New multipurpose experimental complex of the Tien Shan mountain cosmic ray station of LPI incorporates a set of detector subsystems for simultaneous investigation of the various components of extensive air showers (EAS) which arise from interaction of the (1–100) PeV cosmic ray particle in the atmosphere. A wide-spread system of the charged particles detectors is used for the measurement of the local density of EAS electrons, and for estimation of the main EAS parameters by spatial distribution of this density. The radio-antennas and Cherenkov detectors provide an alternative way for investigation of the EAS electron component. The ionization-neutron calorimeter and the neutron monitor give information on EAS hadrons with the energy above 0.1 TeV, while the low-threshold neutron and gamma detectors can be applied for registration of the prolonged flux of thermalized neutrons and soft gamma rays after EAS passage. The underground muon detector is used for investigation of the muonic component of cosmic rays in an exclusively wide range of muon energies, starting from 5 GeV, and up to tens and thousands of TeV, and with estimation possibility of the energy of muon. The multi-component technique practiced now at the Tien Shan station permits to study effectively those aspects of high energy cosmic ray interaction which have been never considered in previous experiments. In particular, it is possible now to adequately register the flow of various EAS components in the very region of EAS core which opens a real opportunity to solve the long-standing problem of the 3-PeV knee in the energy spectrum of cosmic rays. Some examples of the physical results gained lately at the Tien Shan detector complex are presented here for performance illustration of the newly elaborated methods of EAS investigation.
Despite the fact that the history of cosmic rays investigations in the energy range of (1–100) PeV goes back at least half a century, to date much remains unclear about the properties of these particles. First of all, among such obscure questions should be mentioned the origin of the prominent knee in the energy spectrum of cosmic rays which is a sharp change of its power index at energy PeV [[
It should be noted that presently the processes of particle interaction in the energy range of up to PeV can be directly investigated at largest modern accelerators, such as LHC, and so far no fundamental deviation from the Standard interaction model was found among the collisions of high energy protons and ions [[
The general investigation method of elementary particle interaction in cosmic rays is registration of extensive air showers (EAS), that is, of the cascades formed by successive collisions of both the primary cosmic ray particle itself and of numerous high energy products of its interaction with nuclei in the atmosphere. The above mentioned "exotic" effects are characterized by their common tendency to reveal themselves in an extremely narrow, of the order of (5–10) m only, central core region around EAS axis, where the most energetic shower components are concentrated which keep the movement direction of primary particle. Another common feature of the "exotic" effects is their threshold character: they are found mostly among interaction of cosmic rays with sufficiently high energy. As a rule, the minimum threshold of that energy corresponds to position of the -PeV knee in the energy spectrum of cosmic rays. This circumstance implies specific technical requirements for any experimental installation aimed to investigation of such phenomena: its detectors must be capable to saturation-free registration of intensive particle fluxes characteristic for the central region of powerful EASs, and the arrangement of these detectors must be sufficiently dense to register the particle distribution in a tight spatial region of shower core. In addition, such a facility should be able to register simultaneously the electromagnetic, hadronic, and muonic components of EAS, as well as secondary low-energy particles, such as the thermal neutrons and gamma ray quanta which accompany the passage of showers. For a number of reasons, the most suitable placement option for such installation would be an altitude of about (3–4) km above the sea level.
Based on these considerations, the development program of the Tien Shan mountain cosmic ray station of Lebedev Physical Institute adopted in the early 2000s planned to develop there a modern complex of particle detectors with a general purpose of detailed study of EASs created by the cosmic ray primaries with energy (1–100) PeV, and in particular of the particle flow in the central core region of such showers [[
The experimental complex of the Tien Shan station includes a wide-spread system of the detectors for immediate registration of the density of charged shower particles (electrons) [[
The detection system of EAS charged particles at the Tien Shan station is composed by a set of scintillation detectors placed in the nodes of a rectangular framework with the spatial steps of and m in two perpendicular directions. In the moment of EAS passage all detectors of the system operate synchronously which permits to register the 2-dimensional distribution of the density of electron flux in EAS. Such distribution is necessary to determine the main characteristics of the shower: the arrival direction of a cosmic ray particle which has borne the EAS, the position of shower axis in the plane of detector disposition, the total amount of particles in the shower (its "size") , and the energy of the primary particle . Besides, the momentary amplitude of scintillation signal summed over all detectors is used for generation of a shower trigger pulse which synchronizes operation of all detector subsystems of the Tien Shan experimental complex at registration of EAS events.
Detailed description of the EAS particle detectors of the Tien Shan installation and of the algorithms of data operation can be found in [[
Presently, the installation of particle detectors ensures registration of the EASs with primary energy above 0.3 PeV, and with axes passing through a 1000 m large central area of the detector system. The typical determination accuracy of the EAS axis position by the data of the shower installation is (1–3) m, and the dynamic range of scintillation detectors used for the measurement of the charged particles density is about particles/m. The last characteristic currently ensures a saturation-free measurement of particle density in the central region of EAS with primary energy up to PeV. As well, a possibility is anticipated in the design of detector channels to enlarge their dynamic range up to particles/m which would permit efficient investigation of particle flow in the core of PeV EASs.
An example of EAS events presently recorded by the Tien Shan system of particles detectors is shown in Fig. 1. In the right frame of that figure an illustration plot is also presented of the capability of this installation for the measurement of the particles density distribution just within the central core region of EAS. It is seen there that due to the expanded dynamical range of particle detectors correct measurement of is now possible even at distance m from shower axis, which was not achievable in former experiments.
Graph: Fig. 1 Left: an EAS event with primary energy of 20 PeV as it was seen by registration at the Tien Shan installation of shower particle detectors. The height of vertical bars is proportional to the local density of electron flux. Right: the lateral distribution of particle density in the EASs detected by the modern shower installation at Tien Shan (points), and in a previous experiment (continuous lines). The distributions presented here were averaged between the showers with close energies of 20 (
Besides the detectors aimed to measurement of the peak amplitude of scintillation signal for estimation of particles density, the EAS detection system includes also some specialized points with the detectors of enhanced temporal resolution [[
An alternative means for studying the electron component of EAS at the Tien Shan station is a system of radio receivers for detection of the (30–100) MHz electromagnetic emission generated by the charged particles of shower front [[
The ionization-neutron calorimeter INCA is used to register the hadronic component of EAS, particularly in the region of shower core. The geometric area of the calorimeter is m, it comprises layers of cm ionization chambers alternating with the sheets of iron absorber. The chambers, made of chemically pure copper and filled with technical argon, have mutually perpendicular orientation of their long axes in every pair of neighbouring layers. The whole system of ionization chambers is used for detection of electromagnetic cascades which develop within the heavy absorber of the calorimeter after interaction of high-energy cosmic ray hadrons. The total thickness of absorber is 100 cm which corresponds to (5–6) interaction paths of cosmic ray hadrons, in dependence on inclination of their trajectory.
Besides the chambers there are 48 neutron detector modules installed within the internal volume of the calorimeter. Each of them contains a cm large, He filled gas discharge neutron counter for direct registration of the thermalized evaporation neutrons born in interactions of cosmic ray hadrons. As well, 16 cm Geiger–Müller tubes put in every module ensure detection of MeV energy gamma rays emitted at neutron captures by the nuclei of a hydrogen reach material which constitutes a part of internal module construction. Using of gamma ray counters in the modules thus permits to enhance somewhat the efficiency of neutron detection.
The hybrid design of the INCA calorimeter makes it possible to combine a relatively high, of about 0.25 m, spatial resolution which is achieved by detection of electromagnetic cascades in a three-dimensional structure formed by successive rows of ionization chambers, with a wide, of at least (4–5) orders of magnitude, measurement range of energy deposit from interaction of the hadronic particles of EAS core by the means of neutron and gamma ray detectors.
The NM64-type neutron supermonitor placed at the Tien Shan mountain station provides an alternative possibility for detecting the hadronic component in EAS. The monitor consists of three m detector units. There are 6 cm gas discharge counters in each unit which detect the evaporation neutrons created in the heavy lead target of the monitor under the influence of cosmic ray hadrons. The amount of output signals obtained from these counters after passage of an EAS during a fixed time gate (typically, of a few milliseconds order) can be a measure of the energy deposited by EAS hadrons in the monitor. An explicit appearance of the dependency was defined for the Tien Shan monitor both in a series of special calibration measurements [[
In order to qualitatively improve the temporal resolution of neutron detection and correspondingly to increase even more the measurement range of hadronic energy deposits the special boron-enriched scintillators were designed for using in the monitor [[
The system of the neutron and gamma detectors with low registration threshold was created at the Tien Shan station to register the neutrons born under the influence of EAS hadrons, as well as the flux of gamma rays associated with those neutrons. This trend of experimental activity at the Tien Shan station was inspired by a series of publications [28–37], where the flux of thermalized evaporation neutrons detected with some delay after the main front of relativistic EAS particles was proposed as a valuable information source on the properties of high energy hadronic interaction. As it has revealed, the passage of powerful EAS is often accompanied by a prolonged stream of low-energy neutrons arising from interaction of the hadrons of shower core with the material of surrounding environment, primarily of the soil around the detector, which thus plays the role of a kind of nuclear calorimeter.
Up to date, there were three detector points created at the Tien Shan station for the purpose of thermal neutron detection. One of them is situated near the center of the system of shower particle detectors, the other two are located at distances of 50 and 120 m from that center. There are 12 cm gas discharge counters with He filling installed in each point which detect the evaporation neutrons born by EAS hadrons in surrounding matter and thermalized there in the process of random diffusion. Besides neutron counters, there is a NaI scintillator with a 560 cm area of sensitive surface in each detector point for registration of gamma rays with several energy thresholds between (0.3–3) MeV. The latter is an alternative means for detection of the neutron signal from hadronic interactions which uses for the purpose the gamma ray quanta emitted at neutron captures in the surrounding environment.
The registration efficiency of the neutron and gamma detectors used in Tien Shan experiments in the different energy ranges of detected particles was defined by simulations which were made on the basis of the Geant4 toolkit [[
The underground muon detector is aimed to study the muon component of cosmic rays in an extremely wide energy range of detected muons, starting from 5 GeV and up to tens and hundreds of TeV units. Internal design of this detector is made similarly to the NM64 neutron supermonitor, so its operation principle is also based on detection of evaporation neutrons. In contrast to the monitor, the underground detector is installed in an underground room of the Tien Shan station, beneath a 2000 g/cm thick layer of soil absorber. Thanks to this absorber, the flux of ordinary hadrons (such as nucleons and pions) at the depth of detector is only in relation to their intensity at the top of the ground.
The main source of neutron signals in the underground detector constitutes interactions of cosmic ray muons within the internal absorber of the detector, which can proceed there both directly, by the nuclear channel of muon interaction, and through photonuclear reactions caused by the bremsstrahlung gamma rays emitted by muons. Because of such operation principle, an observable signal of muon passage through the detector is a series of electric pulses gained from its neutron counters in a short time interval. The duration of the signal collection interval is defined primarily by the neutron life time in the detector, and is typically of a few milliseconds order. An illustrative example of such dense group of neutron signals is shown in the left panel of Fig. 2.
Graph: Fig. 2 Left: dense group of signals from neutron counters of the underground muon detector registered in a close passage event of an EAS core. Zero point of the time axis corresponds to the trigger from the charged particles of EAS front; the height of vertical bars is proportional to the amount of neutron signals detected in a s-long time interval. Right: the mean multiplicity of detected neutron signals from the counters of the underground detector, in dependence on the energy of interacting muon (the result of a Geant4 simulation). The vertical arrow marks the lower energy threshold of muon registration in the underground detector of the Tien Shan station.
As a result of simulation calculations which were carried out using the Geant4 toolkit, the right-hand plot of Fig. 2 was obtained which connects the mean multiplicity of registered signals from the neutron counters of the underground detector, , and the energy of a passing muon. As it is seen in the figure, a facility based on detection of neutron signals from muon interactions has a rather high energy threshold of assured muon registration (i.e., with ), which occurs to be of the order of tens of TeV units, while the energy range of effective muon detection by such method is practically unlimited from above. As well, the detector provides an opportunity for estimation of the energy of passing muons by the amount of detected neutron signals .
In the range of muon energies essentially below TeV the underground detector still can be used for investigation of dense muon groups [38–41] in which case the insufficient detection probability of each particular muon particle () is compensated by their high multiplicity. The lowest energy threshold of muon detection at the Tien Shan station is defined, in principle, by the thickness of soil absorber above its underground room and amounts to several GeV units.
The low-energy accompaniment of EAS. To date, the neutron and gamma radiation detectors with low registration threshold have been used for investigation of the delayed neutron and gamma ray fluxes in the core region of EAS [[
Graph: Fig. 3 Time distribution of the intensity of thermal neutrons (left), and of the gamma rays with the energy 30 keV (right), as detected at distance m from the EAS center after passage of the showers with ( PeV). Zero point of the time axes in the plots corresponds to the passage moment of the front of relativistic EAS particles, dashed lines indicate the background level of signal intensity.
Preliminary results of the experiment on investigation of the delayed fluxes of low-energy particles connected with EAS is presented in the plots of Fig. 4, for the neutrons, and in Fig. 5 for the 30 keV gamma rays, as dependence of detected radiation fluxes on the size parameter of corresponding EAS, and on the distance from the detector point to EAS center. The duration of the period of signal collection after the passage of EAS front in these measurements was 8.5 ms in the case of neutron detector, and one whole second for gamma rays. In both cases the levels of the background counting rate were subtracted from the average amount of signals which have been detected after an EAS passage.
Graph: Fig. 4 Left: spatial distribution of the average multiplicity of detected neutron signals , and the corresponding local fluence of EAS-connected neutrons at different distances from the shower axis. Points are the experimental data, dotted lines mark their exponential fit, and the numbers beside curves mean the decimal logarithm of the average size parameter of corresponding shower. The gate time of neutron signal collection after an EAS passage was s, the background is subtracted. Right: the average integral fluence of thermal neutrons detected after EAS passage, in dependence on the mean shower size . Dotted lines mark the approximation of the experimental points with a piecewise power function . The upper axis is graduated in the units of primary EAS energy which correspond to the mean values accordingly to simulation [[
Graph: Fig. 5 Same as in Fig. 4, for the flux of the soft gamma radiation with registration threshold keV delayed up to s after EAS passage.
As it follows from the left plot of Fig. 4, the multiplicity of EAS-connected neutrons in the points situated at various distances from the shower center can be represented as combination of two exponents with different means,
Graph
Specific values for the parameters and there can be defined by fitting each experimental distribution curve in the left-panel plot of Fig. 4. The resulting exponential approximations are shown in that plot with dotted lines.
Normalization of neutron multiplicities to the sum sensitive area of neutron counters in detector point and to the efficiency of thermal neutron registration by a counter gives the mean local fluence of thermal neutrons connected with EAS which have passed through the point at distance from shower center. An auxiliary right axis in the plot of Fig. 4 is graduated immediately in the values of average neutron fluence .
At last, by integration of the normalized approximating functions in the whole range of variation, from zero to infinity, an integral fluence can be obtained of the neutrons which have been borne under influence of EAS hadrons:
Graph
As before, the values thus obtained relate to the mean neutron fluence averaged over the showers with close sizes .
The dependence of the mean values of neutron fluence on the average size of corresponding EASs is plotted in the right panel of Fig. 4. As it is seen there, generally this dependence follows to a piecewise power function which changes its power index sharply near the point of : below this border limit, and above it.
From simulation of EAS development it is known that at the altitude of the Tien Shan station the EASs with the sizes generally stem from interaction of primary comic ray particles with the energy PeV [[
In Fig. 5 the results are presented of an analogical analysis which was made for the fluxes of soft gamma radiation detected after the passage of EAS. It is seen there that the dependence of the integral gamma ray fluence on shower size, , is quite analogous to what was found in the case of thermal neutrons, i.e., it experiences doubling of its power index in vicinity to the knee point of the spectrum of cosmic rays. Indeed, such similarity between the behavior of the delayed neutron and gamma ray signals should be supposed since the gamma radiation flux observed after EAS passage originates mostly from capture of diffusing neutrons in the surrounding environment.
Since the original source of detected neutrons and gamma rays is interaction of high energy hadrons of EAS core, the change of the power index in and dependencies means some corresponding modification in the characteristics of the hadronic component of EAS, and this variation takes place just around the knee point of the spectrum of primary cosmic rays.
Relative share of EASs with observable imprint of muon interaction. The newly developed technique for the study of cosmic ray muons has made it possible to obtain some interesting results concerning the muon component of EAS. As illustration, Fig. 6 presents the experimentally measured share of the EASs which were accompanied by neutron signals from muon interaction among all showers whose axes were passing at a small distance, m, from the center of the underground detector. For numerical analysis, this share was represented as a relation , where is the number of shower events with non-zero multiplicity of detected neutron signals , and is the total amount of all EAS with close axis position. In Fig. 6 the values of the considered relation are plotted in dependence on the average size and primary energy of corresponding showers.
Graph: Fig. 6 The average share of the EASs which have generated neutron events in the underground detector, , in dependence on the mean shower size . The counts of shower events with non-zero multiplicity of detected neutron signals are normalized to the total amount of all showers which had the axis distance below 10 m from the center of the underground detector.
Quite analogously to behavior of the low-energy neutron and gamma ray components of EAS it can be seen in Fig. 6 that the dependence of the relation on the average shower size parameter , though remaining generally of a power-law type, is nevertheless not completely uniform, but has a sharp change of its exponent index at the energy close to the point of , i.e., again, near the knee in the cosmic ray spectrum at PeV. Such an increase means the rise of the relative energy deposit left by EAS muons within the internal absorber of the underground detector which effect could be caused either by some non-proportional growth of the average muon number in EAS, or by an increase of the mean muon energy, or by combination of both these reasons.
Generally, the EAS muons originate from decay of the high energy charged pions produced at early development stages of an atmospheric cascade. Any irregularity in behavior of phenomenological characteristics of the muonic component of EAS, like the variation of power index in the distribution of Fig. 6, should reflect some principal change in the process of pion production. Thus, the results obtained so far at the Tien Shan underground detector is another, and independent, evidence of some variation in interaction properties of the high energy EAS hadrons which occurs near the PeV knee point of the primary cosmic ray spectrum.
Presently, a research complex has been created at the Tien Shan mountain station for experimental study of the (1–100) PeV cosmic rays by the method of multi-component investigation of the core region of extensive air showers. A set of detector installations existing now at the station has no comparable analog in the world in what concerns its high-altitude mountain location, diversity of the used detector types, and accessible information on the properties of detected EAS. The results newly obtained at the Tien Shan experimental complex come in line with the general tendency of peculiar behavior which the various components of cosmic rays demonstrate around and above the PeV knee point of their energy spectrum.
This research has been funded by the Science Committee of the Ministry of Education and Science of the Republic of Kazakhstan, grant no. AP09258896.
By A. Shepetov; S. Mamina; V. Piscal; V. Ryabov; N. Saduyev; T. Sadykov; S. Shaulov; L. Vildanova and V. Zhukov
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