We carried out a balloon observation of cosmic rays with a prototype of the CALET (bCALET-1) at the Sanriku Balloon Center of the Japan Aerospace Exploration Agency. The main purpose of the experiment was verification of the CALET. The detector consists of 1024 scintillating fibers for precise imaging and 24 BGO scintillator for total absorption of showers. The observation was carried at an altitude between 35 and 37 km for about 3.5 hours. We measured electrons in the energy region between 1 to 10 GeV. The prototype system was verified in the balloon environment. We have obtained the electron flux which is useful to investigate solar modulation. In combination with the flux between 10 to 100 GeV measured by BETS, rigidity cutoff effect was clearly observed. These results showed good agreement with that of our Monte-Carlo simulation and demonstrated the detection capability of the CALET in the enegy region below 10 GeV. Now we are planning a series of balloon experiments with larger-scale detectors and longer-duration flights, which include one-month observation by a super-pressure balloon.

The CALET (CALorimetric Electron Telescope) is a detector planned to be on board the JEM-EF (Exposed Facility of Japanese Experiment Module) of the International Space Station to investigate high energy universe by observing high energy gamma-rays, electrons and hadronic cosmic rays. The main part of CALET is composed of an IMaging Calorimeter (IMC), a Total AbSorption Calorimeter (TASC), SIlicon Array (SIA) and Anti-Coincidence Detector (ACD). Monte Carlo simulation has been carried out to obtain basic performance of CALET on orbit.

CALET is a detector planned to be on-board the Japanese Experiment Module Exposed Facility (JEM-EF) of the International Space Station. The CALET mission aims at revealing unsolved problems in high energy phenomena of the Universe by carrying out a precise measurement of the high energy electrons in 1GeV–20TeV, the gamma-rays in 20MeV to a few TeV and the nuclei in a few 10GeV–1000TeV. The main detector is composed of imaging calorimeter (IMC), total absorption calorimeter (TASC), silicon pixel array (SIA) and anti-coincidence detector (ACD) to detect various kinds of particles in very wide energy range. The total absorber thickness is 31 radiation lengths for electromagnetic particles and 1.4 interaction mean free paths for protons. Monte Carlo simulation study has been carried out for optimization of the detector performance in observing each kind of particles. We obtained following performance about the observation of very high energy (>100GeV) electrons, which is a main target of the CALET experiment: (1) Effective geometrical factor is about 7000cm2sr. (2) Energy resolution is better than a few %. (3) Angular resolution is better than 0.1°. (4) Proton rejection power is ∼105 with the electron detection efficiency better than 95%. We also present the simulated performance of the CALET experiment in observing other particles.

Since 1996, a hybrid experiment consisting of the emulsion chamber and burst detector array and the Tibet-II air-shower array has been operated at Yangbajing (4300 m above sea level, 606 g/cm2) in Tibet. This experiment can detect air-shower cores, called as burst events, accompanied by air showers in excess of about 100 TeV. We observed about 4300 burst events accompanied by air showers during 690 days of operation and selected 820 proton-induced events with its primary energy above 200 TeV using a neural network method. Using this data set, we obtained the energy spectrum of primary protons in the energy range from 200 to 1000 TeV. The differential energy spectrum obtained in this energy region can be fitted by a power law with the index of -2.97 ± 0.06, which is steeper than that obtained by direct measurements at lower energies. We also obtained the energy spectrum of helium nuclei at particle energies around 1000 TeV. PACS numbers : 98.70Sa, 95.85Ry, 96.40De, 96.40Pq Typeset using REVTEX 2

As described in an accompanying paper (OG1.5/645: S.Torii et al) of this conference, CALET is a versatile detector for exploring high energy universe by observing gamma rays (> 20 MeV), electrons (>GeV) and other charged particles (> ⁄10 GeV). It is planned to be on board the JEM (Japanese Experiment Module, Kibo) of the International Space Station. In earlier papers(1, 2), many results of CALLET detector simulation have been presented. Since then, the detector structure was modified: the area was reduced and Si array is added etc. Some of the earlier results, for example, the energy resolution or angular resolution may not change much. However, we decided to start new simulations which include every details of the CALET structure for much comprehensive and intensive treatment. We will be presenting results based on the new simulations: the effective area, energy resolution, particle identification capability (especially, distinguishability of gamma- rays, electrons and protons), angular resolution. They will cover the energy range of few GeV to 10 TeV for electrons, 20 MeV to a few TeV for gamma-rays, several 10's GeV to 1000 TeV for protons and other heavy ions.

A survey was performed to search for TeV gamma-ray sources in the northern sky. In the search for steadily emitting sources 3TeV air shower data obtained by the Tibet-HD array and Tibet-III(Phase 1) array are analyzed. Nineteen prominent directions, including the direction of Crab Nebula, were found to have ∗M.Amenomori and H.Nanjo Dept. of Phys., Hirosaki University, Hirosaki, Japan S.Ayabe and K.Mizutani Dept. of Phys., Saitama University, Saitama, Japan S.H.Cui, L.K.Ding, H.B.Hu, C.L.Lan, H. Lu, S.L.Lu, J.R.Ren,Y.H.Tan, B.S.Wang, H.Wang, H.M.Danzengloubu, X.H.Ding, H.W.Zhang and J.L.Zhang Laboratory of Cosmic Ray and High Energy Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, China Guo, Labaciren, X.R.Meng, A.F.Yuan and Zhaxisangzhu Department of Math. and Phys., Tibet University, Lhasa 850000, China C.F.Feng, M.He, J.Y.Li, Y.G.Wang, L.Xue, N.J.Zhang and X.Y.Zhang Dept. of Phys., Shangdong University, Jinan 250100, China Z.Y.Feng, Q.Huang, H.Y.Jia, G.C.Yu and X.X.Zhou Institute of Modern Physics, South West Jiaotong University, Chengdu 610031, China X.Y.Gao, Q.X.Geng, J.Mu and X.C.Yang Dept. of Phy., Yunnan University, Kunming 650091, China K.Hibino, T.Sasaki, T.Shirai, N.Tateyama, S.Torii and T.Utsugi Faculty of Engineering, Kanagawa University, Yokohama, Japan N.Hotta, J.Huang, I.Ohta and S.Ozawa Faculty of Education, Utsunomiya University, Utsunomiya 321-8505, Japan F.Kajino, K.Kawata, M.Sakata, M.Takashima and Y.Yamamoto Dept. of Phys., Konan University, Kobe, Japan K.Kasahara Faculty of Systems Engineering, Shibaura Insti. of Tech., Saitama, Japan Y.Katayose and M.Shibata Faculty of Engineering, Yokohama National University, Yokohama, Japan G.M.Le and Z.H.Ye Center of Space Science and Application Research, Chinese Academy of Sciences, Beijing 100080, China M. Nishizawa National Institute for Informatics, Tokyo,Japan M.Ohnishi, T.Ouchi, A.Shiomi, M.Takita and H.Tsuchiya Institute for Cosmic Ray Research, University of Tokyo, Kashiwa, Japan T.Saito Tokyo Metropolitan College of Aeronautical Engineering, Tokyo, Japan H.Sugimoto and K.Taira Shonan Institute of Technology, Fujisawa, Japan T.Yuda Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya, Japan pp. 1–?? c ©2002 by Universal Academy Press, Inc.

We calculate the expected event rate of subhundred-GeV gamma-ray bursts using the Tibet-III air shower array with single particle counting technique. The calculation is done under reasonable assumptions that the attenuation effect of gamma rays in the intergalactic space as well as the distributions of redshift, luminosity and emission time of GRBs.

Several strong TeV γ-ray flares were detected from Mrk 421 in the years 2000 and 2001 by the Tibet III air shower array at a level of statistical significance of 5.1 σ. Mrk 421 was unprecedentedly active at X-ray and TeV γ-ray energies during this period, and a positive correlation was found between the change of the all-sky monitor Rossi X-Ray Timing Explorer X-ray flux and the Tibet TeV γ-ray flux. When a power-law energy spectrum for γ-rays from this source is assumed, the spectral index is calculated to be -3.24 ± 0.69 at the most active phase in 2001. The spectral index observed by the Tibet air shower array is consistent with those obtained via imaging air Cerenkov telescopes.

Data from the Tibet III air shower array (with energies around 3 TeV) and from the Tibet II array (with energies around 10 TeV) have been searched for diffuse gamma rays from the Galactic plane. These arrays have an angular resolution of about 0.degrees9. The sky regions searched are the inner Galaxy, 20degrees less than or equal to l less than or equal to 55degrees, and outer Galaxy, 140 degrees l less than or equal to 225degrees, and \b\ less than or equal to 2degrees or less than or equal to 5degrees. No significant Galactic-plane gamma-ray excess was observed. The 99% confidence level upper limits for gamma-ray intensity obtained are (for \b\ less than or equal to 2degrees) 1.1 x 10(-15) cm(-2) s(-1) sr(-1) MeV-1 at 3 TeV and 4.1 x 10(-17) cm(-2) s(-1) sr(-1) MeV-1 at 10 TeV for the inner Galaxy, and 3.6 x 10(-16) cm(-2) s(-1) sr(-1) MeV-1 at 3 TeV and 1.3 x 10(-17) cm(-2) s(-1) sr(-1) MeV-1 at 10 TeV for the outer Galaxy, assuming a differential spectral index of 2.4. The upper limits are significant in the multi-TeV region when compared to those from Cerenkov telescopes in the lower energy region and other air shower arrays in the higher energy region; however, the results are not sufficient to rule out the inverse Compton model with a source electron spectral index of 2.0.

We searched for multi-TeV counterparts to the gamma-ray bursts (GRBs) at keV energies detected by BATSE during the period from October 2, 1995 to March 18, 1999 using the data of the Tibet Air Shower Array. In the field of view of the array there were 70 GRBs detected by BATSE in this period. The search was done based on the sky survey within error circles of BATSE bursts and with time scales during 10 to several hundred seconds. No significant signals of counterparts to the GRBs were detected.

The Tibet air-shower array operating at Yangbajing (4300 m above sea level) is sensitive to gamma-ray air showers at energies as low as 3 TeV. The observation of the moon's shadow has provided a direct check of the angular resolution, the energy estimation and the systematic pointing error of this air-shower array. Using these data, we have searched for multi-TeV gamma-ray emission from 21 SNRs located within 5 kpc distance in the declination band of 0 degrees to +60 degrees. The signal from the Crab Nebula was detected at 5.5 sigma level as described another paper. No significant DC excess exceeding 5 sigma level was found from any of these SNRs except from the Crab Nebula. The results of nearby eight SNRs piled up gave a flux upper limit slightly lower than the expected values for a simple model of shock acceleration.

Using a high density air-shower array (Tibet-HD array), we succeeded in detecting multi-TeV -ray signals from the Crab Nebula in 1999. In the late fall of 1999, the HD array was enlarged from 5175 m up to 22000 m (TibetIII array). Using the 316 live-day data taken between 1999 November and 2001 May with this enlarged new array, we studied the emission of TeV -rays from the Crab Nebula. A preliminary analysis gives the excess -ray signal at the statistical significance of 4.8 . The energy spectrum of rays observed is consistent with the previous observation. We report the result on the study of the Crab flux.

Tibet II Air Shower Array consisting of scintillation counters with lattice of 15 m spacing has been operated with very high trigger rate of about 200 Hz. The threshold enegy of this array is estimated to be about 8 TeV for proton induced showers. Tibet High Density (HD) Array with 7.5 m spacing has been operated with the trigger rate of 115 Hz. The Mode energy of this array is estimated to be about 3 TeV for proton showers. Angular resolution of the arrays are estimated to be 0.9 degree above 10 TeV for Tibet II array, and 0.85 degree above TeV for HD array, resepectively. The angular resolution of these arrays and other array performances are examined by observing the Moon shadow resulting from the cosmic ray deficit in the direction of the Moon. Using the deflection of the Moon shadow to the east-west direction, the error of the array can be estimated by observing the displacement of the shadow in the north-south direction, because it is free from the effect of geomagnetic field, especially at Yangbaji...