Leading a Wrong, Nuclear Physicist Improve Neutrino Search Accuracy

At the University of Notre Dame, part of the Oak Ridge Deuterated Spectroscopic Array measured a reaction that causes noise in some neutrino detectors. Credit: Michael Febbraro / ORNL, US Department of Energy

Led by the Department of Energy’s Oak Ridge National Laboratory, a new study clears a discrepancy regarding the largest contributor of unwanted background signals to specialized neutrino detectors. Better background characterization can improve current and future experiments to detect real signals from these subatomic particles with poor, neutral interaction and to understand their role in the universe.

“We have identified a reaction with significant discrepancies between our new measurement and historical data,” said Michael Febbraro of ORNL, lead author of a study published in Physical review letters which represents an improved reaction measurement. “Onesht is one of the oldest feedback ever studied, and we’re still discovering new things about it.”

An old measurement from 2005, which was used as a reference standard, was incorrectly analyzed. He considered only the basic state of particles rather than a spectrum of earthly and excited states. The new measurement, obtained using a detector set based on neutron spectroscopy and gamma secondary rays, considered the entire spectrum of particle energies.

February, who conceived the experiment and built the detectors, made the measurement with Richard deBoer of the University of Notre Dame and Steven Pain of the ORNL. Other co-authors represent the University of Surrey; University of Michigan, Ann Arbor; University of Tennessee, Knoxville; and Rutgers University.

These nuclear physicists were not put to study the properties of neutrinos; they usually deal with atomic nuclei and their interactions. But in science, discoveries in one area often have profound implications for other areas.

A known nuclear reaction converts carbon-13 to oxygen-16 and a neutron. The same reaction is a major contributor to the background of experiments measuring neutrinos, whether they are emitted by the sun, the atmosphere, accelerators, nuclear reactors, or the Earth’s nucleus.

The rate of this reaction should be well known to accurately calculate the background in detectors such as the anti-neutrino detector of Japan Kamioka liquid scintillator, or KamLAND. Using an accelerator from the University of Notre Dame, the researchers shot an alpha particle (i.e., helium-4 nucleus) at a carbon-13 target, briefly forming oxygen-17, which dissolved into oxygen-16 and a neutron. . The researchers measured the “cross section”, or probability of a reaction that will occur, which is proportional to the speed of neutron production.

“We found that the current data set in the world is a bit inaccurate because they did not account for the other feedback channels that turn on,” Febbraro said. “We have a special kind of detector that can show what neutron energy is, and it was the key technology that enabled that made this measurement possible.”

Neutrino detectors need to be large to amplify weak signals. KamLAND is packed with a hydrocarbon-based scintillator, an oil that interacts with neutrinos and emits light. Those glows make it easier to distinguish and count elusive neutrinos. However, the products of radon decay, a naturally occurring radioactive gas, combine with carbon-13, a rare isotope of carbon present in the scintillator, to create oxygen-16 and neutrons that mimic signals from neutrinos.

KamLAND weighs approximately one thousand tons. So while carbon-13 makes up only 1.1% of all carbon, KamLAND contains 10 tonnes of it. The radon that enters the detector breaks down into girl elements that have different energies. The alpha particles produced by those decays interact with carbon-13, creating a background that overloads the neutrino signal. “It’s the main source of background in these experiments,” Febbraro said.

Preliminary reference measurement of the reaction had measured the nuclei only at the lowest energy level, or state of the earth. But nuclei also live at higher energy levels, called excited states. Different energy levels affect the likelihood that a reaction will take a specific path.

“We greatly improved the accuracy and accuracy of measurements using a configuration that is sensitive to a spectrum of neutron energies, “Febbraro said.

The global scientific community uses uses evaluated nuclear databases containing reference measurements generated by experts, evaluated by colleagues. To assess KamLAND’s history, KamLAND physicists drew the 2005 reference measurement generated by nuclear physicists from one of these databases, the Japanese Nuclear Database Library. They assumed the measurement was accurate and entered it into their calculations.

“The assumption that excited states do not matter is not true,” Febbraro said. “The inclusion of excited states not only changes the background size it causes in KamLAND, but also affects multiple aspects of the neutrino signal.”

ORNL physicist Kelly Chipps, who helped analyze the data and interpret the results with her ORNL colleague Michael Smith, agreed.

“The background is something you need to understand exactly,” she said. “Otherwise, the number of real events you have seen may be completely wrong.”

Looking for a large detector, filled with scintillator, to distinguish the background from the signal, is like blindfolding, feeding chocolates or a layer of red or green candy, and asking to tell how much dark chocolate you have eaten .

“The problem is, all sweets taste the same,” Chipps said. “To find out how many red cakes you ate, you would count the total number of cakes and call the chocolate maker to ask how many red cakes are usually in a bag.”

Just as knowing this report would let you make an estimate about the quantities of sweets, the reference information in the estimated nuclear databases allows scientists to estimate neutrino numbers.

“It turns out that our experiment received a different response from what the ‘confectioner’ said the report should have been,” Chipps continued. “This is not because the manufacturer intended to give a wrong answer; this is because their sorting machine was programmed with an incorrect value. ”

The new neutron production rate found by Febbraro and his nuclear physics colleagues can now be used by physicists working at KamLAND and other liquid scintillator-based experiments to lower the background with better accuracy and precision.

Since this new measurement, the Febbraro team has used the special detector to measure similar reactions. They have found inconsistencies in neutron production rates for half a dozen isotopes. “Calculations in this massive region are not very reliable,” he said.

Reference: “E re 13C (α, n)16Cross section with implications for neutrino mixing and geoneutrino measurements ”by M. Febbraro, RJ deBoer, SD Pain, R. Toomey, FD Becchetti, A. Boeltzig, Y. Chen, KA Chipps, M. Couder, KL Jones, E Lamere, Q. Liu, S. Lyons, KT Macon, L. Morales, WA Peters, D. Robertson, BC Rasco, K. Smith, C. Seymour, G. Seymour, MS Smith, E. Stech, B. Vande Kolk and M. Wiescher, 7 August 2020, Physical review letters.
DOI: 10.1103 / PhysRevLett.125.062501

The development of detectors was supported by the DOE Science Office. The measurement was made at the University of Notre Dame Nuclear Science Laboratory, which is supported by the National Science Foundation.

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