2009-09-23 03:29:45

Yerevan Physics Institute Cosmic Ray Division History, status and development 2009-2014

Introduction
Information on the particles of highest energies bombarding the Earth’s atmosphere provides vast information on the most violent processes in the Universe. One of the “main players” reflecting physical processes in stellar systems are particles and stripped nuclei reaching the Earth from interstellar space and from Sun, the so called Galactic and Solar Cosmic Rays (GCR and SCR). These “primary” Cosmic Rays (CR) were discovered almost 100 years ago by the ionization effects of the secondary fluxes (particle showers), produced in their interactions with the terrestrial atmosphere. Exploiting different physical processes of shower interaction with atmosphere (particle multiplication, fluorescence, Cherenkov light emission in atmosphere and in water, acoustic waves, and radio waves emissions) different experimental techniques were developed to detect cosmic rays above and on the Earth’s surface, underground and underwater. Fifty years ago, with the launch of first satellite on October 4, 1957, experiments in space directly detected primary cosmic rays and confirmed that our nearest star, the Sun, is a particle accelerator.
Direct measurements of particle fluxes by facilities onboard satellites and balloons provide excellent charge and energy resolution but, due to severe limitation of payload and the weak flux of high energy, CR perform measurements in KeV to GeV energy region. In hundreds of TeV - PeV region now and in the nearest future only surface based techniques of secondary particle showers detection can provide data on energy and types of primary particles, although with an uncertainty inherent to indirect methods, based on the extensive use of numerical models and simulation techniques. One of the first permanent high-mountain research stations was established in Armenia 65 years ago. The Aragats and Nor Amberd research stations of the Cosmic Ray Division (CRD) of the Yerevan Physics Institute (YerPhI) named after A.Alikhanyan are located on the slopes of Mount Aragats, the highest peak of Armenia (see Figure 1 and 4), at 3200 meter and 2000 m elevations respectively. The scientific history of cosmic ray research at Aragats can be traced back to 1934 when a group from Leningrad Physics-Technical Institute and Norair Kocharian from Yerevan State University (YSU)1, measured the East-West cosmic ray anisotropy (Kocharian, 1940). These measurements stimulated the interest of famous physicists the brothers Artem and Abraham Alikhanyan (see Figure 2), who organized a scientific expedition to Aragats in 1942. Since then, expeditions on Aragats have continued uninterruptedly, despite the World War II, insufficient funding, electricity and fuel shortages during the recent history of Armenia.
In the 40’s and 50’s cosmic rays were the main source for information about the properties of elementary particles. Later CR research has lead to new, modern branches of physics named “Astroparticle Physics”, “High Energy Astrophysics” and “Space Weather”. The most important dates and achievements of Cosmic Ray research at Aragats can be itemized as follows:
  • 1942 – First expedition to Aragats
  • 1943 – Establishment of the Physical-mathematical Institute of Yerevan State University; now Yerevan Physics Institute after Artem Alikanyan;
  • 1945-1955 – Foundation of Aragats high-mountain research station. Experiments at Aragats with Mass-spectrometer of Alikhanyan-Alikhanov: investigations of the composition of secondary CR (energies <100 GeV); exploration of the “third” component in CR; observation of particles with masses between μ-meson and proton;
  • 1957 – Installation of the ionization calorimeter, detection of particles with energies up to 50 TeV;
  • 1960 – Foundation of the Nor Amberd high-mountain research station;
  • 1970 – Modernization of the Wide-gap Spark Chambers;
  • 1975 – Experiment MUON: measuring the energy spectrum and charge ratio of the horizontal muon flux;
  • 1975 – Installation of the Neutron supermonitors 18NM64 at Aragats and Nor Amberd research stations;
  • 1977 – Experiment PION: measuring pion and proton energy spectra and phenomenological parameters of CR hadron interactions;
  • 1981-1989 –ANI Experiment: Commence of MAKET-ANI and GAMMA surface detector arrays for measuring cosmic ray spectra in the “knee” region (1014 – 1016 eV);
  • 1989-1992 – Design and tests of the system of Atmospheric Cherenkov Telescopes, introduction of multivariate methods for signal detection from γ-ray point sources;
  • 1993-1996 – Development of new methodology of multivariate, correlation analysis of data from Extensive Air Shower detectors, event-by-event analysis of shower data from KASCADE experiment; classification of primary nucleus;
  • 1996-1997 – Renewal of Cosmic ray variation studies at Aragats: installation of the Solar Neutron Telescope and resumption of Nor Amberd Neutron Monitor;
  • 2000 – Foundation of Aragats Space Environmental Center (ASEC) – for Solar Physics and Space Weather research; measurements of the various secondary fluxes of cosmic rays; inclusion of the large surface arrays in monitoring of the changing fluxes of secondary cosmic rays;
  • 2003 – Detection of the intensive solar modulation effects in September – November in the low energy charged particle, neutron and high energy muon fluxes;
  • 2004 – Measurement of the spectra of heavy and light components of GCR, observation of very sharp “knee” in light nuclei spectra and absence of “knee” in heavy” nuclei spectra;
  • 2005 - Measurements of highest energy protons in Solar Cosmic Rays (GLE 70 on January 20; detection of Solar protons with E>20GeV);
  • 2007 - Launch of SEVAN (Space Environmental Viewing and Analysis Network) - a new type of world-wide network of particle detectors for monitoring of geophysical parameters
  • 2008 - Multivariate analysis and classification of the solar transient events (Ground level enhancements, Geomagnetic effects, Forbush decreases) detected by ASEC monitors during 23rd solar activity cycle.

The mass-spectrometric period of scientific research on Mt Aragats
The history of scientific research on Mt Aragats can be divided into several periods. The first - mass-spectrometric period - lasted about 15 years. Experiments with magnetic spectrometer designed by the Alikhanyan brothers lead to the discovery of protons in CR (Alikhanian et al., 1945) and narrow air showers (Alikhanian, Asatiani, 1945).2 According to the viewpoint of the time, CRs were believed to have a pure electromagnetic origin (Anderson, Neddermeyer, 1937), therefore the presence of protons in CR strongly contradicted the established concepts. The origin of narrow showers could not be electromagnetic because of their great penetrability. Later narrow showers were thoroughly studied with the Aragats Ionization Calorimeter (Grigorov et al., 1958).
Using the Alikhanyan-Alikhanov magnetic spectrometer N. Kocharian obtained the energy spectra of muons and protons with energies up to several GeV (Kocharian et al, 1952). Till now this data remain one of the best measurements of the secondary cosmic ray fluxes at mountain altitudes. Figure 3), performing the simultaneous measurement of the momentum and absorption length of charged particles, provided the effective particle mass analysis. This method presents the first evidence of the existence of particles with masses ranging from μ-meson to proton; however, only some of the many peaks in mass distributions measured at Aragats were later verified to be “real” particles and became known as π- and K-mesons.
The mass spectrometer method (see the picture of memorial magnet on Mt. Aragats) in other “particles” with masses heavier than μ-meson, including so called varitrons (Alikhanian & Alikhanov, 1951), “discovered” using the Aragats mass-spectrometer, turned to be artifacts due to fluctuations in the mass distributions. Nonetheless, the discussion on varitrons led to several excellent experimental and theoretical investigations and Alikhanyan brothers` idea about a variety of elementary particles became very popular among physicists all over the world, making the Aragats research station one of the most important centers of cosmic ray physics. It should be mentioned that defining the reliability of peaks in one- and two-dimensional distributions is still one of the most important and complicated problems in High Energy Physics and Astrophysics. Nowadays there are also many groups using sophisticated mathematical methods that cannot avoid mistakes and who reported discoveries based on the fake peaks (see for example discussion about “discovery” of pentaquark in Seife, 2004).
Calorimetric measurements on Mt Aragats
The second phase of scientific research on Mt Aragats, calorimetric measurements, covers the period from 1958 to 1970. The mass spectrometric method had reached its energy limit by that time. In 1958 a group of scientists from the Institute of Nuclear Physics of Moscow State University and Yerevan Physics Institute (team leader - Naum Grigorov) installed the first ionization calorimeter at Aragats station (Grigorov et al, 1958). Experiments with ionization calorimeter at Aragats proved the energy-dependence of the effective inelastic cross-section of the hadron interaction with nuclei. This fact was later confirmed by direct measurements on Proton satellites (Grigorov, 1970) and accelerator experiments. The ionization calorimeter also detected another interesting result concerning the peculiarities of multiparticle production of high energy pions (Babayan et al., 1965), which was later (1990) registered as a discovery in USSR: in some cases only few π0-mesons, generated in the interaction with atmospheric nuclei, “takes away” almost the entire energy of the primary particle. The authors of this discovery were Kh. Babayan (deputy-director of YerPhI from 1956-1969), Naum Grigorov, Erik Mamijanyan (head of Cosmic Ray Division of YerPhI in 1969-1992) and Vladimir Shestoperov.
The Nor Amberd station, which started its operation in 1960 (see Figure 4) at the altitude 2000 m, considerably enlarged the possibilities for studying high energy cosmic ray hadrons and their interaction with different nuclei (head of laboratory in 1960 - 1986– Gerasim Marikyan).
At that time physicists from various scientific institutions of the Soviet Union participated in the investigations on the Armenian mountains, scientists from the USA, France, Japan and Great Britain also visited high altitude stations.
The method of wide-gap spark chambers was intensively investigated in YerPhI in late 50-s. The prestigious Lenin Prize was awarded to Artem Alikhanyan and Tina Asatiani (head of muon laboratory of YerPhI in 1960-1987) in collaboration with groups of Russian and Georgian physicists for developing the wide-gap spark chamber techniques.
In 1968-69 a system of proportional counters was added to the Aragats ionization calorimeter. Using this facility, the neutron component of cosmic rays at mountain altitude was measured by E. Mamijanyan and his colleagues (Azaryan et al, 1977). K. Babayan in early 70-s started his research of CR variations by installing neutron supermonitors of 18NM64 type at Aragats and Nor Amberd research stations, which served as a basis for creating a unique center of cosmic ray monitoring in the “new history” of Aragats.
High energy astrophysics
During the next period (1970 –1980) the experiments PION (Avakyan et al, 1978) and MUON (Asatiani et al., 1980) measured fluxes of secondary cosmic rays and some phenomenological characteristics of strong interactions. The team leaders of the experimental groups were Vahram Avakyan (head of Aragats station from 1963 till 1993) and Tina Asatiani, respectively. PION was a unique facility (Alikhanian et al., 1975), which includes transition radiation detection system for particle identification, created by Albert Oganesian’s group (head of laboratory from 1978 – till 1996) and an ionization calorimeter for particle energy estimation.
The muon magnetic spectrometer for studying near-horizontal high energy muons was equipped with coordinate measuring systems based on the wire spark chambers and wide- gap spark chambers, thus increasing the range of reliable muon momentum measurement up to ~2.5 TeV/c. Both experiments used modern numerical algorithms and on-line computers for data analysis. One of the first soviet computers М220 was used to calculate horizontal muon energy spectrum. The PION experiment used the first Armenian minicomputer NAIRI-2 for data acquisition.
In 80s it became clear that larger detectors are necessary for the research of primary cosmic ray fluxes. The planned ANI experiment on Mt. Aragats (Danilova et al., 1982) met all these requirements. It was intended to register electrons and muons of Extensive Air Showers (EAS) by a system of surface scintillators; interactions of hadrons from EAS core with the world’s largest calorimeter (surface area 1600 m2); high energy muons by a huge underground muon detector and huge magnetic spectrometer (area 40 m2). The ANI experiment was designed in cooperation with the Lebedev Physics Institute of USSR Academy of Science under the guidance of USSR Ministry of Medium Machinery (presently, Federal Nuclear Energy Agency of the Russian Federation). The experiment leaders were Sergey Nikolsky (director of the Division of Nuclear Physics and Astrophysics of Lebedev Physics Institute) and Erik Mamijanyan.
The ANI complex was not completed because of the collapse of the USSR, followed by the collapse of the Armenian economy, but 2 surface particle arrays MAKET ANI (Figure 5, experiment leader Gagik Hovsepyan, see details in Avakyan et al., 1986) and GAMMA (Figure 6, experiment leader Romen Martirosov, see details in Garyaka et.al., 2002) made significant contribution to the “knee” region physics.
To select the proper model of the CR origin one has to measure the partial energy spectra of the different groups of primary nuclei, i.e. perform the classification of the primary nuclei by highly smeared EAS information content. These very complicated tasks became feasible after developing the nonparametric multivariate methodology of data analysis by Ashot Chilingarian in 1989.
Event-by-event-analysis of EAS data, using Bayesian and Artificial Neural Network (ANN) information technologies (Chilingarian, 1989, 1994) helped to obtain the energy spectra of light and heavy primary nuclei from MAKET ANI experiment and also 3 partial spectra, corresponding to light, intermediate and heavy nuclei groups from KASCADE experiment (Antoni et al., 2003). MAKET-ANI data (Chilingarian, Hovsepyan et al., 2004, 2007) demonstrates the existence of a sharp knee in the light component, and no evidence of knee in the heavy component up to ~3•1016eV (see Figure 7). Analysis of the GAMMA detector data (Garyaka, 2002), located nearby MAKET- ANI and in addition measuring muon content of EAS also demonstrates sharp knee in proton flux (Garyaka et al., 2007). The available world data confirm these results. In the KASCADE experiment, the position of the knee shifts towards higher energies with increasing mass number (Apel et al., 2005). In HEGRA experiment (Horns et al., 2001) a steepening of the light mass group spectrum was detected. In EAS-TOP (Aglietta et al., 2004) the light nuclei group also demonstrate sharp knee.
Therefore, EAS evidence on the galactic CR origin consists in establishing charge dependent acceleration of CR in general agreement with model of shock acceleration in the blast waves of supernovae explosions. Further observations made by orbiting in space gamma-ray observatories and ground-based Atmospheric Cherenkov Telescopes (ACTs) also point on the Supernovae Remnant (SNR) as one of the major cosmic ray sources.
After publishing the final papers the MAKET ANI detector ceased operation in 2007. The scintillators are used now for monitoring changing fluxes of low energy charged CRs. Arrangement was made also for making a test facility for the new precise timing system for a new large EAS array for measuring CRs far beyond the knee, now under consideration at CRD.
Direct evidence of shock acceleration in SNR shells can be deduced from joint detection of young SNRs in X and γ-rays. To prove that the young supernovae remnant RX J1713.7-3946 is a very efficient proton accelerator Uchiyama with colleagues (Uchiyama et al, 2007) include in the analysis information on broadband X-ray spectra (from 0.4 to 40 KeV) measured by the Suzaku satellite (Takahashi et al., 2007) and on high energy γ-ray spectra (extending over 10 TeV) measured by HESS Atmospheric Cherenkov Telescope (Aharonyan et al., 2007). They exclude the inverse Compton origin of detected high energy γ-quanta, and taking into account the Tev-KeV correlations validate the hadronic model of detected γ-rays. Thus, the joint analysis of X-ray maps from Chandra and X-ray spectra from Suzaku satellites with high energy γ-ray spectra measured by HESS ACT provide very strong argument for the acceleration of protons and nuclei of 1 PeV and beyond in young SNR shells.
Armenian physicists have a significant impact in the development of the ACT technique. Pioneering system of ACTs on Canarias (HEGRA) followed by large ACTs HESS in Namibia and MAGIC at Canarias designed and operated by international collaborations with the participation of Armenian physicists.
In 1985 design and construction of the first system of ACTs for the ANI experiment at Aragats started at YerPhI. The telescopes comprised tessellated reflectors of 3m diameter and an imaging camera in the focal plane of 37 pixels based on FEU-130 type Soviet PMTs of bialkali type. High quality glass mirrors with quartz protection, equatorial mounts of the telescopes, the imaging cameras and DAQ electronics also were prepared at YerPhI workshops. The gamma ray group was lead by Felix Aharonian; with leading role of Razmik Mirzoyan and Ruben Kankanian. The group started measuring cosmic ray signals at Nor Amberd research station and calibrating the telescope for the first measurements of the Crab Nebula when the collapse of the former Soviet Union stopped the experimental activities. Fortunately, the Armenian scientists with German physicist O. Alkoffer prepared a proposal to install the same system of ACTs on a newly created HEGRA (High Energy Gamma Ray Astronomy) cosmic ray detector on the Canary island of La Palma. The prepared devices and materials for the construction of the 5 telescopes were shifted from Armenia via Germany to La Palma and the construction started in 1991. In 1992 the first HEGRA telescope measured gamma rays from Crab Nebula (see Mirzoyan et al, 1994).
That was the first significant confirmation of the discovery of the 10m diameter Whipple telescope in Arizona, the USA. In 1993 the second telescope was build and operated in stereo mode with the first one and later on 4 more telescopes were added to the system. The HEGRA telescopes operated until 2002 and provided a rich harvest of gamma sources. The contribution of Armenian physicists in HEGRA was very significant because of their leading role both in the techniques of IACTs as well as their theoretical work on the very frontier of gamma astronomy.
After termination of HEGRA the astrophysicists from the collaboration continued to build new advanced instruments. Already in 1994 the 17m diameter MAGIC telescope, intending to investigate gamma rays below 300 GeV down to energies of 30 GeV was proposed by Razmick Mirzoyan. An international collaboration was formed and in 1998 it became an official project in Max-Planck-Institute Physics (MPI) in Munich. YerPhI and several institutions in Germany, Spain, Italy, Switzerland and Finland became members of the MAGIC collaboration. The first MAGIC telescope was built in La Palma in 2001-2003 and has operated since 2004. The second MAGIC telescope was built on a 85m distance from the first one and will operate together with the first one in fall, 2008.
The other part of HEGRA collaboration continues its research with 10m diameter class telescopes, with advanced optics and electronics. A new array, one of initiators of which was Felix Aharonian, under the name H.E.S.S., is comprised of 4 telescopes of 12m diameter and was built by an international collaboration, mostly from Germany and France, in Namibia in 2001-2003. Scientists from YerPhI also became members in HESS. HESS collaboration intends to complete their array with one 28m diameter very large telescope in 2009.
The number of sources increased from ~20 to more than 80 just in 3-4 years and very interesting publications, more than 70 by now, appeared in peer refereed journals, also in such famous ones as Science and Nature. It is expected that both telescopes together will increase the number of sources to ~100 just in the next 2-3 years and finally long-standing questions of cosmic rays, astrophysics, and astroparticle physics can be understood and answered. Felix Aharonian, Ashot Akhperjanyan and Vardan Sahakian, got the prize of the President of the Republic of Armenia in 2006 , and the YerPhI group got the Descartes’ prize as a part for the HESS collaboration in 2007.
Solar Physics and Space weather research