28th International Cosmic Ray Conference167

A Study of the Primary Composition at ∼ 1014 − 1015 eVwith the GRAPES-2 array at Ooty
S.C. Tonwar1 , S.K. Gupta1 , Y. Hayashi2 , N. Ito2 , Atul Jain1 , A.V. John1 ,S. Kawakami2 , D.K. Mohanty1 , P.K. Mohanty1 , S.D. Morris1 , T. Nonaka2 ,K.C. Ravindran1 , K. Sivaprasad1 , B.V. Sreekantan1 , H. Tanaka2 ,K. Viswanathan1 and T. Yoshikoshi2(1) Tata Institute of Fundamental Research, Mumbai 400005, India(2) Graduate School of Science, Osaka City University, Osaka 558-8585, Japan(The GRAPES Collaboration)

Abstract
Several observations on the electrons, muons and hadrons in air showers
at energies ∼ 1015 eV have suggested that the primary cosmic ray ﬂux may begetting enriched in heavier nuclei with increasing energy. This conclusion, though
in conﬂict with some other observations, particularly, on the characteristics of theCherenkov radiation in showers, has very signiﬁcant implications for the natureof galactic sources generating and accelerating particles to ultra-high energies.
We present here the results from the analysis of data on the muon multiplicity
distributions from observations made with the large 192-module 200 m2 area
muon detector operating in association with the 100-detector air shower arrayof the GRAPES-2 experiment. Our observations also provide support to thehypothesis that the energy spectra for the heavier elements in the primary ﬂux
seems to be ﬂatter relative to spectra for lighter nuclei at energies ∼ 1015 .
1.

Introduction

The presence of ultra high energy (≥ 1014 eV) particles in primary cosmic
rays invites speculations on the nature of very energetic astrophysical sources
which are capable of producing and accelerating particles to energies as high as
1020 eV. Due to the scattering of particles by the chaotic magnetic ﬁelds pervading
most of the interstellar space in our galaxy, it has not been possible to identify
these sources directly. While some success [1] has been achieved in identiﬁcationof a few sources emitting TeV γ-rays, similar success at PeV and higher energies
continues to elude us as it is very diﬃcult to identify showers initiated by primary
γ-rays in the presence of the huge background of showers due to charged cosmic
rays. Therefore, presently, detailed studies on the charged component of cosmic
rays oﬀer the only means to understand the astrophysics of high energy cosmic
rays.
cpp. 167–170 2003
by Universal Academy Press, Inc.

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At energies above 1014 eV the ﬂux of cosmic rays is too small to be detected by satellite or balloon borne detectors ﬂying above the atmosphere due toconstraints on achieving very large exposure factors with moderately heavy payloads. For example, even a detector with a large exposure factor of ∼ 1010 cm2 ssr
would detect only ∼ 1 event above 1015 eV. Though the energy spectra for various
nuclear groups have been well measured with detectors ﬂown aboard long-ﬂying
balloons [2,3] at energies around ∼ 1012 eV, the statistics at energies above 1014
eV are inadequate to extrapolate the measured spectra to the energy of the kneein the energy spectrum of primary cosmic rays at energy ∼ 3 × 1015 eV . Therefore, measurements on the energy spectra of various nuclear groups at energies
above ∼ 1014 eV have to rely presently on indirect observations on the products
of interactions of primary cosmic ray particles in the atmosphere.
Detailed simulations show that measurements on the distribution of thenumber of muons incident over a very large area detector, usually called the muon
multiplicity distribution, along with good measurements on the electron component of the shower accompanying the muons, oﬀer a very promising means to study
the energy dependence of the composition of primary ﬂux at energies above 1014eV. The Gamma Ray Astronomy at PeV EnergieS - Phase 2 (GRAPES-2)
experiment has been designed with this objective in mind.
2.

The GRAPES-2 Experiment

The GRAPES-2 experiment is located within the city of Ooty (11o.4 Nlatitude, 76o .3 E longitude and 2200 m altitude), a popular mountain resort town
in southern India. It consists basically of 100 unshielded ‘electron’ detectors anda 192-module 200 m2 area shielded ‘muon’ detector, shown schematically in theleft panel of Fig. 1. Due to the limitations imposed by the ground topography,
the placement of electron detectors, with inter-detector spacing of ∼ 10 m, ishexagonally symmetric only upto about 40 m from the center, that is, for 61
detectors placed within the inner 4 rings. However, outer detectors, 62-85, provide
good sampling of particle density for core distances upto ∼ 80 m for showers whose
cores are located within ∼ 30 m of the array center. The very compact nature of
the GRAPES-2 array is a unique feature which yields a relatively lower energy
detection threshold and more accurate reconstruction of shower characteristics
including arrival direction.
The large (200 m2 ) area muon detector of the GRAPES-2 array consistsof 192 modules, each 1.04 m2 in area. The center of the muon detector is locatedat a distance of ∼25m from the array center and the distance of individual muon
detector modules from the array center varies from ∼15 m to ∼40 m. A total of
16 tunnels, each 22 m deep, were made by digging into the hillside to accommodate the 192 muon detector modules. The tunnels are covered by a 0.3 m thickreinforced concrete slab which takes the load of 3.5 m thick layer of packed soil

169Fig. 1. Schematic layout of electron detectors and the muon detector in theGRAPES-2 array located within the Raj Bhavan campus at Ooty (left panel) anddistribution of ‘Particle Sum’ for showers satisfying the software selection conditions’ (right panel).

placed above it. The total thickness of the absorber above the detector modulesis ∼ 600gcm−2 which sets the energy threshold of 1 GeV for vertical muons to beable to give signal in the 20 cm deep water Cerenkov detector modules.
3.

Observations

A special feature of the GRAPES-2 experiment is the requirement for thecores of most of the selected showers to lie in the central area of the array. For thispurpose, only a 4-fold coincidence between signals from each of the four centrally
placed detectors, N, E, W and S, is required for generating a shower trigger. The
signal from each detector is required to be larger than ∼ 0.3 times the mean signal
I min expected for the passage of a minimum ionizing particle (MIP) through thedetector. However, the trigger has been designed to disfavor the selection of verysmall local showers by imposing the additional requirement that one of these 4detectors should have a signal ≥ 3 MIPs.
The average rate of the shower trigger has been observed to be 5.69 min−1 .
However it was decided during data analysis to put additional software selection
conditions which required (a) a 4-fold coincidence between signals ≥ 1.0 MIP
from the detectors, N, E, W and S and (b) a signal ≥ 6 MIP from any one ofthese 4 detectors. The shower rate using these additional selection conditions is3.55 min−1 .
There is considerable uncertainty in the reconstruction of lateral distribu-170

Fig. 2. Distribution of the number of triggered muon detector modules for all showers
satisfying the ‘software’ trigger conditions and for three ranges of ‘Particle Sum’values.

tion and determination of shower parameters for small size showers. However,information from these showers can provide information on composition for theenergy range which partially overlaps with the range covered by direct measurements. Therefore it is convenient to use the quantity, ‘Particle Sum’, as themeasure of shower size by summing the number of particle measured in detectorsof the inner 3 rings, that is, detectors numbered 1 to 37. The observed distribution of ‘Particle Sum’, is shown in the right panel of Fig. 1 for showers satisfying
the ‘software’ trigger conditions.
Fig. 2 shows the distribution of the number of triggered muon detector
modules for showers satisfying the ‘software’ trigger conditions and also for threeranges of ‘Particle Sum’ values. Detailed simulations are being carried out, using semi-Monte Carlo procedure, discussed in detail elsewhere [4,5], to obtain
information on primary composition from data shown in Fig. 2.
4.

Summary

The GRAPES-2 experiment has been described brieﬂy and some observational data on ‘Particle Sum’ and muon multiplicity distributions has been presented. Detailed results from the comparison of observations with expectations
from simulations would be discussed at the conference.
5.

References

1. Ong, R.A. 1998, Phys. Rep. 305, 93.
2. Asakimori, K. et al. 1998, Ap. J. 502, 278.
3. Apanasenko, A.V. et al. 2001, Phys. 16, 113.
4. R. Srivatsan 1995, Muons in EAS, Ph.D. Thesis, Bombay University.
5. Gupta, S.K. et al. 2003, Preprint (TIFR-GRAPES-01/2003).