"The sun will rise ..."

In the Borexino experiment , the so -called PP neutrino, which make up the main part of the neutrino from the Sun, but previously slipped away from direct observation, are first registered . This work, recently published in the prestigious multidisciplinary journal Nature, directly demonstrates that the main source of energy of the Sun is a proton cycle that begins to merge two protons into the core of the deater.

Look at the sun. Close it with an index finger. Now every second through your nail (its area of ​​about one square centimeter) flies 60 billion neutrinos that arose in the nucleus of the Sun. And in total, our luminary emits about 1.7 × 1039 of these particles every second.

However, until recently, this value was only the result of theoretical calculations. Those, in turn, relied on the directly measured solar luminosity and on the assumption (which, however, had the status of almost 100 percent confidence) that the energy emitted by the sun arises as a result of certain thermonuclear reactions, which ultimately boil down to the merger of four protons-the nuclei of light hydrogen-into the Helia-4 nucleus, alpha-particle.

This chain of reactions, which serves as the main source of energy not only of the Sun, but also most other stars, is called a proton - about a ton ( PP ) cycle . It should be noted here that the Helia-4 core consists of two protons and two neutrons. So, two protons of four during the PP cycle should turn into neutrons.

The law of conservation of the charge prohibits positive protons into neutral neutrons just like that, without the participation of additional electrically charged particles. The proton charge should either neutralize a negatively charged electron captured from the surrounding plasma, or leave the nucleus along with a positive positron (and it, in turn, is almost instantly annihilate with the electronic environment).

In both cases, the law of preservation of the lepton number comes into force - in any reaction, the disappearance of lepton (electron) or the occurrence of anti -andpton (positron) should be compensated by the occurrence of another lepton. So that the total algebraic amount of leptons (the number of anti -andptons is taken negative) does not change during the reaction. Moreover, in our case, the born lepton should be neutral so as not to violate the already achieved balance of electric charges. The only famous candidate for this role is neutrino.

So, we found out that the formation of a helium nucleus in a proton-proton cycle should be accompanied by the disappearance of two electrons and the formation of two neutrinos.

Energy released

Any reactions accompanied by a change in the electric charge of leptons, including a PP cycle, are due to the so -called weak interaction, one of the four fundamental interactions known in nature. It is not called weak by chance.

The fact is that the photon, which is a carrier of electromagnetic interaction, flies in the plasma of the Sun to scattering on some electrically charged particle, most often an electron, only a fraction of a centimeter. On the contrary, for neutrinos, which can only enter into a weak (and gravitational) interaction, the bowels of the sun are almost transparent. These particles, which almost do not interact with the usual substance, freely leave the sun at a speed, almost equal to the speed of light, flying 700 thousand km from the center to the surface in two seconds, and after another 8 minutes they can already be registered on the ground.

That part of the energy of the PP cycle, which is transmitted to photons and charged particles, ultimately also reaches the surface of the sun and is displayed into space, but the time of passage through the opaque sunny substance is estimated at about 100 thousand years. Stop thermonuclear reactions in the center of the Sun today, we would have learned about it only through millennia - if not for the neutrino.

Complete energy released in a PP cycle is easy to find. To do this, you need to subtract the mass of the final product-the Helia-4 nucleus-from the mass of particles entering the reaction (four protons and two electrons), which are 0.7% heavier in total. This energy is 26.73 MeV. And the measured flow of solar energy on Earth is about 0.136 watts (8.5 × 1011 MeV per second) per square centimeter. Having divided the second value by the first and not forgetting to multiply by two, you can get the number of solar neutrinos passing in a second through the nail of your index finger, i.e. mentioned above 60 billion.

Difficulties of detection

As you can see, by registering solar neutrinos, we can find out if the sun really shines due to the merger of hydrogen into helium, and if so, what is the power of this thermonuclear “firebox” at the moment, and not thousands of years ago. It would seem that everything is simple. However, the very weakness of the interaction of neutrinos with the usual substance, thanks to which these particles freely leave the nucleus of the sun, make it difficult to detect them.

For example, from the innumerable trillions of solar neutrino, flying through your body in the last week, only one particle had a chance to make a composure with any of its electrons. Conventional nuclear radiation detectors are not able to distinguish such a small stream of events from the usual background due to natural and technogenic radioactivity, as well as cosmic radiation.

To register solar neutrinos, a suitable detector must be: a) very massive to increase the frequency of rare events of the interaction of neutrino with the substance of the target, and b) deeply protected from the background so as not to confuse these rare events with the background. Only less than a dozen experiments in the history of research were able to solve this problem and explore part of the spectrum of the solar neutrino (mainly the components of this spectrum with a small stream, but with high energy, the so -called boron and beryllium neutrinos). However, the main component of the spectrum -low -energy PP -neutrino, which occur in the first reaction of the PP cycle, the merger of the two protons into the core of the deater -was only possible to distinguish only recently and so far in only one of the experiments.

Namely, in the Borexino experiment, which is a multi-ton scintillation detector of a solar neutrino, placed at a depth of 1400 m B in the tunnel under the Grand Sasso mountain massif in central Italy. The experiment is conducted by an international team of physicists from Germany, Italy, Russia, Poland, the USA, Ukraine and France.

The thickness of the rocks covering the detector from cosmic radiation is equivalent to a water layer of 3.8 km. However, this is only the first “defense line” that allows you to protect the center of the detector from the background resembling a bulb or nesting doll.

The most outer layer of the “bulbs” - the muon detector - consists of a steel dome filled with 2100 tons of ultra -low water. A layer of water is visible sensitive even to single photons with devices - photovlectronic multipliers (FEU). Residual cosmic muns that managed to fly through the thickness of the rocks and not be absorbed, thanks to the effect of Cherenkov, create a flash in the water along their track, perceived by the detector electronics as a signal to suspend the registration of useful events for a couple of milliseconds. After all, even a fast neutron “a quick neutron“ struck ”along the edge of a sensitive volume can knock out a fast neutron from the nucleus, and it is with some probability to reach the detector’s heart and cause false trigger.

Inside the dome there is a stainless steel sphere with a diameter of 13.7 m, filled with a ultra -low liquid scintiller - an organic fluid that creates a very fast flash when an ionizing particle enters it. The number of highlighted photons in proportion to absorbed energy, so that by counting photons, you can determine the energy of the particle. 2212 photographs are installed to collect light on the inner surface of the sphere.

It is important that the final time of the photons spaces from the scintilization flash to FEU allows you to localize the flash in space with good accuracy - about 10 cm. The so -called buffer layer of the scintiller 2.6 m thick near the walls of the sphere is attached with a special addition to suppress the signal from the background radio activity of photographs and steel; At the same time, there is still enough light from the Muon passing through the buffer to restore its track. The buffer layer itself also protects the inner part of the detector from gamma quanta emitted during the decay of natural radionuclides into FEU and steel. In addition, it contains a thin nylon spherical membrane that protects from the natural radioactive gas of radon, the atoms of which, thanks to chemical inertia, easily diffuse into the detector.

Another transparent nylon sphere with a diameter of 8.5 m separates the buffer from the scintillation target itself, located in the center of the detector. It is in it that solar neutrinos are recorded, interacting with its electrons. The mass of the target is 278 tons, almost five railway tanks. After the elastic scattering of the neutrino on the electron, it acquires a certain kinetic energy absorbed in the scintille and partially highlighted in the form of optical photons. However, only events that occurred no more than three meters from the center go into the overall standings, since the nylon sphere shows some residual radioactivity, which can simulate the desired effect. Therefore, flashes near the walls are discarded.

All this multi-layer “nesting” design that protects against the external background would be useless if radioactive impurities in a more or less significant amount were present in the scintille. However, a specially designed multi -stage cleaning procedure removes dangerous impurities from the liquid almost completely, to an unprecedentedly low level. So, the content of uranium and thorium in it is less than 10-18 g/g.

For comparison, in a ton of any natural substance (including unpeeled scintillyato-ra) usually contains from 0.1 to 1 g of uranium and thorium, i.e. The typical content of these natural radioactive elements is 10–7 ... 10-6 g/g. Thus, the center of the Borek-Sino detector is the most radiation clean place on the planet. Nowhere in nature - neither on earth nor in the modern universe - there is no substance where the content of uranium and thorium would be so little; The only exception is probably the bowels of neutron stars, where atomic nuclei are simply absent as such.

One of the main sources of the background that prevents the detecting neutrino in low energies is the carbon-2-roller-rolled beta-roller, a well-known isotope-chronometer used to dating organics in archeology and paleontology. A typical attitude of 14C/12C in modern organics (for example, in our body) is ~ 10–12, one atom of the radiocalrod on a trillion of atoms of stable carbon. In the scintillator of Borexino, made from oil of ancient deposits, where carbon-14 almost broke up, this is a million times less attitude, ~ 10–18. Nevertheless, it is the radiocarbon who is responsible for most of the events recorded in the target, giving about 40 decays per second per 100 tons of the scintillar.

Conclusions of researchers

The Borexino detector, who began measurements in 2007, was originally intended for the direct registration of one of the small components of the spectrum of the solar neutrino -the so -called Beryllium neutrinos arising with an energy of 0.87 Meav at the side stage of the PP cycle. Not only this task was successfully completed, but for the first time another small component of the spectrum (the so-called PEP -neutrino) was identified, as well as the upper border on the stream of neutrinos that occur in the alternative process of the merger of the four protons in helium-4, the so-called CNO cycle, which should be the main source of energy in the stars is heavier of the sun.

Moreover, in the course of work, the background was so much suppressed by low energies that it became possible to take on a more difficult task -direct registration of PP neutrino arising from the merger of the two protons into the deer nucleus (this is in 99.76% of cases the first reaction in the PP cycle). The energy of the PP neutrino lies in the range from 0 to 420 KEV, in the area of ​​a much higher background. Therefore, although their flow is much higher than the flow of Beryllium neutrinos, their direct registration was previously impossible. (The low energy threshold of the radiochemical gallium -German experiments Sage and Gallex/GNO made them sensitive to part of the PP spectrum, but only the integral stream of neutrino with energy above the threshold was recorded, without the possibility of direct separation of various stream components.)

After careful accounting for all the remaining component of the background, the measured signal from the desired effect, i.e. From the scattering of PP -neutrino on the target electrons, it was 144 ± 13 events per day per 100 tons of the scintille. This signal corresponds to the flow of the PP neutrino equal to 66 ± 7 billion neutrinos per second through a square centimeter, which is consistent with the predicted stream, 60 billion.

In addition to access to the physics of the Sun, confirming the standard solar model, the result is also significant for neutrinum physics. As you know, neutrino on the way from the Sun experiences oscillations: part of the electronic neutrinos (their varieties associated with electrons and born in a PP cycle) turns into muon and tau-neurino (neutrino varieties associated with the corresponding heavy leptons).

In the described experiment, the probability of survival of an electronic neutrino on the way from the Sun to Earth in the PP spectrum was first measured; This value is 0.64 ± 0.12 and is quite consistent with similar probabilities for the neutral spectrum components neighboring energy.

At the same time, more high-energy neutrinos that occur in the sun during the decay of BOR-8 survive with a probability of about 0.3. This dependence of the probability of energy survival is described by the so-called decision of Mikheev-Smirnov-Wolfenstein with a large angle of mixing (MSW-LMA): high-energy neutrino oscillopolists, flying through a layer of substance with a slowly changing density in the convective zone of the Sun (MSW effect), and low-energy neutrinos experience vacuumed neutrino oscillations.

So, the collaboration of Borexino, for the first time performing direct measurements of the main component of the spectrum of the solar neutrino, confirmed the validity of the standard solar model and showed that the current energy intensity in the PP cycle coincides with the modern luminosity of the sun. This means that our luminary on a temporary scale of about 100 thousand years has been in a stable state and at least in the next hundred millennia we should not expect big surprises from it. It is unlikely that anyone was very doubted in this, but still the result adds confidence in the future: no matter what happens, and the sun will rise tomorrow just like today.