30 TH I NTERNATIONAL C OSMIC R AY C ONFERENCE

Nuclear composition of primary cosmic rays around the knee region observed withGRAPES-3
H. TANAKA1 , S.K. GUPTA1 , K. HAYASHI2 , Y. H AYASHI2 , N. I TO2 , A. I YER1 , P. JAGADEESAN1 ,A. JAIN1 , S. K ARTHIKEYAN1 , S. K AWAKAMI2 , H. KOJIMA3 , T. M ATSUYAMA2 , M. M INAMINO2 ,P.K. MOHANTY1 , S.D. M ORRIS1 , P.K. NAYAK1 , T. N ONAKA2 , S. O GIO2 , T. O KUDA2 , A. O SHIMA2 ,B.S. R AO1 , K.C. RAVINDRAN1 , M. S ASANO2 , N. S HIMIZU2 , K. SIVAPRASAD1 , S.C. T ONWAR1 ,T. YOSHIKOSHI2
1Tata Institute of Fundamental Research
2Osaka City University
3Nagoya Women’s University
tanaka@tifr.res.in
Abstract: We have measured the primary cosmic ray spectra of various nuclear groups by analyzing
the relationship between muon multiplicity distribution and air shower size, and we have also estimatedtheir mean mass as a function of primary energy. The shower data were obtained from the two years ofobservations with the GRAPES-3 air shower experiment, which has a high-density air shower array ofplastic scintillation detectors and a large area muon detectors located at Ooty in southern India. We will
present brief description about our experiment, data analysis and discuss the implications of the results.

Introduction
Power index of primary cosmic ray (PCR) energy
spectrum changes from ∼ −2.7 to ∼ −3.1 ataround 1015 ∼ 1016 eV and it is called knee. Somemodels of knee expect change of PCR composition
through this energy range. It is hoped to provide
information on the origin of PCR.
Here we report GRAPES-3 observations of the nuclear composition and energy spectrum of PCR obtained from size and muon multiplicity distribution(MMD) of air showers.

Experiment
GRAPES-3 Experiment is located at Ooty in India
at an altitude of 2,200 m above sea level. Figure 1
shows GRAPES-3 air shower array.
Shower detector array consists of nearly 300 plastic scintillation detectors (SDs), each contains 1 m2
area, with 8 m separations as shown in figure 1.
Signal for each SD is taken to ADC and TDC.

Figure 1: GRAPES-3 EAS Array. Shower detectors and Muon detectors are plotted with N and .
Shower triggering requires more than 10 SDs firing. [1]There are large area muon track detectors (total560 m2 ) to observe muons accompanying withEAS. Each detector consists of a 6 m long proportional counter with cross section of 10 cm × 10 cm.
58 counters are placed side by side on a concrete
platform. Four layers of counters are arranged incrossed configuration to identify the track of individual muon and covered with 2 m thick con-N UCLEAR COMPOSITION OF PRIMARY COSMIC RAYS AROUND THE KNEE REGION OBSERVED WITH GRAPES-3

crete slab. Threshold energy of vertical throughmuons is about 1 GeV. The anode signal from eachcounter is shaped to an exponential form and discriminated at ∼ 0.2 mip. Width and timing of thepulse are recorded for each counter. [2]Simulation
CORSIKA [3] EAS Monte Carlo (MC) code isused for interpretation of observations. To evaluate influence of hadronic interaction models on
this analysis, MCs with SIBYLL 2.1 [4] (CORSIKA v6.50), QGSJET-II [5] (CORSIKA v6.50)
and QGSJET01 [6] (CORSIKA 6.02) are performed for various primary nuclear groups namely
H, He, N, Al and Fe. These results have been
calculated and compared with the direct measurements obtained from balloon and satellite borne experiments. [7]geometrical track overlapping has been consideredthrough MC.

Muon Multiplicity Distribution
MMD analysis has been done for showers that satisfy the following conditions. Zenith angle of theair shower should be less than 25◦ . Core location
should be inside areas as shown in figure 1 (a) forNe ≥ 105.2 and (b) for Ne < 105.2 to limit distance between shower core and muon detectors.

Shower Reconstruction
A total of 6 × 108 EAS collected over a live-timeof 4.71 × 107 s for two years, 2000 and 2001, havebeen analyzed. Each shower size is estimated fromthe lateral distribution of charged particles with fitting NKG function to them.
The followings are free parameters and estimated by NKG fitting using Maximum Likelihood
Method with MINUIT [8].
• Shower Core Location Rcore = (X, Y )• Shower Size Ne• Lateral Age sThe linearity of the PMT response is studied withdensity spectrum of each detector. Detector has
good response within ∼ 200 particles for each. After first NKG fitting, second is carried out with ignoring detectors where particle number exceed 200
particles in fitted function.
The detected number of muons is decided for eachmuon detector. In order to reject the accidental
muons, pulse timings of proportional counters areconsidered and the number of tracks are counted
to have the same direction to the shower. Effect ofFigure 2: Observed and simulated (SIBYLL 2.1)
distribution of multiple muons in size region of106.0 ≤ Ne < 106.2 .
Selected showers are classified with Ne in intervals of 0.2 of log10 (Ne ). MCs are also selected
and classified with the same conditions. Ne has
been converted to vertical value with consideringof shower attenuation. MMDs of MC are fitted
to observations using MINUIT to estimate relative abundance of each nuclear group in each size
bin. Here, abundance ratio of Al to Fe is fixed to0.8 based on direct measurements. Figure 2 shows
MMDs of 106.0 ≤ Ne < 106.2 . Red, green, lightblue, purple and blue lines show MC results of H,He, N, Al and Fe respectively after fitting. Black
point and line mean observation and sum of MC.

30 TH I NTERNATIONAL C OSMIC R AY C ONFERENCE

Energy Spectrum
Size spectrum initiated from each nuclear groupis estimated with total size spectrum and relative
abundance obtained from MMD fitting. Relations
between shower size and primary energy are calculated on MC for each mass groups.
Energy spectra for each groups have been
estimated with the above calculations using
SIBYLL 2.1, QGSJET-II and QGSJET01 models.
They are shown in figure 3. SIBYLL 2.1 andQGSJET-II have almost same results, though there
are visible differences from QGSJET01. The Bentin proton spectrum can be seen around 1 ∼ 3 PeV
for all models, though other spectra don’t have thisstructure in our energy range.
All-particle energy spectra is also shown in figure 4. All the models give similar flux and havegood agreement with direct observations here.

Mean Mass Number
Mean mass number hln Ai = Σ(ln A)/N of cosmic ray will be derived from energy spectra. Estimated hln Ai = Σ(ln A)/N is shown in figure 5.
hln Ai with SIBYLL 2.1 is 0.1 ∼ 0.2 larger (e.g.
0.15 ± 0.03 larger at 1015 eV) than QGSJET-II.
However, this difference is rather small and won’t
have much influence on study of composition.

Summary and Outlook
EAS data of GRAPES-3 of 2000 and 2001 are analyzed. With observed muons accompanying withshowers, relative abundance in the size spectrum
is estimated for each mass groupsH, He, N andAl+Fe with MMD fitting.
It was performed using three hadronic interaction
models, SIBYLL 2.1, QGSJET01 and QGSJET-II.
New model, namely QGSJET-II, and SIBYLL 2.1
have given very similar results in this analysis.
Proton spectrum becomes steeper at higher energy.
An extension of the spectrum to higher energy side
may show the break of the spectrum for heavier
components. New scintillation detectors with dual
PMT have been already installed to have large dynamic range (up to 2000 ∼ 3000 particles). TheseFigure 3: Energy spectra using SIBYLL 2.1 (•),
QGSJET-II () and QGSJET01 (N). (a) - (e) panels show H, He, N, Al and Fe spectra respectively.

N UCLEAR COMPOSITION OF PRIMARY COSMIC RAYS AROUND THE KNEE REGION OBSERVED WITH GRAPES-3References

Figure 4: All-particle spectra from direct measurements N Grigorov [9], • SOKOL [10],JACEE [11],  RUNJOB [12] and GRAPES-3 (•SIBYLL 2.1,  QGSJET-II, N QGSJET01).

Figure 5: Mean Mass Number hln Ai as a functionof energy of PCR using SIBYLL 2.1 (•), QGSJETII () and QGSJET01 (N). JACEE [11], RUNJOB [12], KASCADE [13] and CASA-MIA [14]results are also shown.
are expected to provide more reliable estimate ofthe energy. It is also proposed to expand GRAPES3 shower array over an area of 1 km 2 . It would permit to study about nuclear composition to 1018 eV.
[15]

Acknowledgement
We thank persons assisting in GRAPES-3 experiment.

[1] S.K. Gupta et al. Nuclear Instruments andMethods in Physics Research A, 540:311–323, 2005.
[2] Y. Hayashi et al. Nuclear Instruments andMethods in Physics Research A, 545:643–657, 2005.
[3] D. Heck, J. Knapp, J.N. Capdevielle,G. Schatz, and T. Thouw. CORSIKA: Amonte carlo code to simulate extensive airshowers. Technical report, Forschungszentrum Karlsruhe, 1998. FZKA 6019.
[4] R. Engel, T.K. Gaisser, P. Lipari, andT. Stanev. In 26th International Cosmic Ray
Conference, volume 1, pages 415–418, Salt
Lake City, Utah, 1999. HE 2.5.03.
[5] S. Ostapchenko. Nuclear Physics B Proceedings Supplements, 151:143–146, 2005.
[6] N.N. Kalmykov, S.S. Ostapchenko, and A.I.
Pavlov. Nuclear Physics B Proceedings Supplements, 52B:17–28, 1997.
[7] H. Tanaka et al. In 30th International Cosmic Ray Conference, Merida, Mexico, 2007.
HE3.1, 1233.
[8] F. James. MINUIT: Function Minimizationand Error Analysis. CERN, 94.1 edition,
1998. CERN Program Library Long Writeup
D506.
[9] N.L. Grigorov et al. In 12th International
Cosmic Ray Conference, volume 5, pages
1746–1768, Hobart, 1971.
[10] I.P. Ivanenko et al. In 23th International Cosmic Ray Conference, Calgary, 1993.
[11] Y. Takahashi for the JACEE Collaboration.
Nuclear Physics B Proceedings Supplements,
60B:83–92, 1998.
[12] V.A. Derbina et al. The Astrophysical Journal
Letters, 628:L41–L44, 2005.
[13] H. Ulrich et al. In 27th International Cosmic Ray Conference, volume 1, pages 97–100, Hamburg, Germany, 2001.
[14] M.A.K. Glasmacher et al. Astroparticle
Physics, 12:1–17, 1999.
[15] S.K. Gupta et al. In 30th International Cosmic Ray Conference, Merida, Mexico, 2007.
HE1.5, 1058.