A spectrum of cosmic rays within energy range 10^15 - 3x10^17 eV was derived from the data of the small Cherenkov setup, which is a part of the Yakutsk complex EAS array. In this, work a new series of observation is covered. These observations lasted from 2000 till 2010 and resulted in increased number of registered events within interval 10^16 - 10^18 eV, which in turn made it possible to reproduce cosmic ray spectrum in this energy domain with better precision. A sign of a thin structure is observed in the shape of the spectrum. It could be related to the escape of heavy nuclei from our Galaxy. Cosmic ray mass composition was obtained for the energy region 10^16 - 10^18 eV. A joint analysis of spectrum and mass composition of cosmic rays was performed. Obtained results are considered in the context of theoretical computations that were performed with the use of hypothesis of galactic and meta-galactic origin of cosmic rays.
Energy spectrum of cosmic rays (CR) in energy range 3 ×(10 15 -10 18 ) eV could not be studied in detail with compact arrays due to their small acceptance at energy above 10 17 eV. At the same time this area of the spectrum is of a great interest, since local irregularities are manifested there: production of kinks (thin structure at 3 × 10 15 -10 17 eV) arising from non-uniform distribution of heavier CR components in our Galaxy. On the other hand, this effect is smoothed by addition of a new component (of meta-galactic or other origin) to the cosmic ray flux near Earth. As a result, presence/absence of significant irregularities in spectra measured by various compact arrays allows one to speculate on the CR origin and propagation in our Galaxy [1,2].
The Yakutsk array in this sense appears as a unique scientific tool. It is related to medium-sized arrays, capable of effective measuring of cosmic rays flux in a wide energy range (10 15 -10 19 eV).
Other important traits of the array are its model-independent technique of energy estimation of extensive air showers (EAS) and the ability to track longitudinal EAS development by detecting the Cherenkov light emission. Factors mentioned above enable adopting the unique method, combining the studies of CR spectrum and mass composition aimed at exploration of astrophysical aspect of cosmic rays [3,4].
For more than 15 years the small Cherenkov setup has been operating as a part of the Yakutsk array. It measures Cherenkov light emission from EAS of lower energies (see Fig. 1) using standard detectors which are designed to operate in winter conditions. The area of modern prototype was significantly increased in comparison with the original setup, its border forms a circle of 500 m radius. The number of optical detectors was also increased (see Fig. 1). Table I presents the information on operation of the setup (on annual basis) combined with mean spectral atmosphere transparency at wavelength 430 nm.
All the information on each shower is stored in the database, which is controlled by a software complex. This program includes units for gathering, sorting and storing of the experimental data.
It also includes mathematical units for data processing and statistical analysis. The results of the analysis are presented below. In order to reconstruct cosmic ray spectrum we used following criteria for shower selection:
- atmospheric transmittance p λ ≥ 0.65; 2. shower axis must lie within 250 m from the center of array (for showers with primary energy E 0 ≤ 3 × 10 16 eV) and within 500 m (for showers with E 0 ≥ 10 16 eV); 3. zenith angle of shower arrival direction θ ≤ 50 • ; 4. chance probability of shower detection w r ≥ 0.9. Introduction of these criteria was mainly defined by the parameters of detectors used in the experiment (mostly by aperture and thresholds of Cherenkov light detectors) and by atmospheric conditions during optical observation. Since shower events were selected by different triggers, during transition from one trigger to another a threshold effect appears. It manifests itself by sudden local increase of intensity in the spectrum. According to our calculations the magnitude of this effect can reach ∼ 30 %. Technically, the softening of this effect has been achieved by extension of the area controlled by the trigger-50 and by its overlapping with other triggers. For example when triggers coincided, having been activated simultaneously by the same shower. In this case, the number of skipped showers was minimized. Another method is introduction of corrections obtained in simulation of measurement.
According to the criteria described above, the data bank of showers was formed by the parameter Q(150), a density of Cherenkov light flux at r = 150 m from shower axis. This parameter was derived from readings of Cherenkov detectors located within 80 -200 m from shower axis.
The structure of the data bank was defined by the task -it was formed strictly by those periods of observation, which confirmed to adopted shower selection criteria mentioned above. We suppose that chosen conditions are sufficient to avoid distortions related to experimental and methodical errors in reconstruction of cosmic rays energy spectrum.
It is believed that photon losses in clear atmosphere arise from Relay scattering (5 % from total flux). In real conditions there is significant loss in received light due to Mi-aerosol of various size.
In winter (in the region where array is located the climate is sharp-continental) the atmosphere above the array is non-standard, its parameters change significantly from autumn to winter and vice-versa. According to work [5] all this factors should be tracked on an operational basis and taken into consideration when analysing different observational periods. On Fig. 2 perennial data are presented on Cherenkov light transmission in atmosphere during different periods of optical observations. These data were utilized during generation of sh
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