Borexino – one of the world’s most sensitive neutrino detectors on our planet – has helped scientists create the first ‘snapshot’ of the complete spectrum of neutrinos emitted by the Sun.
Sun’s energy emitted as neutrinos is produced through nuclear reaction sequences initiated by proton-proton (pp) fusion in which hydrogen is converted into helium. According to physicists at he University of Massachusetts Amherst, the neutrinos emitted by the Sun are a unique tool to understand solar and neutrino physics.
These components include not only the pp neutrinos, but others called Beryllium-7 (7Be), pep and Boron-8 (8B) neutrinos. The pp fusion reaction of two protons to produce deuteron, nuclei of deuterium, is the first step of a reaction sequence responsible for about 99 percent of the Sun’s energy output.
For earlier studies of pp, 7B, pep and 8B neutrinos, the team had focused on each one separately in targeted analyses of the collected data in restricted windows of energy, “like trying to characterize a forest by taking one picture each of many individual types of trees,” notes one of the authors of the study. “Multiple pictures give you an idea of a forest, but it’s not the same as the photo of the entire forest.”
Solar neutrinos stream out of the star at the center of our system at nearly the speed of light, as many as 420 billion hitting every square inch of the earth’s surface per second. But because they only interact through the nuclear weak force, they pass through matter virtually unaffected, which makes them very difficult to detect and distinguish from trace nuclear decays of ordinary materials.
The Borexino instrument detects neutrinos as they interact with the electrons of an ultra-pure organic liquid scintillator at the center of a large sphere surrounded by 1,000 tons of water. Its great depth and many onion-like protective layers maintain the core as the most radiation-free medium on the planet. It is the only detector on Earth capable of observing the entire spectrum of solar neutrino simultaneously, which has now been accomplished, he notes.
The UMass Amherst physicist, one principal investigator on a team of more than 100 scientists, is particularly interested in now turning his focus to measure yet another type of solar neutrino known as CNO neutrinos, which he hopes will be useful in addressing an important open question in stellar physics, that is the metallicity, or metal content, of the Sun.
“There are two models that predict different levels of elements heavier than helium, which for astronomers is a metal, in the Sun; a lighter metallicity and a heavier model,” he notes. CNO neutrinos are emitted in a cyclic fusion reaction sequence different from the pp chain and subdominant in the Sun, but thought to be the main source of power for heavier stars. The CNO solar neutrino flux is greatly affected by the solar metallicity.
“Our data is possibly showing some slight preference for heavy metallicity, so we’ll be looking into that because neutrinos from the Sun, especially CNO, can help us disentangle this.”