Borexino detector catches neutrinos from the Sun’s CNO cycle

For most of their lives, stars are fueled by the fusion of hydrogen into helium. Fusion proceeds via two separate processes. One, known as the proton–proton (pp) chain, begins with the fusion of two protons in the solar plasma to form deuterium as an intermediate and is the source of 99% of the Sun’s energy. The other, known as the CNO cycle, comprises a set of reactions in which four hydrogen nuclei ultimately combine to form ⁴He with carbon, nitrogen, and oxygen as catalysts and intermediate products. The relative importance of the two mechanisms depends mostly on the stellar mass and on the metallicity—the abundance of elements in the core that are heavier than helium. Because the CNO cycle relies on those heavy elements, the flux of its neutrinos scales with the abundance of metals in the solar core. That scaling makes the flux an indicator of the chemical composition of the Sun at the time of its formation.

Neutrinos are exceedingly difficult to detect, though. Some 700 million CNO-cycle neutrinos pass through a square centimeter of Earth per second but with a mean free path of about a light-year through rocky matter. The Borexino detector, located 3 km under the Apennine mountains at Italy’s Gran Sasso National Laboratory, has now caught those CNO neutrinos for the first time. To see them, a collaboration of nearly 100 scientists looked for flashes of light produced by the scattering of neutrinos from electrons in 780 tons of petroleum-based scintillator. The researchers measured the count rate as just a handful ( ${7.0}_{-1.7}^{+3.0}$) per day per 100 tons of scintillator.

Key to the achievement was distinguishing CNO neutrinos from other solar neutrinos and low-energy radioactive contaminants whose decays create light flashes in the same energy window. The collaboration had already measured the flux of pp neutrinos in 2014 and spent years purifying the scintillator liquid (see Physics Today, November 2014, page 12 ). To hunt for the far less numerous CNO neutrinos, they embarked on another years-long campaign. Catching those few elusive particles would require the innermost volume of scintillator to remain as free of temperature fluctuations—and thus convection currents—as possible. To that end, in 2015 the collaboration wrapped the detector in a giant wool blanket, shown here, and installed a new air-conditioning system in the same room. Those advances reduced the drift of the worst background contaminant, bismuth–210, and kept it to a mere 20 cm per month—relatively far from the detector’s inner volume. The ²¹⁰Bi comes from the decay of lead–210, which is concentrated on the vessel wall that contains the scintillator.

Unfortunately, the newly measured flux of CNO neutrinos was not precise enough to resolve the abundance of “metals” in the Sun’s core. Previous helioseismology measurements of the Sun suggest a metal-rich core, whereas photoabsorption measurements suggest a metal-poor one. The collaboration is currently working to make yet another measurement before the Borexino experiment comes to an end—possibly sometime this year. (Borexino collaboration, Nature 587, 577, 2020 .)