2004 Vol. 28, No. 11
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The integrated luminosity for the two sets of BES data,collected at s=3.650,3.686GeV in 2002 and 2003,are carefully studied,and measured to be (6.42±0.24)pb-1 and (19.72±0.86)pb-1.The result provides basic input parameter in the measurements of cross sections at the above two energy points.
The identification of quark and gluon jets produced in e＋e－collisions using the artificial neural network method is addressed.The structure and the learning algorithm of the BP(Back Propagation)neural network model is studied.Three characteristic parameters—the average multiplicity and the average transverse momentum of jets and the average value of the angles opposite to the quark or gluon jets are taken as training parameters and are inputed to the BP network for repeated training.The learning process is ended when the output error of the neural network is less than a pre-set precision(σ=0.005).The same training routine is repeated in each of the 8 energy bins ranging from 2.5—22.5 GeV,respectively.The finally updated weights and thresholds of the BP neural network are tested using the quark and gluon jet samples,getting from the non-symmetric three-jet events produced by the Monte Carlo generator JETSET 7.4.Then the pattern recognition of the mixed sample getting from the combination of the quark and gluon jet samples is carried out through applying the trained BP neural network.It turns out that the purities of the identified quark and gluon jets are around 75%—85%,showing that the artificial neural network is effective and practical in jet analysis.It is hopeful to use the further improved BP neural network to study the experimental data of high energy e＋e－ collisions.
The random phase approximation (RPA) method and the Feymann-Hellman theorem are used to calculate the seventh order glueball mass of the 2+1-D SU(2) lattice gauge theory. As an approximation, the trial wave function is constructed only by single-hollow graphs. The calculated result of the seventh order glueball mass shows good scaling behavior at the weak coupling region of 1/g2=1.0-1.8(m/e2≈1.20±0.01).
The density dependent relativistic hadron field theory(DDRH) containing meson δ is applied to very asymmetric nuclear matter. Because of the interaction between the nucleons and the scalar-isovector meson δ,the property of the asymmetric nuclear matter is so different from symmetric nuclear matter.The relations between scale density ρS and vector density ρB have been calculated. We also investigate the effective masses of nucleons in symmetric and asymmetric nuclear matter by using DDRH.The density dependence of in-medium kaon and antikaon effective masses has been obtained and then the effects of both the density dependence of the coupling constant Γi(ρ)(i=σ,ω,δ,ρ) and the scalar-isovector meson δ were discussed.
The formulas for calculating the energy balance and the intensity balance in ε decay are presented.As an application,the ε decay of 52Mn is given.
The thermal stability for hot nucleus is investigated based on limiting temperatures. We calculate the isotope distributions of limiting temperatures for finite nuclear systems by using Skyrme interaction. The isotope distribution of limiting temperatures appears to be an upside-down parabolic shape, which looks very similar with the isotope distribution of the products of multifragmentation in intermediate energy heavy ion collisions. We systematically calculated the isotope distributions of limiting temperatures for a series of nuclei from light to heavy nuclei, and obtain a thermal stability line for hot nuclei. The thermal stability line is more neutron-rich compared with the β-stability line.
The pseudorapidity distribution of shower particles produced in the 24Mg-emulsion collisions at 4.5A GeV/c is reported in this paper. The dependences of the distribution width and the peak position on the target size are observed.The pseudorapidity distribution of shower particles for the events with low target multiplicity (light target) is narrower than that with high target multiplicity (heavy target). The maximum probability pseudorapidity for light target is greater than that for heavy target. The experimental data is analyzed by using the cylinder model suggested by Liu et al. The Monte Carlo results based on Liu's cylinder model are approximately in agreement with the experimental tendency and fluctuation.
In TOF measurement, the factors that can increase the time resolution include noise, amplitude measurement error, and the difference of pulse amplitudes and shapes. This article discusses how these factors affect time resolution, and performs quantitative analysis based on the GEANT4 simulation result of the BESⅢ's TOF detector. The result shows that the discriminator level must be selected carefully. To get better time resolution, pulse waveform digitizing is necessary.
A study is performed on the characteristic impedance of readout strip of the Resistive Plate Chamber (RPC). An empirical formula to calculate the characteristic impedance of microstrips is used for RPC readout strip. We find the formula fits our measurement very well. This can give a guide in the design of the RPC Readout Strip.
A comic-ray test system has been built for testing the performance of MRPC modules. Some methods have been studied to improve the time resolution of the cosmic-ray test based on this testing system. The time resolutions of about 84ps and 75ps can be achieved for MRPC and its reference time, respectively. The temperature effects of MRPC have also been researched and some useful results are obtained.
A new detecting system of the beam profile used in HIRFL(Heavy Ion Research Facility in Lanzhou) is introduced. The detecting system is based on the residual gas ionization principle and consisted of electrostatic collecting plates, micro-channel plates and a resistive anode. The principle and the construction of the system is described. The on-line experiment results are presented. The non-interceptive diagnostic method by using residual gas ionization to detect beam profile is verified.
Correction of longitudinal energy leakage for a lead scintillating fiber sandwiched electromagnetic calorimeter was studied. Electron data from AMS beam test in 2002 was analyzed. Γ-Function-Fit and Last-Layer-Energy-Deposit methods were used to correct longitudinal energy leakage. The linearity and resolution of energy were clearly improved after the correction. And the Last-Layer-Energy-Deposit method is more effective on energy resolution improving.
With different gas mixtures and high voltage, the temperature dependence of the noise and dark current for a 6-gap Multi-gap Resistive Plate Chamber (MRPC) is measured, and they are found to increase exponentially with the increase of temperature.
The beam-gas backgrounds simulation of the BESⅢ, including the theory and the results is introduced. The results show that, after masking on the storage ring, the detector backgrounds are in safe level. The results can guide the design and construction of the BEPCⅡ-BESⅢ.
The transverse stability is a critical issue in circular accelerator. In this paper, we analysed the effect of tune modulation on a FODO lattice with sextupole nonlinear through estimating the dynamic aperture including the influence of the distortion along the phase. It turned out that the tune modulation decreases the stability of particle in storage ring, the extent of this decrease depends largely on the amplitude and tune of modulation.
Longitudinal coupled bunch instability in fractionally filled storage rings is studied in this paper with analytic method to derive the expression for the growth rate of the instability as well as the synchrotron oscillation frequency shift. An interesting phenomenon has been found that imaginary part of impedance makes contribution to the growth rate of coupled bunch instability. This phenomenon is contrary to that in symmetrical bunch filling cases.
BEPCⅡ—an upgrade project of Beijing Electron Positron Collider (BEPC) is a factory type of e+e- collider. It requires its injector linac to have a higher beam energy (1.89 GeV) for on-energy injection and a higher beam current (40 mA e+ beam) for a higher injection rate ( 50 mA/min). The low beam emittance (1.6π mm·mrad for e+ beam, and 0.2πmm·mrad for 300 mA e- beam) and the low beam energy spread (±0.5%) are also required to meet the storage ring acceptance. Thus the original BEPC injector linac must be upgraded to have a new electron gun with its complete tuning system, a new positron source with a flux concentrator, a new RF power system with its phasing loops and a new beam tuning system with orbit correction and optics tuning devices. These new components have been designed, fabricated, tested and will be installed in their final positions in this spring and summer, which are described in this paper.
As is well known, the EM field of TE11 mode at the wall of the circular waveguide changes as sine (or cosine) function azimuthally. So when we attach two perpendicular waveguides to the wall of the circular waveguide and rotate them around the axis of the waveguide, we can distribute the input power between the two waveguides with arbitrary distribution proportion. We have designed a new power divider following this idea.The 3D electromagnetic simulation software HFSS is used in the design. And a new type circular TE11 mode launcher is developed.
Electron cloud instability (ECI) may take place in a storage ring when the machine is operated with multi-bunch positively charged beam. According to the actual shape of vacuum chamber in BEPCⅡ, a computer simulation program has been developed. With the code, the electron density in the chamber with the different widths of antechamber and the different secondary electron yields, respectively, can be obtained. The possibility to put clearing electrodes in the normal chamber or ante-chamber to reduce the electron density in the central region of the chamber is also investigated. In the simulation, the coherent oscillation of bunches and the formation of electron cloud (EC) are tracked simultaneously. The sideband distribution and growth rate can be obtained through tracking and dealing the bunch oscillation amplitude with FFT. Based on the head-tail model, the single bunch instability induced by EC is also studied. By the simulation, for BEPCⅡ the threshold density for the single bunch instability is estimated to be～10 12 m -3 and the restraining measures that might be adopted in BEPCⅡ can control the EC density under the threshold. According to the simulation results, a relatively big positive chromaticity should be used to suppress the bunch blow up.
In our SLAC type of RF pulse compressor (SLED), the detuning device works not as well as expected. It happend that the detuning needles sticked to the sleeves when working at detuning mode. Due to this reason, we can not measure its energy multiplication factor. To solve the sticking problem of SLED needles, a prototype of the new SLED detuning system has been developed and it has been operated with high power successfully on BEPC Linac.
The X-ray emission from the highly charged Ar17+ ion interacting with Mo surface are measured on the 14.5GHz Electron Cyclotron Resonance ion source facility at Institute of Modern, the Chinese Academy of Sciences in Lanzhou. Ar17+ ions came from Electron Cyclotron Resonance ion source and impacted on Mo surface.The incident ion with vacancy of inner shell captures electrons emitted from surface into highly excited Rydberg states, outer shell electron cascades into K vacancy giving rise to Kα X-ray emission of the incidentionsions.
Recently, a new digitalized 2-Detector Doppler Broadening Spectrometer is developed. 10 pure metal samples are studied with this system. The results show the relationship between 2-D Doppler broadening spectra and electronic structure of these elements. This article introduces the spectrometer, data analysis,the Doppler broadening of 10 pure metal samples, and the discussion about these results.
Variety of X-ray facilities are widely used in many fields. One of its developing trend is to gain harder and more intense X-rays which are yielded by electron beam with energy above 10MeV and current above kA impacting on thick-target. Dependence of bremsstrahlung characteristics on beam parameters and target thickness are much concerned. The paper studies the selection of the thickness of the target, the bremsstrahlung photons intensity distribution along emergence angle as well as the bremsstrahlung energy spectra. Also, the dependence of X-ray intensity on beam emittance and size of beam-target interaction spot is given. The result reveals at the usual emittance, the decreasing rate of the X-ray intensity with decreasing spot sige is much slowe than that claimed by other references. The consistency between the simulation with experimental results shows that the EGS4 codes can be effectively used in High Energy X-ray facility study.
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