The world is building two detectors capable of registering geoneutrinos - particles generated by radioactive decay in the interior of the Earth. This will help to understand the mechanism of formation of deep heat, and in the future - to predict natural disasters. Perhaps "terrestrial neutrinos" will also clarify the question of how exactly our planet was formed.
Antineutrinos called to account
Neutrinos and antineutrinos are elementary particles that have long been considered elusive. They are formed during beta decay, a type of nuclear fission. On Earth, they are produced by nuclear reactors.
Natural neutrinos come from the Sun as a result of self-sustaining thermonuclear reactions. They are born in the atmosphere under the influence of cosmic rays. In space, relic neutrinos are carried, which appeared in the first moments of the Big Bang. And, finally, the source of neutrinos are radioactive isotopes scattered in the bowels of the planet.
The idea of using antineutrinos to test geological hypotheses originated with physicists in the 1960s. They were registered for the first time only in 2005 at the KamLAND underground detector (Japan) as a side result of the study of solar neutrinos. In 2010, the existence of particles was reliably confirmed in the Borexino experiment in Italy.
Terrestrial antineutrinos will help to reveal the fundamental mysteries of science: how many radioactive elements are in the interior of the planet and where they are localized, how much heat they generate, which models of the structure and composition of the Earth are more consistent with observations.
However, this is not so easy to do: matter, by and large, is transparent to neutrinos (which is reflected in the name of the particle). Particles do not participate in electromagnetic and strong interactions, they hardly feel gravity, they respond only to weak forces acting on scales less than the diameter of a proton. A neutrino can fly in space for tens of light years, piercing stars, clouds of gas, planets, never colliding with any other particle.
Over the entire period, Borexino and KamLAND have registered signals from about 190 geoneutrinos - the decay products of uranium-238 and thorium-232. On the one hand, this is proof that direct observation of the flux of terrestrial neutrinos is possible, and preliminary data are in agreement with generally accepted geological models; on the other hand, this statistics is not enough for unambiguous scientific conclusions. It will take hundreds of years to collect it in existing experiments.
Beta decay converts a neutron in an atomic nucleus into a proton. This is accompanied by the emission of an electron and an antineutrino. The energy of the electron turns into thermal energy, and the antineutrino, without interacting with anything, is carried away into space
Giant detectors at the service of geophysicists
The Borexino and KamLAND detectors are huge tanks filled with liquid hydrocarbons that act as a scintillator. When interacting with neutrinos, they emit photons, which are registered by photomultipliers. The installations are placed in mines deep underground to reduce the effects of cosmic rays.
The detectors under construction will operate on the same principles as the current ones. To register more events, the mass of the scintillator will be significantly increased, and the liquid itself will be cleaned of radioactive impurities (carbon-14, radon) that create noise. In addition, it is important to locate detectors as far away from operating nuclear reactors as possible.
One of the facilities, SNO +, is under construction at the Sudbury Neutrino Observatory in Canada. It has already begun to be filled with a liquid scintillator. The world's largest 20-kiloton detector designed, among other things, for the study of terrestrial neutrinos - JUNO - is being built in southern China. It will start collecting statistics by 2021.
The development of a large scintillation detector with a target mass of ten kilotons at the Baksan Neutrino Observatory of the INR RAS in the North Caucasus is being discussed.
As the authors of the project write, "the geographic features of the location of the observatory make it possible to substantially suppress the background associated with antineutrino fluxes from operating reactors of nuclear power plants, and register antineutrino fluxes that carry information about the structure of the earth's crust in this region."
SNO + neutrino detector in Canada
What heats the bowels of the planet
34 long-lived isotopes are responsible for the natural radioactivity of the Earth, the largest contribution is made by only three: uranium-238, thorium-232 and potassium-40. According to the generally accepted model of the Earth - silicate (Bulk Silicate Earth) - most of the radionuclides are contained in the upper shell of the Earth - the lithosphere, about half of them are scattered in the mantle, and practically none in the core.
This distribution of radionuclides was a consequence of the formation of the planet. Immediately after birth from a dense cloud of gas and dust, the Earth was a molten ball. This was facilitated by two conditions: a very high content of radionuclides (in particular, then there was twice as much uranium-238, its half-life is equal to the lifetime of the Earth - 4.5 billion years) and intense bombardment by meteorites.
As it cooled down, the matter of the planet began to stratify. Iron and nickel sank inside, forming a core, a silicate melt accumulated on top, which absorbed lithophilic elements, including potassium, thorium, uranium.
During beta decay, the energy carried by electrons is converted into heat, and antineutrinos carry their part of the energy into outer space. If you know their parameters, you can calculate the concentration of parent radionuclides in the crust and mantle and clarify how much heat they generate.
Sources of internal heat of the planet. The generally accepted model of the Earth states that radionuclides are dispersed in the earth's crust and mantle and are absent in the core.
Current estimates of the total heat flow of the Earth and the share of each of the sources vary greatly depending on the calculation method. On average, the contribution of radiogenic heat is about 20 percent. The rest is due to the secular cooling of the mantle (which was originally melted and has been cooling since then) and the heat of the planet's core.
Due to internal sources of heat, mixing (convection) of the mantle occurs, plumes are formed and, as a result, tectonic activity manifests itself on the planet's surface: the movement of plates of the earth's crust, the formation of large faults and mountain systems, earthquakes and volcanism.
Another fundamental task is to establish the ratio of the isotopes of thorium and uranium. Analysis of chondrite meteorites and comparison of samples taken in the earth's crust made it possible to calculate that thorium-232 is 3, 9 times more than uranium-238. To understand the early evolution of the Earth, an accurate estimate is needed, which can be obtained by studying geoneutrinos.
However, the preliminary calculated mass of thorium and uranium in the crust and mantle does not explain the entire radiogenic heat flux. In this regard, in the 1990s, a hypothesis appeared that at the initial stage of the formation of the Earth, part of the radionuclides entered the core. This natural georeactor is the source of energy for the mantle plumes and the planet's magnetic field. The JUNO detector will help you check this assumption.
Where did potassium-40 go?
In calculations of the heat flux of the planet, the contribution from the decay of potassium-40 is usually not taken into account. It is believed that it is an order of magnitude less than uranium-238 and thorium-232, and it is all concentrated in the earth's crust. However, these assumptions may turn out to be erroneous, according to scientists from INR RAS and INEOS RAS.
They use an alternative, rejected by the scientific community, the model of an initially hydride Earth, based on the fact that the composition of planets is influenced by their distance from the Sun. The fundamental difference between this model and the generally accepted one is that it allows for the content of radionuclides in the core, and the mass of potassium-40 is two orders of magnitude greater than the masses of uranium and thorium. Due to this, the total heat flux turns out to be huge - about 304 terawatts against 47 terawatts calculated from measurements in superdeep wells.
According to the authors of the article, observations of geoneutrinos can resolve this paradox and check the model of the initially hydride Earth. Moreover, it is critically important to isolate the signal from the decay of potassium-40. However, so far existing technologies do not allow this to be done.