Proceedings of ICRC 2001: 111 c Copernicus Gesellschaft 2001

ICRC 2001

THE AVERAGE MASS NUMBER OF PRIMARY COSMIC RAYS AROUND THE KNEEREGION DERIVED FROM GRAPES III ARRAY AT OOTY

The average mass number of primary cosmic rays around the knee
Y.Hayashi (1), S.K.Gupta (2), N.Ito (1), A.Jain (2), S.Kawakami (1), H.Kojima (3), D.K.Mohanty (2), T.Nonaka (1), S.Noto (1),regionderived from Grapes III array at Ooty
K.C.Ravindran (2), K.Satomi (1), K.Sivaprasad (2), B.V.Sreekantan (2), H.Tanaka (1), S.C.Tonwar (2), T.Toyofuku (1),K.Viswanathan
T.Yoshikoshi(1)
2
Y.
Hayashi1 , S. (2)K. andGupta, N. Ito1 , A.
Jain2 , S. Kawakami1 , H. Kojima3 , D. K. Mohanty2 , T. Nonaka1 , S. Noto1 , K. C.
2
1
Ravindran , K. Satomi , K. Sivaprasad2 , B. V. Sreekantan2 , H. Tanaka1 , S. C. Tonwar1 , T. Toyofuku1 , K.
Viswanathan, and T. Yoshikoshi(1) Osaka City2 University,OSAKA 1
1 (2) Tata Institute of Fundamental Research, MUMBAI

Osaka City University, Osaka
TataNagoya
Instituteof Fundamental(3)
Women'sUniversity, Research,NAGOYA Mumbai
3Nagoya Women’s University, Nagoya
2ABSTRACT
8We analyzed three months data on air showers (total 1046showers in 10 <Ne<10 ) observed with the GRAPES III
experiment during 2001. The shower array consists of 257
2scintillation detectors (1m each) and 16 muon detectors
22(35 m each). Very large area (560m ) of the muon
detectors gives good statistics for estimating the number ofmuons even for small air showers. We present thecorrelation between muon multiplicity distribution andshower size. This result is compared with simulation(Corsika code with QGS Jet model) for primary energy
1315
between 10 eV and 10 eV. We find the average mass
number in this energy region follows gradual increase andmixing of proton and heavy nuclei model fits well ourobserved data.

it reveal the environmental condition from where thecosmic rays are produced and same time it contains theinformation of propagation in our gala xy. It has been
known for a long time that a simultaneous study on theelectron and muon components in EAS has good potentialof yielding significant results on the composition of theprimary cosmic rays.

Ooty Air Shower Array
60

20West - East[m]40

1 Introduction
Our aim is to establish the chemical composition of14
16
primary cosmic rays between 10 to 10 eV. So far quite afew experiments have been conducted in the field of Air
shower to investigate the composition. But some results
have shown even opposite feature on the chemicalcomposition of primary cosmic rays, one favors for thelight element abundance, another favors to the heavy.
Number of muons associated with Air showers are quitesensitive to the primary mass so that we can deduce thetype of primary cosmic rays. Though the absolute muon
numbers in an Air shower is interaction model dependent,
now we can calculate them fairly well with the help ofaccelerator physics in this energy region.
Chemical composition is important, since it is thought that

0-20-40-60-10 -80 -60 -40 -20 020
North - South [m]0Fig.1 Ooty Air Shower Array

Correspondence to: hayashi@sci.osaka-cu.ac.jp

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2Air Shower Array and Muon Detectors(c) Threshold energy for proton is 1 TeV and for iron 5
TeV.
(d) The slope of the primary cosmic rays is –2.7.
Each simulated shower is randomly distributed within anarea of 200 m radius from center of the array and weadopted the showers which satisfy the trigger condition
exactly same as experiment.
The process and accuracy of the determination of muon
number is reported in previous ICRC, Hayashi et.
al.(1999) With those results, we are confident of around
15 muons in one module as a maximum number ofdetectable muons. For the discussion of primary
6composition of the shower size Ne < 10 , this will give
sufficient accuracy.

Air shower array of GRAPES III is located at Ooty
(latitude 11.4 longitude 76.7). it consists of 257
scintillation detectors and 16 unit of proportional counters.
2
Each of these scintillation detectors (SDs) is 1 m area,
5cm thick plastic scintillation detector viewed by a fast
5cm diameter photomultipliers placed 65cm above thescintillater. SDs are meant to measure the electron andgamma ray component in the Air shower. They are placedin hexagonal shape with a separation of 8m between theadjacent detectors. This arrangement can give good
triggering efficiency over the entire range of interest in
13
16
energy of 10 to 10 eV
We have installed large area muon detectors. Thedetector element is the 6m long proportional counter witha cross section of 10cm by 10 cm. 58 counters are placedside by side on a concrete platform and covered with 15cm
thick concrete slabs. Four layers of counters are arrangedin crossed configuration to obtain the track and angle ofindividual muon. In total 16 modules of muon detectors,
2
2
area of each module is 35 m , covering total area of 560 m
have been fully operational since February 1998.
Threshold energy of through muons is about 1.0 GeV (5502g/cm of concrete above the top counters, about 20 r.l.).
Trigger rate is 14Hz with selection of any 10 detectors
out of 120 SDs.
All the electron detectors are instrumented formeasurement of both density and arrival time of particles.
We determine the shower size and arrival angle forindividual showers. The accuracy in the determination of
the arrival direction of showers is estimated as 1.5degrees
5for shower of the size of around 10 .

3Analysis of Data

Detailed analysis has been carried out for showers
which satisfy the following conditions(a) More than 10 SDs firing out of 120 SDs inner array.
(b) In order to select the air showers which core falls
inside the SDs array, we adopted the following
conditions. Sum of the detected particles of hit SDs out
most and one line inside should be less than 35% of total
number of detected particle.
(c) Zenith angle of the air shower should be less than 25degrees.
7With these conditions, we selected 3x10 showers inthree months.
The followings are the conditions of simulation.
(a) CORSIKA V5.6240 with QGS-Jet model + full EGS
is used.
(b) Minimum energy for electron is one MeV and forhadron is one GeV.

Fig.2 muon multiplicity distribution
With different primary energy
blue:Iron, black:Observed data, red: Proton
4Experimental Results

There are several possible ways to study for the primarycosmic ray composition through muon components, forexample, the multiplicity dis tribution, the Ne-Nmurelation etc. We adopted here the multiplicity
distribution with shower size in order to estimate theprimary composition. This method of analysis has anadvantage that we need not use the assumed muon’slateral distribution. Fig.2 shows the muon multiplicity

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distributions obtained by the Ooty experiment for
detected number of particles 2.0 < logNe < 3.0. (corepositions from the muon detectors are 10m < r < 100m).
and the results of the simulations. We can obtain quite
high statistics on muon due to the compact but hugearray of muon detectors..

Fig.3 Muon Multiplicity distribution
Blue : Iron
Black: Observation
Red : proton
We could finish only 3moths data so far and another 1.5
years data is still to be analyzed. Though our present results
5Discussion and Conclusion

is preliminary, we obtained ratio of proton and iron, around
3 to 1 for the primary cosmic rays in the energy range of1415. Even though present observations show that the large area

10 to 10 eV from the data shown in Fig.2.

muon detectors in the GRAPES III array permit accurate
determination of muon numbers with high statistics andAcknowledgements. We are thankful to the Ministry ofthese observations are expected to give us reliable

Education, Japan for partial financial support for thisinformation on primary composition of cosmic ray flux.

experiment. We are also happy to acknowledge valuable

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contributions of G. Paul Francis, K.Manjunath, B.S.Rao,C.Ravindran, V.Viswanathan and T.Matsuyama during theinstallation, operation and maintenance of theinstrumentation. The help and cooperation of the Radio
Astronomy Center for providing site facilities for theGRAPES III array are gratefully acknowledged.

ReferencesthHayashi Y., 1999, 26 ICRC HE-2.2.22 276