2009 Vol. 33, No. 7

Particle and field theory
Study of the data taking strategy for a high precision τ mass measurement
WANG You-Kai, ZHANG Jian-Yong, MO Xiao-Hu, YUAN Chang-Zheng
2009, 33(7): 501-507. doi: 10.1088/1674-1137/33/7/001

To achieve a high precision τ mass measurement at the high luminosity experiment BESIII, Monte Carlo simulation and sampling technique are utilized to simulate various data taking cases for single and multi-parameter fits by virtue of which the optimal scheme is determined. The optimized proportion of luminosity distributed at selected points and the relation between precision and luminosity are obtained. In addition, the optimization of the fit scheme is confirmed by scrutinizing a variety of fit possibilities.

B→f0(980) (π,η(′)) decays in the PQCD approach
ZHANG Zhi-Qing, XIAO Zhen-Jun
2009, 33(7): 508-515. doi: 10.1088/1674-1137/33/7/002

Based on the assumption of a two-quark structure of the scalar meson f0(980), we calculate the branching ratios and CP-violating asymmetries for the four B→ f0(980)π and B→f0(980)η(′) decays by employing the perturbative QCD (pQCD) factorization approach. The leading order pQCD predictions for branching ratios are, Br(B→f0(980)π)~2.5×10-6, Br(B0→f0(980)π0)~2.6×10-7, Br(B0→f0(980)η)~2.5×10-7 and Br(B0→f0(980)η′)~6.7×10-7, which are consistent with both the QCD factorization predictions and the experimental upper limits.

Nuclear and ion Physics
A pilot experiment for mass measurement at CSRe
TU Xiao-Lin, XU Hu-Shan, XIA Jia-Wen, WANG Meng, MAO Rui-Shi, YUAN You-Jin, HU Zheng-Guo, LIU Yong, ZHANG Hong-Bin, ZANG Yong-Dong, ZHAO Tie-Cheng, ZHANG Xue-Ying, FU Fen, YANG Jian-Cheng, MAO Li-Jun, XIAO Chen, XIAO Guo-Qing, ZHAO Hong-Wei, ZHAN Wen-Long
2009, 33(7): 516-520. doi: 10.1088/1674-1137/33/7/003
A pilot experiment of mass measurement was performed at CSRe with the method of isochronous mass spectrometry. The secondary fragments produced via RIBLL2 with the primary beam of 400 MeV/u 36Ar delivered by CSRm were injected into CSRe. The revolution periods of the stored ions, which depend on the mass-to-charge ratios of the stored ions, were measured with a time-of-flight detector system. The results show that the mass resolution around 8×10-6 for Δm/m is achieved.
Multiplicities of forward-backward particles in 16O-emulsion interactions at 4.5 AGeV/c
LI Hui-Ling, ZHANG Dong-Hai, LI Jun-Sheng
2009, 33(7): 521-527. doi: 10.1088/1674-1137/33/7/004

An exclusive study of the characteristics of interactions accompanied by backward emission (θ lab≥90°) of shower and grey particles in collisions of a 4.5 AGeV/c 16O beam with emulsion nuclei is carried out. The experimental multiplicity distributions of different particles emitted in the forward (θlab<90°) and backward hemispheres due to the interactions with the two emulsion components (CNO, AgBr) are presented and analyzed. The correlations between the different emitted particles are also investigated. The results indicate that there are signatures for a collective mechanism, which plays a role in the production of particles in the backward hemisphere. Hence, the backward multiplicity distribution of the emitted shower and grey particles at 4.5 AGeV/c incident momentum can be represented by a decay exponential law formula independent of the projectile size. The exponent of the power was found to increase with decreasing target size. The experimental data favor the idea that the backward particles were emitted due to the decay of the system in the latter stages of the reactions.

Surveying the role of excitation energy in probing nuclear dissipation
YE Wei
2009, 33(7): 528-531. doi: 10.1088/1674-1137/33/7/005

A dynamical Langevin model is employed to calculate the excess of the evaporation residue cross sections of the 194Pb nucleus over that
predicted by the standard statistical model as a function of nuclear dissipation strength. It is shown that large excitation energy can increase the effects of nuclear dissipation on the excess of the evaporation residues and the sensitivity of this excess to the dissipation strength, and that more higher excitation energies have little contribution to further raising this sensitivity. These results suggest that on the experimental side, producing those compound systems with moderate excitation energy is sufficient for a good determination of the pre-saddle nuclear dissipation strength by measuring the evaporation residue cross
section, and that forming an extremely highly excited system does not considerably improve the sensitivity of evaporation residues to the dissipation strength.

Elastic scattering of 6He from 12C at 38.3 MeV/nucleon
DONG Hong-Fei, MA Yin-Qun, MA Zhong-Yu
2009, 33(7): 532-537. doi: 10.1088/1674-1137/33/7/006

The microscopic optical potential of nucleus-nucleus interaction is presented via a folding method with the isospin dependent complex nucleon-nuclear potential, which is first calculated in the framework of the Dirac-Bruecker-Hartree-Fock approach. The elastic scattering data of 6He at 229.8 MeV on 12C target are analyzed within the standard optical model. To take account of the breakup effect of 6He in the reaction an enhancing factor 3 on the imaginary potential is introduced. The calculated 6He+12C elastic scattering differential cross section is in good agreement with the experimental data. Comparisons with results in the double-folded model based on the M3Y nucleon-nucleon effective interaction and the few the body Glauber-model calculations are discussed. Our parameter free model should be of value in the description of nucleus-nucleus scattering, especially unstable nucleus-nucleus systems.

Low-lying states and isospin excitation in the Ge isotopes
Falih H. Al-Khudair
2009, 33(7): 538-546. doi: 10.1088/1674-1137/33/7/007

The level structure of 64-70Ge isotopes has been studied within the framework of the interacting boson model-3 (IBM-3). The symmetry character in the proton and neutron degrees of freedom of the energy levels has been investigated. The isospin excitation states (T>Tz) have been assigned for the 64Ge (N=Z) nucleus. Some intruder states in these nuclei have been suggested. The calculated energy levels and transition probabilities are in good agreement with recent experimental data. The study indicates that the Ge isotopes are in transition from γ-unstable to vibrational.

Decomposition of the equation of state of asymmetric nuclear matter into different spin-isospin channels
ZUO Wei, ZHOU Shan-Gui, LI Jun-Qing, ZHAO En-Guang, Scheid W
2009, 33(7): 547-551. doi: 10.1088/1674-1137/33/7/008

We investigate the equation of state of asymmetric nuclear matter and its isospin dependence in various spin-isospin ST channels within the framework of the Brueckner-Hartree-Fock approach extended to include a microscopic three-body force (TBF). It is shown that the potential energy per nucleon in the isospin-singlet T=0 channel is mainly determined by the contribution from the tensor SD coupled channel. At high densities, the TBF effect on the isospin-triplet T=1 channel contribution turns out to be much larger than that on the T =0 channel contribution. At low densities around and below the normal nuclear matter density, the isospin dependence is found to come essentially from the isospin-singlet SD channel and the isospin-triplet T=1 component is almost independent of isospin asymmetry. As the density increases, the T=1 channel contribution becomes sensitive to the isospin asymmetry and at high enough densities its isospin dependence may even become more pronounced than that of the T=0 contribution. The present results may provide some microscopic constraints for improving effective nucleon-nucleon interactions in a nuclear medium and for constructing new functionals of effective nucleon-nucleon interaction based on microscopic many-body theories.

Critical behavior of a dynamical percolation model
YU Mei-Ling, XU Ming-Mei, LIU Zheng-You, LIU Lian-Shou
2009, 33(7): 552-556. doi: 10.1088/1674-1137/33/7/009

The critical behavior of the dynamical percolation model, which realizes the molecular-aggregation conception and describes the crossover between the hadronic phase and the partonic phase, is studied in detail. The critical percolation distance for this model is obtained by using the probability P of the appearance of an infinite cluster. Utilizing the finite-size scaling method the critical exponents γ/ν and τ are extracted from the distribution of the average cluster size and cluster number density. The influences of two model related factors, i.e. the maximum bond number and the definition of the infinite cluster, on the critical behavior are found to be small.

Detection Technology and Methods
Studies of a scintillator-bar detector for a neutron wall at an external target facility
YU Yu-Hong, XU Hua-Gen, XU Hu-Shan, ZHAN Wen-Long, SUN Zhi-Yu, GUO Zhong-Yan, HU Zheng-Guo, WANG Jian-Song, CHEN Jun-Ling, ZHENG Chuan
2009, 33(7): 557-561. doi: 10.1088/1674-1137/33/7/010

To achieve a better time resolution of a scintillator-bar detector for a neutron wall at the external target facility of HIRFL-CSR, we have carried out a detailed study of the photomultiplier, the wrapping material and the coupling media. The timing properties of a scintillator-bar detector have been studied in detail with cosmic rays using a high and low level signal coincidence. A time resolution of 80~ps has been achieved in the center of the scintillator-bar detector.

Study of BESⅢ MUC offline software with cosmic-ray data
LIANG Yu-Tie, MAO Ya-Jun, YOU Zheng-Yun
2009, 33(7): 562-566. doi: 10.1088/1674-1137/33/7/011
Cosmic-ray data of 90 M events have been collected and used for calibration, alignment as well as detector tuning. A special tracking algorithm for the BESⅢ muon counter is developed and verified with Monte-Carlo simulation and then further confirmed with the cosmic-ray data. The obtained strip resolutions are in good agreement with the design values. A new alignment approach for the BESⅢ muon counter is confirmed with the cosmic-ray data and proposed to be used in future analysis of experimental data.
Circulation model for water circulation and purification in a water Cerenkov detector
LU Hao-Qi, YANG Chang-Gen, WANG Ling-Yu, XU Ji-Lei, WANG Rui-Guang, WANG Zhi-Min, WANG Yi-Fang
2009, 33(7): 567-571. doi: 10.1088/1674-1137/33/7/012

Owing to its low cost and good transparency, highly purified water is widely used as a medium in large water Cerenkov detector experiments. The water circulation and purification system is usually needed to keep the water in good quality. In this work, a practical circulation model is built to describe the variation of the water resistivity in the circulation process and compared with the data obtained from a prototype experiment. The successful test of the model makes it useful in the future design and optimization of the circulation/purification system.

Bunch transverse emittance increase in electron storage rings
2009, 33(7): 572-576. doi: 10.1088/1674-1137/33/7/013

In this paper a theoretical framework to estimate the bunch transverse emittance growing in electron storage rings due to short range transverse wakefield of the machine is established. New equilibrium emittance equations are derived and applied to explain the experimentally obtained results in ATF damping ring. This equation will be useful for linear collider damping ring

Tuning of RF amplitude and phase for the drift tube linac in J-PARC
SHEN Guo-Bao, Masanori Ikegami
2009, 33(7): 577-582. doi: 10.1088/1674-1137/33/7/014

The J-PARC linac has three DTL tanks to accelerate the negative hydrogen ions
from 3 MeV to 50 MeV. The RF phase and amplitude are adjusted for each
cavity with a phase scan method within the accuracy of 1° in phase and
1% in amplitude. The experimental results show a remarkable agreement
with the numerical model within a sufficient margin in the tuning of the last two DTL tanks. However, a notable discrepancy between the experiment and the numerical model is seen in the tuning of the first DTL tank. After studying with a three-dimensional multi-particle simulation, the generation of the low energy component and the pronounced filamentation are identified as the main causes of the discrepancy. The optimization of the tuning scheme is also discussed to attain the tuning goal accuracy for the first DTL tank.

Study of the design of CSNS MEBT
OUYANG Hua-Fu, LIU Hua-Chang, FU Shi-Nian
2009, 33(7): 583-589. doi: 10.1088/1674-1137/33/7/015

The design of CSNS MEBT has two objectives: (1) to match the beam both in the transversal direction and the longitudinal direction from RFQ into DTL; (2) to further chop the beam into the required time structure asked by RCS. It is very difficult and critical to control well the emittance growth and in the meantime to match and chop the beam. Firstly, the optical design is done and optimized, and the multi-particle simulations show that the maximum emittance growth is successfully controlled within 14%. Secondly, based on the different beam envelopes obtained by TRACE-3D and PARMELA, the least deflecting angle of the chopper is determined by TRACE-3D. At last, the field of steering magnet is determined through simulations.

Preliminary application of turn-by-turn data analysis to the SSRF storage ring
CHEN Jian-Hui, ZHAO Zhen-Tang
2009, 33(7): 590-593. doi: 10.1088/1674-1137/33/7/016

There is growing interest in utilizing the beam position monitor turn-by-turn (TBT) data to debug accelerators. TBT data can be used to determine the linear optics, coupled optics and nonlinear behaviors of the storage ring lattice. This is not only a useful complement to other methods of determining the linear optics such as LOCO, but also provides a possibility to uncover more hidden phenomena. In this paper, a preliminary application of a β function measurement to the SSRF storage ring is presented.

FPGA-based amplitude and phase detection in DLLRF
LIU Rong, WANG Zheng, PAN Wei-Min, WANG Guang-Wei, LIN Hai-Ying, SHA Peng, ZENG Ri-Hua
2009, 33(7): 594-598. doi: 10.1088/1674-1137/33/7/017

The new generation particle accelerator requires a highly stable radio frequency (RF) system. The stability of the RF system is realized by the Low Level RF (LLRF) subsystem which controls the amplitude and phase of the RF signal. The detection of the RF signal's amplitude and phase is fundamental to LLRF controls. High-speed ADC (Analog to Digital Converter), DAC (Digital to Analog Converter) and FPGA (Field Programmable Gate Array) play very important roles in digital LLRF control systems. This paper describes the implementation of real-time amplitude and phase detection based of the FPGA with an analysis of the main factors that affect the detection accuracy such as jitter, algorithm's defects and non-linearity of devices, which is helpful for future work on high precision detection and control.

Synchrotron radiation,applications of nuclear techniquees,etc
X-ray beam hardening correction for measuring density in linear acceleratorindustrial computed tomography
ZHOU Ri-Feng, WANG Jue, CHEN Wei-Min
2009, 33(7): 599-602. doi: 10.1088/1674-1137/33/7/018

Due to X-ray attenuation being approximately proportional to material density,it is possible to measure the inner density through Industrial Computed Tomography (ICT) images accurately. In practice, however, a number of factors including the non-linear effects of beam hardening and diffuse scattered radiation complicate the quantitative measurement of density variations in materials. This paper is based on the linearization method of beam hardening correction, and uses polynomial fitting coefficient which is obtained by the curvature of iron polychromatic beam data to fit other materials. Through theoretical deduction, the paper proves that the density measure error is less than 2% if using pre-filters to make the spectrum of linear accelerator range mainly 0.3 MeV to 3 MeV. Experiment had been set up at an ICT system with a 9 MeV electron linear accelerator. The result is satisfactory. This technique makes the beam hardening correction easy and simple, and it is valuable for
measuring the ICT density and making use of the CT images to recognize