Measurement of air shower cores to study the cosmic ray composition in the knee energy region

Abstract

Since 1996, a hybrid experiment consisting of an emulsion chamber and a burst
detector array and the Tibet-II air shower array has been operated at
Yangbajing (4300 m above sea level) in Tibet. This experiment can detect
air-shower cores, called burst events, accompanied by air showers in excess of
about 100 TeV. Using the burst event data observed by this experiment, we
discuss the primary cosmic ray composition around the knee in comparison with
the Monte Carlo simulations. In this paper, we show that all the features of
burst events are wholly compatible with the heavy enriched composition in the
knee energy region.

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arXiv:astro-ph/0008043v2 3 Aug 2000
Measurement of air shower cores to study the cosmic ray
composition in the knee energy region
M. Amenomori1, S. Ayabe2, Caidong3, Danzengluobu3, L.K. Ding4, Z.Y. Feng5, Y. Fu6,
H.W. Guo3, M. He6, K. Hibino7, N. Hotta8, Q. Huang5, A.X. Huo4, K. Izu9, H.Y. Jia5,
F. Kajino10, K. Kasahara11, Y. Katayose12, Labaciren3, J.Y. Li6, H. Lu4, S.L. Lu4,
G.X. Luo4, X.R. Meng3, K. Mizutani2, J. Mu13, H. Nanjo1, M. Nishizawa14, M. Ohnishi9,
I. Ohta8, T. Ouchi7, Z.R. Peng4, J.R. Ren4, T. Saito15, M. Sakata10, T. Sasaki10, Z.Z. Shi4,
M. Shibata12, A. Shiomi9, T. Shirai7, H. Sugimoto16, K. Taira16, Y.H. Tan4, N. Tateyama7,
S. Torii7, T. Utsugi2, C.R. Wang6, H. Wang4, X.W. Xu4,9, Y. Yamamoto10, G.C. Yu3,
A.F. Yuan3, T. Yuda9,17, C.S. Zhang4, H.M. Zhang4, J.L. Zhang4, N.J. Zhang6,
X.Y. Zhang6, Zhaxiciren3, and Zhaxisangzhu3
(The Tibet ASγ Collaboration)
1Department of Physics, Hirosaki University, Hirosaki 036-8561, Japan
2Department of Physics, Saitama University, Urawa 338-8570, Japan
3Department of Mathematics and Physics, Tibet University, Lhasa 850000, China
4Laboratory of Cosmic Ray and High Energy Astrophysics,
Institute of High Energy Physics, Academia Sinica, Beijing 100039, China
5Department of Physics, South West Jiaotong University, Chengdu 610031, China
6Department of Physics, Shangdong University, Jinan 250100, China
7Faculty of Engineering, Kanagawa University, Yokohama 221-8686, Japan
8Faculty of Education, Utsunomiya University, Utsunomiya 321-8505, Japan
9Institute for Cosmic Ray Research, University of Tokyo, Kashiwa 277-8582, Japan
10Department of Physics, Konan University, Kobe 658-8501, Japan
11Faculty of Systems Engineering, Shibaura Institute of Technology, Omiya 330-8570, Japan
12Faculty of Engineering, Yokohama National University, Yokohama 240-0067, Japan
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13Department of Physics, Yunnan University, Kunming 650091, China
14National Institute of Informations, Tokyo 101-8430, Japan
15Tokyo Metropolitan College of Aeronautical Engineering, Tokyo 116-0003, Japan
16Shonan Institute of Technology, Fujisawa 251-8511, Japan
17Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya 464-8601, Japan
(February 1, 2008)
Abstract
Since 1996, a hybrid experiment consisting of an emulsion chamber and a
burst detector array and the Tibet-II air shower array has been operated at
Yangbajing (4300 m above sea level) in Tibet. This experiment can detect
air-shower cores, called burst events, accompanied by air showers in excess of
about 100 TeV. Using the burst event data observed by this experiment, we
discuss the primary cosmic ray composition around the knee in comparison
with the Monte Carlo simulations. In this paper, we show that all the features
of burst events are wholly compatible with the heavy enriched composition in
the knee energy region.
PACS numbers : 95.85Ry, 98.70Sa, 96.40Pq, 96.40De
Typeset using REVTEX
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I. INTRODUCTION
It is commonly understood that the knee of the primary cosmic ray spectrum has its
origin in the acceleration and propagation of high energy cosmic rays in the Galaxy. The
model of the shock acceleration by supernova blast waves leads to the formation of a power-
law spectrum of particle energies with a differential index of about -2 at sources [1]. The
plausible propagation models of their confinement by galactic magnetic fields and of their
eventual escape from our Galaxy can explain well a steeper power-law spectrum than that
at the source region [2], suggesting a rigidity-dependent bending for different cosmic ray
composition. Within the framework of this picture the average mass of primary cosmic rays
before the knee should increase with increasing primary energy. In other words, the knee
composition should be heavily dominant as the proton spectrum may bend at an energy
of about 100 TeV corresponding to a maximum energy gained by shock acceleration at
supernova remnants.
While there is no consensus on the origin of cosmic rays with energies beyond the knee,
observations of cosmic rays in such a high energy region may naturally stand in need of
another acceleration mechanisms [3,4] or new cosmic ray sources [5,6]. Among those, one of
the most promising models may be that the cosmic rays come from extra-galactic sources
such as active galactic nuclei [6], although the evidence is far from convincing. However,
such an extra-galactic source model should predict proton enriched primary composition
around and beyond the knee.
Thus, measurements of the primary cosmic rays around the knee are very important and
its composition is a fundamental input for understanding the particle acceleration mechanism
that pushes cosmic rays to very high energies. However, because of extremely low and steeply
decreasing flux at high energies, direct measurements of primary cosmic rays on board
balloons are still limited in the energy region to below a few hundred TeV [7,8]. To date,
the knee composition of primary cosmic rays has been studied by observing air showers with
a large aperture ground-based apparatus. For air shower observations, a surface detector
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array is commonly set up to measure the lateral distributions and the arrival times of
shower particles (mostly electrons and photons), which enable us to locate the core position,
determine the arrival direction, and deduce the shower size and shower age [9]. These are
important basic parameters to describe an air shower, although they are not very sensitive to
the primary composition. In order to study the composition around the knee, measurements
of muon content in each air shower [10] or muons in deep underground [11,12], measurements
of lateral distribution of air shower Cherenkov lights [13], or maximum depth of shower
development using air Cherenkov telescopes [14], and multiparameter measurements of air
showers [15] have been carried out and devoted to making the conclusion about the knee
composition. In spite of great efforts so far, however, there is a divergence of conclusions
on the composition from experiment to experiment and the knee composition is still the
question at issue. Another approach may be required to get more direct information about
the composition.
Within the ground-based air shower experiments those set up at higher altitudes are
preferable for the physics studies at the knee region. The reasons include ; first, maximum
development of showers with the knee energies is closer to the observation level, so that the
shower is less fluctuated and the energy determination is more precise and less dependent
upon the unknown composition [9] ; second, the energy flow in the core region of air showers is
less attenuated and easier to observe with conventional calorimeters. High energy air-shower
cores are sensitive to the composition of the primary cosmic rays around the knee. Air-shower
cores contain a large part of the primary energy in the early stage of shower development, but
with increasing atmospheric depth they are rapidly diffused with dissipation of their energies.
Hence, the measurement of air-shower cores should be implemented at high altitude with a
detector having a reasonably large area. In general a thick shower detector is required when
one wants to record all (or most) hadronic components in the core, but this needs enormous
heavy materials to absorb them in the detector. At high altitude, however, this can be
achieved by observing the electromagnetic components in the core with a thin detector,
since these are mostly the cascade products induced by high energy π0-decay γ rays which
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are produced in the air shower cores. Therefore, the electromagnetic components in the core
well reflect the major behavior of the whole hadronic components, keeping the sensitivity to
the primary composition.
In this paper we report our study of the primary composition in the knee region using
data of the Tibet burst detector and the air-shower array II. The experiment, including the
apparatus and its performance, data set and background analysis, is introduced in Sec. II.
The air-shower simulation and detector response calculation are described in Sec. III. The
results and discussions are described in Sec. IV. A brief summary is given in Sec, V.
II. EXPERIMENT
A. Apparatus
We started a hybrid experiment of the emulsion chamber, the burst detector and the
air shower array (Tibet-II) at Yangbajing (4300 m above sea level), Tibet in 1996 [16].
The Tibet-II array consists of 221 scintillation counters of 0.5 m2each which are placed
on a 15 m square grid and has been operated since 1995. Any fourfold coincidence in the
detectors is used as the trigger condition for air-shower events. Under this condition the
trigger rate is about 200 Hz with a dead time of about 12% for data taking. The energy
threshold is estimated to be about 7 TeV for proton induced showers. The precision of
the shower direction determination is about 1◦, which has been confirmed by observing
the Moon’s shadow [17]. The main aim of Tibet-II is to search for γ-ray point sources at
energies around 10 TeV. But it can also be used for the measurement of the spectrum of
the total cosmic ray particles [9], and for the study of topics in the knee region by providing
information of the shower size, direction, core position, and arrival time of each air-shower
event to the core detectors [16].
The emulsion chambers and the burst detectors are used to detect high-energy air-shower
cores accompanied by air showers induced by primary cosmic rays with energies above 1014
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