Riddle of a neutrino from Supernova 1987A

    Supernova 1987A: during and before the outbreak

    Not so long ago there was a good article by Bars21 about neutrinos from supernovae . I really liked it, and some moments (for example, about the urka-process) led to the fact that we not only read or listened to the same teachers, but maybe we knew each other personally :)

    One of the paragraphs of this article (for a better understanding of what will be said in the future, I recommend rereading it) was devoted to the registration of neutrinos from Supernova 1987A, and I would like to supplement this moment. The fact is that the detectors that detected the neutrino burst were not 3, but 4, and the bursts themselves were 2. But this is practically unknown even to experts, not to mention the general public. It is even more insulting that Russian (or Soviet, as you like) scientists played a significant role in this discovery (though, is it a discovery?).

    And it was like that.

    In 1984, the forces of the Soviet-Italian collaboration led by Academician Georgy Timofeevich Zatsepin (mentioned in the original article by his student Vadim Alekseevich Kuzmin, for all his merits, was not the pioneer in this field) in the tunnel between France and Italy under Mont Blanc was built Liquid Scintillation Detector (LSD).

    About personalities
    Due to the fact that I knew the main characters of this story personally, because of the age difference, I will call everyone by their first name and patronymic.

    The abbreviation, ahem, led many to all sorts of frivolous thoughts, but when I asked about this the professor of the University of Turin Piero Galeotti, who was credited with the authorship of the name, he somehow jokingly joked.

    The detector circuit and the main antineutrino reaction underlying its operation (the one that was expected during construction)

    The detector contained 72 scintillation counters measuring 1 * 1 * 1.5 m in iron containers. A scintillator is a substance, usually based on petroleum products (in our case it was white spirit), in which charged particles generate flashes of light that are captured by photoelectronic multipliers. As a shield against external radioactivity, sheets of iron were placed on all sides of the installation, so that approximately 200 tons of iron were produced per 90 tons of scintillator. Initially, the main goal of the installation was to search for neutrino radiation from supernovae.

    February 23, 1987 already at 2:52 UTC (5 hours earlier than the KII, IMB and BSTT mentioned in article Bars21) the LSD detector recorded the expected signal: 5 events with an energy release of 6 - 11 MeV, very similar to the neutrino interaction, for 7 seconds.

    The printout that the team discovered in the morning of February 23, 1987, and this team itself

    At 7:36 UTC in LSD, along with three other detectors, 2 more similar events were recorded with an energy release of 8-9 MeV. During the first signal in LSD, two gravitational antennas were triggered in Rome and Maryland (USA), which were massive cylinders suspended on thin threads. Monsters like the current LIGO or Virgo installations have not yet been built. And in Kamiokand II, a signal consisting of two events was also recorded.

    New chronology;)

    But how could a huge detector containing more than 2000 tons of water “lose” the number of events in a small installation with 90 tons of an active substance - a scintillator? And where did the second set of events come from? What happened did not fit into the theory of standard star collapse so much that the signal detected by LSD was explained by a random background event (a similar one, however, was not observed during the entire operation of the installation, until 1999), and preferred to forget about it. Probably, the traditional slowness of Soviet scientists, who sought to double-check everything at the moment when it was necessary to forge the iron, while it was hot, also affected.

    Here it is necessary to delve a little into those conditions under which the standard collapse model is valid. In fact, this is a literal “spherical horse in a vacuum”: a star should not rotate, have a magnetic field, but be spherically symmetrical. In the years when this model was developed, systems of differential equations with more complex boundary conditions, I suspect, simply could not be solved - even numerically. However, in this model, no one was able to get a dump of the star’s shell, which we will perceive as a supernova flash.

    Famous astrophysicist image of the remnant SN1987A

    But in reality, stars are not at all spherically symmetrical and, as a rule, they rotate. Even modern images of the remnant of Supernova 1987A are not at all like a spherically symmetric picture. So there is every reason to believe that in nature the outbreak of the Supernova occurs due to some more complex processes. But which ones?

    In 1995, Vladimir Sergeyevich Imshennik, with the help of Dmitry Konstantinovich Nadezhin, finished developing a model, which he called the theory of rotating collapsar. Its essence is as follows.

    If the iron core of the star (and we know that stars are producing atoms from hydrogen starting from helium to iron, the formation of heavier nuclei is energetically disadvantageous) rotates on the threshold of gravitational compression, which is caused by the “inheritance” of rotation of the whole star and the law of conservation of torque , then from the calculations it follows that the period of its rotation is a thousandth of a second. Naturally, the core is flattened in the axial direction and instability occurs. A dumbbell arises from a flattened disk, which is torn to pieces (in the simplest case, into two). At this moment, mainly electron neutrinos are emitted (and not neutrinos of all types, as follows from the standard collapse model).

    The binary system begins to rotate around a common center of mass, actively emitting gravitational waves, due to which both energy and the moment of rotation are carried away from the system. The fragments of the nucleus come together, so that the moment of mass transfer comes: the lighter component begins to dump the substance onto the heavier one, continuing the rotation. When the mass of the light component becomes about 10% of the solar, it becomes unstable and explodes, and the heavier one collapses, presumably according to the standard scenario (this moment personally always seemed like a big stretch to me in the whole model).

    Despite the fact that the density of matter in the star’s core in both scenarios — a rotating collapsar and a standard one — is close to nuclear, in the second case the temperature in the center of the nucleus is two orders of magnitude higher. Because of this, neutrinos are born with rather high energies - 100-200 MeV, but at this density of matter even neutrinos will interact repeatedly. Scattering and reradiating, neutrinos of all types with energies of 10–20 MeV come to the surface. Due to the low temperature, the main reaction of neutrino formation in the rotating collapsar is the “indentation” of electrons into protons:

    e - + p → n + ν e

    The neutrino energy in this case will be approximately 30 - 40 MeV, the amount of substance that the neutrino needs to be overcome near polar directions is much less. Similar neutrinos can reach the surface of a star without interaction, retaining their energy of 30–40 MeV.

    To detect electron neutrinos emitted during the first outbreak, nuclei such as deuterium, carbon, and heavy neutron-rich elements such as iron, lead, and others are well suited. A sufficient number of such elements existed only in the LSD (the BPST also included iron, but there it was relatively small and in a not so successful configuration). Thus, this setup turned out to be the only one that could reliably “see” something during the first flash. The interaction of neutrinos with oxygen contained in the water formula would also give several events (it did, but the Kamiokande II team did not advertise it), but much less than iron, if we count the effect per unit mass.

    The fact is that as a result of the interaction of an electron neutrino with iron, cobalt and an electron are formed.

    ν e + 56 Fe → e - + 56 Co *

    The cobalt-56 core (due to purely nuclear reasons) is always born in an excited, not ground state. This excitation is removed by the emission of one or more gamma rays. And if the electrons born in the iron may not come out of it, then neutral gamma rays (with characteristic energies of 1.7, 1.8, 4, or 7 MeV) have greater penetrating power and will almost certainly fall into the scintillation layer.

    Scheme of interaction of neutrinos with iron in an LSD scintillation detector.

    The energy spectrum in the scintillator will be described by the formula dE / E with an additional maximum of about 7 MeV. The main contribution to it will be made by gamma rays from the removal of cobalt excitation and gamma rays generated by an electron as a result of its inhibition in iron.

    It seemed that the riddle of Supernova 1987A was solved using a model of a rotating collapsar. Olga Georgievna Ryazhskaya, another student of Zatsepin, who was actually responsible for the Soviet LSD experiment, spoke at several conferences with Imshennik trying to convince the world of the discovery (it seems to me that its scale pulled the Nobel Prize). However, the scientific community’s distrust of the long-standing result of the LSD experiment was so great (indeed, “as you name the yacht, it will float”), and time was lost (it was possible to compare the experimental data with the calculation only in the early 2000s, after 15 years after the outbreak itself), so this explanation did not receive wide recognition. Only in Russia alone were several more competing theories developed pretending to explain the mechanism of the supernova explosion and the generation of neutrinos. In the absence of experimental evidence, all of these theories have remained models or, if you like, nothing more than hypotheses.

    The only conclusion that can be drawn from this story for sure is the need to build such devices that could record not only the “universally recognized” interaction of the electron antineutrino with the proton, but the interaction of all types of neutrinos. For this, not only Cherenkov detectors using water, or purely scintillation installations are needed, but it is desirable to have a cellular configuration — some sort of neutrino calorimeters with the ability to measure energy — using heavy elements like iron or lead.

    LVD For lack of space in the mine, he was always photographed from one angle and only when there was no other installation in front of him

    A similar installation was the successor of the LSD experiment - the LVD detector (Large Volume Detector), located in the hall next to the Borexino underground laboratory of Gran Sasso in the Italian Apennines. It contained about 1000 tons of the same scintillator and the same amount of iron as load-bearing modules and could successfully record up to 1000 purely neutrino events in the event of a Supernova outbreak in the center of our Galaxy. Alas, this phenomenon is quite rare, and over the years of its work (in 2001 it was completely built, but observations began several years earlier) to this day he was not lucky. I write about it in the past tense, because, unfortunately, next year it will be planned to be decommissioned. Perhaps humanity will forever miss the opportunity to solve one of the mysteries of the universe.

    But I still believe in the best.

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