Massive underground ‘ghost particle’ detector uncovers final secret in our sun’s fusion cycle

A hyper-sensitive instrument, deep underground in Italy, has finally succeeded in the nearly impossible task of detecting CNO neutrinos (tiny particles indicating the presence of carbon, nitrogen and oxygen) from our sun’s core. These little-known particles reveal the last missing detail in the fusion cycle powering our sun and other stars.

In the results published on November 26, 2020 in the journal Nature (and featured on the cover), investigators from the Borexino collaboration report the first detections of this rare type of neutrino, called “ghost particles” because they pass through most materials without leaving a trace.

The neutrinos were detected by the Borexino detector, a huge underground experiment in central Italy. The multinational project is supported in the United States by the National Science Foundation as part of a shared grant supervised by Frank Calaprice, professor of physics emeritus at Princeton; Andrea Pocar, 2003 graduate of Princeton and professor of physics at the University of Massachusetts-Amherst; and Bruce Vogelaar, professor of physics at Virginia Polytechnical Institute and State University (Virginia Tech).

The detection of “ghost particles” confirms predictions of the 1930s that part of our sun’s energy is generated by a chain of reactions involving carbon, nitrogen and oxygen (CNO). This reaction produces less than 1% of solar energy, but it is believed to be the main source of energy for large stars. This process releases two neutrinos – the lightest elementary particles of matter known – along with other subatomic particles and energy. The more abundant hydrogen-helium fusion process also releases neutrinos, but their spectral signatures are different, allowing scientists to tell them apart.

“The confirmation that CNO burns in our sun, where it operates at just 1%, strengthens our confidence in our understanding of how stars work,” said Calaprice, one of the initiators and principal researchers of Borexino.

CNO Neutrinos: Windows to the Sun

For much of their life, stars obtain energy by fusing hydrogen into helium. In stars like our sun, this happens mainly through proton-proton chains. However, in heavier and hotter stars, carbon and nitrogen catalyze the combustion of hydrogen and release CNO neutrinos. Finding neutrinos helps us scan the cogs deep inside the sun; when the Borexino detector discovered proton-proton neutrinos, the news shed light on the scientific world.

But CNO neutrinos not only confirm that the CNO process is at work in the sun, they can also help solve an important open question in stellar physics: to what extent the interior of the sun is made up of ‘metals’, that astrophysicists define as arbitrary elements. heavier than hydrogen or helium, and whether the “metallicity” of the core matches that of the sun’s surface or outer layers.

Unfortunately, neutrinos are extremely difficult to measure. More than 400 billion of them hit every square inch of the Earth’s surface every second, but virtually all of these “ghost particles” traverse the entire planet without interacting with anything, forcing scientists to use very large instruments. and very carefully protected to detect them. .

The Borexino detector is located 800 meters below the Apennine mountains in central Italy, at the Laboratori Nazionali del Gran Sasso (LNGS) of the Italian National Institute of Nuclear Physics, where a giant nylon balloon – about 30 feet in diameter – filled with 300 tons of ultra – pure liquid hydrocarbons are kept in a multilayer spherical chamber which is submerged in water. A tiny fraction of the neutrinos passing through the planet will bounce off electrons from these hydrocarbons, producing flashes of light that can be detected by photon sensors lining the water reservoir. The great depth, size and purity make Borexino a truly unique detector for this type of science.

The Borexino project was started in the early 1990s by a group of physicists led by Calaprice, Gianpaolo Bellini at the University of Milan and the late Raju Raghavan (then at Bell Labs). Over the past 30 years, researchers around the world have contributed to the discovery of the proton-proton chain of neutrinos, and about five years ago the team began the hunt for CNO neutrinos.

Background removal

“The last 30 years have been spent removing the radioactive background,” said Calaprice.

Most of the neutrinos detected by Borexino are proton-proton neutrinos, but a few are recognizable as CNO neutrinos. Unfortunately, CNO neutrinos look like particles produced by the radioactive decay of polonium-210, an isotope escaping from the gigantic nylon balloon. Separating the sun’s neutrinos from polonium contamination required a careful effort, led by scientists at Princeton, which began in 2014. Since the radiation could not be prevented from escaping the balloon, the scientists found a another solution: ignore the signals from the contaminated exterior. edge of the sphere and protect the deep interior of the ball. This required them to significantly slow down the rate of movement of the fluid in the balloon. Most of the fluid flows are driven by differences in heat, so the US team worked to get a very stable temperature profile for the tank and the hydrocarbons, to make the fluid as still as possible. The temperature was accurately mapped by a set of temperature probes installed by the Virginia Tech group, led by Vogelaar.

“If this movement could be reduced enough, then we could observe the expected five low-energy recoils per day that are due to CNO neutrinos,” Calaprice said. “For reference, a cubic foot of ‘fresh air’ – which is a thousand times less dense than hydrocarbon fluid – undergoes about 100,000 radioactive decays per day, mainly from radon.

To ensure fluid stillness, scientists and engineers at Princeton and Virginia Tech developed hardware to isolate the detector – essentially a giant blanket to wrap it around – in 2014 and 2015, then they added three circuits of heating that maintain a perfectly stable temperature. These were successful in controlling the temperature of the detector, but the seasonal temperature changes in Hall C, where Borexino is located, still caused tiny streams of fluid to persist, obscuring the CNO signal.

So two Princeton engineers, Antonio Di Ludovico and Lidio Pietrofaccia, worked with LNGS staff engineer Graziano Panella to create a special air handling system that maintains a stable air temperature in Hall C. Le Active Temperature Control System (ATCS), developed at the end of 2019, finally produced sufficient thermal stability outside and inside the balloon to calm the currents inside the detector, finally preventing the Contaminating isotopes from being transported from the walls of the balloon to the core of the detector.

The effort has paid off.

“Removal of this radioactive background created a Borexino low background region which made the measurement of CNO neutrinos possible,” said Calaprice.

“The data is getting better and better”

Prior to the discovery of CNO neutrinos, the lab had planned to shut down Borexino’s operations at the end of 2020. Now it looks like data collection could extend until 2021.

The volume of hydrocarbons still at the heart of the Borexino detector has grown steadily since February 2020, when the data from the Nature article was collected. This means that beyond the revelation of the CNO neutrinos that are the subject of this week’s Nature article, there is now potential to help solve the problem of “metallicity” too – the question of knowing. so the nucleus, the outer layers and the surface of the sun are all have the same concentration of elements heavier than helium or hydrogen.

“We continued to collect data as the core purity continued to improve, making a new result focused on metallicity a real possibility,” said Calaprice. “Not only are we still collecting data, but the data is getting better and better.”

To learn more about this research:

Reference: “Experimental evidence of neutrinos produced in the CNO fusion cycle in the Sun” by The Borexino Collaboration, November 25, 2020, Nature.
DOI: 10.1038 / s41586-020-2934-0

Other Princetonians in the Borexino team include Jay Benziger, professor emeritus of chemical and biological engineering, who designed the super-purified detector fluid; Cristiano Galbiati, professor of physics; Paul LaMarche, now vice-provost for programming and space planning, who was the original project manager of Borexino; XueFeng Ding, postdoctoral research associate in physics; and Andrea Ianni, project manager in physics.

Like many scientists and engineers from the Borexino collective, Vogelaar and Pocar started the project in Calaprice’s laboratory in Princeton. Vogelaar worked on the nylon balloon as a researcher and later assistant professor at Princeton, as well as calibration, detector monitoring, fluid dynamic modeling and thermal stabilization at Virginia Tech. Pocar worked on the design and construction of the nylon balloon and the commissioning of the fluid handling system at Princeton. He then worked with his students at UMass-Amherst on data analysis and techniques to characterize the history of CNO and other solar neutrino measurements.

This work was supported in the United States by the National Science Foundation, Princeton University, the University of Massachusetts and Virginia Tech. Borexino is an international collaboration also funded by the Italian National Institute of Nuclear Physics (INFN) and funding agencies in Germany, Russia and Poland.