• Physics 15, 190
A examine of neutrinos from the Solar has measured the sign from the so-called CNO cycle, providing a direct measure of the fundamental abundances within the Solar’s core.
Photo voltaic neutrinos are copiously produced by hydrogen fusion reactions within the Solar’s core. Due to this fact, they’re the direct proof that the Solar is powered by nuclear reactions. Measurements of photo voltaic neutrinos have offered details about the temperature and density of the photo voltaic inside, however uncertainties stay in regards to the chemical elements. Now the Borexino Collaboration studies a brand new measurement of the neutrino flux produced by the so-called CNO hydrogen burning cycle within the Solar . This cycle—which requires the presence of carbon (C), nitrogen (N), and oxygen (O)—produces neutrinos that carry monumental diagnostic energy referring to the properties of the photo voltaic inside. By measuring these neutrinos, the collaboration gives a valuable piece of details about the fundamental make-up of the Solar, bringing us nearer to resolving an issue that has plagued photo voltaic physics for over 20 years .
Stars spend about 90% of their lifetimes fusing hydrogen into helium, producing two neutrinos within the course of. The pp chain—or proton–proton chain—and CNO cycle are the 2 basic modes by which stellar fusion happens. Whether or not a star is dominated by the pp chain or the CNO cycle is dependent upon its core temperature, which is primarily decided by the mass of the star. Within the Solar and related low-mass stars, the pp chain generates virtually all of the nuclear vitality; the CNO cycle is the primary energy supply for extra huge stars. The pp chain is a collection of nuclear reactions that require no extra nuclei in addition to hydrogen as gas. In contrast, the CNO cycle depends on the presence of C, N, and O nuclei as catalysts within the manufacturing of helium (Fig. 1). Within the Solar, this catalytic course of introduces a linear dependence between the quantity of C, N, and O and the flux of CNO neutrinos. Thus, CNO neutrinos are a strong software for probing the chemical composition within the Solar’s core.
Figuring out the Solar’s chemical composition is necessary not only for photo voltaic physics however for all of astronomy, because the photo voltaic abundances for various components present a “chemical ruler” for measuring the fundamental abundances in different stars and fuel clouds . There has, nevertheless, been a long-standing controversy surrounding the photo voltaic abundances. The principle method for measuring the Solar’s chemical composition is observing absorption traces within the photo voltaic spectrum. Early spectroscopic research discovered comparatively excessive ranges of key components reminiscent of C, N, and O, however within the early 2000s superior strategies started to measure abundances that had been 30 to 40% decrease than the preliminary estimates. When these decrease abundance values are used as inputs for the usual photo voltaic mannequin (SSM)—the bodily framework for calculating the Solar’s construction and evolution—the predictions strongly disagree with helioseismology measurements of the Solar’s inside construction . Alternatively, the older photo voltaic abundances—that are primarily based on simplistic and outdated modeling—result in SSM predictions which might be remarkably in-line with helioseismology knowledge .
This 20-year-old paradox, dubbed the photo voltaic abundance drawback, has defied a passable answer. The newest spectroscopic measurements appear to be shifting towards higher settlement with the helioseismology observations . Nonetheless, the photo voltaic physics group has lengthy been looking out for an impartial option to measure the chemical abundance of the Solar. And it’s right here the place neutrinos and the brand new Borexino outcomes come into play.
Positioned deep underground in Italy’s Gran Sasso Nationwide Laboratory, the Borexino Collaboration measures the vitality spectrum of the photo voltaic neutrino flux right down to very low energies (round 100 keV) and separates the person contributions originating from completely different nuclear reactions within the Solar’s core. From its evaluation of information from the experiment’s last part III (January 2017 to October 2021), the collaboration estimated 4.8 counts per day of CNO neutrinos within the detector’s quantity of 71.3 tons. The collaboration beforehand reported a detection of CNO neutrinos in 2020  however have now decreased the measurement errors by an element of two, permitting the crew to make inferences about photo voltaic abundances.
The CNO flux measurement could be changed into a measurement of C plus N abundances by together with measurements of one other neutrino flux: that from boron-8 decay, which strongly is dependent upon temperature . The collaboration used this flux to constrain the Solar’s core temperature to a precision of about 0.1%. The crew utilized this temperature to the relation between the CNO flux and the C, N, and O abundances, arriving on the first-ever measurement of chemical abundances within the Solar’s core. Relying solely on neutrino measurements, this system presents a really impartial willpower of the photo voltaic composition in opposition to which spectroscopic determinations, new and outdated, could be checked. The outcomes strongly favor the Solar having excessive C, N, and O abundances, opposite to the estimates from fashionable, state-of-the-art spectroscopic research (except E. Magg et al. ).
The discrepancy between the core abundances estimated by Borexino and the floor abundances measured by spectroscopic research might indicate that the Solar’s chemical composition varies with depth. Such a nonuniform distribution of components could possibly be defined, for instance, by the Solar having shaped in an inhomogeneous surroundings, or by the Solar’s outer layers having been enriched in sure components by accretion of planetary materials [9, 10].
Testing these hypotheses would require extra refined CNO-neutrino measurements. Sadly, Borexino shut down in 2021, and there’s presently no experiment that may observe photo voltaic neutrinos at low sufficient energies. Borexino has, nevertheless, left us with a powerful legacy, as its measurements of the CNO flux and different photo voltaic neutrino indicators symbolize landmarks for future neutrino experiments. The Borexino crew developed applied sciences, notably in radioactive purification, which might be and might be of nice profit for different experiments, such because the SNO+ neutrino experiment in Canada. The collaboration additionally developed data-analysis strategies for background subtraction that may definitely discover future functions in neutrino research in addition to darkish matter searches. There aren’t any plans but for a devoted photo voltaic neutrino experiment, but when one had been to be constructed utilizing the experience acquired by Borexino, it will have a vivid future and would offer invaluable details about the Solar.
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