaging,” said Juliana Whitmore, Fermilab scientist and URA Early Career Award Committee co-chair.
Skipper CCD instruments, for example, could detect the neutrinos emitted from a nuclear reactor and potentially determine whether the reactor was producing weapons-grade plutonium. Such instruments also might reveal new information about light neutrinos that have never been seen before, or neutrino oscillations. Tiffenberg’s achievements already have spawned new research at Fermilab. Alex Drlica-Wagner is leading Fermilab’s effort to develop astronomical applications of skipper CCDs. Applications in quantum science are being investigated by Fermilab’s Juan Estrada.
Fermilab is managed by the Fermi Research Alliance LLC for the U.S. Department of Energy Office of Science. FRA is a partnership of the University of Chicago and Universities Research Association Inc.
Fermilab dark matter and particle detector technology research is supported by the DOE Office of Science.
Fermilab is supported by the Office of Science of the U.S. Department of Energy. 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, visit science.energy.gov.