When we consider the origins of life in our Solar System, a remarkable discovery has to be taken into account –the Solar System is substantially over-abundant in metals compared with average interstellar abundances at the time of its formation 4.6 billion years ago. These solar abundances are similar to present interstellar abundances, an anomaly that remains a mystery. One possibility scientists suggest is that the Sun formed much closer to the galactic center than its current position, which may have resulted in a plentiful supply of raw materials in the solar nebula from which to form the Earth and its biosphere.
Our Unusual ‘Metal-Rich’ Solar System
Our Solar System, reports A & G Views, “could be one of relatively few in the Milky Way known to have so far produced an evolved lifeform due scientists believe to availability of heavy elements needed to form planets and life “as we know it” –to the extent that it originates on an Earth-like planet and is carbon-based (or at least composed of chemical elements heavier than helium).
“It is well known that the elements essential to life, together with the silicon and other metals needed to make terrestrial planets, were synthesized from primordial hydrogen and helium by nuclear processes in stars. The earliest populations of stars to form in our Galaxy must have lacked these elements. Some heavy elements will have been injected into the interstellar medium by the first generations of massive stars, which become supernovae on timescales very short compared with the age of the Galaxy, and subsequent generations thus became progressively metal-enriched.”
The bulk of the heavy elements, and notably much of the carbon, come primarily from nuclear processes in stars of no more than a few solar masses, which evolve relatively slowly and release their products in red-giant winds and planetary nebulae rather than in supernova events.
Ghostly Subatomic Particles –Zero Charge, Zero Radius, and Possibly Zero Mass
Which brings us to our Sun –a hyper-sensitive instrument– the Borexino detector, an enormous deep underground experiment in central 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, reports Princeton University. 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.
Neutrinos give us a more complete understanding of the universe, and important new insights into the most powerful objects and events in the sky.
The Borexino Collaboration
The investigators of the Borexino collaboration –detecting the highest energy neutrinos requires a massive particle detector–reported the first detections of this rare type of neutrinos, called “ghost particles” because they pass through most matter without leaving a trace. These ghostly subatomic particles travel to Earth unhindered for billions of light years from the most extreme environments in the universe. They have zero charge, zero radius, and very possibly zero mass. These little-known particles reveal the last missing detail of the fusion cycle powering our sun and other stars.
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 –“Final Secret of Fusion”
“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.
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.
Resolves How Much of the Sun’s Interior is Made up of “Metals”
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.
A Truly Unique Detector
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 (shown at top of page) 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 Princeton physicist Frank 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.
“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 Bruce 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.”
“A Giant Blanket” –Stymied Contaminating Isotopes
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.
Princeton engineers worked with LNGS staff 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 elimination of this radioactive background created a low background region of Borexino that made the measurement of CNO neutrinos possible,” Calaprice said.
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.
Resolving the Metallicity Problem
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 the 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.
Like many of the scientists and engineers in the Borexino collective, reports Princeton, Vogelaar and Andrea 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.
Source: “Experimental evidence of neutrinos produced in the CNO fusion cycle in the Sun,” by the Borexino Collaboration, appeared in the Nov. 25 issue of Nature (DOI: 10.1038/s41586-020-2934-0).