The Complex of Experimental Facilities for the Cosmic Ray Investigation at the Tien Shan Mountain Station

Abstract

The study describes the experimental complex of the station located in the Tien Shan mountains at an elevation of 3340 m above sea level. The complex consists of detectors of different types scattered across the station area, such as scintillation particles detectors, Cherenkov detectors, radio emission detectors for the measurement of the electron component of extensive air showers (EAS) created by the (1–1000) PeV cosmic ray particles, an ionization calorimeter and neutron detectors for the study of the nuclear-active component of EAS cores, and the underground particle detectors for the detection of cosmic ray muons. The data acquisition system allows the simultaneous recording of parameters from various stand-alone detectors registering an EAS, and storage of the acquired data in the database. As an illustration of research capability, the results of the EAS study are presented here which were obtained during the last few years at the different experimental set-ups constituting the Tien Shan complex.

1. Introduction

In the 1960s an experimental research complex of the Tien Shan mountain cosmic ray station was developed by the Institute of Physics and Technology of the Ministry of Education and Science of the Republic of Kazakhstan and of the P. N. Lebedev Physical Institute of the Russian Academy of Sciences. The experimental site is located at a height of 3340 m above the sea level in the mountains of Zailiyskiy Alatau (Northern Tien Shan), in the vicinity of Almaty city [1]. The complex provides the means to solve many actual problems related to different areas of the cosmic ray particle physics, astrophysics, physics of atmospheric electricity, and experimental geophysics. The list of scientific research at the Tien Shan station includes the following topics:

• Investigation of the processes of high-energy particle interaction measuring extensive air showers (EAS) which are the large cascades consisting of (10⁵–10⁸) secondary particles, mostly electrons, created by the (1–1000) PeV cosmic rays in the atmosphere [2].
• Investigation of the high energy hadronic and muonic EAS cores where the most energetic cascade products are concentrated that keep the movement direction of a primary cosmic ray particle [3,4].
• Study of the large-scale distribution of cosmic ray primaries’ directions over the celestial sphere and the search for possible sources of high energy cosmic rays, in dependence on their energy, using the anisotropy effects of this distribution [5,6].
• Investigation of the cosmic ray particles’ interaction with the matter of the Earth and of the role which such interactions play in a number of the atmospheric electricity and seismology connected phenomena [7,8].

Accordingly, the experimental complex of the Tien Shan mountain station nowadays includes a HORIZON-T system of charged particles detectors aimed at registering the widely scattered electron component of high-energy EAS; the ionization-neutron calorimeter INCA, the neutron super monitor NM64, and the separate neutron counters for the detection of the interaction products of collimated hadronic cores in the central region of EAS; the underground muon and acoustic detectors for the search of the effects which the passage of high-energy cosmic ray muons might cause in the deep seismically stressed regions of the Earth’s core. The coordinated operation of the different independent sub-systems of the Tien Shan research complex is ensured by means of the high-speed local computer network.

This work studies the keystone components of the Tien Shan experimental complex and illustrates some scientific results which were recently obtained in the experiments currently active at the station.

2. The HORIZON-T Installation for Detection of High-Energy EAS and the Multi-modal Shower Events

The electron component of secondary cosmic rays at the Tien Shan mountain station is detected by a set of synchronously operating large-sized plastic scintillators and Cherenkov detectors which both constitute the HORIZON-T installation for registration of the high-energy EAS [9]. The general layout of this installation is shown in Figure 1. As it follows from the figure, the system of charged particles detectors currently comprises 12 detector points distributed over an area of ~3 km². The lower energy threshold of EAS is ~10 PeV, while the upper limit of its effective operation is defined by the steeply diminishing intensity of the energy spectrum of cosmic rays, and for an installation of such geometrical sizes it is close to ~(3–5) × 10³ PeV.

As an active medium for the detectors of the installation (1 × 1) m² sized polystyrene scintillator plates are used. The thickness of the plates is 10 cm in the points No (1–5), and 5 cm in the points No (6–12) (refer to Figure 1). Besides the scintillator, there is an additional Cherenkov detector in the point No 1.

One of the unusual results of the HORIZON-T installation due to an enhanced temporal resolution of its detectors is bi- and multi-modal EAS, which is shown in Figure 2. As it is seen there, instead of a single scintillation corresponding to the passage of the main particles in front of EAS there are three closely allocated pulses of comparable amplitude in the high-resolution time series of signal amplitude of the scintillation detectors. Absolute amplitudes of the pulses correspond to the passage of bunches of 316, 327, and 363 relativistic particles which are successively hitting the detector with the mutual time delays of a few hundreds of nanoseconds. Such a time raw of the scintillation signal can be naturally interpreted as a passage of three particle disks during an extremely narrow time gate of the order of a microsecond. Nevertheless, because of a low general intensity of the EAS with primary energy of ~100 PeV, the probability for a number of such independent EAS to hit one and the same time window of microsecond duration is quite negligible, as it can be easily estimated by applying the modern simulation tools of EAS development, such as the CORSIKA package [10]. According to simulations, which were made using this package for the HORIZON-T experiment, all the detected events should consist of a single scintillation flash which corresponds to the passage of a unique particle disk of the shower. Consequently, successive separate showers in the cases like the event shown in Figure 2 must be somehow casually connected with each other.

The relative frequency to meet a multi-modal EAS among the flux of all detected showers can be estimated from the general statistics of all observed events. Thus, in the period from 21 March to 12 May 2018, during 1137 operation hours of the HORIZON-T installation it had registered 15,725, showers with the energy above 20 PeV, 13.8 events per hour. Of them, more than 500 showers were found to belong to the multi-modal type. All the multi-modal showers had primary energy above 100 PeV which in fact can be an indication of the threshold nature of the discussed effect.

Another peculiarity of multi-modal showers is the exceedingly prolonged lateral distribution of the density of charged particles in those events. An illustration of this effect is shown in Figure 3, where the distributions of the particle density measured in 217 multi-modal EAS, shown with points, are compared with the distribution functions resulting from a CORSIKA simulation based on the standard EAS development models. It is seen that in the experimental events the intensity of the particles flux falls down much slower than it should be according to the simulation.

3. The Tien Shan Ionization Calorimeter and Its Applicability for Selecting the Pure Electromagnetic Showers

Currently, almost all experiments in the field of cosmic rays create or design installations for research in the field of gamma astronomy using the Cherenkov radiation method. This is due to the fact that the explosive processes of the formation and acceleration of protons and nuclei are accompanied by the generation of gamma quanta and neutrinos, which do not disperse, but fly in a straight line in the magnetic fields of the Universe. This in turn makes it possible to determine the place and study interactions with the release of tremendous energy in the Universe.

Basically, the energy range of primary gamma quanta ≤10¹² eV is studied using installations that register Cherenkov radiation [11,12,13,14,15,16]. In addition, various models and theories of the development of the Universe are created and tested based on experimental data obtained in the field of gamma astronomy. From the above mentioned, it becomes clear, that first of all, it is necessary to develop gamma-astronomical experimental research and methods for their observation.

Hadron-55 is an experimental setup that has been modernized to improve the accuracy of registration of gamma quanta and can be used for research in the field of gamma astronomy. The principle of operation of this setup is to select events from interaction only in the gamma block, i.e., pure electron-photon components are selected. A network of high-resolution scintillation detectors (with accuracy to within nanoseconds) is used to increase the reliability of tracing the trajectory of primary particles.

Upgraded ionization calorimeter with an array of scattered scintillation detectors is used for research in the region of high-energy gamma astronomy. To determine the zenith and azimuth angles of the EAS trajectory, scintillation detectors located around the calorimeter in a radius of 25, 40, and 100 m are used. The accuracy of determining the angles of a terrestrial gamma-ray flash is~0.2 degrees (refer to Figure 4).

The introduction of an air gap between the upper and lower layers into the composition of this calorimeter makes it possible to increase the size of the shower. This, together with the scintillation detectors introduced at the level of the gamma block, increases the efficiency of the installation and makes it possible to use it for research in the field of high-energy gamma astronomy.

The novelty of the research lies in carrying out gamma-astronomical studies, by the method of extensive air showers, i.e., without expensive Cherenkov detectors. It should be noted that the SHALON [17] installation with Cherenkov detectors is operating literally 50 m away, and the ongoing research will be compared with its results.

This setup has the following advantages over the standard registration of Cherenkov radiation from the atmosphere:

-

if the observation time of Cherenkov radiation from the atmosphere is limited to night, moonless, cloudless periods (5–10% of the calendar), then the observation time of showers by an integrated installation can reach almost 100% of the calendar year;

-

the energy and geometric distributions of the electron-photon component in the gamma block of the installation can be obtained when studying the same gamma source with the accuracy of the width of the ionization chambers;

-

portable scintillation detectors have an area of 0.25 to 1 m² and are very compact.

The gamma block registers (i.e., absorbs) the electron-photon component (EPC) of cosmic rays, while the hadronic component, due to the small thickness of it, passes through the gamma block without interactions and begins to interact and generate particles in the hadron block.

4. The Search for Interconnection Effect between Cosmic Ray Muons and Seismological Processes

Forecasting the level of seismic activity and, in particular, dangerous earthquakes, is an urgent problem of modern seismology. An unresolved problem of traditional seismology thus far remains the separation from the stream of information recorded by numerous seismic sensors of a strictly defined signal related to the approach of a specific, localized in time and space, catastrophic earthquake.

At the edge of the 1980s and 1990s, scientists from the Institute of Physics named by P.N. Lebedev and the Institute of Physics of the Earth had developed a preliminary concept of a new perspective direction in seismology: the use of a signal from elastic vibrations in the acoustic frequency range for predicting earthquakes, which, presumably, can be generated under the influence of local ionizations, which formed at the time of the passage of penetrating particles of space radiation (high energy muons) through a seismically stressed medium in the deep layers of the Earth’s crust [18]. In the work by the authors of [19], a method was proposed for the short-term forecasting of earthquakes, where high-sensitive detectors were used [20,21,22,23]. One of the directions for the development of effective methods for forecasting earthquakes is the study and analysis of the temporal characteristics of high-frequency seismic noise. The paper [24] proposes a method for such a forecast based on the idea that the processes preceding the earthquake cause abnormal behaviour of the intensity of acoustic noise. The role of a trigger provoking the generation of elastic oscillations in the acoustic frequency range can be played by a short-term increase in ionization from the passage of high-energy cosmic ray muons through seismically stressed areas of the lithosphere.

The Tien Shan high-altitude station is located in a mountainous area, directly in the zone of deep fractures of the lithosphere, and the system of shower detectors enables one to directly monitor the moments of the passage of powerful EAS and the associated flux of energetic muons. At the initial stage of the development of the acoustic system, a highly sensitive microphone (with a sensitivity of 20 mV/Pa in the acoustic frequency range of 500–10,000 Hz) was installed and has been in operation since the fall of 2017 (Figure 5). The microphone was placed in the well drilled through the rocky soil on the territory of the Tien Shan station at a depth of 52 m below ground level. The distance between the well and the “Elling” storm detector system is about 200 m. Acoustic detector signals are recorded in a special building, which is located directly at the upper edge of the well, and in which the main units of the signal-forming equipment are located: a differential amplifier and a low-frequency envelope selector of the microphone’s signal. To record the signals of the acoustic detector, a special, low-power, small-sized analogue-to-digital converter (ADC) system has been developed, which was placed together with the analogue signal generation scheme directly at the upper of the well’s edge. The basis of the ADC system is a single-board computer Raspberry Pi B + on an ARM microprocessor such as Broadcom BCM2835 with a clock frequency of 700 MHz.

The recording of input analogue signals via two information channels of a small-sized ADC system is performed continuously with a period of 2 ms, and the data obtained as a result of measurements are accumulated for subsequent off-line analysis in a file on a local disk, which is connected to the Raspberry Pi B + microcomputer via the built-in USB interface. At the end of the session, the accumulated data is transmitted to the server via Wi-Fi.

In Figure 6 and Figure 7, the time point as on 16 November 2017, 01:42:58 UT corresponds to an earthquake with a magnitude of 5.1 and an epicentre at a distance of 130 km from the station location.

A test recording of the level of signals coming from the acoustic detector was implemented at the Tien Shan station at the end of 2017, and since then it has been continuing all the time. Since there was no indication of the expected type of signal induced on this detector by the effect of seismic activity, the recording with a sampling frequency of 2 ms (500 Hz) was carried out continuously from the very beginning and all acquired data was stored on server disks.

Further studies presuppose a systematic and continuous recording of acoustic signals in combination with the registration of the moments of EAS passage and with the determination of their parameters based on data from the storm installation as well as the subsequent comparison of the data obtained in all series of measurements.

Figure 8 shows the registered acoustic signals and distributed density of the particles flux during the EAS event that occurred on 15 November 2017.

An analysis of these data concludes that the acoustic response was observed for the EAS events with primary particles energy E₀ > 10¹⁵ eV (N_µ ≥ 4, N_µ—quantity of registered muons). The amplitude of the acoustic response was found to be dependent on the cumulated energy of the muonic component, i.e., EAS energy.

Collecting more statistical data of registered acoustic events is required in order to assess and/or confirm the critical cumulated EAS energy and EAS parameters that can initiate an acoustic event.

5. An Informational Complex of the TIEN Shan Mountain Station

Experimental installations of the type “Hadron-55”, “Storm installation”, “Horizon -T”, “EAS Radio Emission”, “Groza”, “MAC2” (installation for registration of earthquakes), and others, which contain a scintillation carpet, a neutron monitor, an underground monitor, a calorimeter with a gamma block and neutron detectors, Cherenkov and remote scintillation detectors, a scintillation spectrometer, and a number of other subsystems, are currently operating at the station. They are combined into the following items, for which a general scheme of the network infrastructure has been developed with the ability to connect them to a single dedicated local network:

-

Point “Dormitory”, where the center of the local network is located with network and server equipment.

-

Point “Elling”, the center of the storm water installation.

-

Point «IPT», a center of the «Hadron-55» installation with a calorimeter, gamma-block, and neutron detectors.

-

Point «Horizon», the registration center of the «Horizon-T» installation.

-

Point “Bunker” site, here the remote detectors of the “Horizon-T” installation with an autonomous registration system are located.

-

Point “Stone Flower” site, with the remote detectors of the “Horizon-T” installation with an autonomous registration system.

-

Point “Neutron Super Monitor”.

-

Point “Underground” with muon detectors and an underground neutron monitor located at a depth of 20 m of water equivalent.

The communication between the subsystems is performed by combining fiber-optic lines into a network, which increases the reliability of communication and the speed of data transmission over the network. The application of a fiber-optic line has increased the resistance of the local network to adverse environmental influences such as precipitation, lightning, and static electricity.

The application programs for working from a local network are subroutines, which by using special libraries form queries to databases, transfer the requested records to the program content, and process them in accordance with the specified criteria. The processed data are represented in text and/or graphical form. Using the Linux operating system, programming languages C, C++, Python, Java, and Java script, the programming interfaces and modules have been developed and debugged, allowing for the transfer of database records into program content.

Developed and debugged software support ensures the normal functioning of the entire infrastructure and uninterrupted client access to the database of the combined system. The complete information obtained in the course of the experiments carried out at the Tien Shan station is accumulated in a common database which has an open Internet access for general use of these data for analysis by the participants of third-party scientific organizations [25].

6. Conclusions

The study provides consolidated findings obtained from the complex of experimental units for the cosmic ray investigation at the Tien Shan mountain station. The HORIZON-T unit is used for registering of EAS created by the (10–1000) PeV cosmic ray particles with accuracy to within nanoseconds. This helps to quantitatively identify characteristics of bimodal pulses. The Hadron-55 unit is used for the continuous registering of cosmic particles flux, and the effective area for registering is 30,000 m². The study shows the interrelation of high-energy cosmic ray muons’ passage in the seismically stressed region and registered seismic events.

Integration of the stand-alone experimental units in one complex is assured by the development of a special interface and utilization of the high-speed local computer network. All acquired experimental data are collected and stored in a database which can be openly accessed via the Internet by third-party research organizations.