1997 Vol. 21, No. S3
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Two tracking atmospheric Cherenkov telescopes have been operated at the Xing Long Station, Beijing Astronomical Observatory, for the observation of atmospheric Cherenkov radiation produced by extensive air showers. They are used to search for point sources of very high energy (VHE) gamma-rays and to study their emission characteristics as well as to detect the spatial distribution of VHE gamma-ray intensity. The structure and the performance of the telescopes as well as the testing results under observation conditions are presented in this paper.
A search for gamma-ray bursts at the 10 TeV energy region was made using data of the Yangbajing air shower experiment in Tibet. About 4×108 events were analyzed to search for shower clusters appearing in a given time interval and a given small sky bin. An equal-zenith angle method is used to estimate the background. Some clusters show the excess to the background but with less significance as the evidence of gamma-ray bursts. The much-higher sensitivity of the Yangbajing Phase Ⅱ array to the detection of the 10 TeV gamma-ray bursts is discussed.
Based on the existing concept design and the expected performance of the detector at the future τ-charm factory, a fast Monte Carlo simulation program using the BES software frame and its data structure has been developed. Various important and/or interesting physics issues have been simulated using this method and a series of important physics results have been attained. The design idea, the program frame, and the advantage of this method have been described. This method can greatly save CPU and disk space and is specially suitable for BES users. The physics results obtained from the simulation demonstrate the physics importance of building the BTCF.
Coincidence measurements of fission fragments and light-charged particles have been performed for the reactions of 40Ar+natAg, 209Bi at E/A=25 MeV using 4 PPAC and 11 sets of △E-E telescopes. Angular correlations of fission fragments are plotted as a function of the folding angle between the two detected fission fragments. The linear momentum transfer distributions are derived by measured angular correlation. The backward spectra of light particles detected in coincidence with fission fragments having different average <LMT> are analyzed with a Maxwell distribution. After some corrections the initial temperature of the hot nuclei is determined from the energy spectra. The excitation energies corresponding to the different average <LMT> are obtained considering the reaction Q values and pre-equilibration emission. In the central collision of the 40Ar+natAg, 209Bi reactions, excitation energies are measured to be about 4.2 MeV/u, 2.4 MeV/u and temperatures about 6.1 MeV, 5.5 MeV, respectively. In a semi-central collision, the measured excitation energies and temperatures are about 3.5 MeV/u, 1.9 MeV/u, and 5.8 MeV, 4.8 MeY, respectively.
The fragment angular distributions for 35.9, 41.0, 45.6, and 50.0 MeV/u 16O+197Au, 50 MeV/u 16O+159Tb, and 50 MeV/u 16O+209Bi reactions have been measured by the solid state nuclear track detectors. After subtraction of the contribution of intermediate mass fragment (IMF), the fission angular distributions have been fitted by the transition state statistical model (TSM) and the single-spin standard theory to extract the spins of the fissioning nuclei. The relation between spins and bombarding energies is also discussed.
Multifragmentation is studied in correlation with energetic protons. A cold multifragmentation pattern is proposed based on the numerical simulation of the stochastic Boltzmann-Langevin model to describe the intermediate heavy ion collisions, i.e, a sizeable fraction of the available energy is released from the system by fast non-equilibrium proton emission, in which the explosion is caused by a violent but cold expansion of the system. Some typical examples simulated for 40Ca+40Ca central collisions at 90 MeV/u are presented. A possible experimental signature which could be detected using a 4π detector system is suggested.
Influence of the central collective flow on a two-particle correlation function is discussed. The Monte Carlo calculation was used to describe how the single-particle and two-particle spectra from a hot emission source are affected by the central collective flow. The two-proton correlation function for the 100 MeV/u Ni+Ni system at b=0 fm was calculated using the QMD model. The particle interchange method was developed and used to prove the existence of a central collective flow in central collisions of the 100 MeV/u Ni+Ni system. The results show that the two-particle correlation functions are indeed sensitive to the central collective flow. This might provide a method to study the central collective flow formed in central collisions for medium heavy system at energies above 100 MeV/u.
The single-particle level densities, calculated under the local density approximation and based on the Fermi gas model, are used to obtain the partition functions. The results show that the internal excitation of the intermediate mass fragments (IMF) decayed from a hot nucleus have strong effect upon the extraction of the isotope nuclear temperature.
Using the QMD model, the time evolution of the Au-h Au system at 100 MeV/u has been studied. For very central collisions, the conditions for freeze-out and how the different physical quantities approach equilibrium are investigated. The calculation results show that for 100 MeY/u Au+Au b=0 fm, the maximum density can reach 1.6ρ0, after which the system expands to a low density region. A position correlated central flow develops during the expansion stage. Through study of the two-particle relative momentum spectrum, it is possible to separate the central flow from random thermal motion. The dependence of this central flow on the equation of state (EOS) is also discussed.
The spectra of 207-210Tl and 207,208Hg are calculated in terms of the shell model with different interactions. The one-particle one-hole (1p-1h) excitation is taken into account for 207Tl and 208Tl. The calculated results for low-lying states agree with the available experimental data very well, which shows that those effective interactions are still valid to the nuclei such as 208Tl. Some levels of 208,209Tl whose spins and parties have not been determined in the experiment are predicted. The spectra of 207Hg and 208Hg are calculated and can be used to guide the feature experiments.
A recombination factor is introduced to describe the nuclear shadowing and anti-shadowing effects in the small x region. Based on the consideration of the nuclear momentum conservation, a unified description of the EMC effect, the nuclear Drell-Yan process, and an inelastic J/ψ production are given in terms of the x-rescaling model with the recombination factor.
The necessity and possibility of beam polarization in the Beijing electron-positron collider (BEPC) and the methods for beam polarization measurements are introduced briefly. The design of a Compton scattering polarimeter used for the measurements of beam polarization of BEPC beam via a synchrotron radiation pipeline and the experiments under way are presented.
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