The proposed beam energy measurement system at BEPC II is composed of three parts： the laser source and optics system, the laser-electron interaction system and the HPGe detector system. The working principles of each system are expounded together with the calculation for preliminary design. The normaliza- tions of laser and electron beams are put forth and used for the evaluation of intensity of the backscattering photon. The simulation of HPGe detector is also performed for understanding the working properties.
Understanding the radiation background at the north crossing point （NCP） in the tunnel of BEPCII is crucial for the performance safety of the High Purity Germanium （HPGe） detector, and in turn of great significance for long-term stable running of the energy measurement system. Therefore, as the first step, a NaI（T1） detector is constructed to continuously measure the radiation level of photons as background for future experiments. Furthermore, gamma and neutron dosimeters are utilized to explore the radiation distribution in the vicinity of the NCP where the HPGe detector will be located. Synthesizing all obtained information, the shielding for neutron irradiation is studied based on model-dependent theoretical analysis.
Resorting to Hessian matrix, the analytical formula is obtained to determine the optimal luminosity proportion for the experiment of τ mass scan. Comparison of numerical results indicate the consistency between the present analytical evaluation and the previous computation based on the sampling technique.
A gamma spectrum of a Pu-C source is measured using a p-type HPGe detector, whose three peaks （full energy, single-escape and double-escape peak） can be used as a calibration source for the beam energy measurement system of BEPCII. The effect of fast neutron damage on the energy resolution of the HPGe detector is studied, which indicates that the energy resolution begins to deteriorate when the detector is subject to 2×107 n/cm^2 fast neutrons. The neutron damage mechanism and detector repair methods are reviewed. The Monte Carlo simulation technique is utilized to study the shielding of the HPGe detector from the fast neutron radiation damage, which is of great significance for the future commissioning of the beam energy measurement system.
The technique details for measuring radiation dose are expounded. The results of gamma and neutron radiation levels are presented and the corresponding radiation shielding is discussed based on the simplified estimation. In addition, the photon radiation level move as background for future experiments is measured by a NaI（T1） detector.
We have examined the parametrization of the e^＋e^- → ωπ0 cross section in the vicinity of the φ resonance and the extraction of the branching fraction of the isospin violating process φ → ωπ0 from experimental data. We found that there are two possible solutions of the branching fraction： one is 4 × 10-5, and the other is 7×10^-3. The latter is two orders of magnitude higher than the former, which is the commonly accepted one.
Scenarios for the τ mass measurement at the upgraded Beijing Electron-Positron Collider （BEPC- II ） are studied. A nested minimization procedure is used to optimize the data taking plan. It is found that by using five energy points with the total integrated luminosity of 100 pb-1, the τ mass can be determined with a statistical error of 50 keV.
M. N. AchasovV. E. Blinov蔡啸傅成栋F. A. HarrisQ. Liu莫晓虎N. Yu. MuchnoiI. B. Nikolaev秦庆A. G. ShamovK. Yu. Todyshev王贻芳张建勇
The beam energy measurement system at BEPCII is composed of there parts： laser source and optics system, laser-electron interaction system and High Purity Germanium （HPGe） detector system. The special components and construction of each part are introduced, especially about radiation background measurement in the storage ring, which is of great importance for the safe commissioning of HPGe detector.
It has been conjectured that the relative phase between strong and electromagnetic amplitudes is universally -90° in charmonium decays. ψ′ decaying into a pseudoscalar pair provides a possibility to test this conjecture. However, the experimentally observed cross section for such a process is depicted by the two-fold integral, which takes into account the initial state radiative （ISR） correction and energy spread effect. Using the generalized linear regression approach, a complex energy-dependent factor is approximated by a linear function of energy. Taking advantage of this simplification, the integration of ISR correction can be performed and an analytical expression with accuracy at the level of 1% is obtained. Then, the original two-fold integral is simplified into a one-fold integral, which reduces the total computing time by two orders of magnitude. Such a simplified expression for the observed cross section usually plays an indispensable role in the optimization of scan data taking, the determination of systematic uncertainty, and the analysis of data correlation.
The kinematic properties of two-body decay near τ threshold are studied according to the special capacity of the BEPC accelerator and the BESⅢ detector.Explicitly presented are the transformations of energy and momentum of hadronic particles between different reference frames,and the corresponding distributions.A brand new method is proposed to obtain the energy spread of the accelerator by fitting the energy distribution of hadron from τ semi-leptonic decays.
