Neutrino astronomy is very young - it is only about two decades old. Scientists believe that the study of the smallest and very difficult to detect particles can provide us with new information about much larger objects that we could not have obtained otherwise.
Neutrinos are one of the particles predicted by physicists even before experimental detection. There is in elementary particle physics such a concept as "beta decay", in which the nucleus of an atom emits a β-particle, that is, an electron well known to us. The phenomenon itself was discovered at the end of the 19th century, and in 1914, James Chadwick (the future discoverer of the neutron) recorded its energy spectrum. It became clear that the resulting electrons are emitted into space, carrying any kinetic energy, and, as a rule, less than expected.
This was a serious challenge for scientific minds: the energy disappeared into an unknown direction. The law of conservation of energy, the basis of the foundations of modern physics, was called into question.
In 1930, Wolfgang Pauli proposed a decay model, as a result of which, in addition to the electron, another particle was formed. She carried away the excess energy. To answer the question "Why this particle has not yet been discovered by experimenters?" it was required to assume that it hardly interacts with matter.
It was a very bold idea, apparently, that is why Pauli presented it not in a journal article, but in the form of an open letter to the participants of the symposium in Tübingen. A few years later, Enrico Fermi developed an extensive theory of β-decay. He also introduced the modern name into circulation: "neutrino" in Italian means "little neutron". Pauli himself called the particle a neutron, but this name was later assigned to another object of the microworld. He also argued with his friend, astronomer V. Baade, on a bottle of champagne, that the predicted particle would not be experimentally detected during the life of the disputants. It is unclear how he planned to get the winnings, but anyway, he bet a bet. Neutrinos were first recorded in 1953, and Pauli died five years later. History is silent as to whether he bought champagne for his friend.
Object of study
Neutrino is a very small particle. Until recently, it was generally unclear whether she had mass. In recent years, it has become clear that there is, but very small. Its exact value is unknown at this time, and the available estimates in general boil down to the fact that neutrinos are about 10 orders of magnitude lighter than a proton. The weight of a grasshopper (about 1 gram) correlates in approximately the same way with the displacement of the modern nuclear aircraft carrier George Bush (about 100 thousand tons).
The particle has no, or almost no, electric charge - experiments have not yet given an unambiguous answer, and of all fundamental physical interactions, it reliably participates only in weak and gravitational ones.
Neutrinos are subdivided into three generations (in the literature there are variations of this designation, such as "flavors"): electron, muon and tau neutrinos. They are usually listed in smart books in this order, and this is no coincidence - this is how the sequence of their opening is displayed. In addition, there are also antineutrinos - antiparticles of three different types, corresponding to "ordinary" ones. Neutrinos of different generations can spontaneously transform into each other. Scientists call this "neutrino oscillations" and have been awarded the 2015 Nobel Prize in Physics for their discovery.
There is a hypothesis that, in addition to the three listed generations of neutrinos, there is also a fourth - sterile neutrinos, whose privilege is not to participate in weak interactions. Perhaps they are the ones that make up the dark matter that we have not yet discovered. It is not known whether such neutrinos actually exist, but if they exist, then their detection promises to be a truly nontrivial task.
What are they?
Neutrinos are the result of nuclear (and thermonuclear, we will not separate them further) reactions. There are a lot of them, elusive. According to the calculations of theoretical physicists, there are about 109 neutrino. Nevertheless, living in this "soup", we do not notice it at all. Particles pass through us as if we were not there.
If a neutrino happens to once fly through a wall of lead, then the free path of a particle in it will, on average, be 1015 km. This distance is quite galactic in scale - from our planet to the center of the Galaxy is only ten times greater. Of course, such a value means that registration of individual neutrinos in a detector of technically possible sizes is realistic if there are many particles. Let one of them come across. This is not surprising if you consider their real number. So, on Earth, about 6x10 flies through a square centimeter of area every second6 neutrinos formed on the Sun. And the usual statistics of neutrino events for modern detectors, much more than a centimeter in size, are a few or the first dozen per year.
The enormous penetrating power of neutrinos, in addition to difficulties with registration, also means obvious benefits. A neutrino is a particle that flies directly from the place where it was formed, without deviating anywhere. In most cases, the direction of arrival can be determined with some accuracy, and by the neutrino energy it is often (but not always) possible to say what kind of reaction the particle has become. The first of these properties favorably distinguishes neutrinos from all other cosmic particles, which on their way to us are influenced by external factors in the form of magnetic and gravitational fields, as well as matter that is opaque to them.
Difficulties and charms
Modern detectors do not register neutrinos themselves - this is still impossible. The object of registration is the results of the interaction of the particle with the substance filling the detector. It is chosen so that neutrinos of certain energies of interest to developers react with it. Since the energy of neutrinos depends on the mechanism of their formation, we can assume that the detector is designed for particles of a certain origin.
Here we see an analogy with the "electromagnetic" astronomy we are used to. An optical telescope is even visually noticeably different from its radio sibling, both of them are from an X-ray telescope, etc. The difference is even more noticeable than in the "neutrino" case, where all devices look formally similar. The parallel, however, is not entirely correct - neutrinos of different energies are formed in the course of processes occurring in different celestial bodies, and waves of different frequencies - at the same ones.
A common feature of all modern neutrino telescopes is the measures aimed at shielding the equipment from all foreign particles. Neutrinos, although there are a lot of them in nature, are very rarely detected by detectors. Any extraneous noise from cosmic or terrestrial particles will probably drown them out. Therefore, the standard placement of a neutrino observatory is in a mine or, in some cases, under water, so that the overlying stratum blocks unnecessary radiation. This stratum is also carefully selected - rocks, for example, should be as less radioactive as possible. Granites will not work for us, nor will clays. A good location for a detector is a clean limestone mine.
Another important requirement is to be as far away from nuclear power plants as possible.A working nuclear reactor is a very powerful source of antineutrinos, which are superfluous in this case.
The best direction for a neutrino observatory is to receive particles from below through our planet. For neutrinos it is transparent, for everything else it is not. A kind of natural filter.
Modern detectors determine a neutrino event by its "destructive effect". When an elusive particle nevertheless interacts with the substance of the detector, it causes the destruction of the original atomic nucleus with the formation of some other particles. They are then found in the detector. To cause such a reaction, a neutrino must have its own energy not lower than a certain level required for a given detector. Therefore, modern technology always has a lower limit - it registers neutrinos with energies above a certain level. In this order, we will consider them.
Shards of the Big Bang
Once upon a time, the universe was small and very opaque. The future matter in it was placed so densely that even neutrinos could not fly through it. This epoch lasted, according to standard concepts, for a very short time: about 1-3 seconds. Then the space became quite large, its contents were placed more freely, and since then to the present day the Universe is practically transparent for neutrinos.
During the Big Bang and the events that followed it, a lot of our particles were formed, most likely about the same as the number of photons. The latter, now constituting relic radiation, are abundant around us. If you count in pieces, then there are about a billion times more of them than protons with neutrons.
Like photons, neutrinos gradually cooled down as the Universe expanded, and now their temperature is about 3-4 K. More precisely, it should be like this, but it has not yet been verified.
The fundamental difference between relict photons and relict neutrinos is that the former are easily recognized by modern technology, while the latter are not. We are talking about ultra-low energy neutrinos, and what kind of detector can “catch” them is a big question. Modern technology is not capable of such an achievement, and it is widely believed among professionals that it will not exist at least until the end of this century.
In 2010, a team of scientists from the Massachusetts Institute of Technology was reported trying to detect relic neutrinos by observing the decay of tritium nuclei. This isotope of hydrogen is very unstable, and to "push" its nucleus to decay, the impact of any particle with non-zero energy is enough. Not to mention the fact that it can disintegrate itself, without any external influences (half-life - 12 years). Tracking the energy of the resulting fragments and remembering the law of conservation of energy, one can distinguish among them those that were obtained from spontaneously decayed nuclei, and those that were acted upon by some external forces. In the case of a well-shielded detector, these will in most cases be neutrinos. The latter can be divided into high-energy neutrinos, about which we already know a lot, and low-energy neutrinos - the required relics.
Everything would be fine, but for the implementation of this idea, a technique that is supersensitive for the present times is needed. Probably, it was for this reason that no news of decaying tritium was received in the following years. This is unfortunate - the discovery of relict neutrinos and the possibility of at least roughly counting them would greatly help cosmologists in understanding how the universe was formed.
Strictly speaking, our star is a source of exactly the same neutrinos, like any other star. The main difference is that the Sun is much closer, which means that there are much more solar neutrinos around us. Accordingly, the likelihood of their detection is much higher. The energies of the sought particles are in the range from hundreds of keV to tens of MeV.
These neutrinos were first discovered in 1967 at a detector housed in the former Homestake gold mine in South Dakota.
The operation of this neutrino detector was based on the chlorine-argon method: the detector was a three hundred and seventy liter tank located at a depth of 1400 m and filled with tetrachlorethylene (C2Cl4). In addition to the "usual" isotope 35Cl also included 37Cl, which, interacting with neutrinos, turned into radioactive argon (37Ar) with a half-life of 5 days. Then the equipment recorded its decay, by the fact of which the detection of neutrinos was determined. Such a pretentious path was inevitable when using the technology of that time with its measurement accuracy, but it was very irrational. The hit of the neutrino in the detector was recorded long after the fact itself and in a way that did not allow determining the direction in which the particle flew.
Now the search for solar neutrinos is carried out in several observatories. The most famous of these is the Borexino neutrino observatory in Italy. We will talk about it, all the more since its design is typical in many ways.
The observatory's detector is located at a depth of 1400 m in a tunnel under the Gran Sasso massif. The rock mass above the station in terms of screening capacity is equivalent to 3.8 km of water.
The installation is made in multi-layer. Outside is a steel dome filled with 2,100 tons of ultrapure water. Its thickness is viewed by photomultiplier tubes and plays the role of a fuse against cosmic radiation. Relatively few cosmic muons that have managed to overcome a rock mass, falling into water, move faster than the speed of light in it (note that we are talking about the speed of light in a certain medium, in this case, in water). This means that the energy of the particles is spent on Cherenkov radiation in the optical range. Having recognized the flash, the automatic device turns off the detection system for two milliseconds, avoiding false alarms.
This is not a new idea, the protection against cosmic particles was arranged in about the same way in the very first experiment to detect neutrinos in 1953.
The core of the installation is a large (13.7 meters in diameter) round steel tank filled with a scintillating (that is, glowing when ionizing particles hit) liquid. The number of photons emitted during a flash is proportional to the absorbed energy, so that by counting the photons, you can determine the particle energy. To collect light, 2212 photomultipliers are installed on the inner surface of the sphere.
The outer layer of the scintillator (2, 6 meters) acts as another screen that blocks radiation from steel, which inevitably contains a certain amount of radioactive elements.
The next layer of the "onion" is a nylon sphere with a diameter of 8.5 meters, inside which there are 278 tons of scintillating liquid. Since nylon also contains radioactive elements, only those flashes that can be detected within a radius of three meters from the center of the trap are included in the "total count". It is believed that the probability of penetration of foreign particles there is no longer very high.
The scintillator itself is thoroughly cleaned, as a result of which the content of uranium and thorium in it is about 10-18 y / y. This is very small. For comparison, a ton of any natural substance (including an unpurified scintillator) usually contains from 0.1 to 1 g of uranium and thorium.
The neutrino telescopes in use today may differ markedly in details, but their general outlines are approximately the same: a dungeon and an "onion" design that provides shielding from all sides.
The Borexino observatory was built to "catch" solar neutrinos with energies of about 870 keV, which are formed during the reverse beta decay of beryllium in the course of one of the reactions predicted by theorists. As established by measurements, such a reaction is actually taking place in the interior of the Sun.
The achieved level of interference suppression made it possible to switch to the registration of neutrinos of lower energies - from zero to 420 keV. Such particles are formed when two protons combine to form the nucleus of a deuterium atom. There are significantly more of them, but interference is also stronger in this range. Because of this, neutrino data have practically not been recorded so far. It turned out that their real number (66 ± 7 billion neutrinos per square centimeter per second) is in good agreement with predictions (60 billion). These are, of course, calculated figures; in reality, the installation recorded an average of 144 neutrinos per 100 tons of its own mass per day.
One might ask, how important is all this if theorists predicted everything correctly anyway? Alas, one cannot look directly into the depths of the Sun, one can only observe the particles emitted by them. Theoretical models, of course, are a good thing, but they can be different, and in this case you have to choose between them. At any moment, any of them may turn out to be incorrect, and then the real picture will need to be explained somehow. This has already happened with the solar neutrino flux, the first measurements of which showed that its density is approximately three times different from the predicted one. As a result, neutrino oscillations were discovered, which require the presence of mass in neutrinos, this mass logically leads us to the assumption of the existence of sterile neutrinos, and those (if any) may turn out to be dark matter.
Aliens from the bowels of the earth
Neutrino geophysics is not formally the topic of our article, but how not to talk about it, since we have already started, especially since our planet, strictly speaking, is also a celestial body no worse and no better than all others.
In the bowels of the Earth, there are radioactive elements that got there during the formation of the planet and have not yet decayed. As is commonly believed, the largest proportion of them are three isotopes: 238U, 232Th and 40K. All three undergo decay to form, among other products, an electron antineutrino. These particles are further scattered from the place of their formation through the earth's thickness, which is transparent to them.
Unfortunately, antineutrinos from potassium decay are not caught by modern detectors, but the study of the other two cases is possible and very interesting. Recall that our planet has been more or less studied by drilling about 10 kilometers deep with a radius of about 6370 km. Everything that is deeper, we know only from the data of seismology, which allows us to trace the reflective and refractive boundaries in the rock mass. What they are and how they were formed is decided on the basis of theoretical models.
The study of neutrinos emitted by the Earth can help us at least understand how much radioactive elements are in the earth's substance and where they are mainly located. As for the latter, there are different versions, ranging from the fact that uranium with thorium is an attribute of the lower part of the earth's crust, and ending with the fact that radiation sources during the formation of the planet "drowned" to its center, and there is something like a nuclear reactor, and periodically acting.
The accumulated decay products, when there are enough of them, stop the chain reaction. Then, in a hot environment, they slowly diffuse upward (they are lighter), making room for new portions of fissile material, after which the process starts again. If this is the case, then such a cyclicity could help in explaining the changes in the magnetic polarity of the Earth and, one must think, in many other ways.
The question of the share of nuclear reactions in the total heat release of the Earth is also interesting. Recall that the earth's interior gives out a total of about 47 TW of heat per year, but scientists still vaguely imagine how much of this energy comes from radiogenic heat, and how much from the residual heat that was released once during the gravitational differentiation of terrestrial matter.
Geoneutrinos were first reliably detected at the aforementioned Borexino neutrino observatory ten years ago.In 2015, scientists working with the obtained data published an overview of the results. It turned out that the total thermal power of decays of uranium and thorium is somewhere in the range from 23 to 36 terawatts. Radioactive decay and, accordingly, the decaying elements themselves, are found both in the earth's crust and in the mantle. Both are generally consistent with the data of some theoretical models and help to make the right choice between them. So far, the high content of uranium in the earth's interior seems unexpected - it is about twice as much as it was thought. It is too early to say that these data refute something. For six years, the detector has recorded 77 "terrestrial" neutrino events, of which about two-thirds are reactor neutrinos from nuclear power plants, that is, interference. More data needed.
The last part of our story is devoted to neutrinos of high and ultrahigh energies - from tens of teraelectronvolts and above. “How so? - the reader asks. - Solar neutrinos have an upper threshold of tens of meV, while here they are many orders of magnitude higher. Where did the missing thing go? " There is no mystery here. The "hole" in the range falls on the area in which there are many neutrinos of atmospheric origin, which are formed when high-energy cosmic rays (consisting of protons, electrons, etc.) hit the air. There are a lot of high-energy particles in space, and they bombard the Earth continuously. Cosmic neutrinos of the same energies also reach us, but against the background of "debris" they are lost, and it is impossible to isolate them at the current level of technology development.
Raising the lower limit of the range of interest to us to teravolts, we find ourselves in an area where there is relatively little interference. Neutrinos of such high energies are most often of cosmic origin, in many cases even extragalactic. A long time ago, in a distant galaxy, a supernova exploded or something else similar happened - these are the traces of this event that reached us through billions of light years. Actually, the first reliable case of registration of astrophysical neutrinos in 1987 was timed to coincide with a supernova explosion in the Large Magellanic Cloud.
On the other hand, there are also very few ultrahigh-energy neutrinos in the space around us. This means that a larger detector is needed to register them. Meters and even tens of meters will not work, we will talk about kilometer-sized devices. It is not yet possible to make a tank of this size. And why?
The scheme implemented today in operating and under construction installations is very simple in its principles. Garlands of photosensitive elements are lowered into ordinary water to a depth of a couple of kilometers, forming an array with a given vertical and horizontal step. The substance of the detector is the surrounding water itself. Interacting with an atom of any of its constituent substances, a high-energy neutrino generates particles, the speed of which, to match the speed of the neutrino itself, is very high - more than the speed of light in water. A particle moving at this speed emits Cherenkov radiation, which is detected by detectors / photomultipliers.
The visual effect depends on what kind of neutrino we came across. Muons usually generate thin straight tracks, electron and tau neutrinos - broad cascades formed by many electrons and positrons scattering in different directions. In the first case, the direction of motion of the original particle is restored with an accuracy of about half a degree, in the second, the error in its determination can reach about 15 degrees. The neutrino energy is determined by the number of Cherenkov photons emitted by the fragments.
Now there are very few installations of this type in the world - three. The IceCube Observatory has been operating at the South Pole for several years. As you might guess, in this case, Antarctic ice is used instead of water.Wells were drilled (more precisely, thawed with a thermal drill) in it, and garlands of photomultipliers were lowered into them, which were then frozen into the ice. Its transparency at a depth of a couple of kilometers is even better than thought, which makes it easier both to collect data on today's installation and to formulate plans for its improvement. It is quite possible that the initial volume per cubic kilometer will be increased tenfold in the future. There are many places in Antarctica.
Tracking of neutrino flares is carried out automatically on IceCube. If the station registers two or more neutrinos arriving at small intervals from approximately the same place (the flight directions of the particles differ by no more than 3.5 degrees), the search for a probable source is automatically launched by means of electromagnetic astronomy operating in different ranges of electromagnetic radiation - from optical (including the "MASTER" network) to X-ray (Swift) and gamma radiation (VERITAS). So far, it has never been possible to find such space attractions.
In February 2016, the "cube" detected three neutrinos at once. Such an event is statistically expected about once every 13 years, so there is reason for caution. Unfortunately, the directions of neutrino movement diverged by a tenth of a degree more than the automation needed, so the search for the source was started manually only after 22 hours. Nothing was found.
In July 2018, it was announced the registration of ultrahigh-energy neutrinos emitted by the blazar TXS 0506 + 056, which is located 4.33 billion light years from Earth. Astrophysicists hope that this discovery will help them understand the nature of ultra-powerful cosmic rays and improve methods for observing them.
In recent years, several works have been published, the authors of which have tried to compare the sources of astrophysical neutrinos with sources of cosmic rays and other objects known to science. So far, there has been no obvious success, but this does not mean that it will not continue in the future.
In the Mediterranean Sea, the KM3NeT telescope (KM3 Neutrino Telescope) is being completed, of which ANTARES, built back in 2007, will become an integral part. Baikal GVD is under construction on Lake Baikal. In both cases, it is too early to speak of full-fledged results.
Summing up, it should be noted that neutrino astronomy is still very young. She is about twenty years old, and her most promising directions are even less. Therefore, one should not expect full-scale results from her yet, but those that already exist look good.