DETECTION OF CNO-NEUTRINO IN SUN’S CORE
- Jadavpur University Science Club
- Dec 21, 2020
- 3 min read
-Ashesh Kumar Gupta
Physics Department, UG2
The first detection of neutrinos produced by the Sun’s secondary solar-fusion cycle paves the way for a detailed understanding of the structure of the Sun and of the formation of massive stars. At the recent “Neutrino 2020” virtual meeting, the Borexino Collaboration has announced the first detection ever of neutrinos from the CNO cycle in the Sun, an astounding experimental achievement, which closes a chapter of physics commenced in the years the ‘30s of the past century.
It was in 1938 when Bethe and Carl Friedrich Freiherr von Weizsäcker independently proposed that hydrogen fusion in the Sun might be catalyzed by the heavy nuclei carbon, nitrogen, and oxygen, according to a cyclic sequence of nuclear reactions. This second mechanism of hydrogen burning into helium in the Sun’s core complements the main energy-generating process constituted by the pp chain of reactions, initiated by the direct fusion of two protons into a deuteron.
The Borexino Collaboration has detected solar neutrinos produced by a cycle of nuclear-fusion reactions known as the carbon–nitrogen-oxygen (CNO) cycle. Measurements of these neutrinos have the potential to resolve uncertainties about the composition of the solar core and offer crucial insights into the formation of heavy stars.
Inside view of the Borexino detector
Neutrinos are tiny, subatomic particles. They were first postulated to exist by Wolfgang Pauli in 1930, to account for the energy that was apparently missing during β-decay, a process in which energetic electrons are emitted from an atomic nucleus. Pauli’s explanation for why neutrinos had never been observed was that they interact incredibly weakly with matter. Subsequent decades of research have yielded a wealth of information about Pauli’s ‘ghost particle’, including the Nobel-prizewinning discovery that neutrinos do, in fact, have a mass, one so small as to be beyond the reach of current measurements.
Fusion reactions in the Sun produce an astonishing number of neutrinos: roughly 100 billion solar neutrinos pass through each of your eyelids every second. Because of the weakness of their interactions, they are barely deterred from their path even when they have to pass through the entire body of the Earth
Neutrinos are therefore both challenging to observe and yet able to offer insights into other-wise unreachable regions of the Universe, such as distant supernovae or the interiors of stars. The energy produced in the center of the Sun in the form of photons takes tens of thousands of years to escape, but a solar neutrino can escape the Sun and reach Earth in just eight minutes.
The Sun is powered by fusion reactions that occur in its core: in the intense heat of this highly pressurized environment, protons fuse together to form helium. This occurs in two distinct cycles of nuclear reactions. The first is called the proton-proton chain (or pp chain) and dominates energy production in stars the size of our Sun. The second is the CNO cycle, which accounts for roughly 1% of solar power, but dominates energy production in heavier stars.
The Borexino Collaboration now reports a 2ground-breaking achievement from its experiment: the first detection of neutrinos from the CNO cycle. This result is a huge leap forward, offering the chance to resolve the mystery of the elemental composition of the Sun’s core. In astrophysics, any element heavier than helium has termed a metal. The exact metal content (the metallicity) of a star’s core affects the rate of the CNO cycle. This, in turn, influences the temperature and density profile — and thus the evolution — of the star, as well as the opacity of its outer layers.
The metallicity and opacity of the Sun affect the speed of sound waves propagating through its volume. For decades, helio-seismological measurements were in agreement with SSM predictions for the speed of sound in the Sun, giving confidence in that model. However, more-recent spectroscopic measurements of solar opacity produced results that were significantly lower than previously thought, leading to discrepancies with the helio-seismological data. Precise measurements of CNO-cycle neutrinos offer the only independent handle by which to investigate this difference. Such measurements would also shed further light on stellar evolution.
Although the chief obstacles to making these measurements are the low energy and flux of CNO neutrinos, and the difficulty of separating the neutrino signal from sources of background signals, such as radioactive-decay processes. The Borexino Collaboration carried out a multi-year purification campaign to ensure unprecedentedly low levels of radioactive contaminants in the scintillator.
Hopefully, future experiments will work to improve the results by The Borexino collaboration in reducing the background noise caused by the radioactive contamination. Till then The Borexino Collaboration gives new sets of information to unlock new discoveries of the interior of the stars.
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