The installation of the SEVAN CRO particle detector at Zagreb Astronomical Observatory was finished in December 2008. It is the first instrument designed for detecting cosmic rays in Croatia and its installation greatly promotes solar physics research in this country. The detector is a fully autonomous unit, with the capability to send data via the Internet, and it is part of the SEVAN (Space Environmental Viewing and Analysis Network). The SEVAN network units are located at different latitudes, longitudes and altitudes and they are use for simultaneous measurements of flux of most species of secondary cosmic rays born in atmospheric cascade caused by primary ions and solar neutrons. The main scientific aim is to the improve research of solar particle acceleration in the vicinity of the Sun by detecting highest energy solar cosmic rays giving additional secondaries detected by surface particle detectors and to improve researches of the space environment conditions. The detectors are sensitive to very weak and poorly explored fluxes of high-energy solar cosmic rays above 10 GeV. Decices are also used for exploration of solar modulation effects on galactic cosmic rays. SEVAN started in the framework of the International Heliophysical Year and United Nations Basic Space Science projects (UNBSS) /(see also International Space Weather Initiative (ISWI)/
Particle detector SEVAN – schematic view
The SEVAN network of particle detectors (Space Environmental Viewing and Analysis Network) aims to improve fundamental research on solar physics and space weather conditions. Ground based particle detectors measure time series of secondary particles born in cascades in the Earth’s atmosphere by primary galactic cosmic rays. By detecting solar modulation effects it is possible to predict upcoming geomagnetic storms hours before the arrival of Interplanetary Coronal Mass Ejection’s (ICMEs) at the space-borne instruments. Huge magnetized plasma clouds and shocks initiated by Coronal Mass Ejections (CME) travel in the interplanetary space with mean velocities up to 2500 km/sec (the so called Interplanetary Coronal Mass Ejection (ICME), are known as major drivers of severe space weather conditions when arriving at the Earth. On their way to Earth ICMEs also “modulate” the flux of Galactic Cosmic Rays (GCRs) introducing anisotropy and changing energy (rigidity) spectra of the previously isotropic population of protons and stripped nuclei accelerated in the numerous galactic sources. Changes in the rather stable flux of GCR are detected by space-born spectrometers (rigidities up to ~1GV) and by world-wide networks of particle detectors (rigidities up to ~100GV) located at different latitudes, longitudes and altitudes.
The ICME is a major modulating agent, interacting with GCR, and introducing anisotropy in their flux. These anisotropies of GCR manifested themselves as peaks and deeps in time series of secondary cosmic rays, detected by surface particle detectors.
Therefore, the measurements of secondary fluxes can be used for “probing” ICMEs, providing highly cost-effective information on the key characteristics of these interplanetary disturbances. The size and magnetic field strength of ICMEs are correlated with the ICME modulation effects on the energy spectra and the direction of GCRs. At the same time the presence of strong and long-duration southward magnetic field component in the sheath region of ICMEs is the primary requirement for their geoeffectiveness. Thus, strong magnet field of the ICMEs is both modulation agent of GCR and driver of GMS.
The large Bz value associated with approaching ICMEs is a best known diagnostics of GMS strength. Therefore, appropriate observations of the variations of the primary and secondary cosmic rays can be a proxy of Bz value available long before IMCEs reach the L1 libration point where Bz is measured directly.
The ICME takes less than one hour to travel from the ACE or SOHO satellite to the magnetosphere. It is a too short time to take effective mitigation actions.
To establish a successful forecasting service by a network of ground based particle detectors we need to measure the time series of secondary particles (neutrons, low energy charged particles and high-energy muons) and to simulate and compare the correlation between changing fluxes and direction of detected particles. To meet this goal a new-type of particle detector (SEVAN detector) was designed and fabricated in Aragats Space Environmental Center (ASEC) of the Alikhanyan Physics Institute, Armenia. ASEC has a great experience with data analysis of multivariate time-series from ASEC monitors.
The SEVAN network is compatible with currently operating neutron monitor and muon telescopes networks.
The installation of a SEVAN particle detector (SEVAN CRO) at Zagreb Astronomical Observatory (latitude 45.82 N; longitude 15.97 E; altitude 220 m; vertical cut-off rigidity ≈ 4.9 GV) was finished at the end of the year 2008. All necessary equipment, which includes scintillators, photo-multiplier tubes and electronics were provided by the Alikhanyan Physics Institute. The housing was made in Croatia. The installation was supported by the European Office of Aerospace Research and Development and the City government of Zagreb.
The threshold energy of the SEVAN module was estimated using three different methods: by calculation of ionization losses of muons in lead; by computer simulation of atmospheric cascade and response of the monitor to the secondary particles and by comparing a simulated spectrum and experimentally measured count rates. Analytical calculations are in good agreement with the simulation. The selective sensitivity of the SEVAN modules allows us to probe different populations of the primary cosmic ray flux from 7 GeV to 20-30 GeV and to classify Ground Level Enhancements (GLEs) in neutron or proton initiated events.
A SEVAN detector is assembled from standard slabs of 50·50·5 cm3 plastic scintillators. Two identical assemblies of four slabs scintillators are separated with two lead absorbers of size 100·100·5 cm3. Between the lead absorbers there is a thick scintillator assembly of the size 50·50·25 cm3 (five standard slabs). Each scintillator assembly is inside a light protected iron-made shielding with a photo-multiplier tube. So we have a 3-layer detector in which is possible to occur different combinations of the signals and absence of signals.
Plastic scintillators 50·50·5 cm3
If the signal appears only in the upper layer it represents the flux of low energy charged particles (mostly electrons and muons) filtered by 5 cm thick lead below the upper scintillators (energy of charged particles below 100 MeV). If we have a signal in the upper and lower layer (or in all three layers) it represents the transit of a high energy muon with minimal energy of about 250 MeV. Neutral particles undergo nuclear reactions in the 25 cm thick scintillators of the middle layer and produce protons and other charged particles. There is not enough substance in the upper 5 cm thick layer for nuclear reactions of neutral particles. If the signal appears only in the middle layer, it represents the neutral component of secondary cosmic ray fluxes. Some of the other possible combinations are when the signal appears in the upper and middle layer (transit of higher energy charged particle stopped in the second lead absorber) and when the signal appears only in the lower layer (represents inclined incidence of charged particles). SEVAN electronics (Arakelyan et al., 2009) provides registration and storage of all possible combinations of the signals for further analysis. Atmospheric pressure data are collected with a pressure sensor designed and fabricated in the Yerevan Institute of Physics.
One-minute measurements are stored in an output file in nine columns:
D1 – time
D2 – number of events (appearing signal) in the upper layer
D3 – number of events in the middle layer
D4 – number of events in the lower layer
D5 – atmospheric pressure
D6 – signal in the upper and middle layer (110)
D7 – signal in the upper and lower layer (101)
D8 – signal in the middle and lower layer (011)
D9 – signal in all three layers (111)
Count rates of low energy, high energy and neutral particles can be found by a simple calculation:
Low energy particles (100) = D2 – (D6 + D7 + D9)
Neutral particles (010) = D3 – (D6 + D8 + D9)
High energy muons (111&101) = D7 + D9
The measured counting rates of different species of secondary particles by SEVAN CRO are approximately:
6 500 low energy charge particles (100)
300 neutral particles (010)
4 000 high energy muons (111&101).