Научная аппаратура «НУКЛОН»
Where are born and how are accelerated the galactic cosmic rays? Scientist of SINP MSU want to find an answer to this question by means of scientific instrument "NUCLON". The Head of the Laboratory of the Galactic Cosmic Rays, PhD. Dmitry Podorozhny told us about this project in details.
- Please, let's start from the term "cosmic rays". What does it mean?
- Cosmic rays are the fluxes of charged particles. In the near-Earth space cosmic rays are separated into solar (injected by the Sun), galactic and extragalactic. This classification first of all takes into account th difference of cosmic rays energies. The energy of the solar cosmic rays is realtively low, from several keV up to several Gev (as a rule particles' energy is measured in electron-Volts and derived units keV=103 eV, MeV=106 eV, GeV=109 eV, TeV=1012 eV). The energy of the galactic cosmic rays starts from several hundreds MeV (the particles with the lower energy are declined by the heliospheric magnetic field). The energetic boundary between galactic and extragalactic cosmic rays is a subject for discussion and according to different models varies within wide tange from 1015 eV up to 1018 eV. Ultimate energy of the cosmic rays can reach fantastic values - 1020 – 1021 eV.
The goal of our research is to study the galactic cosmic rays in the range of high energies - 1011 - 1015 eV. This flow mainly consists of nuclei: from the protons up to the heaviest nuclei from the periodic table (surely, stable isotopes, because the age of the galactic cosmic rays is evauated at the level of several millions of years). There are also electrons and positrons in small numbers.
- How we can determine where the cosmic rays were born?
- Historically formed term "cosmic rays" is not fully appropriate. The reason is that this flow of particles of interstellar matter propagates not along the direct line. The flow is declined by magnetic fields from the primary trajectory, because the particles of interstellar matter are electrically charged. Because of such declination of cosmic rays we can't see their source, as we see, for instance, the source of the electromagnetic radition. Therefore the sources of the cosmic rays are identified due to indirect data, such as composition, energy spectrum shape and possible anisotropy.
- What objects are the sources of the galactic cosmic rays?
- Power density of the galactic cosmic rays is significantly more (~1 eV/cm3) than the density of the total electromagnetic radiation of the stars in Galaxy, and than the power density of the magnetic field of the Galaxy. Therefore, the first attempts to explain the origin of the galactic comsic rays based on energy balance of the Galaxy. Supernovae outbursts, very powerful energy sources in out Galaxy were determined as the first, most probable sources of the galactic cosmic rays. With the development of astrophysics another candidates appeared. Among them are neutron stars. Many questions are still unanswered in spite of long history of the galactic cosmic rays studies, especially in the field of high energy. First of all, we'd like to get answer: is there a single mechanism of particles flows generation, or are there several different mechanisms for different energy ranges?
- What do we know today about the galactic cosmic rays production?
- Mainly space is filled with protons. Admixtures of heavy elements in the Universe are very small. At the periphery of the Galaxy heavy elements give only 0.01-0.5% from the total amount of the particles. In the galactic disc where the supernovae are concentrated and the star formation is in progress this portion is not more than ~2-3%. But this portion is larger for orders of magnitude for the cosmic rays composition! We extract cosmic rays just due to large amount of the heavy elements.
What processes produce heavy elements? Excluding light elements H, He and Li (in a very limited amount), which are produced during the creation of the Universe, all the elements in the Universe up to iron are produced during the stars evolution processes of burn and fusion of the light elements. The nucleosynthesis process (the light nuclei combine with the heavier ones during theermonuclear reactions inside the stars with separation of energy providing the star's luminosity) runs continuously during all stages of the star's evolution. Due to gravitational forces the plasma temperature in the internal areas of the stars is very high, and it is enough for the heavy elements synthesis. At the end of its evolution the massive star looks like a bulbus, where content of heavy elements grows in the direction to its center. When the energy separated during the nucleosynthesis can't withstand the pressing forces the star collapses. An explosion similar to the atomic bomb explosion but dozens times more powerful happens.
- And what is khown about the galactic cosmic rays acceleration?
- Cosmic rays are accelerated additionally according to the mechanism offered by the American physicist Fermi. Under the influence of the magnetic field the particles of interstellar matter many times cross the shock wave front, getting additional energy with every crossing. This process is of probability nature, therefore energy spectrum of cosmic rays going out to the space is a power function. So we can be sure that all nuclei heavier thatn carbon and available at the Earth passed through thermonuclear heating of some star.
By the way, the elements with the charge of the nucleus over 26 (iron) are roduced in more exotic heatings than thermonuclear one. According to the up-to-date ideas they can be produced as a result of neutron capture, which is possible only under conditions of high neutron densities and very high temperatures. These conditions are available in the moment of explosion of the second type supernovae (which results in a creation of a neutron star) or combination of two neutron stars, when large amount of neutrons at high temperature is emitted.
But not all elements can be produced during nucleosyntheis with equal probability. According to the nuclear physics laws probability of production of such elements as Li, Be, B, Sc, Ti, V, Cr, Mn during thermalnuclear reaction is low. These elements presumably are formed during the processes of the nuclear reactions of heavy nuclei and the nuclei of the interstellar matter. Because they are fragments of more heavy nuclei they are customary called "secondary nuclei". Content of such nuclei in the cosmic rays is several orders of magnitude higher than in the average interstellar matter. The studies of the secondary nuclei is an instrument for the studies of the cosmoc rays propagation in the Galaxy. Such a "main track" of the galactic cosmic rays production is accepted by te mojority of the scientists. But there are still no model description. Probably, in the reality enerything is much more interesting.
- What other processes will be studied by means of "NUCLON" except galactic cosmic rays production and acceleration?
- "NUCLON" will consider more or less essentially all problems of high-energy cosmic rays physics, including exotic problems which can provide experimental proof of dark matter particles parameters and search for strange matter particles existance. But specific attention is paid to such an anomaly as "the knee" of cosmic rays spectrum (in the energy range of Е~3х1015 eV). This phenomenon concludes sharp diminishing of particles flux with energy increasing. "The knee" was discovered by SINP scientists G.B.Khristiansen and G.V.Kulikov about 55 years ago by means of indirect methods at the ground-based experimental equipment. It was numerously checked at the dozens of experimental units on all continents and all experiments proved its presence. But still nobody could explain its nature. I know about 30 theoretical interpretations of this phenomenon - from the most extreme which change physics as a whole, up to incrimination of non-accurate measurements. None of them can be proved till safe experimental data is obtained.
- Howdo you want to providesafe experimental data?
- Ideally it is necessary to conduct statistically approved research indirectly in "the knee" region, but nobody has such an opportunity: due to very low intensity of particles it is very expensive and difficult in methods. Then we decided to collect data in the region direclty close to "the knee". So we'll closely reach "the knee" region. We suppose that we'll get a possibility to get an interpretation of the phenomenon by this way.
- Dmitry, how "NUCLON" looks like?
- The scientific equipment is a monoblock with overall size less than 1 meter. It's weight is 300 kg. Due to its light weight "NUCLON" will be mounted as an additional payload onboard serial spacecraft "Resurs-P2". It will be fixed to the spacecraft by an arm and will get power supply and control from it. Power consumption is 160 W. Scientific data volume is not over 10 GBytes per day. In spite of small size of the eqipment "NUCLON" consists of 12 thousands of sensors, and each of them has its own program and digitizing. The launch of the experimental equipment is planned for December 2013, and we plan to expose it during a long period of time - not less than 5 years.
- What is the difference of "NUCLON" from other scientific instruments, used for the studies of the high-energy cosmic rays?
- Our equipment is additional payload for a serial Earth's remote sensing satellite. It's a so-called small weightage of a big spacecraft. Under conditions of limited fnancing of fundamental sacientific research this approach is particularly advantageous, and in our case it is the only possible. There is concurrence among the scientists and the programs. Our approach allowed to refuse from the development of an expensive specialized spacecraft for our experiment. It is well known that every kilogram of the payload at the orbit is twice more expensive than kilogram of pure gold.
- What instrument will directly detect high-eneg cosmic rays?
- We know that cosmic rays intensity sharply decreases with energy increasing, and the main subject of our interest is to study the range high energies. Currently the only universal instrument which can detect such particls within wide energy range is ionisation calorimeter. It was developed just here, in this room by three SINP scientists: N.L. Grigorov, V.S. Murzin and I.D. Rapoport in 1950s. Till now it is the best instrument for the studies of high-energy particles both at the Earth's surface and in space. t is interesting, that in the laboratories all over the world they suppose that the ionisation calorimeter was appeared on its own. As a bycicle. Probably, the reason is that the idea of the instrument is very simple. In very simple terms the ionization calorimeter is a "metalware" filled with particles sensors. Entering densed matter primary particle produces cascade of secondary particles, and their amount provides us information about the energy of the primary particle. Although the idea is very simple, the instrument itself can be extremely complicated due to requirements on the measurements accuracy.
- The weight of the ionisation calorimeter must be large. How did you diminish the total weight of the "NUCLON" instrument?
- We simply recollected blast from the past and filled it with innovative content. In the beginning of 1950s is was noted, that during nuclear interaction opening angle of the produced secondary particles depends on energy. Such methods were called "kinematic". Energy of the primary partcile was evaluated by means of measurements of the mean opening angle of the produced particles, ususally in nuclear emulsions. In this method errors were high. Therefore kinematic methods were changed with the ionisation calorimeter, where the primary particle's energy is evaluated with higher accuracy by means of the total amount of secondary particles, but it requires more heavy instrument. We combine using of a very thin ionisation calorimenter (microcalorimeter) with kinematic methods. In order to realize this idea we had to use the most innovative micro-technology, for instance, as I've already todl you, there will be over 12 thousands of independent sensors in our equipment. As a result we'll get light and precise instrument.
- Do any foreign colleagues take part in this research?
- "NUCLON" is an absolutely native, Russian astrophysical project. It is included into Federal Russian space program. The ordering customer is the federal space agency and Russian academy of science. I'd like to add that for SINP MSU, the leading manufacturer of the "NUCLON" equipment, the studies in the field of high-energy cosmic rays astrophysics is a traditional field of interest, we had great achievements in past, and would like to conform them in future.