One of the technical wonders at the Institute of High-Energy Physics (IHEF) at Protvino is a unique research unit-a tracer, or tagged neutrino complex (TNC). It was built in the late 1980s-early 1990s by a large team of scientists from Russia, Italy and Germany for research on one of this country's biggest accelerators of the Institute of High-Energy Physics which "speeds up" protons to 70 BeV.
Sergei DENISOV, RAS Corresponding Member, department head, Institute of High-Energy Physics (Protvino);
Vladimir LIPAYEV, Cand. Sc. (Phys. & Math.), senior researcher;
Anatoly PETRUKHIN, Dr. Sc. (Phys. & Math.), Professor of the Moscow Institute of Physics and Engineering;
and Rostislav KOKOULIN, Cand. Sc. (Phys. & Math.), senior researcher
Neutrino is born in the process of bombardment of a special target with an accelerated beam of protons during the decay of charged pi- and K- mesons * . If a mu- meson (muon) appears thereby, this neutrino is called muonic; and if an electron is produced, it is called electronic. There are definite distinctions between the two.
Meson decays occur in a long tube in which high vacuum is maintained in order to rule out the particles' interaction with the air. Usually a thick steel shield, weighing thousands and even tens of thousands of tons, was placed right behind it to absorb all generated particles except neutrinos. These are known for very weak interactions with matter, which means that a shield of tens or hundreds of meters is no barrier for them. Placed next was a neutrino detector combining the function of a target, which is bombarded by neutrinos, and that of a tracer of the products of their interactions. This being so, the unit had to be very massive to capture at least some of the "invisibles".
The above standard layout of the experiment, however, has some serious drawbacks. It does not provide for reliable determination of such key neutrino parameters as energy, flight trajectory and type (electronic or muonic, neutrino or anti-neutrino), for registered in this kind of experiments is only the fact that the particle interacts with the target's matter, but no data on the decay which produced this particle.
The unit developed at our Institute is essentially different from all of its predecessors in that it registers
* See: L. Smirnova, "Stepping into the Twenty-First Century", Science in Russia, No. I, 1996 . - Ed.
not only the neutrino from the K-meson decay, but also the muons or electrons as well as other particles. Judging by the end products of such a reaction, one can reconstruct its whole picture as well as determine the type of the neutrino, its energy and flight trajectory. And that means that we can "tag" (with the help of a special tagging station placed after the decay tube) the required particle by associating it with a certain decay process. This amounts to an absolutely new approach in the science of neutrino.
Apart from the tagging station, an important feature of TNC is that it is equipped with a large liquid-argon spectrometer (LARS) used for neutrino registration. Detectors working on this principle display good energy resolution and high stability of parameters. Widely used in high-energy physics, especially in experiments with colliding beams, this is the first time they are employed for neutrino registration.
The LARS unit consists of two cylindrical cryogenic vessels measuring 20 m in length and 4 m in diameter. And one should bear in mind that the temperature of liquid argon at vapour pressure of 1 atm approaches - 190 0 C. Each of the two vessels contains about 300 tons of it, with impurities making up no more than a millionth fraction of the total amount. Immersed into the vessels is a system of electrodes- thin aluminum plates- placed perpendicular to the LARS axis. The odd ones are "grounded", and the even ones carry a current of 6 kV. The charged particles (hadrons, muons, etc.) generated during the neutrino flight collide with the atoms of argon and ionize them, knocking out electrons. The latter, drifting towards the high- voltage electrode, generate upon it an electric signal whose strength is proportional to the number of electrons in the gaps between the high-voltage (signal) and the grounded plates, which makes it possible to assess the energy losses of particles on ionization. And since the high-voltage plates are not monolithic and consist of 48 separate strips, there is a possibility for each of them to measure the amplitude of the induced signal. Besides, these strips in the adjacent electrodes are turned at an angle of 120 against one another. And since LARS contains 288 high-voltage plates and, accordingly, 13,824 electronic channels for the registration and measuring of the amplitude of the signals (all this makes it possible to reconstruct the spatial picture of neutrino interaction), it turns out that in all of its parameters our detector is the world's biggest liquid-argon spectrometer. Another asset which makes it really unique is that it is also an indispensable tool of research in other areas of science.
In recent years there have been clear tendencies for using detectors of this kind for studies ofhigh-energy
cosmic ray particles. And LARS has become the first detector like that to be used for such studies on a regular basis. These studies were begun in 1995 and reached a particularly large scale after a teaching and research center was set up within the framework of the Special Federal Program launched by a presidential decree in 1997.
As a result scientists were able to measure the energy parameters of muons which, on the one hand, carry full information on the spectra of rays emanating from the depths of the universe and which, on the other hand, make it possible to study the interactions of high and super-high energy particles from space with the upper atmosphere of the Earth.
The new method of research was initiated and developed by experts of the Moscow Institute of Physics and Engineering, and it can be briefly described as follows. A high-energy muon, passing through a detector, collides with the atoms of the matter inside-which is argon in this particular case-to produce either a photon or an electron-positron pair. The latter interact in their turn with the surrounding atoms and generate again photons, electrons and positrons with the whole process becoming avalanche-like. All that combined causes what are called cascade showers; their parameters, measured by a spectrometer, make it possible to assess the initial energy of the muon. The technical potential of LARS (impressive thickness, reliable registration of low- energy showers and a high transmission factor) is far superior to the same parameters of all of the previous units of this kind. This conclusion was borne out by an experiment conducted in 1996-1998 during which hundreds of thousands of muons were registered with the formation of cascade showers, and new data obtained on the spectra of these particles in the energy range above 1 Tev.
And now let us take a look at yet another type of collision of high-energy muons with the nuclei of detector material during which more muon pairs (muon-antimuon) are produced. These, together with their "parents", give rise to "tridents". The probability of this process is scores of times lower than that of the electron-positron pairs formation. Scientists were trying to isolate tridents in a whole number of
experiments, but the insufficient resolution of the detectors used did not allow to get any reliable results. The LARS unit eliminates this drawback. A much smaller number of tridents turn to be generated in reality than has been expected in theory.
Finally, we come to the third type of interaction of muons with matter which is called inelastic, or non-elastic collision. In this case a nucleus simply "disintegrates" with the formation of secondary hadrons (pi-mesons and K- mesons, etc.). Studies of this phenomenon will provide important information on the structure of nucleons (protons and neutrons). At the same time the inelastic interaction of muons with nuclei results in the formation of nuclear- electromagnetic cascades. These differ strongly by their longitudinal and transverse dimensions from the above cascade showers, electron-positron pairs and photons, and can be reliably identified with the help of LARS. A quantitative analysis of such events is very important in that it helps to verify various theoretical predictions.
And more. Our "superspectrometer" also makes it possible to study the structure of what are called extensive air showers (EAS), or Auger showers * . These are formed with the development in the Earth's atmosphere of nuclear-electromagnetic cascades initiated by interactions of protons and nuclei of primary cosmic rays of very high energies. Thus, with the primary proton energy of 1016 eV the surface of our planet is struck simultaneously by emissions of charged particles, and the size of the area covered by an extensive air
* See: G. Zhdanov, "Puzzles of Cosmic Radiation", Science in Russia, No. 2, 1999. - Ed.
shower reaches hundreds of thousands of square meters.
Conventional detectors for the studies of extensive air showers usually consist of a lattice of tens and even hundreds of widely spaced scintillation counters (devices for the registration and spectrometric measurements of particles). The measuring instruments used for the registration of the central, most energized region of this phenomenon also include this or that spectrometric detector. By comparison, LARS is much superior in terms of spatial resolution, sensitivity, the possibility of registration of individual particles as well as cascade showers and tracing their development in depth of matter while covering the same area of observation. This opens up unique opportunities for the studies of spatial and energy parameters of extensive air showers. To check on the range of LARS potentialities in combination with an EAS registration complex, a pilot version of this unit consisting of 8 counters was designed and built by a joint team of scientists. It included experts from the Institute of High-Energy Physics, the Moscow Institute of Physics and Engineering and the RAS Institute of Nuclear Research. And their measurements confirmed the expectations of physicists right from the start.
Obviously, by its potentialities and technical parameters LARS is a world's unique spectrometer. And one can hardly overestimate its importance in the current studies of the physical picture of the universe.
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