To correct a mistake, nuclear physicists improve the accuracy of neutrino studies

Led by the Department of Energy’s Oak Ridge National Laboratory, a new study resolves a discrepancy regarding the largest contribution of unwanted background signals in specialized neutrino detectors. Better characterization of the background could improve current and future experiments to detect real signals from these weakly interacting, electrically neutral subatomic particles and understand their role in the universe.

“We identified a response with significant differences between our new measurement and the historical data,” said Michael Febbraro of ORNL, lead author of a study published in Physical Review Letters this gives an improved measure of the response. “It’s one of the oldest responses ever studied, and we’re still discovering new things about it.”

An older measurement from 2005, which had been used as a reference standard, was incorrectly analyzed. It considered only the ground state of particles rather than a spectrum of ground and excited states. The new measurement, performed using a detector array based on neutron spectroscopy and secondary gamma rays, took into account the entire spectrum of particle energies.

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

These nuclear physicists had no intention of studying neutrino properties; they are mostly concerned with atomic nuclei and their interactions. But in science, discoveries in one area often have profound implications for other areas.

A well-known nuclear reaction turns carbon-13 into oxygen-16 and a neutron. That same reaction is a major contributor to the background of experiments measuring neutrinos, whether emitted from the sun, the atmosphere, accelerators, nuclear reactors or the core of the Earth.

The speed of that reaction must be well known to accurately calculate the background in detectors such as Japan’s Kamioka liquid scintillator anti-neutrino detector or KamLAND. Using a University of Notre Dame accelerator, the researchers fired an alpha particle (ie, helium-4 core) at a target of carbon-13, briefly forming oxygen-17, which decayed into oxygen-16 and a neutron. The researchers measured the “cross-sectional area,” or the probability of a reaction occurring, which is proportional to the rate of neutron production.

“We found that the current world data collection is quite inaccurate, because they did not take into account other response channels that are catching on,” Febbraro said. “We have a special type of detector that can tell what the neutron energy is, and that was the main technology that made this measurement possible.”

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

KamLAND weighs about a thousand tons. So, while carbon-13 makes up only 1.1% of all carbon, KamLAND contains 10 tons of it. Radon entering the detector decays into daughter elements of different energies. The alpha particles produced by that decay interact with carbon-13, creating a background that overwhelms the neutrino signal. “It is the main source of background information for these experiments,” Febbraro said.

The earlier reference measurement of the reaction only had nuclei measured at the lowest energy level or ground state. But nuclei also live at higher energy levels called excited states. Different energy levels affect the likelihood that a response will follow a specific path.

“We have significantly improved the precision and accuracy of measurements by using a set-up that is sensitive to a spectrum of neutron energies,” said Febbraro.

The global scientific community uses evaluated nuclear databases of expert-generated, peer-reviewed reference measurements. To estimate KamLAND’s background, KamLAND physicists pulled the 2005 reference measurement generated by nuclear physicists from one of these databases, the Japanese Evaluated Nuclear Data Library. They assumed the measurement was correct and used it in their calculations.

“The assumption that the excited states don’t matter is untrue,” Febbraro said. “Including the excited states, not only changes the size of the background it causes in KamLAND, but it also affects multiple aspects of the neutrino signal.”

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

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

To ask a large scintillator-filled neutrino detector to distinguish the background of the signal is like being blindfolded, given chocolates with a red or green candy coating, and asked how many red chocolates you ate.

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

Just as knowing this ratio would allow you to estimate the quantities of candy, so the reference information in evaluated nuclear databases allows scientists to estimate the number of neutrinos.

“It turns out that our experiment got a different response than what the ‘candy maker’ said the ratio should be,” Chipps continued. “This is not because the manufacturer wanted to give a wrong answer, but because their sorting machine was programmed with the wrong value.”

The new neutron production that Febbraro and his colleagues in nuclear physics found can now be used by the physicists working on KamLAND and other liquid scintillator-based neutrino experiments to more accurately and accurately subtract the background.

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

The title of it Physical Review Letters paper is “New ¹³C (α, n)¹⁶O Cross-section with implications for neutrino mixing and geoneutrin measurements. The development of the detector was supported by the DOE Office of Science. The measurement was conducted at the University of Notre Dame Nuclear Science Laboratory, which is supported by the National Science Foundation.