Earlier this year, a series of very carefully packed boxes left a shipping facility in Fort Collins, headed for Switzerland.
The precious cargo was 60 photon detectors, the result of many years of research and late nights in a Colorado State University physics lab. The detectors are now playing a key role in an international research experiment called ProtoDUNE, taking place at CERN, the Geneva-based European laboratory for particle physics.
ProtoDUNE is the lead-up to a massive international neutrino experiment called DUNE (Deep Underground Neutrino Experiment), hosted by the U.S. Department of Energy’s Fermilab. CSU researchers led by Norm Buchanan, associate professor of physics, are among 1,100 scientists from more than 160 institutions working on DUNE. Others at CSU include Professor Robert J. Wilson, who chairs the institutional board overseeing the DUNE collaboration, and Assistant Professor Michael Mooney, who works on simulations and calibrations of the time projection chamber technology used to capture neutrino-nucleus interactions.
The purpose of DUNE is to study neutrinos – elusive subatomic particles that scientists hunger to understand, because their odd behavior could explain once and for all the matter-antimatter imbalance of our universe. DUNE will involve a beam of neutrinos that starts at Fermilab in Illinois and smashes into a target 800 miles away in South Dakota.
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Preparing for DUNE
As its name suggests, ProtoDUNE is the critical testing and validation precursor to DUNE, which is scheduled to start taking neutrino data in approximately 2026. The CSU researchers are working on the first of two ProtoDUNE cryostats, which are like mini versions of the eventual DUNE neutrino detectors. “Mini” is a relative term; each ProtoDUNE cryostat is the size of a three-story house.
Over the last five years, Buchanan’s team led the mechanical design, testing and assembling of the ProtoDUNE photon detector modules. They were shipped to CERN over the summer and are now installed inside a single-phase liquid-argon time projection chamber. The chamber is a type of particle detector that uses extremely high-voltage electric fields in combination with the liquid argon to perform three-dimensional reconstructions of particle trajectories resulting from interactions in the liquid argon.
Senior engineer David Warner and senior technician Jay Jablonski, who work in Buchanan’s lab, traveled to CERN to help install the photon detectors, which hang suspended from rails inside the cryostat. Over the years, the CSU team worked with Fermilab, Indiana University, University of Northern Illinois, Caltech, Argonne National Laboratory, and MIT for research, development and testing of the photon detectors.
Just a few weeks ago, those years of work paid off for Buchanan, Warner, Jablonski, and the rest of the lab: the ProtoDUNE detector began recording its first particle tracks. Over the next several months, an international team of scientists will operate the detector, which is filled with 800 tons of extremely cold liquid argon, to test all the technologies inside it.
Neutrinos from supernovae
The photon detectors are key for determining the precise moment an interaction has occurred in the liquid argon. They identify individual particles of light – or photons – emitted from these interactions. Without this information, the scientists would have no way of accurately knowing when such an event occurred – only that it did. Applying this technology to DUNE, several years from now when its much-larger detectors start recording neutrino interactions, the scientists hope to gain new insights into how these mysterious particles behave.
“They’re working beautifully,” Buchanan said of the ProtoDUNE photon detectors. “The photon detector group is seeing great data. This is the culmination of many years of hard work.”
The photon detectors consist of light guides and silicon photomultipliers that are sensitive to even one particle of light. That level of sensitivity is essential for capturing data from neutrinos the researchers aren’t necessarily expecting – like those that rain down from exploding stars, or supernovae. The detectors will also be on the lookout for extremely rare, and as of yet undetected, events from the argon atoms, known as proton decay. “This is also really important for physics beyond the Standard Model,” Buchanan said.
‘The more we know’
Anne Christensen, a physics master’s student, has spent three years working on the ProtoDUNE photon detectors, including writing computer code for characterizing how the light guides were performing. She has enjoyed working on a project with international significance, and the potential to change physics as we know it. Now, she is continuing to contribute by developing electronic simulations for the photon detectors to be used in DUNE.
“It’s a huge experiment, so you get to meet a lot of people who know a lot about physics,” Christensen said. “Neutrinos are cool because they’re one of those small bits of matter we really don’t know much about. And the more we know, the more we can analyze stuff going on in the universe, in places we really can’t see otherwise.”
Neutrinos are among the most abundant subatomic particles in the universe, but scientists know little about them because they hardly ever interact with matter. Understanding how they behave, using miles-long, deep-underground particle detectors to observe them, could allow scientists to better understand the origins of the universe. In particular, they want to know why we see so much more matter – planets, people, stuff – than antimatter, and relatedly, why we exist at all.