A Detector for a Rare Phenomenon

Nicholas Scielzo

A small subset of postdocs comes to Lawrence Livermore National Laboratory as part of the Lawrence Fellowship Program. These individuals have the freedom to choose the projects they work on during their three‑year term. One such postdoc is Nicholas Scielzo, who received a Ph.D. in physics from UC Berkeley. At the Laboratory, he splits his time between three nuclear physics projects, one of which involves detecting an extremely rare radioactive process known as neutrinoless double-beta decay.

In standard double-beta decay, two neutrons in a nucleus are converted to two protons, emitting two beta particles and two neutrinos that share the energy generated from the decay. In neutrinoless decay, the neutrinos annihilate each other instead of being emitted, and the full energy—a little over 2 megaelectronvolts—is carried away by the beta particles. “However,” says Scielzo, “this decay can only occur if a neutrino and its antimatter, the antineutrino, are the same particle.” In a project funded by the Department of Energy’s Office of Science and LDRD, Scielzo is working with U.S. and Italian collaborators to build an extremely sensitive detector to identify this rare decay mode.

CUORE, the Cryogenic Underground Observatory for Rare Events, will be a 1-ton detector located within Italy’s Gran Sasso mountain group. The detector will contain an array of nearly 1,000 tellurium dioxide crystals, each a 5-centimeter cube. Tellurium-130 is one of the few isotopes that emit two neutrinos through doublebeta decay and thus could theoretically undergo the neutrinoless decay process. The crystals will be cooled to 0.01 kelvins above absolute zero using dilution refrigeration. “At this temperature, each crystal’s heat capacity is small enough that the energy from a single radioactive decay within the crystal will be detected,” says Scielzo. Sensitive thermometers outside the crystals will indicate a change in temperature, which the team will then use to calculate the decay energy.

As part of his research, Scielzo tests the raw materials used to make the crystals and the shielding for the detector. He also works with vendors to ensure that crystals meet the team’s strict specifications. “We look for the most radio-pure materials, those with little to no radioactive background,” says Scielzo. Excess radioactive decay would overshadow the unusual signal they are trying to detect. The detector is surrounded by 1,400 meters of rock overburden to protect it from cosmic muons. Radiopure shielding must be added to eliminate background radiation from the surrounding environment. Scielzo also applies his knowledge of particle physics to help CUORE researchers interpret the data from the device and develop new detection methods. “Currently, I am researching tellurium-120, which could also undergo the decay,” says Scielzo. CUORE must run for up to five years to collect enough data for accurate analysis. However, researchers on the project know that what they may find is well worth the wait. “The neutrinoless double-beta decay experiments at CUORE have the potential to reveal interesting properties of neutrinos that no other experiments have been able to show,” says Scielzo. In addition to proving that the neutrino and its antimatter are the same particle, these experiments could help identify the neutrino mass hierarchy and scale and provide details to explain why matter dominates over antimatter in the universe. “The experiments won’t tell us everything we want to know about the formation of our universe,” says Scielzo, “but they could provide one component of the larger explanation.”

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