Massive Underground “Ghost Particle” Detector Finds Final Secret of Our Sun’s Fusion Cycle

The Borexino detector, a hyper-sensitive instrument deep underground in Italy, has finally succeeded at the nearly impossible task of detecting CNO neutrinos from our sun’s core. These little-known particles reveal the last missing detail of the fusion cycle powering our sun and other stars, and could answer still-outstanding questions about the sun’s composition. Credit: Borexino Collaboration

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

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

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

The “ghost particle” detection confirms predictions from the 1930s that some 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 the sun’s energy, but it is thought to be the primary energy source in larger stars. This process releases two neutrinos — the lightest known elementary particles of matter — as well as other subatomic particles and energy. The more abundant process for hydrogen-to-helium fusion also releases neutrinos, but their spectral signatures are different, allowing scientists to distinguish between them.

“Confirmation of CNO burning in our sun, where it operates at only a 1% level, reinforces our confidence that we understand how stars work,” said Calaprice, one of the originators of and principal investigators for Borexino.

CNO neutrinos: Windows into the sun

For much of their life, stars get energy by fusing hydrogen into helium. In stars like our sun, this predominantly happens through proton-proton chains. However, in heavier and hotter stars, carbon and nitrogen catalyze hydrogen burning and release CNO neutrinos. Finding any neutrinos helps us peer into the workings deep inside the sun’s interior; when the Borexino detector discovered proton-proton neutrinos, the news lit up the scientific world.

But CNO neutrinos not only confirm that the CNO process is at work within the sun, they can also help resolve an important open question in stellar physics: how much of the sun’s interior is made up of “metals,” which astrophysicists define as any 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 exceedingly difficult to measure. More than 400 billion of them hit every square inch of the Earth’s surface every second, yet virtually all of these “ghost particles” pass through the entire planet without interacting with anything, forcing scientists to utilize very large and very carefully protected instruments to detect them.

The Borexino detector lies half a mile beneath the Apennine Mountains in central Italy, at the Laboratori Nazionali del Gran Sasso (LNGS) of Italy’s National Institute for Nuclear Physics, where a giant nylon balloon — some 30 feet across — filled with 300 tons of ultra-pure liquid hydrocarbons is held in a multi-layer spherical chamber that is immersed in water. A tiny fraction of the neutrinos that pass through the planet will bounce off electrons in these hydrocarbons, producing flashes of light that can be detected by photon sensors lining the water tank. The great depth, size and purity makes Borexino a truly unique detector for this type of science.

The Borexino project was initiated 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 finding the proton-proton chain of neutrinos and, about five years ago, the team started the hunt for the CNO neutrinos.

Suppressing the background

“The past 30 years have been about suppressing the radioactive background,” Calaprice said.

Most of the neutrinos detected by Borexino are proton-proton neutrinos, but a few are recognizably CNO neutrinos. Unfortunately, CNO neutrinos resemble particles produced by the radioactive decay of polonium-210, an isotope leaking from the gigantic nylon balloon. Separating the sun’s neutrinos from the polonium contamination required a painstaking effort, led by Princeton scientists, that began in 2014. Since the radiation couldn’t be prevented from leaking out of the balloon, the scientists found another solution: ignore signals from the contaminated outer edge of the sphere and protect the deep interior of the balloon. That required them to dramatically slow the rate of fluid movement within the balloon. Most fluid flow is driven by heat differences, so the U.S. team worked to achieve a very stable temperature profile for the tank and hydrocarbons, to make the fluid as still as possible. The temperature was precisely mapped by an array of temperature probes installed by the Virginia Tech group, led by Vogelaar.

“If this motion could be reduced enough, we could then observe the expected five or so 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 the hydrocarbon fluid — experiences about 100,000 radioactive decays per day, mostly from radon gas.”

To ensure stillness within the fluid, Princeton and Virginia Tech scientists and engineers developed hardware to insulate the detector — essentially a giant blanket to wrap around it — in 2014 and 2015, then they added three heating circuits that maintain a perfectly stable temperature. Those succeeded in controlling the temperature of the detector, but seasonal temperature changes in Hall C, where Borexino is located, still caused tiny fluid currents 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. The Active Temperature Control System (ATCS), developed at the end of 2019, finally produced enough thermal stability outside and inside the balloon to quiet the currents inside the detector, finally keeping the contaminating isotopes from being carried from the balloon walls into the detector’s core.

The effort paid off.

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

“The data is getting better and better”

Before the CNO neutrino discovery, the lab had planned to end Borexino operations at the close of 2020. Now, it appears that data gathering could extend into 2021.

The volume of still hydrocarbons at the heart of the Borexino detector has continued to grow in size since February 2020, when the data for the Nature paper was collected. That means that, beyond revealing the CNO neutrinos that are the subject of this week’s Nature article, there is now a potential to help resolve the “metallicity” problem as well — the question of whether the core, outer layers and surface of the sun all have the same concentration of elements heavier than helium or hydrogen.

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

For more on this research:

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

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

Like many of the scientists and engineers in the Borexino collective, Vogelaar and Pocar got their start on the project while in Calaprice’s lab at Princeton. Vogelaar worked on the nylon balloon while a researcher and then assistant professor at Princeton, and the calibration, detector monitoring, and 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 later worked with his students at UMass-Amherst on data analysis and techniques to characterize the backgrounds for the CNO and other solar neutrino measurement.

This work was supported in the U.S. 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 for Nuclear Physics (INFN), and funding agencies in Germany, Russia and Poland.

AstrophysicsNeutrinosParticle PhysicsPopularPrinceton UniversitySun
Comments ( 17 )
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  • FredTex

    You say they have discovered the final secrets of the Suns subatomic particles. I’ve got news for you, you have yet to discover the sum-neutriono particles.

    • Torbjörn Larsson

      They had predicted that the CNO cycle would stand for 1 % of the energy in Sun (and so potentially much more in more massive stars), and that is what they found.

      They use neutrino-electron scattering, which AFAIU is sensitive only to left-handed neutrinos of the Standard Model but doesn’t distinguish between them. Therefore it captures all the 3 generations of neutrinos (electron, muon and tau).

  • Dennis Zamudio Flores

    How come an instrument beneath under the mountain describe the nuclear production of sun?
    It is apparent that these personalities doesn’t knows exactly how a sun, an offspring of galaxy works. If someone is really serious in knowing the nuclear production of sun or galaxy try to read the books 1.ALL BUT THE WORLD IS LOVING 2.THE THEORY OF EVERYTHING AND THE MAN WHO COULD SAVE THE EARTH all but the world is loving 2 ISBN 9781482855388

    • Torbjörn Larsson

      If you opened any encyclopedia (or scientific work) on neutrinos you would know that they rarely interact with other particles. Therefore they can be captured beneath rock, ice or water – which is preferred since it lowers the confusing “noise” from cosmic ray interaction.

      Science is a peer review process, so pointing to anonymous – and from your description pseudoscientific – books aren’t useful but a distraction for people that are curious about the world.

  • Skitter!

    I can do it ghost 👻

  • rassalas

    Why didn’t they just use something other than Nylon? I presume there is a reason, but the article lacking an explanation for the obvious is poor journalism.

    • Torbjörn Larsson

      Publishing a university press release is not journalism as such – apart from vetting the source and providing the service to interested readers – and the office workers at universities press offices are of varying standards. Caveat emptor – better read the paper if it is an interesting topic (but then also check the journal, scientists et cetera for yourself – lots of pseudoscience vanity press ‘papers’ out there).

      They worked quite a lot on Borexino. From “The Borexino detector at the Laboratori Nazionali del Gran Sasso”, arxiv, 2008:

      “Within this sphere, two nylon vessels separate the scintillator volume in three shells of radii 4.25 m, 5.50 m and 6.85 m, the latter being the radius of the SSS itself. The inner nylon vessel (IV) contains the liquid scintillator solution, namely PC (pseudocumene, 1,2,4-trimethylbenzene C6H3(CH3)3) as a solvent and the fluor PPO (2,5-diphenyloxazole, C15H11NO) as a solute at a concentration of 1.5 g/l (0.17 % by weight). The second and the third shell contain PC with a small amount (5 g/l) of DMP (dimethylphthalate, C6H4(COOCH3)2) that is added as a light quencher in order to further reduce the scintillation yield of pure PC (8).”

      “The Inner Vessel is made of 125 µm thick Nylon6 carefully selected and handled in order to achieve maximum radiopurity (11). Since the PC/PPO solution is slightly lighter (about 0.4 %) than the PC/DMP solution, the Inner Vessel is anchored to the bottom (south pole of the SSS) with a set of nylon strings. The outer nylon vessel (OV) has a diameter of 11 m and is built with the same material as the inner one. The OV is a barrier that prevents 222Rn emanated from the external materials (steel, glass, photomultiplier materials) to diffuse into the fiducial volume.”

      “The buffer fluid between the Inner Nylon Vessel and the SSS (PC/DMP solution) is the last shielding against external backgrounds. The use of PC as a buffer is convenient because it matches both the density and the refractive index of the scintillator, thus reducing the buoyancy force for the nylon vessel and avoiding optics aberrations that would spoil the spatial resolution.”

  • Amye Keith

    To the authors of the snarky know it all comments…. When was the last time ANY work ,project theory, or scientific discovery of YOURS published or made news? If you had all of the answers then you are late for work fixing the entire universe.

  • Tim Kliesch

    Out of curiosity…why is a Borexino detector used to find Neutrinos? Isn’t that like saying a Gold detector is used to find Silver? (No disrespect intended)

    • Torbjörn Larsson

      That is a good question! It seems to be special designed to capture solar neutrinos – and incidentally geoneutrinos – and I guess the deep underground placement makes a smaller detector (with e.g. IceCube much larger) more economical.

      From “The Borexino detector at the Laboratori Nazionali del Gran Sasso”, arxiv, 2008:

      “Borexino is a large volume liquid scintillator detector whose primary purpose is the real-time measurement of low energy solar neutrinos. It is located deep underground (‘ 3800 meters of water equivalent, m w.e.) in the Hall C of the Laboratori Nazionali del Gran Sasso (Italy), where the muon flux is suppressed by a factor of ≈ 10^6.

      The main goal of the experiment is the detection of the monochromatic neutrinos that are emitted in the electron capture decay of 7Be in the Sun (1).”

      “Besides solar physics, the unprecedented characteristics of its apparatus make Borexino very competitive in the detection of anti-neutrinos (¯ν), particularly those of geophysical origin. The physics goals of the experiment also include the detection of a nearby supernova, the measurement of the neutrino magnetic moment by means of a powerful neutrino source, and the search for very rare events like the electron decay (4) or the nucleon decay into invisible channels (5).”

      • Torbjörn Larsson

        Re geoneutrinos they help establish how much of mantle heat flux is core primordial and how much is radiogenic. Wikipedia [“Geoneutrino”] refers to Borexino:

        “Analysts from the Borexino collaboration have been able to get to 53 events of neutrinos originating from the interior of the Earth.[1]”

        “As of 2016 geoneutrino measurements at two sites, as reported by the KamLAND and Borexino collaborations, have begun to place constraints on the amount of radiogenic heating in the Earth’s interior. A third detector (SNO+) is expected to start collecting data in 2017. JUNO experiment is under construction in Southern China. Another geoneutrino detecting experiment is planned at the China Jinping Underground Laboratory.”

        It is exciting to me, since it have implications on Earth formation and early vs late plate tectonics et cetera.

        [FWIW: The fashionable geodynamo speculations may be a dud. Recent thinking of astrophysicists is that observations may say it helps atmosphere vanish from Earth by concentrating the solar wind to the polar regions. From Astrobiology:

        “Surprisingly Little Water Has Escaped from Venus …

        “In my thesis I have calculated how much water has escaped from Venus in the past. I have looked at how the ion escape is affected by the solar wind variations today and how the solar wind has changed over time,” says Moa Persson.

        The results of the thesis can be compared to similar studies of Mars and Earth. The comparisons between the three sibling planets give a more comprehensive picture of the solar wind effects on planetary atmospheres. For example Earth, with its strong magnetic field, has a larger loss of atmosphere to space than both Venus and Mars.”

        There is references in Persson’s thesis. Also, gratuitous plug for Swedish science!]

  • Michael Vreeland

    Thank you very much for your additional comment descriptions beyond CNO confirmation. It’s encouraging to hear from those sharing knowledge of the why with the thirsty.

  • Don Bodine

    What about smoke lite shows up well in smokevery we’ll. Solo, liquid, gas, of some sort may help detect these super fast particals.

    • Torbjörn Larsson

      Early particle detectors were designed in that way – cloud chambers that showed traces in mists, spark chambers that showed traces by making them pass sparks, bubble chambers that showed traces by making them nucleate bubbles [“Particle detector” @ Wikipedia] – but they were superseded by modern electronics. In most particle accelerators they look at rapidly repeating events with many traces. Here they look at faint signals. But of course YMMV.

      • Torbjörn Larsson

        Those old detectors were exclusively for ionizing particles though. And I meant to say that cloud chambers showed traces by nucleating mists (in supersaturated vapors), analogous to how the bubble chambers worked.

  • Sekar Vedaraman

    Very Interesting. New area for me.

    Some out of the box thoughtfor considearion.

    1. Solid is more stablethan Liquid andGas and using solids as detectors could be considered? Maybe Solid state Physics experts and Materialss cientists can come up with the engineering to create such detectors and whether CNO Neutrinos or Proton-Proton Neutrinos?

    2. Cannot believe 400 billion”Ghost Partcles” hitting the planet cannot be detected.Figure out ways for neutrinos toointeract and become detectable. On a Gross level we as a species are 7 bilion going to 10 billion!

    3. Hope the British space ship headed for the sun canhelp providedata for a better unferstanding of the Sun and fusion going on before we try tocreate mini suns on theplanet. Fusuin saety controls to control chain reactionsare always a worry if they cannot be stopped or controlled.

    These are topof the mind ideas. Hope some of this is useful. Theory alwas should preecede practise. Re-look at current theories ?

  • xABBAAA

    … ∉ ♠ …