The 28th International Cosmic Ray Conference

The Telescope Array Experiment:
Physics Aims

1

An Overview andY.Arai,1 J.Beltz,2 M.Chikawa,3 H.Fujii,1 M.Fukushima,4 K.Hashimoto,5 Y.Hayashi,6
N.Hayashida,4 K.Hibino,7 K.Honda,5 N.Inoue,8 C.Jui,9 K.Kadota,10 F.Kakimoto,11
K.Kasahara,12 H.Kawai,13 S.Kawakami,6 K.Martens,9 T.Matsuda,1 T.Nakamura,14
S.Ogio,11
M.Ohnishi,4
H.Ohoka,4N.Sakurai,4M.Sasano,15
S.Schnetzer,16
4
9
4
17
1
H.Shimodaira,P.Sokolsky, M.Takita,
K.Tanaka,
M.Tanaka, Y.Tanaka,18
19
16
4
20
M.Teshima,
G.Thomson,
R.Torii,
S.Uchihori,
N.Yasuda20 S.Yoshida,13
H.Yoshii,21 T.Yoshikoshi,6(1) Institute of Particle and Nuclear Studies, KEK, Tsukuba 305-0801, Japan(2) Montana State University, Bozeman, MT 59717, USA(3) Kinki University, Osaka 577-8502, Japan(4) Institute for Cosmic Ray Research, University of Tokyo, Chiba 277-8582, Japan(5) Yamanashi University, Kofu 400-8510, Japan(6) Osaka City University, Osaka 558-8585, Japan(7) Kanagawa University, Yokohama 221-8686, Japan(8) Saitama University, Saitama 338-8570, Japan(9) University of Utah, Salt Lake City, Utah 84112, USA.
(10) Musashi Institute of Technology, Tokyo 158-8557, Japan(11) Tokyo Institute of Technology, Tokyo 152-8551, Japan(12) Shibaura Institute of Technology, Saitama 337-8570, Japan(13) Chiba University, Chiba 263-8522, Japan(14) Kochi University, Kochi 780-8520, Japan(15) Communications Research Laboratory, Tokyo 184-8795, Japan(16) Rutgers University, Piscataway, NJ 08854-8019, USA(17) Hiroshima City University, Hiroshima 731-3194, Japan(18) Nagasaki Institute of Applied Science, Nagasaki 851-01, Japan(19) Max Planck Institute for Physics, 80805 Muenchen, Germany(20) National Institute of Radiological Sciences, Chiba 263-8555, Japan(21) Ehime University, Matsuyama 790-8577, Japan

Abstract
The Telescope Array (TA) experiment plans to deploy an array of 10 telescope stations in the west desert of Utah, USA and observes extremely high energy
cosmic rays (EHECRs) by the atmospheric ﬂuorescence. Its purpose is to study
super-GZK (E > 1020 eV) cosmic rays discovered by AGASA. In order to identify
the origin of super-GZK events, TA has ∼30 times larger aperture than AGASA
and provides a particle identiﬁcation by the shower proﬁle measurement. The ﬁrst
step of construction will start as a hybrid detector with an AGASA×10 groundarray and a part of ﬂuorescence telescopes, which will unambiguously establish
pp. 1–4c
2003
by Universal Academy Press, Inc.

2

the existence of super-GZK and cluster events with much improved statistics andsystematics. This paper describes the physics of TA and its overall planning.
1.

Physics of Telescope Array

The AGASA so far observed 11 super-GZK cosmic rays in 12 years ofoperation. The energy spectrum continues toward high energy without showing
an indication of the GZK-cutoﬀ[1]. The arrival directions are isotropic and does
not show a correlation with the galactic plane or the super-galactic plane of nearby
galaxies. A few point-like clusters were observed in the sky, from which 2 or more
EHECRs were arriving[2]. High energy astronomical objects have been searchedbehind these events but no apparent candidates were found. A correlation withquasar remnants[3] and the BL Lac objects[4] had been suggested, but none seemdeﬁnitive at this moment. The latest results of AGAGA are reported elsewherein this conference. Several hypotheses have been proposed to explain super-GZK
events and clusters[5];• The production of EHECRs by the decay of long-lived, super-heavy relicparticles concentrated in the halo of our galaxy and with some level ofclumping to produce clustered events.
• The production of Z◦ by the collision of cosmological neutrino backgroundwith extremely high energy neutrinos (E≥ 1021 eV) produced deep in theuniverse. The EHECR is produced as a decay product of Z◦ . The primordial
neutrinos may be concentrated in the local super-cluster of galaxies by its
gravitational interaction with the dark matter.
• The Lorentz invariance breaks down at a very high Lorentz factor suchthat the high interaction cross section of pion photoproduction via ∆(1232)
resonance is avoided.
• The generation of EHECRs by the yet unknown dark stars which are overpopulated by a factor of ∼10 in the vicinity of our galaxy.
Note that the Z-burst and the violation of Lorentz invariance explain the extensionof the spectrum above GZK cutoﬀ, but they leave the question of acceleration
itself unanswered. It is certain that super-GZK events and clusters are diﬃcult
to explain in the standard framework of astrophysics and particle physics.
2.

Prospects

The models with super-heavy relics and the Z-burst have a clear experimental signature to be tested, i.e. EHE gamma rays and neutrinos are an order ofmagnitude more abundant than protons in these models. The super-heavy relics

3Fig. 1. The Hybrid TA (left panel) is composed of 576 plastic scintillation counters
covering the ground area of 760 km2 and 3 fluorescence telescope stations (rightpanel) surrounding the ground array and looking inward.

model also expects a concentration of the sources in the center of the galaxy. A
realistic test of Lorentz invariance becomes possible when we identify the source
objects of EHECR and measure the distance to the source. The detection of heavy
nucleus as a major composition of the EHECR, on the other hand, excludes mostof the particle physics origin and directly leads us to the shock wave acceleration.
Thus, we expect the TA and the other next generation EHECR experiment will
give a clear answer to many of these hypotheses.
The experimental situation around the GZK cutoﬀ became unclear when
the HiRes group submitted a paper[6] recently on the ﬁrst measurement of theenergy spectrum and concluded that it is consistent with the existence of the
GZK cut-oﬀ. Discussions after the submission of the paper can be summarized
as follows;• Taking a systematic uncertainty of the energy determination by two groups,
25% for HiRes[6] and 18% for AGASA[7], the claim of neither group on theexistence of the GZK cutoﬀ has a statistical signiﬁcance higher than 3 σ.
• Below 1020 eV where the comparison can be made with enough statistics, theenergy spectra of both agree perfectly well, if we remake the HiRes spectrum
by increasing the energy of all HiRes events by 25%, or by decreasing the
AGASA energy by the same amount.
It is clear that the key of resolving the issue is the higher statistics and better
understanding of the systematics. This can be best achieved by a large hybrid
experiment making a simultaneous measurement with an AGASA type air shower

4
Table 1.

Expected Acceptance, Resolution and Number of Events of Hybrid TA

Detector

Acceptance(km2 sr)
AGASA
162
1371
TA:Ground Array
TA:Fluorescence
670
165
TA:Hybrid

AngularResolution
1.6◦
1.0◦
0.6◦
0.4◦

# of Events per Year
E > 1019 eV E > 1020 eV
100
1
700
9
300
4
80
1

array and a HiRes type air ﬂuorescence telescope. The good statistics can beobtained by building a large ground array, which is relatively inexpensive and its
stable operation and uniform coverage over the sky have been guaranteed by theexperience of AGASA and other arrays. The better understanding of systematics
can be made by the ﬂuorescence measurement, which allows us a calorimetricmeasurement of the shower energy, a direct observation of the arrival direction
and a determination of primary particle species by the shower longitudinal proﬁle.
We therefore propose to urgently build a hybrid TA, an AGASA×10
ground array with a set of ﬂuorescence stations making a simultaneous measurement with the ground array (see Figure 1). It will be built in the West desertof Utah, USA, and will become a ﬁrst step of constructing the full TA[8]. The
performance of the hybrid TA detector is summarized in Table 1 and further details are given elsewhere in this conference. The plan is to complete the hybrid
TA by the end of 2006. We expect the full sky survey of EHECRs will be started
by the southern hemisphere Pierre Auger observatory in Argentina in 2005, andthe hybrid TA will start contributing to the northern hemisphere survey in 2007.
We hope that astonishing discoveries of AGASA are conﬁrmed, and the origin of
super-GZK and cluster is identiﬁed by 2010.
3.

References

1. Takeda, M. et al., 1998, Phys. Rev. Lett. 81, 1163
2. Chikawa, M. et al. 2001, Proceedings of 27th ICRC (Hamburg) HE1.4-3
3. Torres, D. F. et al. 2002, Phys. Rev. D66, 023001
4. Gorbunov, D. S. et al. 2002, Astrophys. J. 577, L93
5. Nagano, M., & Watson, A. A. 2000, Rev. Mod. Phys. 72, 689 and references therein
6. T.Abu-Zayyad et al. 2002, astro-ph0208301 and astro-ph0208243
7. Takeda, M. et al. 2002, astro-ph/0209422 submitted to Astropart. Phys.
8. The telescope Array Project: Design Report, July 2000

The 28th International Cosmic Ray Conference

1

The Telescope Array Experiment: Hybrid Measurement of Ultra HighEnergy Cosmic Rays in Northern Hemisphere
Fumio Kakimoto1 , Shoichi Ogio1 , Ryuichi Minakawa1 , Masaki Fukushima2 ,Nobuyuki Sakurai2 , Naokai Hayashida2 , Hideaki Shimodaira2 , Reiko Torii2 ,Masato Takita2 , Yasuo Arai3 , Takeshi Matsuda3 , Hirofumi Fujii3 , Manabu
Tanaka3 , Yoshito Tanaka4 and TA group(1) Department of Physics, Tokyo Institute of Technology, Tokyo 152-8551, Japan(2) Institute for Cosmic Ray Research, University of Tokyo, Kashiwa, Chiba 2778582, Japan(3) Institute of Particle and Nuclear studies, KEK, Tsukuba 305-0801, Japan(4) Nagasaki Institute of Applied Science, Nagasaki 851-01, Japan

Abstract
The result of AGASA shows the extension of the cosmic ray energy spectrum above the GZK cut-oﬀ. Several new generation experiments are plannedor under construction to identify the origin of such ultra high energy (UHE) cosmic rays exceeding the cut-oﬀ. We report here the design of hybrid Telescope
Array (TA) and introduce plans of improving the accuracy of measuring the energy spectrum by the hybrid measurement of ground array and air ﬂuorescencetelescope.
1.

Introduction

The AGASA energy spectrum shows that there is no indication of GZK
cut-oﬀ expected by the photo-pion production of the ultra high energy cosmic
rays. There is also an indication of point sources in the arrival direction of suchcosmic rays [1]. If the origin of these cosmic rays is some type of already known
astronomical object, it must exist within several tens of Mpc from our galaxy.
Until now no astronomical object is identiﬁed in the directions of the observed
UHE cosmic rays. Several top-down models are proposed as a remedy, in which
UHE cosmic rays are produced via anomalous physics processes or by unknown
astronomical objects [2]. In order to get a deﬁnite answer on the origin of UHE
cosmic rays, it is crucially important to measure the energy spectrum, the arrival
distribution and the composition more accurately and compare them with thepredictions of these models. We therefore proposed the Telescope Array as thenext generation air shower experiment [3] and planned to observe air showers by
the air ﬂuorescence technique. On the other hand, HiRes group recently reported
that there is a GZK cut-oﬀ in their observed energy spectrum [4]. It seems clear
pp. 1–4c
2003
by Universal Academy Press, Inc.

2

that a part of the inconsistency is due to the systematic error of both experimentsin the determination of primary cosmic ray energies [4]. Taking this criticalsituation into account, we now intend to observe air showers with an AGASA typeground array and with an air ﬂuorescence detector simultaneously as the ﬁrst stepof the TA project. In this paper, we present a simulation study to conﬁrm how
much the systematic error is improved in the primary energy estimation by using
our hybrid detector.
2.

Experimental Plan

In our design of thehybrid-TA, an array of 24×24
plastic scintillators covering the
ground area of 760 km2 issurrounded by three air ﬂuorescence stations. The West
Desert in Utah, USA is taken as
1. The prototype camera and telescope reflector forthe experimental site. The to- Fig.thefluorescence measurement.
tal ﬁeld of view of each stationis 3◦ − 34◦ in the elevation angle and 120◦ in the azimuthal angle, with all of
the three stations looking toward the center of the ground array. The separationof the station is 30 − 40 km and the stereo acceptance is ∼670 km2 sr for UHE
cosmic rays falling within 45 km of the station. The ﬂuorescence acceptance alone
is 4 times larger than that of AGASA assuming 10% duty factor. Approximately
60% of the events for E > 1020 eV is observed by all 3 stations.
The ﬁeld of view of each telescope is 18.0◦ in azimuth and 15.5◦ in elevation.
The telescope has a spherical dish of 3.3 m diameter composed of 18 hexagonal
segment mirrors (see Fig.1). The focal length is 2960 mm and the spot size on the
focal plane is ∼30 mm in FWHM according to a ray tracing calculation. The air
shower image is recorded by a camera composed of 16×16 2-inch PMTs placed on
the imaging plane (see Fig.1). Each PMT pixel covers 1.1◦ ×1.0◦ patch of the sky.
A signal from the PMT is ampliﬁed by a factor of 4 with a pre-ampliﬁer and issent to a Charge Successive Integrators (CSI) circuit. The CSI integrates a signal
every 200 ns and send it out successively to a 12-bit pipeline ADC by means of 3
capacitances operated in a rotational sequence. A self-ranging H/L gain scheme
is adopted to cover a dynamic range corresponding to 1−64k photoelectrons.
The digitized signal is fed to a Digital Signal Processor (DSP) to search for a
ﬂuorescence signal by maximizing the S/N ratio in 25.6 µs time window.
By one year operation of the hybrid-TA, we will collect ∼12 events with
E > 1020 eV and more than 900 events with E > 1019 eV, of which ∼80 eventsare measured simultaneously by both the scintillation detector array and the air
ﬂuorescence telescopes.

3

3.

Simulation Procedure

The simulation of UHE cosmic rays by the full Monte-Carlo is practicallyprohibited by the computation time. In our analysis of the hybrid event, we
ﬁrst generate UHE events using a thinning method. The proton is assumed as a
primary and the ﬁrst interaction depths are sampled by the Monte Carlo simulation. The longitudinal development (LD) proﬁles are calculated analytically witha modiﬁed Gaisser-Hillas function, in which 6 parameters characterizing LDs are
estimated with empirical formulas. These formulas are derived from the results ofa small amount of simulated events with Corsika Monte Carlo package. Fluorescence and Cherenkov photons are generated according to the LD function and thetransmission of photons in the atmosphere is simulated taking eﬀects of Rayleighand Mie scattering into account. For the determination of the primary energy,
it is important to estimate the eﬀects of these scatterings accurately. Note thatin the event generation we adopted 1.2 km for the aerosol scale height, h, and
20 km for the attenuation length for the Mie scattering. In the ﬁnal stage of thesimulation, PMT output signals are generated by taking optical properties of thetelescopes and cameras into account. The produced shower signal was analyzed,
and its energy and arrival direction were reconstructed by the χ2 minimization
method. Details of the analysis procedure are described in our design report [3].
4.

Method to Improve Systematic Error

Following is one of the ways we are considering to improve the systematic
error of the energy determination using the hybrid event. (a) First a primaryenergy, E, and an air shower size at the array altitude, NF , are estimated witha corresponding LD proﬁle reconstructed by the air ﬂuorescence measurement.
(b) Then the obtained E is corrected by multiplying a factor NG /NF , where NG
is the measured shower size by the ground array. In the AGASA experiment,
the primary energy is estimated from the observed local particle density at thecore distance of 600m. In this procedure, a major part of 18% total systematic
error comes from the uncertainty in the adopted Monte Carlo simulation notably
from the diﬀerences of used hadronic interaction models and the assumed primary
composition. On the other hand, the measurement of NG itself can be as goodas 10% or better, and this is used for the correction of E in our method. In thefollowing analysis, we artiﬁcially produced ∼20% systematic error in the ﬂuorescence energy measurement by reconstructing the LD proﬁle using a“wrong scaleheight of 2.0 km and 0.4 km against 1.2 km used in the event generation. Whereas
the value of NG at the array altitude is generated by smearing the correct shower
size by 10%.

4

5.

ResultThe simulation is performed for primary energies of 1019 , 1020 and 1021 eVwith zenith angles less than 60◦ and with impact parameters from the center of theground array less than 10 km. For each primary energy, 200 events are generated.
The result for the events with E = 1020 eV
is shown in Figure 2. Before the correction, the estimated energies are ∼14% lessor ∼21% more than the generated value of1020 eV by adopting the wrong scale heightof 0.4 km or 2 km. This is shown in theupper panels of Figure 2. After the correction, the systematic shift of the measured
energy is reduced as seen in the lower twopanels of the Figure 2. The present results
are summarized in Table 1. It is shown
Fig. 2. Distributions of the energy measure- that even when there is ∼20% systematic
ment errors for E = 1020 eV by using a shift in the air ﬂuorescence energy measure“wrong scale height of 0.4 km (a and c)
ment, we can reduce the error within ∼10%
and 2 km (b and d).
by adopting our method with a minor sacriﬁce in the resolution. Now we are improving our reconstruction program in order to reduce the systematic error evenfurther.
Table 1.

The improvement of the systematic error.

E
Adopted h av. E shift ± resol.
(eV)
(km)
before correction
1019
0.4−9.5 ± 6.0%
20
10
0.4
−13.9 ± 6.3%
21
10
0.4
−17.7 ± 6.8%
19
10
2.0
+21.7 ± 6.0%
20
10
2.0
+21.2 ± 4.8%
21
10
2.0
+21.8 ± 5.8%

av. E shift ± resol.
after correction+14.9 ± 16.7%+10.1 ± 18.4%+8.6 ± 17.1%−4.1 ± 12.5%+4.8 ± 13.1%+2.9 ± 11.7%

References[1] Takeda, M. et al., 1998, Phys. Rev. Lett. 81, 1163.
[2] For example, P. Bhattacharjee et al., 1992, Phys. Rev. Lett. 69, 567.
[3] The Telescope Array Project: Design Report July 2000.
[4] T. Abu-Zayyad et al., 2002, astro-ph0208310 and astro-ph0208243

The 28th International Cosmic Ray Conference----------1

The Telescope Array Experiment: The Search for The
Clusters in The Northern Hemisphere Sky with A Large
Scintillator Array
M.Chikawa,1 M.Fukushima,2 K.Hashimoto,5 Y.Hayashi,3 N.Hayashida,2K.Hibino,4 K.Honda, 5 H.Kawai,11 S.Kawakami,4 T.Matsuda,6 T.Nakamura,7
M.Ohnishi,2 H.Ohoka,2 N.Sakurai,2 M.Sasano,8 K.Tanaka,9 M.Teshima,10
S.Yoshida,11 T.Yoshikoshi3 and TA Collaboration.
(1) Kinki University, Osaka 577-8502, Japan(2) Institute for Cosmic Research, University of Tokyo, Chiba 277-8582, Japan(3) Osaka City University, Osaka 558-8585,Japan(4) Kanagawa University, Yokohama, 221-8686, Japan(5) Yamanashi University, Kofu 400-8510, Japan(6) Institute of Particle and Nuclear Studies, KEK, Tsukuba 305-0801, Japan(7) Kochi University, Kochi 780-8520, Japan(8) Communication Research Laboratory, Tokyo 184-8795, Japan(9) Hiroshima City University, Hiroshima 731-3194, Japan(10) Max Plank Institute for Physics, 80805 Muenchen, Germany(11) Chiba University, Chiba 263-8522, Japan

Abstract
In addition to the existence of extremely high energy cosmic rays (EHECRs)
above GZK cutoff, the results of AGASA shows the clustering of EHECRs above
1019.6 eV. Confirming such clusters and identifying the corresponding astronomical object is crucial to understand the origin of EHECRs. The northern hemispheresky is ideal for this purpose since the deflection by the galactic magnetic field is
expected to be small and uniform compared to the southern hemisphere. In thispaper, we report a design of the large ground array placed at the center of the hybrid-TA experiment. The array has an acceptance 9 times larger than AGASA.
Using this detector, the cluster in the northern hemisphere will be searched withan angular resolution better than 1 degree.
1. Introduction
So far AGASA has obtained nice results about the existence of super-GZK
particles. The results suggest a uniform distribution in the arrival direction of
EHECRs and at the same time the existence of clustering in some directions (see

2 -----------The 28th International Cosmic Ray Conference

Fig.1). The candidate astronomical objects should have a large enough volume
to accelerate the nucleons up to 1020eV orhigher, but we so far have not been able
to locate any such object within 50 Mpc
of the Earth as sources of those EHECRs
and clusters. This introduces serious
problems to understand the origin andacceleration mechanism of EHECRs.
Several models have been proposedto try to give answer for these issues,
such as a decay of super massive relicparticles or cosmic strings1, a breakdown
Fig.1 Distribution of spatial angle between twoof special relativity at the energy above
cosmic rays measured by AGASA. All possible
1020eV2 and so on. But none of them
combinations are included.
seems to be able to convince people.
Considering these problems we have keen interest to confirm the existence of super-GZK cosmic rays and especially their clustering.
2. Performance of Hybrid-TA Ground Array
The overall plan of hybrid-TA is presented elsewhere in this conference. Oneof the major aims of the hybrid-TA is to quickly settle the issues around the clustering of EHECRs with higher statistics. To achieve this target, we have decided to
add conventional scintillation detectors as a ground array in addition to the fluorescent detectors originally planned in the TA project. Here we would like to concentrate on presenting the design and the performance of the ground detector array.
Total 576 detectors are to be distributed in 24x24 matrix covering the area of 760
km2 in 1.2 km mesh. Its acceptance is 9 times larger than that of present AGASA.
Each detector is a plastic scintillator of 3 m2 area and 2 cm thickness. The scintillation light is collected to a photomultiplier using wave length shifter bars installed in a groove cut on the surface of the scintillator. We calculated the detection efficiency for the present arrangement; it is around 50% at 1018.5 eV and becomes full above 1019 eV.
The plastic scintillator mainly measures the electrons and positrons in the air
shower. It is easier than measuring other components such as muons and hadrons,since the total number of electrons and positrons is roughly 10 times larger and no
special preparation is required. Better statistics in terms of the number of charged
particles in the detector can be achieved with smaller detector size compared to
the water Cherenkov detector. In addition, the measurement of electrons and posi-The 28th International Cosmic Ray Conference----------3

trons are less affected by the primary composition and the details of the hadronicinteraction at UHE because nearly 90% of the total energy is converted to theelectromagnetic component near the shower maximum where the ground detector
samples the air shower. This means the plastic scintillator has inherently better
energy resolution and less systematic uncertainty compared to other types of particle detectors. It should also be noted that, starting as low as 1014 eV, a series ofenergy spectrum measurements has been made by the plastic scintillator array, including AGASA, and valuable experiences have been accumulated. The continuation and consistency is a good asset for establishing a reliable energy spectrum atextremely high energy.
We adopt two approaches to achieve a good angular resolution with the present ground detector system; One is to obtain a good timing standard for all the
576 counters and another is to obtain a good timing resolution for the hitting timeof each counter, which is achieved by recording the time profile of all the charged
particles hitting the counter by the flash ADC. As the time standard, we found an
accuracy of 20ns can be rather easily achieved with ordinary GPS, and a better
time resolution can be expected with the differential GPS system or by some othermeans such as selecting the same set of satellites for all the counters. These techniques are now being tested. We eventually aim at reaching 10ns accuracy byadopting these methods.
The Flash ADC board developed for the ground array isshown in Fig.2. It has a 50 MHz
12-bit ADC, a comparator for
generating a local trigger, a
CPU and a TCP/IP network interface. The connection to the
GPS is made by a RS232 serial
port. With a conventional fast
timing system, one measures theFig.2 Prototype Flash ADC Board
timing of first arriving particle
at the front of the shower disk. The arrival direction of the air shower is determined by combining all the timing information and fitting it to the shape of theshower front. In such a case, there can be a large fluctuation in timing measurement, especially where the particle density is sparse, and achieving a good angularresolution is limited. Using the flash ADC system, a complete history of ~10µs
after the arrival of the shower front will be recorded with 20ns time resolution. We
then can observe the distribution of arriving particles with time and determine the4 -----------The 28th International Cosmic Ray Conference

structure of the shower disk. It will give us a great advantage to have better resolution in the angle measurement. We are expecting the accuracy of 1 degree can beachieved with the present FADC system.
In terms of the energy
resolution of present design,major contribution comes fromthe fluctuation in the samplingof shower particles. We presently estimate that the error forthe determination of primaryenergy is around 30% at
Fig.3 Time profile of 30 m2 scintillation counter signal ob1019.5eV and 25% at 1020eV. On
served 2 km away from the shower core of a large air shower
event (E=2x1020 eV) observed by AGASA.
the other hand, we expect thatthe systematic uncertainty of theenergy determination will improve because the time structure of the shower disk
will be better understood for the new array. An example is shown in Fig.3. This
time profile is obtained by a large (30m2) scintillator near the core of AGASAEHECR event. The signals 4,6 and 8 µs after the shower front is considered to bedelayed neutrons associated with the shower. In the new flash ADC system, we
can explicitly exclude these signals from the energy calculation and improve thesystematics, though the energy shift of AGASA arising from these effects is estimated to be only 5%3.
The whole system will be operated stand alone using a solar panel power
generation. Since a large amount of detectors are required, stable and reliable detectors are essential. We believe the present system is robust and easy to deploy
and maintain in the desert.
3. References
1.

V.Kuzmin and I.Tkachev, JETP Lett. 68 (1998) 271.
V.Berezinsky, P.Blasi and A.Vilenkin, Phys. Rev. D58 (1998) 103515
K.Hamaguchi, I.Izawa, Y.Nomura and T.Yanagida, Phys. Rev. D69(1999) 125009
V.Berezinsky, Nucl. Phys. Proc. Suppl. 81(2000) 311

2.

H.Sato and T.Tati, Prog. Theo. Phys. 47(1972) 1788
S.Coleman and S.L.Glashow, Phys. Rev. D59(1999) 116008

3.

M.Takeda et al., Astroparticle Phys. 19(2003) 447