How does the Sun shine? (Final part: The search for the proton-proton neutrinos)

Let’s return to the original goal of this series of posts on the Sun’s source of energy. After the post on the proton-proton chain model, it became quite clear that it was very difficult to experimentally prove that this mechanism was really happening inside the Sun. Because observing the low energy neutrinos from this first branch of the fusion process is itself an experimental challenge due to the fact that at these energies, there are many other things which can interfere with the results. Attempts to measure pp neutrinos directly over the past 30 years have been hindered by the presence of radioactive backgrounds in this low-energy region (0 to 420 keV).

It is to overcome this problem that the Borexino detector was designed. Borexino is very good at observing neutrinos of all known flavors and at all energies. Located deep beneath Italy’s Apennine Mountains, Borexino is designed to minimize backgrounds from radioactive isotopes both within, and external to, the liquid scintillator (also called the target). In the liquid scintillator, neutrinos interact with the electrons of this ultra-pure organic liquid scintillator at the center of a large stainless steel sphere surrounded by tons of water. Its great depth and multiple onion-like protective layers keep the core free from most radiation.


The Borexino experiment detects solar neutrinos by measuring the energy deposited in the liquid scintillator target by recoiling electrons undergoing neutrino-electron elastic scattering. That is, a neutrino of one of the three known neutrino flavours, collides with a free electron of the target/scintillator and gives it some of its kinetic energy (for more info on that, I’ll have to learn QFT, and Z & W bosons…). The target/scintillator converts the kinetic energy of electrons into photons (i.e., re-emit the absorbed energy in the form of photons/light). This light is detected and converted into electronic signals by photon multipliers mounted on a concentric stainless steel sphere. Despite extreme precautions against external sources of radiation from the earth and the equipment used by the instrument, the Borexino has to cope with the internal Carbon-14 beta radioactivity present inside the liquid scintillator and the cosmic ray muons which interfere with the detection of low-energy ppI neutrinos. (One can read about the statistical methods used to account for these interferences in the source below).

The primary aim of the experiment is to make a precise measurement of the beryllium-7 neutrino flux from the sun (coming from another branch of the fusion process) and comparing it to the Standard solar model prediction. This will allow scientists to further understand the nuclear fusion processes taking place at the core of the Sun and will also help determine properties of neutrino oscillations, including the MSW effect (The Mikheyev-Smirnov-Wolfenstein effect often referred to as matter effect, is a particle physics process which can act to modify neutrino oscillations in matter). Other goals of the experiment are to detect boron-8, pp, pep and CNO solar neutrinos as well as anti-neutrinos from the Earth and nuclear power plants. The project may also be able to detect neutrinos from supernova within our galaxy. (NOTE: Neutrinos are also important as early indicators of exploding stars, because neutrinos emitted by the dying star arrive before the light/photons, neutrinos still do not travel faster than light, it is only because they are produced earlier, before the photons that we often see on fantastic supernova pictures).

Finally, Borexino is currently the only detector on Earth capable of observing the entire spectrum of solar neutrino simultaneously. As we’ve seen previously, neutrinos come in three types, or flavors. Those from the Sun’s core are of the electron-flavor, and as they travel away from their birthplace, they oscillate or change between two other flavors, muon- to tau-. Since 2007, Borexino has, strongly confirmed this behavior of the neutrinos, and has proved with increased precision, that 99 per cent of the power of the Sun, 3.84 × 10^33 ergs per second, is generated by the proton-proton fusion process (the first branch or ‘ppI’ branch of the proton-proton chain).

This post is the last one on this series on the Sun’s source of energy, but the Sun has still many more mysteries like for example: What is responsible for the 11 years long solar cycle? Why is the corona’s temperature higher than the Sun’s surface? Why is the solar cycle interrupted every 70 years by a Maunder minimum? Why is the heliosphere shrinking and the solar wind flow that inflates it decreasing? The heliosphere is a bubble of magnetism springing from the sun and inflated to colossal proportions by the solar wind. Every planet of the solar system is inside it. The heliosphere is our shield against galactic cosmic rays, high-energy particles from black holes and supernovas, entering the solar system, but most are deflected by the heliosphere’s magnetic fields.

Sources: Wikipedia and “Neutrinos from the primary proton-proton fusion process in the Sun”, Borexino Collaboration (27 August 2014), Nature 512 (7515): 383-386.

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