by Boris KHRENOV, Dr. Sc. (Phys. & Math.), leading researcher, Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University
Heavenly phenomena have beckoned to humankind since time out of mind, with overawed man fumbling for spiritual affinity with cosmos, the universe, which has become woof and fabric of human culture. The nightly messengers in the form of meteors, or "falling stars", are one intriguing phenomenon that still commands much attention.
There are meteors and meteors, however. The "falling stars" differ according to their origin. Some are just particles of cosmic dust (meteors proper) that, zooming into the atmosphere at a velocity of dozens of kilometers a second, collide with molecules of the air, come apart and burn up, releasing heat and light.
But the light produced by elementary particles from deep space is of different nature. Such particles, accelerated to superhigh energies, are known as cosmic rays. In this case the burning up proper does not take place, for the bonds keeping their substance together are far stronger than the chemical bonds. That's why this substance does not disintegrate and bum up.
What is the scenario of the process?
Interacting with the nuclei of atmospheric atoms (mainly those of nitrogen and oxygen), the space voyagers produce new, secondary particles. The latter, too, collide with the nuclei of atmospheric atoms to give rise to yet another generation of new particles. Thus what we call a "nuclear cascade" is triggered off. Playing an essential role here are the secondary (pi)-mesons, or pions, and K-mesons, or kaons, that rapidly disintegrate into high-energy photons (gamma-ray quanta).* Interacting with the atmosphere - rather, with its components - these particles give birth to electron/positron pairs which, colliding with the nuclei of the atmosphere's atoms, reproduce their "parental" particles. What we get as a result is a cascade of electrons, positrons and gamma-ray quanta. This intensive particle flux propagating at the velocity of light is confined within an arbitrarily limited space in the form of a disk; its thickness and width are determined by particle scattering in the atmosphere and changes depending on its density. Say, at sea level and in the
* See: L. Smimova, "Symmetries and Their Distortions in the Microworld", Science in Russia, No. 6, 1997. - Ed.
mountains the disk is only a few meters thick, and its radius is about a hundred meters. However, some of the particles break free from the confines of this disk and travel as far as several kilometers. Because of its considerable width the particle cascade is called a "broad atmospheric shower" (BAS). We might as well note a priori: it is the large area of the disk that has made it possible to register cosmic rays of immense energy above 1 joule (j).
The Russian physicist Pavel Cherenkov, way back in 1934, made a breakthrough discovery: charged particles, when flying within substance (not in vacuum) at a rate higher than the phase velocity of light, produce emission directed along the pathway of their movement. This phenomenon is known as the Cherenkov (Cerenkov) effect, or radiation. The electrons and positrons of the cascade obey this effect. Studying the Cerenkov light (radiation) with the aid of detectors over a large area, we can obtain a quantitative value for the energy of primary cosmic rays and determine their pathway.
Cerenkov radiation is not the only source of light from the BAS disk. Colliding with the atmosphere, its charged particles "excite" - or rather, energize - the atmospheric atoms and molecules (i.e. electrons move into higher energy levels) or even ionize them (with a free electron and a positive ion thus obtained). All these processes are akin to burning, which means that the cascade of BAS particles ultimately "bums up" in the air too. The excited (energized) atoms or molecules of the air (their lifetime is very short - just a few millesimal fractions of a microsecond) produce an isotropic light Hash, a phenomenon known as fluorescence of the atmosphere. And so the disk of the BAS particles flashes like a regular meteor, though the velocity of this flash is much higher, equal to that of light. The BAS disk is described as a "rela-tivistic meteor". Understandably, it is
not visible to the naked eye. To register a like phenomenon we should employ a chamber with a mirror no less than 1 m 2 large (concentrator). The Russian scientist, Academician Alexander Chudakov, was one of the first to point, in 1962, to the possibility of registering the fluorescence of ultrahigh energy BAS.
Such studies are not an end in itself, of course. By measuring the vector of the movement of the BAS disk and the brightness of its fluorescence, we can determine the main parameters of the primary cosmic particle - its energy and its inflight trajectory.
The present level of science and technology has enabled us to design and build optical chambers for observing what we call "relativistic meteors" and detecting weak light fluxes. Our panoply includes photo-multipliers (with the signal amplified millions of times over), devices for registering fast processes (and obtaining images with a scanning rate of dozens of millions of sequences a second), and oversize mirrors (concentrators).
The fluorescent track left in the atmosphere by a high-energy cosmic particle is a unique object of observation. It cannot be simulated and confused with anything else - say, with the discharge of lightning, for the velocity of this discharge is hundreds of times as low.* This singular feature enables us to identify the comparatively weak fluorescence of the BAS disk from among the other phenomena in the night sky.
The first essential measurements of BAS were carried out with the aid of special fluorescent detectors composed of highly directional elements so arranged that the whole setup looks like a fly's eye. That's how physicists of Utah State University (USA) have called i - just Fly's Eye. Compared with other BAS detectors, this setup allows to scan a far larger area. Its advantages and the proper location (desertland with a more or less transparent atmosphere devoid of light interferences) have made it possible to register an elementary particle of 30j, the most energized particle detected thus far.
Despite good progress made in BAS studies with the use of such setups, their possibilities happened to be rather limited. For one, the transparency of the atmosphere in the horizontal direction above the terrestrial surface (that's where the instruments are placed) is rather poor after all. Therefore, to boost the monitoring efficiency the costly detectors should be spaced apart all too close to one another, every 30 kilometers. And then our planet is not one solid desertland alone, there is something else to it as well. So this line of studies is unaffordable.
However, there came a windfall as a spinoff from space exploration. If a fluorescence detector is placed aboard an orbital satellite to scan "relativistic meteors" thence, it can open up boundless opportunities to researchers. The point is that the atmosphere of our planet is quite transparent when observed from circumterrestrial space (particularly, the upper layer of the atmosphere, above the cloud cover where BAS releases the larger part of its energy); the overview area depends only on the altitude of the satellite's orbit and on the detector's angle of aspect. For instance, a 120 0 angle-of-aspect detector on board the International Space Station (the altitude of its orbit averaging 400 km) can "visualize" an area of about 1 mln sq. km.
Some countries are working on the design of orbital detectors; this work is on at our Institute too. We have prepared designs of two detector modifications to be employed in two separate experiments: one dubbed Tracking Setup (TS, or TUS in Russian), and the other - Cosmic Rays of Maximum Energies (CRME, or KLPVE, respectively). Both experiments are meant for studying UH energy space radiation, i.e. above 1j.
The centerpiece of our setup are Fresnel mirrors (concentrators) with a photodetector in the focus. These mirrors, folded compact on earth, are unfolded in orbit to large size (TUS to 1.5 m 2 , and KLPVE - to 10 m 2 ).
Now, a quick overview of such a detector in operation. The light captured by the mirrors is collected in their focus-that's where the photodetector is fixed; it is composed of fast photoelectronic multipliers, or gadgets in which light beams are converted to electric signals. The amplitude and time of such signals are transformed into a digital code fed into a computer. The latter evaluates the dataflow and reconstructs the picture of the BAS movement within the detector's field of vision.
With this orbital "eye" one can watch "relativistic meteors" over an area of dozens of thousands of square kilometers - true, on the night side of the earth. Incidentally, such setups are capable of registering not only a track left by the fluorescing BAS disk, but also the Cerenkov
1 - mirror elements;
2 - light beams.
* See: A. Perunov et al., "Taming the Lightning", Science in Russia, No. 2, 2000 - Ed.
light of BAS reflected from the clouds. The data thus obtained complement the information on the primary particle and allow to determine the position of a "relativistic meteor" track with respect to the terrestrial surface.
In fact, the "relativistic meteor" phenomenon is of exceptional interest for understanding fundamentals of the structure of micro- and macroworld. Cosmic particles - observed as "meteors" - possess energy (or mass, in the "relativistic" terminology) millions times as high as one generated at the most up-to-date particle accelerators. We cannot tell yet how their "cosmic counterpart" operates. Only a few objects in deep space that have a sufficient mass and sufficiently strong magnetic fields are able to keep back accelerating particles with the energy of several joules. How could one do that here on earth is a sealed book to us.
Besides, the very existence of cosmic particles energized to 8 j and higher (over ten such particles have been registered) runs counter to the idea of the even distribution of their generating sources in the universe. In 1966 experimental proof was obtained concerning the electromagnetic radiation, a holdover of the Big Bang creation of the universe (relict radiation), filling it in a more or less uniform pattern. Simultaneously, two Russian scientists, G. Zatsepin and V. Kuzmin (as well as C. Graysen of the United States, working independently) showed that protons energized above 8 j interact with relict photons and are absorbed fast. The distances where this process does not occur are short on the cosmic scale, a mere 50 megaparsecs*. But the extension of what we know of the universe is about 1,000 megaparsecs and, therefore, more distant sources of radiation are beyond our ken.
Yet there are two enigmas to this story. First, at such distances it is hard to find a cosmic object capable of accelerating the particles to such energies. Second, the inflight trajectory of registered protons does not fit in with any of the objects, the putative particle accelerators.
Not so long ago physicists in Russia and elsewhere came to the paradoxical conclusion: the registered particles have never been accelerated at all. They are the product of the decay of precursor particles, the proparticles so-called, possessing an enormous mass of 10 24 eV (by way of comparison: bosons, the heaviest particles known to date, have a mass of 10 11 eV). Such particles were predicted long ago by a theory postulating a single origin of all interactions in the universe ("great unification" particles). Even though the bulk of matter after the Big Bang changed into the present-day state, it still persists in the primordial state in the remote parts of the universe.
In the light of the data on "relativistic meteors" physicists are inclined to agree on this very scenario. Space laboratories enable us to undertake a fantastic project - carry out experimental studies of matter at the initial moments of its existence.
Besides our institute of Nuclear Physics, the experiments Tracking Setup (TUS) and Cosmic Rays of Maximum Energies (KLPVE) also involve the Joint Institute of Nuclear Research at Dubna and universities in Mexico.
TUS detector in shorthand:
1,2 - photoelectronic multipliers;
3 - rows of photoelectronic multipliers;
4 - photodetector in the mirror focus.
* Megaparsec - a unit for measuring space distances equal to a million parsecs; one parsec is equal to 3.26 light years, or 3.086x10 13 km. - Ed
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