Detecting the Sun’s Hidden Neutrinos
Neutrinos are nearly massless particles that carry away some of the energy of high-powered events such as atomic fission or fusion inside a star. They travel through space at the speed of light and only rarely interact with most matter – they are only affected by the weak nuclear force (and gravity, although that force is even weaker). Thus, although they are produced throughout the universe and can travel with high energies, they are extremely difficult to detect.
This is a photo of the Borexino neutrino detector, built beneath the ground in Italy.
It is a giant tank shielded from the outside by multiple layers including the covering you see in this shot; all of those are to filter out and block neutrinos other than those of certain energies from entering the tank. When a neutrino with the correct energy enters the tank, it interacts with electrons in an organic solution loaded into the inner tank, eventually producing an electromagnetic signal response (light) that can be detected by instruments on the outer edge of the tank.
This instrument previously has been used to detect a variety of neutrinos, including ones produced by radioactive decay inside the Earth and neutrinos produced by other stars out in the universe. There are 2 mechanisms known through which fusion can happen – direct merger of two protons to create helium, or a process called the CNO process where carbon atoms capture 2 separate protons, converting into nitrogen and then oxygen, before a helium atom is released and the carbon atom is restored. In this mechanism, carbon effectively serves as a catalyst for the nuclear reaction.
The direct merger mechanism is thought to have occurred when carbon abundances were low in the early universe and in smaller stars, but our star should have enough carbon to produce some of its energy via the CNO reaction. However, because the neutrinos of the CNO process had, within error, the same energy as some fission reactions that occur on Earth, the first decade of operation at Borexino was unable to isolate specific neutrinos from our sun.
Recent upgrades to the detector allowed the background neutrino level for the correct energies to be pushed down to the point where the solar CNO neutrinos should have been detected, and in a recent paper the device’s science team announced that they had in fact detected the sun’s CNO neutrinos.
These neutrinos from the CNO process are the first true experimental confirmation that the CNO fusion catalyst mechanism actually works; it was hypothesized over 30 years ago but prior to now humans have never been able to verify its occurrence in nature. Understanding the energy released by this mechanism is also important for understanding the makeup of the sun, as there have been several difficulties in fitting observations of motions inside the sun to those predicted by the measured abundances of various elements in our star.
-JBB
Image credit: https://en.wikipedia.org/wiki/Borexino#/media/File:Laboratori_Nazionali_del_Gran_Sasso,_Borexino_detector.png
References:
https://arxiv.org/pdf/2006.15115.pdf
https://arxiv.org/pdf/0806.2400.pdf
