I.V. Rakobolskaya

Composition and energy spectra of cosmic ray primaries in the energy range 1013 - 1015 eV/particle observed by Japanese-Russian joint balloon experiment

Abstract We report experimental results obtained by the emulsion chambers on board of the long duration balloon. We have been carrying out the trans-Siberian-continental balloon flight since 1995, and the results from 1995 to 1996 experiments are presented here. Total exposure of these two years amounts to 231.5 m 2 h at the average altitude of ∼32 km. The energy range covers 10–500 TeV for proton-primary, 3–70 TeV/n for helium-primary, and 1–5 TeV/n for Fe-group ( Z =26–28), though statistics of heavy components is not yet enough. Our preliminary data show that the spectra of the proton and the helium have nearly the same power indices ∼2.80, while those of heavier ones become gradually harder as the mass gets heavier, for instance the index is ∼2.70 for CNO-group and ∼2.55 for Fe-group. It is remarkable that a very high energy proton with multi-PeV is detected in 1995 experiment, and the estimated flux of this event coincides with a simple extrapolation from the energy spectrum with the power index 2.8 observed in the range 10–500 TeV. It indicates that there is no spectral break at around 100 TeV, in contrast to the maximum energy predicted by the current shock-wave acceleration model. This evidence requires some modification on the acceleration and/or propagation mechanism. Also we present all-particle spectrum and the average primary mass in the energy range 20–1000 TeV/particle. Our preliminary data show no drastic change in mass composition over the wide energy range, at least up to 1 PeV/particle, though the statistics is not yet enough to confirm it concretely. The flight performance and the procedure of the analysis, particularly the energy determination methods and the detection efficiency calculation are also given.

Cosmic-Ray Spectra and Composition in the Energy Range of 10-1000 TeV per Particle Obtained by the RUNJOB Experiment

This is a full report on the cosmic-ray spectra and composition obtained by the emulsion chambers on board 10 long-duration balloons, launched from Kamchatka between 1995 and 1999. The total exposure of these campaigns amounts to 575 m2 hr, with an average flight altitude of ~32 km. We present final results on the energy spectra of two light elements, protons and helium nuclei, and on those of three heavy-element groups, CNO, NeMgSi, and Fe, covering the very high energy region of 10-1000 TeV particle-1. We additionally present the secondary/primary ratio, the all-particle spectrum, and the average mass of the primary cosmic rays. We find that our proton spectrum is in good agreement with other results, but the intensity of the helium component is nearly half that obtained by JACEE and SOKOL. The slopes of the spectra of these two elements obtained from RUNJOB data are almost parallel, with values of 2.7-2.8 in the energy range of 10-500 TeV nucleon-1. RUNJOB heavy-component spectra are in agreement with the extrapolation from those at lower energies obtained by CRN (Chicago group), monotonically decreasing with energy. We have also observed secondary components, such as the LiBeB group and the sub-Fe group, and present the secondary/primary ratio in the TeV nucleon-1 region. We determine the all-particle spectrum and the average mass of the primary cosmic rays in the energy region of 20-1000 TeV particle-1. The intensity of the RUNJOB all-particle spectrum is 40%-50% less than those obtained by JACEE and SOKOL, and the RUNJOB average mass remains almost constant up to ~1 PeV.

Observation Of Very High Energy Cosmic-ray Families In Emulsion Chambers At High Mountain Altitudes (i)

Characteristics of cosmic-ray hadronic interactions in the 1015 − 1017 eV range are studied by observing a total of 429 cosmic-ray families of visible energy greater than 100 TeV found in emulsion chamber experiments at high mountain altitudes, Chacaltaya (5200 m above sea level) and the Pamirs (4300 m above sea level). Extensive comparisons were made with simulated families based on models so far proposed, concentrating on the relation between the observed family flux and the behaviour of high-energy showers in the families, hadronic and electromagnetic components. It is concluded that there must be global change in characteristics of hadronic interactions at around 1016 eV deviating from thise known in the accelerator energy range, specially in the forwardmost angular region of the collision. A detailed study of a new shower phenomenon of small-pT particle emissions, pT being of the order of 10 MeV/c, is carried out and its relation to the origin of huge “halo” phenomena associated with extremely high energy families is discussed as one of the possibilities. General characteristics of such super-families are surveyed.

Alignment in gamma -hadron families of cosmic rays.

Alignment of main fluxes of energy in a target plane is found in families of cosmic ray particles detected in deep lead X-ray chambers. The fraction of events with alignment is unexpectedly large for families with high energy and large number of hadrons. This can be considered as evidence for the existence of coplanar scattering of secondary particles in interaction of particles with superhigh energy,

Observation of attenuation behaviour of hadrons in extremely high energy cosmic ray interactions: New hadronic state?

E_0>10^{16}

Observation of a high-energy cosmic-ray family caused by a Centauro-type nuclear interaction in the joint emulsion chamber experiment at the Pamirs

eV. Data analysis suggests that production of most aligned groups occurs low above the chamber and is characterized by a coplanar scattering and quasiscaling spectrum of secondaries in the fragmentation region. The most elaborated hypothesis for explanation of alignment is related to the quark-gluon string rupture. However, the problem of theoretical interpretation of our results still remains open.

Large transverse momenta in nuclear interaction at E 0 > 1016 eV detected in stratosphere

Abstract Experimental results are presented on high energy cosmic-ray hadron interactions recorded in homogeneous-type thick lead chambers (total thickness being 60 cm and 110 cm) exposed at the Pamirs (atmospheric depth 595 g/ cm 2 ). High energy cosmic-ray hadron flux is measured. The attenuation mean free path of the arriving cosmic-ray hadrons of E h γ ≥ 6 TeV measured in the chamber is obtained as 252 ± 30 g/cm 2 of lead. However, for the high energy hadrons ( E ( γ ) ≥ 10 TeV) constituting cosmic-ray families of the highest energy range, ΣE ( γ ) ≥ 700 TeV, which have been accumulated so far in the series exposures at the Pamirs and analysed by MSU group, the attenuation mean free path of hadrons in lead has turned out to be as short as 170 −26 +47 g/cm 2 with 95% CL by the maximum likelihood method. The present experimental result of such a short attenuation length of hadrons in cosmic-ray families is essentially consistent with that obtained for high energy hadrons ( E ( γ ) ≥ 10 TeV) constituting the “Chiron-type” families of ΣE ( γ )>100 TeV in the Chacaltaya two-storeyed chamber experiments. The anomalous transition characteristics of high energy hadrons in lead is found and examples are presented. The physical significance of the experimental results on extremely high energy hadron interactions is discussed.

Energy determination of the cascade shower by means of a new type of emulsion chamber with diffuser module

Abstract An exotic cosmic-ray family event is observed in the large emulsion chamber exposed by the joint at the Pamirs (4360 m above sea level). The family is composed of 120 γ -ray-induced showers and 37 hadron-induced showers with individual visible energy exceeding 1 TeV. The decisive feature of the event is the hadron dominance: ΣE γ , ΣE ( γ ) h , 〈 E γ , 〈 E ( γ ) h 〉, 〈 E γ · R γ 〉 and 〈 E ( γ ) · R h 〉 being 298 TeV, 476 TeV, 2.5 TeV, 12.9 TeV, 28.6 GeV m and 173 GeV m, respectively. Most probably the event is due to a Centauro interaction, which occured in the atmosphere at ∼700 m above the chamber. The event will constitute the second beautiful candidate for a Centauro observed at the Pamirs.

First results obtained by RUNJOB campaign

A gamma-hadron superfamily of cosmic-rays created by a primary cosmic-ray particle with energy above 1016 eV was detected at an altitude of 30 km by a stratospheric balloon-borne emulsion chamber. Being of superhigh energy, this event is the unique example in the world statistics of practically pure nuclear interactions in the energy range unattainable for modern accelerators. The present analysis allowed one to estimate the interaction height above the chamber and transverse momenta of the secondaries produced in the interaction. The mean value of transverse momenta appears to be very large (〈pt〉 > 2.5 GeV/c).

High-Energy Gamma Rays in the RUNJOB Experiment

Abstract An account is given of a new type of emulsion chambers which have been in our use since 1997 in our RUNJOB program (RUssia–Nippon JOint Balloon-program). Each chamber is equipped with an additional “diffuser module” placed under the usual set of modules. We have made the experiments using 4 cm thick diffuser modules composed of several photo-sensitive layers (X-ray films and/or nuclear emulsion plates) sandwiched with spacers. The result is as follows. Even in the case where the path length of only 6 radiation lengths is available within the calorimeter module placed above, the visible energy sum is determined with an accuracy better than σ∼0.2 for a group of electromagnetic cascade showers induced by a proton of energy up to several tens of TeV, or by an iron nucleus of energy up to one hundred TeV. If the available path length in the calorimeter module is 9 radiation lengths, we can estimate the energy sum up to ∼100 TeV within an accuracy of σ∼0.2 for a proton-induced cascade shower group. It means that the use of our new-type emulsion chamber can reduce the detector payload dramatically, which is essentially important for the high-energy cosmic-ray observations made on board the vehicles such as balloons, satellites and so on.