A mile or so beneath the surface of a repurposed South Dakota gold mine, scientists have shown they can successfully shield a detector from background radioactivity, opening the door to deeper discoveries about the nature of the universe.
Today in Physical Review Letters, collaborators with the MAJORANA DEMONSTRATOR have published data from the construction, commissioning, and full operation of an array comprising 44 kilograms of germanium detectors. Their goal is to detect neutrinoless double-beta decay, the observation of which would indicate that neutrinos are Majorana particles, meaning neutrinos can be their own antiparticles. Understanding this matter/anti-matter phenomenon is of a piece with understanding how the universe gave the advantage to matter over anti-matter after the Big Bang. In the results published today, the scientists working with the MAJORANA DEMONSTRATOR have met a milestone by showing the detector array can be shielded to block any background that might mimic the signal they’re looking for.
Physics Professor Yuri Efremenko explained that the UT Neutrino Group has been involved in the MAJORANO experiment from the very beginning. Their responsibilities were to supervise selection of extremely low radioactive materials for detector components and the construction of the active muon veto system, which is crucial for reducing background signals. Efremenko was the principal investigator for both activities.
“Both tasks were imperative to achieve the goal of the MAJORANA DEMONSTRATOR: to prove that we can build an extremely low background double beta decay experiment,” he said.
The active veto system consists of 32 large plastic scintillator panels with the total active area of 400 square feet. All veto panels were built, calibrated and tested on the UT campus in the clean room at the Science and Engineering Research Facility (SERF). Former UT postdoc Sergey Vasiliev played a decisive role in the construction of veto system. After construction, all panels were vacuum packed in the clean room and delivered to the mine in South Dakota by former physics graduate student Brandon White (PhD, 2012). Professor Emeritus Bill Bugg took an early role in the design of these panels. Efremenko explained that all panels were deployed at the experiment and have been working flawlessly ever since.
Current graduate student Andrew Lopez took an active part in the commissioning of panels on site, doing data quality testing and actively analyzing data. The importance of the active muon veto system on the MAJORANA experiment and recommendations for future experiments will be the subject of his PhD thesis. Efremenko also acknowledged the important contributions to veto system construction by craftsmen from the physics department machine shop: Rick Huffstetler, Joshua Bell, and Alvin Peak.
The first searches for neutrinoless double-beta decay date back to 1948 and several experiments have continued that work in the years since. Observing the process is a national physics priority, not only because of what it could reveal about the origins of matter but also because of what it might tell scientists about neutrinos themselves. These ghostly, neutral particles are legion, with trillions passing through our bodies every day. Born in the core of the sun and nuclear reactions on earth, they travel just shy of the speed of light and help physicists understand more about subatomic particles and the Standard Model that governs them. That’s a gold mine all in itself.