Understanding the “hydrogen burning” power of our sun – happened after more than 80 years

The Borexino collaboration, in which scientists from the TU Dresden also participate, has succeeded after more than 80 years in experimentally confirming the Bethe-Weizsäcker cycle.

Stars produce their energy through nuclear fusion by converting hydrogen to helium, a process known to researchers as “burning of hydrogen”. There are two ways to accomplish this fusion reaction: on the one hand, the so-called pp cycle (proton-proton reaction) and the Bethe Weizsäcker cycle (also known as the CNO cycle, derived from the elements carbon (C), nitrogen (N) and oxygen (O)) on the other hand.

The pp cycle is the predominant energy source in our Sun, only about 1.6 per mil of its energy comes from the CNO cycle. However, the standard solar model (SSM) predicts that the CNO cycle is probably the predominant reaction in much larger stars. Already in the 1930s, the cycle was theoretically predicted by physicists Hans Bethe and Carl Friedrich von Weizsäcker and later named after these two gentlemen. While the pp cycle could already be experimentally tested in 1992 at the GALLEX experiment, even in the Gran Sasso massif, the experimental test of the CNO cycle has not been successful so far.

Both the pp cycle and the CNO cycle produce countless neutrinos: very light and electrically neutral elementary particles. The fact that neutrinos hardly interact with other matter allows them to leave the interior of the sun almost at the speed of light and to transport information about their origin on Earth unhindered. Here the ghost particles have nothing more than to be captured. This is a rather complex undertaking, which is only possible in a few large-scale experiments around the world, as neutrinos show up as tiny flashes of light in a huge reservoir filled with a mixture of water, mineral oil and other substances. also called scintillators. Evaluating the measured data is complex and resembles searching for a needle in a haystack.

Compared to all previous and ongoing solar neutrino experiments, Borexino is the first and only experiment in the world capable of measuring these different components individually, in real time and with high statistical power. This week, the Borexino research collaboration was able to announce a great success: in the renowned scientific journal Nature, present their results on the first experimental detection of CNO neutrinos, a milestone in neutrino research.

Dresden physicist Professor Kai Zuber is an avid neutrino hunter.

He is involved in many different experiments around the world, such as the SNO collaboration in Canada, which received the Nobel Prize for its discovery of a neutrino mass. The fact that with Borexino, he and his colleagues Dr. Mikko Meyer and Jan Thurn have now been able to experimentally prove CNO neutrinos for the first time is another milestone in Zuber’s scientific career: “Actually, now I’ve got it all. what I had imagined and hoped for. I (almost) no longer believe in great new discoveries in solar neutrino research for the rest of my life. However, I would like to continue working on the optimization of the experiments, in which the Felsenkeller accelerator here in Dresden plays an extremely important role. For sure, in the future we will be able to have even more precise measurements of the Sun. “

Read Neutrinos Yield First Experimental Evidence of the CNO Energy-Production Mechanism of the Universe 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