We have already reported the first result on the all-particle spectrum around the knee region based on data from 2000 November to 2001 October observed by the Tibet-III air-shower array. In this paper, we present an updated result using data set collected in the period from 2000 November through 2004 October in a wide range over 3 decades between 10(14) and 10(17) eV, in which the position of the knee is clearly seen at around 4 PeV. The spectral index is -2.68 +/- 0.02(stat.) below I PeV, while it is -3.12 +/- 0.01(stat.) above 4 PeV in the case of QGSJET+HD model, and various systematic errors should be carefully done in the very near future. (c) 2007 COSPAR. Published by Elsevier Ltd. All rights reserved.

We searched for steady 100 TeV gamma-ray point sources from -10 degrees to +70 degrees in declination based on six year data obtained from Nov. 1999 to Nov. 2005 by the Tibet air shower array. No new source was found and no significant excess of events could be detected from known TeV sources. we set flux upper limits above 100 TeV on several nearby objects located inside Our Galaxy. All of them are slightly higher than extrapolation from lower energies by a factor of 1 to 20. Our new project named Tibet MD will improve our sensitivity by a factor of similar to 10 in this energy region. we would discover similar to 10 new sources and be able to measure cutoff energy of known and unknown sources, which might lead to identification of the origin of cosmic rays.

We report on the spectral index of cosmic rays around 10 TeV energy region obtained by observation of the Compton-Getting anisotropy due to the terrestrial orbital motion around the Sun with the Tibet III air shower array during the period from 1999 to 2005. The spectral index has turned out to be 2.99 +/- 0.54, which is consistent with 2.74 by the direct energy spectrum measurement. This method is a new approach to measure the spectral index and is complementary to the conventional energy spectrum measurement.

We present an updated all-particle energy spectrum using data collected in the period from 2000 November through 2004 October by Tibet III air-shower array. The energy determination of the air showers is made by fitting the lateral density distribution of the shower particles to the modified NKG function which is optimized by simulation calculation using interaction models of QGSJET01c and SIBYLL2.1 taking into account of the detector configurations. It is shown that the model dependence in the energy determination is not significant, being less than 5% in the absolute flux value, and we obtained the cosmic ray energy spectrum in a wide range over 3 decades between 10(14) eV and 10(17) eV, in which the position of the knee is clearly seen at around 4 PeV. The spectral index is -2.68 +/- 0.02(stat.) below 1PeV, while it is -3.12 +/- 0.01(stat.) above 4 PeV. The various systematic errors are under study now.

We have observed the shadowing of galactic cosmic ray flux in the direction of the moon, the so-called moon shadow, using the Tibet-III air shower array operating at Yangbajing (4300 m a.s.l.) in Tibet since 1999. Almost all cosmic rays are positively charged; for that reason, they are bent by the geomagnetic field, thereby shifting the moon shadow westward. The cosmic rays will also produce an additional shadow in the eastward direction of the moon if cosmic rays contain negatively charged particles, such as antiprotons, with some fraction. We selected 1.5 × 1010 air shower events with energy beyond about 3 TeV from the dataset observed by the Tibet-III air shower array and detected the moon shadow at ∼40σ level. The center of the moon was detected in the direction away from the apparent center of the moon by 0.23° to the west. Based on these data and a full Monte Carlo simulation, we searched for the existence of the shadow produced by antiprotons at the multi-TeV energy region. No evidence of the existence of antiprotons was found in this energy region. We obtained the 90% confidence level upper limit of the flux ratio of antiprotons to protons as 7% at multi-TeV energies.

We propose to build a large water-Cherenkov-type muon-detector array (Tibet MD array) around the 37 000 m2 Tibet air shower array (Tibet AS array) already constructed at 4300 m above sea level in Tibet, China. Each muon detector is a waterproof concrete pool, 6 m wide × 6 m long × 1.5 m deep in size, equipped with a 20 inch-in-diameter PMT. The Tibet MD array consists of 240 muon detectors set up 2.5 m underground. Its total effective area will be 8640 m2 for muon detection. The Tibet MD array will significantly improve gamma-ray sensitivity of the Tibet AS array in the 100 TeV region (10–1000 TeV) by means of gamma/hadron separation based on counting the number of muons accompanying an air shower. The Tibet AS+MD array will have the sensitivity to gamma rays in the 100 TeV region by an order of magnitude better than any other previous existing detectors in the world.

We show that the large-scale anisotropy of similar to 5 TeV galactic cosmic ray (GCR) intensity observed by Tibet Air Shower experiment can be reproduced by the superposition of a bi-directional and uni-directional flows (UDF and BDF) of GCRs. The heliosphere is located inside the local interstellar cloud (LIC) very close to the inner edge of the LIC. If the GCR population is lower inside the LIC than outside, the BDF flow is expected from the parallel diffusion of GCRs into LIC along the local interstellar magnetic field (LISMF) connecting the heliosphere with the region outside the LIC, where the GCR population is higher. A type of the UDF, on the other hand, is expected from the Bx del n drift flux driven by a gradient of GCR density (n) in the LISMF (B). The LISMF orientation deduced from the best-fit direction of the BDF is almost parallel to the galactic plane and more consistent with the suggestion of Frisch (1996) than that of Lallement et al. (2005). We note that the model, if holds, yields the LISMF polarity together with its orientation.

We obtained new upper limits on the diffuse gamma rays from the inner Galactic (IG) and outer Galactic (OG) planes in 3–10 TeV region, using the Tibet air shower data and new Monte Carlo simulation results. A difference of the effective area of the air-shower array for observing gamma rays and cosmic rays was carefully taken into account in this analysis, resulting in that the flux upper limits of the diffuse TeV gamma rays were reduced by factors of 4.0–3.7 for 3–10 TeV than those in our previous results (Amenomori, M., Ayabe, S., Cui, S.W., et al. Observation of multi-TeV diffuse gamma rays from the Galactic plane with the Tibet air shower array. Astrophys. J. 580, 887–895, 2002.). This new result suggests that the inverse power index of the energy spectrum of source electrons responsible for generating diffuse TeV gamma rays through inverse Compton effect should be steeper than 2.2 and 2.1 for IG and OG planes, respectively, with 99%C.L.

A hybrid experiment consisting of emulsion chambers, burst detectors and the Tibet-II air-shower array was carried out at Yangbajing (4300 m a.s.l., 606g/cm2) in Tibet to obtain the energy spectra of primary protons and heliums. From three-year operation, these energy spectra are deduced between 1015 and 1016eV by triggering the air showers associated with a high energy core and using a neural network method in the primary mass separation. The proton spectrum can be expressed by a single power-law function with a differential index of −3.01±0.11 and −3.05±0.12 based on the QGSJET+HD and SIBYLL+HD models, respectively, which are steeper than that extrapolated from the direct observations of −2.74±0.01 in the energy range below 1014eV. The absolute fluxes of protons and heliums are derived within 30% systematic errors depending on the hadronic interaction models used in Monte Carlo simulation. The result of our experiment suggests that the main component responsible for the change of the power index of the all-particle spectrum around 3×1015eV, so-called “knee”, is composed of nuclei heavier than helium. This is the first measurement of the differential energy spectra of primary protons and heliums by selecting them event by event at the knee energy region.

The flux upper limits of the diffuse gamma rays, from the inner and outer Galactic planes, are revised by fact ors of 4.0∼3.7 for mode energies 3 ∼10 TeV, respectively, by using the simulation results of the effective area ratios for gamma-ray induced showers and cosmic-ray induced ones in the Tibet air shower array. In our previous work, (Amenomori et al., ApJ, 580, 887, 2002) the flux upper limits were deduced only from the flux ratio of air showers generated by gamma rays versus cosmic rays. The details of the simulation are given in the pa per (Amenomori et al., Advances in Space Research, 37, 1932, 200 6). The present result using the same data as in ApJ suggests that the spectral index of source electrons is steeper than 2.2 an d 2.1 for the inner and outer Galactic planes, respectively.

The intensity of Galactic cosmic rays is nearly isotropic because of the influence of magnetic fields in the Milky Way. Here, we present two-dimensional high-precision anisotropy measurement for energies from a few to several hundred teraelectronvolts (TeV), using the large data sample of the Tibet Air Shower Arrays. Besides revealing finer details of the known anisotropies, a new component of Galactic cosmic ray anisotropy in sidereal time is uncovered around the Cygnus region direction. For cosmic-ray energies up to a few hundred TeV, all components of anisotropies fade away, showing a corotation of Galactic cosmic rays with the local Galactic magnetic environment. These results have broad implications for a comprehensive understanding of cosmic rays, supernovae, magnetic fields, and heliospheric and Galactic dynamic environments.

The Tibet hybrid experiment composed of emulsion chambers, burst detectors and the Tibet-II air-shower array was done at Yangbajing (4,300m above sea level, 606 g/cm2) in Tibet. For 3-year operation, we have observed 177 γ-families (ΣEγ > 20 TeV) accompanying air showers with the shower size Ne > 2 × 105. Using this data set and a neural network method to select proton and helium induced events, the primary proton and helium energy spectra are deduced between 1015 and 1016eV. The proton spectrum, thus obtained, can be expressed by a single power-law function with a differential index of −3.01±0.11 and −3.05±0.12 based on the QGSJET+HD and SIBYLL+HD models, respectively, which are steeper than that extrapolated from the direct observations of −2.74±0.01 in the energy range below 1014 eV. Our result suggests that the main component responsible for making the knee structure of the all-particle spectrum should be composed of nuclei heavier than helium.

Simultaneous observation of γ-families and their accompanying air showers was performed at Yangbajing (4300m above sea level) in Tibet with the emulsion chambers and burst detectors which were linked with the Tibet-II air-shower array. For three years operation, we observed 177 γ-families (∑Eγ>20TeV) accompanying air showers with the size Ne>2×105. Using this data and the simulation code CORSIKA, we obtained the primary proton spectrum in the energy range of 1015–1016eV. The slope of the observed spectrum is steeper than that extrapolated smoothly from the lower energy region below 1014eV, irrespective of the interaction models assumed in the simulation. The uncertainty of the absolute flux of protons due to the interaction model dependence involved in the simulation codes is at most 30%, which is much weaker than results of other air-shower experiments. Our experiment shows low intensity of light components (proton and helium) independent of the interaction models suggesting that the main component responsible for making the knee structure of the all-particle spectrum is composed of nuclei heavier than helium. This is the first measurement of the differential energy spectra of primary protons by selecting them event by event at the knee energy region.

According to many solar observations, the solar activity, during the Solar Cycle 23, is gradually declining to the quiet phase from the high state in 2000–2004. The Tibet air shower array has been continuing the observation of the shadow that cosmic rays make in the direction of the sun or the moon since 1991. We have shown the influence of the magnetic field around the sun, the interplanetary and the geomagnetic fields for the sun shadow at the multi-TeV region. We discuss the variation of the Sun’s shadow with the Gnevyshev gap (GG) in the maximum phase of the Solar Cycle 23. Unfortunately, the bottom in GG corresponds to off-observation period. Nonetheless, we found the anti-correlation between the significance of the Sun’s shadow and the variation of the magnetic field intensity around GG, although the statistical significance is not so high. It is expected that a detailed analysis of the Sun’s shadow will reveal interesting features of the anti-correlation.

A hybrid experiment was carried out to study the cosmic-ray primary composition in the 'knee' energy region. The experimental set-up consists of the Tibet-II air shower array(AS), the emulsion chamber (EC) and the burst detector (BD) which are operated simulteneously and provides us information on the primary species. For three-year operation, we have observed 177 gamma-families (Sigma E-gamma > 20 TeV) accompanying air showers with the shower size N-e > 2 x 10(5). Using this data set and a neural network method to select proton and helium induced events, the primary proton and helium energy spectra are obtained between 10(15) and 10(16) eV. The proton spectrum, thus obtained, can be expressed by a single power-law function with a differential index of -3.01 +/- 0.11 and -3.05 +/- 0.12 based on the QGSJET+HD and SIBYLL+HD models, respectively, which are steeper than that extrapolated from the direct observations of -2.74 +/- 0.01 in the energy range below 10(14) eV. Our experiment suggests that the main component responsible for making the knee structure of the all-particle spectrum is composed of nuclei heavier than helium.

We have been observing multi-TeV cosmic rays by the Tibet III air shower array since November 1999. This continuous observation enables us a detailed analysis of the sidereal and solar daily variations of the galactic cosmic-ray intensity. The observed solar daily variation is compared with the expected variation that includes the Compton-Getting effect due to the revolution motion of the earth around the sun. The variation in the higher-energy event samples (log mean energy 6.7 TeV) is consistent with the expected anisotropy, while the variation in the lower-energy event samples (log mean energy 3.8 TeV) suggests an additional diurnal anisotropy superposed, probably due to the solar modulation. This is the highest-precision measurement of the Compton-Getting anisotropy ever made.

We searched for steady PeV gamma-ray emission from the Monogem, Ring region with the Tibet air shower array from February 1997 to October 2004. No evidence for statistically significant gamma-ray signals was found in a region 111 degrees <= right ascension <= 114 degrees, 12.5 degrees <= declination < 15.5 degrees in the Monogem Ring where the MAKET-ANI experiment recently claimed a positive detection of PeV high energy cosmic radiation. We set a 99% confidence-level integral flux upper limit of 4.0 x 10(-12) cm(-2) s(-1) sr(-1) above 1 PeV on diffuse gamma-rays extended in the 3 degrees x 3 degrees region, which corresponds to a 10% of the flux estimated by the result from the MAKET-ANI experiment.

The two-dimensional large scale anisotropy of cosmic-ray intensity in a magnitude about 0.1% is observed in TeV energy range using data taken from Tibet-III (Nov. 1999-Oct. 2003). The anisotropy due to the terrestrial orbital motion around sun is discussed simultaneously.

J.L. Zhang for the Tibet AS Collaboration M. Amenomori ,S. Ayabe , D. Chen , S.W. Cui , Danzengluobu , L.K. Ding , X.H. Ding , C.F. Feng , Z.Y. Feng , X.Y. Gao , Q.X. Geng , H.W. Guo , H.H. He , M. He , K. Hibino , N. Hotta , Haibing Hu , H.B. Hu , J. Huang , Q. Huang , H.Y. Jia , F. Kajino , K. Kasahara , Y. Katayose , C. Kato , K. Kawata , Labaciren , G.M. Le , J.Y. Li , H. Lu , S.L. Lu , X.R. Meng , K. Mizutani , J. Mu , K. Munakata , A. Nagai , H. Nanjo , M.Nishizawa , M.Ohnishi , I. Ohta , H. Onuma , T. Ouchi , S. Ozawa , J.R. Ren , T. Saito , M. Sakata , T. Sasaki , M. Shibata , A. Shiomi , T. Shirai , H. Sugimoto , M. Takita , Y.H. Tan , N. Tateyama , S. Torii , H. Tsuchiya , S. Udo , H. Wang , X. Wang , Y.G. Wang , H.R. Wu , L. Xue , Y. Yamamoto , C.T. Yan , X.C. Yang , S. Yasue , Z.H. Ye , G.C. Yu , A.F. Yuan , T. Yuda , H.M. Zhang , J.L. Zhang , N.J. Zhang , X.Y. Zhang , Y. Zhang , Yi Zhang , Zhaxisangzhu and X.X. Zhou (a) Department of Physics, Hirosaki University, Hirosaki 036-8561, Japan (b) Department of Physics, Saitama University, Saitama 338-8570, Japan (c) Faculty of Engineering, Yokohama National University, Yokohama 240-8501, Japan (d) Key Lab. of Particle Astrophys., Institute of High Energy Physics, Chinese Academy of Sciences,Beijing 100049,China (e) Department of Mathematics and Physics, Tibet University, Lhasa 850000, China (f) Department of Physics, Shandong University, Jinan 250100, China (g) Institute of Modern Physics, South West Jiaotong University, Chengdu 610031, China (h) Department of Physics, Yunnan University, Kunming 650091, China (i) Faculty of Engineering, Kanagawa University, Yokohama 221-8686, Japan (j) Faculty of Education, Utsunomiya University, Utsunomiya 321-8505, Japan (k) Institute for Cosmic Ray Research, the University of Tokyo, Kashiwa 277-8582, Japan (l) Department of Physics, Konan University, Kobe 658-8501, Japan (m) Faculty of Systems Engineering, Shibaura Institute of Technology, Saitama 337-8570, Japan (n) Department of Physics, Shinshu University, Matsumoto 390-8621, Japan (o) Center of Space Science and Application Research, Chinese Academy of Sciences, Beijing 100080, China (p) Advanced Media Network Center, Utsunomiya University, Utsunomiya 321-8585, Japan (q) National Institute for Informatics, Tokyo 101-8430, Japan (r) Tokyo Metropolitan College of Aeronautical Engineering, Tokyo 116-0003, Japan (s) Shonan Institute of Technology, Fujisawa 251-8511, Japan (t) Advanced Research Institute for Science and Engineering, Waseda University, Tokyo 169-8555, Japan (u) RIKEN, Wako 351-0198, Japan Presenter: J.L. Zhang (zhangjl@mail.ihep.ac.cn), chn-zhang-J-abs1-og22-poster

We searched for steady PeV gamma-ray emission from the Monogem ring region with the Tibet air shower array from 1997 February to 2004 October. No evidence for statistically significant gamma-ray signals was found in a region 111° ≤ R.A. < 114°, 12.°5 ≤ decl. < 15.°5 in the Monogem ring where the MAKET-ANI experiment recently claimed a positive detection of PeV high-energy cosmic radiation, although our flux sensitivity is approximately 10 times better than MAKET-ANI's. We set the most stringent integral flux upper limit at a 99% confidence level of 4.0 × 10-12 cm-2 s-1 sr-1 above 1 PeV on diffuse gamma rays extended in the 3° × 3° region.