Cosmic-ray energy spectrum around the knee observed with the Tibet air-shower experiment
The energy spectrum of cosmic rays around the knee measured by Tibet air-shower experiment is summarized and its characteristic features are discussed under two possible scenarios. The result of Tibet experiment measured over wide range of10−10 eV with high statistics provided details of the knee at the energy around 4×10 eV. The study of the chemical composition based on measurements of proton and helium spectra obtained from airshower core detection indicates the dominance of heavy nuclei around the knee. Such feature can be explained either by contribution of nearby sources with source composition dominated by heavy nuclei or by nonlinear effect in diffusive shock acceleration mechanism. 1 Tibet Air-shower array and core detector The Tibet air-shower (AS) experiment is characterized by high altitude of the observation site (4300 m a.s.l., 606 g/cm), which enables us to determine the primary energy less dependent upon the primary mass and models of hadronic interactions in the energy range over PeV because of the observation of the shower size at nearly maximum development irrespective of the primary mass. The second merit of the high altitude is that it is possible to select the shower events of light nuclei origin such as protons and helium through the detection of high-energy secondary particles (electromagnetic component) located at the AS core, because the probability of association of high energy secondaries depends on primary mass by the difference of the interaction cross section in the atmosphere. The study of the primary mass composition is made by simultaneous operation of the AS array and the core detector. The AS array consists of 761 fast timing (FT) counters, surrounded by 28 density (D) counters with 36 900 m coverage as shown schematically  The AS array is used to measure the energy and arrival direction of each AS. The details of the installation are described in ( Amenomori et al. , 2008a). The primary energy of each event is determined by the shower size Ne, which is calculated by fitting the lateral particle density distribution to the modified NKG structure function. The energy resolution is less than 20% for the knee energy range. The arrival direction of an air shower is determined as follows. The direction cosine of the shower axis is determined using a least square method in which the difference is minimized between the arrival time signals of each FT counter and the expected values on the assumed cone with a given direction cosine. This procedure determines the arrival angle with an accuracy smaller than 0.2◦ at energies above 10 eV, which is calibrated by observing the Moon’s shadow ( Amenomori et al., 2000a, 2003). The core detector was designed to detect the high-energy particles at the AS core by converting them into electromagnetic cascade showers (burst) using lead absorber of 160 cm×50 cm in area and 7 cm (in Phase I) or 3.5 cm (in Phase II) in thickness. The burst size under the lead plate was measured by scintillation counters of the same size as the lead plate in area and 2 cm in thickness. Four photodiodes were attached to the each corner of the scintillator whose attenuation length for the scintillation light was calibrated with electron beam. From the 4 photo-diode signals, we can estimate the burst size and the position of the burst center with spatial resolution of 10 cm. One hundred core detectors were used (total area is 80 m ) to detect burst size greater than5×10. The study of the primary mass composition was made through two experimental phases. In the first phase, 6 layers of X-ray films were placed between lead plates with 1 cm depth interval to register the development of shower spots induced by high-energy γ and electrons (hereafter abbreviated as γ-rays) over a few TeV which are incident upon the detector, and detailed analysis on the structure of the high energy core has been made with use of image scanner ( Ozawa et al. , 2004) to obtain proton and helium spectra ( Amenomori et al. , 2006). The energy determination of the individualγ-rays was made by the comparison of the transition curve of the optical density of the shower spots with electromagnetic shower theory. Such procedure of the energy determination using X-ray films was calibrated using 200 GeV electron beam at FNAL ( Hotta et al. , 1980). A bundle of the high energy γ-rays originating from successive nuclear interactions of the primary cosmic rays in the atmosphere is called γ-family, which can be detected by spatial reconstruction of the shower spots registered on X-ray films with angular resolution of 2.5◦. They are reconstructed as groups of parallel tracks with typical lateral spread of at most several cm among uncorrelated single tracks, which are reduced as background. The assignment of a γ f mily to an accompanied AS was made through the positional correlation between the X-ray film and the scintillator, and the correlation between the burst and AS was made by their time stamps. The arrival directions obtained from AS and the reconstruction of the shower spots were also used to uniquely determine the AS candidate. Suchγ-family events are more efficiently generated by light primaries than the case of heavy primaries because of the penetrating nature in the atmosphere. Hence, the AS core detector can select the air showers of light primary origin naturally. The contamination of the events of heavy nuclei origin was estimated using Artificial Neural Network (ANN) based on extensive Monte Carlo simulation of air showers (MC) assuming interaction models and primary models using CORSIKA code. Two interaction models of QGSJET and SIBYLL are used in estimating the efficiency of the γ family generation, and also two mass composition models of proton dominant (PD) and heavy dominant (HD) are used to appreciate the contamination rate of heavy primaries. The efficiency of the family generation for various primary nuclei and its energy dependence is calculated by MC and used for obtaining the primary proton and helium fluxes. For exAstrophys. Space Sci. Trans., 7, 15– 20, 2011 www.astrophys-space-sci-trans.net/7/15/2011/ M. Amenomori et al.: Cosmic-ray energy spectrum with the Tibet air-shower experiment 17  ample, the efficiencies by protons at the knee energy 4 PeV in phase I experiment are about 7% and 10% by QGSJET model and SIBYLL model, respectively. ANN is also used in procedures to obtain proton and helium spectra. Phase I experiment selected 177 γ family events with core energy ∑ Eγ >20 TeV and AS sizeNe>2×10. In the second phase experiment, X-ray films were not used and proton+helium (P+He) spectrum was studied without separating them but with higher statistics than that of phase I. The core detector was tuned to decrease the detection threshold energy of the AS core to have several times higher efficiency. The details of the analysis are described in (Amenomori et al. , 2007, 2010). 2 Experimental results The energy spectrum of all particles was obtained as shown in Fig. 2 together with other data (Cited data: Grigorov (Grigorov et al. , 1971), JACEE (Asakimori et al. , 1998), KASCADE (Antoni et al., 2005), BASJE (Ogio et al. , 2004), CASA (Fowler et al. , 2001), Akeno (Nagano et al. , 1984), Akeno array1 and Akeno array20 ( Nagano et al. , 1992)). The uncertainty of the absolute intensity due to the interaction models used in the analysis is shown by QGSJET and SIBYLL (10% difference at most). The uncertainty due to the assumed primary mass composition is also shown by HD and PD. Although the composition is fairly different between HD and PD models at energies above 10 eV, the difference of the absolute intensity is 20% at most between the two models and it decreases with increasing primary energy. Note that the shapes of the spectra from different models are almost the same and the position of the knee is clearly found at the energy around 4 PeV. From the second phase experiment of AS core observation, the energy spectrum of P+He was obtained with high statistics and in good agreement with phase I as shown in Fig. 3, in which QGSJET model was used for the analysis. Use of 10 2 10 3 10 4 10 4 10 5 10 6 10 7 10 8 10 9 E 2 .5 d J /d E [ G e V 1 .5 m − 2 s e c − 1 s r − 1 ] Primary Energy [GeV/particle] Grigorov JACEE KASCADE-QGS KASCADE-SIB BASJE2004 CASA/MIA Akeno Akeno array1 Akeno array20 Tibet3-QGSJET+HD Tibet3-QGSJET+PD Tibet3-. The results from three models of QGSJET+HD, QGSJET+PD, and SIBYLL+HD used in the analysis are shown. SIBYLL model results in about 30% lower intensity than that of the QGSJET analysis There is a problem of discrepancy between Tibet and KASCADE data as shown in Fig. 3, e.g., the flux of the light nuclei by KADCADE is higher than Tibet by more than factor 2. However, if SIBYLL model is used in the analysis of KASCADE, the difference between two data is much reduced. This fact does not necessarily mean that SIBYLL model is more preferable to explain the data at this energy range. KASCADE reported that QGSJET model explains high energy data better than SIBYLL, on the other hand, SIBYLL model is better at lower energies. This problem is related to the interaction model dependence of MC calculations and also to the different experimental methods used in two experiments. Tibet experiment observes high energy part of the air showers, namely, the most forward region of the center of momentum system (CMS) of interacting particles, while KASCADE experiment observes Ne−Nμ correlation, in which muon component is sensitive to the central region of the CMS. The model dependence in forward region www.astrophys-space-sci-trans.net/7/15/2011/  2011 18 M. Amenomori et al.: Cosmic-ray energy spectrum with the Tibet air-shower experiment 10 1 10 2 10 3 10 4 10 5 10 0 10 1 10 2 10 3 10 4 10 5 10 6 10

title: New Estimation of the Spectral Index of High-Energy Cosmic Rays as Determined by the Compton-Getting Anisotropy
abstract: The amplitude of the Compton-Getting (CG) anisotropy contains the power-law index of the cosmic-ray energy spectrum. Based on this relation and using the Tibet air shower array data, we measure the cosmic-ray spectral index to be –3.03 ± 0.55stat ± <0.62syst between 6 and 40 TeV, consistent with –2.7 from direct energy spectrum measurements. Potentially, this CG anisotropy analysis can be utilized to confirm the astrophysical origin of the "knee" against models for nonstandard hadronic interactions in the atmosphere.

title: First results on the Spaghetti Calorimeter projective prototype
abstract: The performances of a projective lead/scintillating fibres (“spaghetti”) calorimeter made out of rectangular cross-section (32 × 32 mm 2 ) straight modules alternated with tapering modules at 2.5° are described. Beam test results on energy resolution, constant term suppression, lineshape, uniformity of response, shower position resolution, and electron-pion discrimination are presented. The readout of the fibres in the straight modules, which are sensitive to most of the electromagnetic showers and to part of the hadronic showers, is kept separate from the readout of the fibres of the wedged sections, which are sensitive only to hadronic showers. The lead-to-fibre volume ratio is kept at about 4 to 1 in order to obtain an equal-amplitude response from electrons and hadrons (usually called “compensation”). The origin of the energy resolution constant term has been better understood. This allowed the reduction of the constant term to 0.5%, down from 1.2% measured in older prototypes. The probability of faking abnormally large signals because of channelling effects is also discussed. The position resolution has been measured to range from 0.5 to 1 mm depending on the hit position of the particle. A comparison with the larger hexagonal module design is also made. The efficiency of a hardware thresholding of the wedges' signal as a level one trigger pion veto is discussed along with off-line electron/pion discrimination performances. Since the wedges start at 26 X 0 from the calorimeter front face they should show no signal for electromagnetic showers. It was found that electromagnetic showers of 50 GeV generate wedge signals of a maximum of a few tens of MeV. It was also found that the discriminating power is angle dependent, mainly because at small particle-to-fibre angles (≤ 1°) some electrons can penetrate the calorimeter through channeling, thus exciting a signal in the wedges. At slightly higher tilt angle it is possible to use a lower wedge threshold without loss in electron efficiency. With a beam-to-fibre angle of 3°, 99.6% of pions are rejected with a 98.5% electron efficiency. More recent and preliminary results on energy resolution at energies up to 100 GeV, and on radiation damage of scintillating fibres are also presented.

title: Electron, pion and multiparticle detection with a lead/scintillating-fiber calorimeter
abstract: We report on the performance of a fine-grained 13-ton lead/scintillating-fiber calorimeter, in particular on its response to electrons, pions and multiparticles (reaction products from pions interacting in a target upstream of the detector). The detector signals were studied for particles in the energy range 5–150 GeV. The energy resolution was measured to be 12.9% √E for electrons, plus a constant term dependent on the angle θ Z between the particle's direction and the fiber axis. This term, which is 1.2% for θ z = 3°, is shown to be due to anomalous sampling in the early shower stage. It is greatly reduced when only electrons entering the detector in the lead are considered. A 1.7 X 0 thick preshower detector, installed 12 cm in front of the calorimeter, only affected the signal linearity for electrons at low energy. The effect on the energy resolution was negligible. Single pions were detected with an energy resolution of ∼ 30%/√ E plus a constant term, which turned out to be mainly due to the effects of light attenuation in the fibers. Knowing the impact point of the particles, these effects could be efficiently removed for single pions. For jets (multiparticles), the effects of light attenuation are much less important, leading to considerably better on-line energy resolutions. The e π signal ratio was measured to range from 1.03 at 80 GeV to 1.10 at 5 GeV, for a detector with an effective radius of 49 cm. After correcting for the instrumental effects, we found the intrinsic e h value of this detector (with our particular choice of fibers and sampling fraction) to be 1.15±0.02. Detailed results are given on the detector performance (energy resolution, e π signal ratio, e/jet signal ratio) as a function of the lateral detector size and as a function of the jet multiplicity.

title: Aging test of a real-size high rate MRPC for CBM-TOF wall
abstract: A new kind of low-resistivity glass has been developed. Its volume resistivity is on the order of 1010 Ω cm and multi-gap resistive plate chambers (MRPCs), once assembled with it, can be operated at a charged particle flux in excess of 25 kHz/cm2, with very small charge build-up at the plates. This new technology has a wide range of application in high energy physics experiments such as FAIR-CBM, LHC-ATLAS or Jlab-SOLID, to mention some. In this paper we report on results related to its long-term behavior (aging). A 6 × 2-pad CBM module has been irradiated by X-rays at a mips-equivalent flux of 15 kHz/cm2, for 300 hours and for a released charge totaling 0.22 C (50 mC/cm2). Tested in an electron beam before and after exposure, no degradation of the detector performances could be appreciated. As expected, compared to common glass MRPCs, the newly developed high rate counter also responds faster to sudden irradiation.

title: Evidence for the isoscalar giant dipole resonance in Pb-208 using inelastic alpha scattering at and near 0 degrees
abstract: The isoscalar giant dipole resonance (ISGDR) in Pb-208 has been investigated using inelastic scattering of 200 MeV alpha particles at and near 0 degrees where the angular distribution of the ISGDR can be clearly differentiated from those of other modes. The ''difference of spectra'' technique was employed to separate the ISGDR from the high-energy octupole resonance (HEOR). These data provide the dearest evidence yet for the ISGDR adjacent to the HEOR. With these results, all expected isoscalar lambda less than or equal to 3 resonances in Pb-208 have been identified.

title: Is the energy generation rate of nuclear reactions in hot accretion flows important
abstract: The temperature of hot accretion flows around black holes is sufficiently high for the ignition of nuclear reactions. This is potentially an important nucleosynthesis mechanism in the universe. As the first step in studying this problem, we need to measure physical quantities such as density and temperature of the accretion flow. In usual studies of the hot accretion flow, viscous dissipation is considered to be the only heating mechanism, while the heating caused by nuclear reactions is not considered. In this paper, we investigate whether the energy generation rate of nuclear reaction is important compared to the viscous heating. Our calculation indicates that the former is at most one percent of the latter and thus is not important. The dynamics of accretion flow can be therefore calculated in the usual way, without the need to consider heating due to nuclear reactions.