2013 Vol. 37, No. 1
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We report an improved measurement of the neutrino mixing angle θ13 from the Daya Bay Reactor Neutrino Experiment. We exclude a zero value for sin22θ13 with a significance of 7.7 standard deviations. Electron antineutrinos from six reactors of 2.9 GWm th were detected in six antineutrino detectors deployed in two near (flux-weighted baselines of 470 m and 576 m) and one far (1648 m) underground experimental halls. Using 139 days of data, 28909 (205308) electron antineutrino candidates were detected at the far hall (near halls). The ratio of the observed to the expected number of antineutrinos assuming no oscillations at the far hall is 0.944± 0.007(stat.) ± 0.003(syst.). An analysis of the relative rates in six detectors finds sin22θ13=0.089± 0.010(stat.)±0.005(syst.) in a three-neutrino framework.
The singlet-triplet mixing of 1P1-3P1 is studied via calculating the branching ratios of semileptonic decay Bs→Ds1lν and B→D1lν by means of the instantaneous Bethe-Salpeter method. Special attention is paid to the relativistic corrections, since they are large for the P-wave states. Using the Mandelstam Formalism, we compute the transition form factors not only in the high-energy lepton end-point region but also in the full Q2 region. And the non-perturbative QCD effects are taken care of in the overlapping integral over the relativistic wave functions of the initial and nial states.
We consider the annihilation process of an electron-positron pair into a pair of heavier fermions when the initial electron and position beams are polarized. By calculating the polarization of the final-state particles, we discuss in detail the effect due to the produced particle masses in the τ-charm energy region at BEPC/BES, and also compare the effect with that at the B-factory. Such a study is useful for the design of possible polarization investigation at the BEPC/BES facility and the B-factory.
We calculate the branching ratios of pure annihilation type decays B0→Ds- K2*+ and Bs→Da2 using the perturbative QCD approach based on kT factorization. The branching ratios are predicted to be (60.6-16.5-10.4-2.1+17.3+4.3+3.2 )× 10-6 for B0→Ds- K2*+, (1.1-0.4-0.2-0.1+0.4+0.1+0.1)×10-6 for Bs→D0a20 and (2.3-0.8-0.4-0.1+0.8+0.2+0.1)×10-6 for Bs→D-a2+. They are large enough to be measured in the ongoing experiment. Due to the shortage of contributions from penguin operators, there are no direct CP asymmetries for these decays in the Standard Model. We also derive simple relations among these decay channels to reduce theoretical uncertainties for the experiments to test the accuracy of theory and search of new physics signal.
Two-quasiparticle bands and low-lying excited high-K four-, six-, and eight-quasiparticle bands in the doubly-odd 174, 176Lu are analyzed by using the cranked shell model (CSM) with the pairing correlations treated by a particle-number conserving (PNC) method, in which the blocking effects are taken into account exactly. The proton and neutron Nilsson level schemes for 174, 176Lu are taken from the adjacent odd-A Lu and Hf isotopes, which are adopted to reproduce the experimental bandhead energies of the one-quasiproton and one-quasineutron bands of these odd-A Lu and Hf nuclei, respectively. Once the quasiparticle configurations are determined, the experimental bandhead energies and the moments of inertia of these two- and multi-quasiparticle bands are well reproduced by PNC-CSM calculations. The Coriolis mixing of the low-K (K=|Ω1-Ω2|) two-quasiparticle band of the Gallagher-Moszkowski doublet with one nucleon in the Ω = 1/2 orbital is analyzed.
We propose a five-parameter dumbbell model to describe the fusion and fission processes of massive nuclei, where the collective variables are: distance ρ between center-of-mass of two fusing nuclei, neck parameter ν, asymmetry D, two deformation variables β1 and β2. The present model has macroscopic qualitative expression of polarization and nuclear collision of head to head, sphere to sphere, waist to waist and so on. The conception of "projectile eating target" based on open mouth and swallow is proposed to describe nuclear fusion process, and then our understanding of the probability of fusion and quasi-fission is in agreement with some previous work. The calculated fission barriers of a lot of compound nuclei are compared with the experimental data.
Electric and magnetic screenings of the thermal gluons are studied by using the background expansion method in a gluodynamic model with gauge invariant dimension-2 gluon condensate at zero momentum. At low temperature, the electric and magnetic gluons are degenerate. With the increase of temperature, it is found that the electric and magnetic gluons start to split at certain temperature T0. The electric screening mass changes rapidly with temperature when T > T0, and the Polyakov loop expectation value rises sharply around T0 from zero in the vacuum to a value around 0.8 at high temperature. This suggests that the color electric deconfinement phase transition is driven by electric gluons. It is also observed that the magnetic screening mass keeps almost the same as its vacuum value, which manifests that the magnetic gluons remain confined. Both the screening masses and the Polyakov loop results are qualitatively in agreement with the Lattice calculations.
In this paper, elliptic flow is studied at fixed centrality in Au+Au collision at √sNN=200 GeV in the AMPT model. It is observed that with the participant increasing, elliptic flow has an increase or a decrease at different fixed impact parameter, but it does not have a trivial fluctuation. It is analyzed that the initial space anisotropy dominates the participant dependence of elliptic flow in near-central collisions(b=5 fm) and mid-central collisions(b=8 fm), while the interaction between particles can mainly answer for the behavior of elliptic flow with participant in peripheral collisions (b=12 fm). To distinguish the pure geometrical effect, elliptic flow scaled by initial eccentricity is studied. It is found that the ratio v2/ε increases with participant and reaches a saturation when the participant is large enough, indicating that the collision system may reach the local equilibrium.
Differential and integrated directed flow and elliptic flow of light charged particles (z ≤ 2) are studied systematically for semi-central (b = 5 fm) 197Au+197Au collisions at incident energies from 25 to 250 MeV/nucleon by the isospin-dependent quantum molecular dynamics model. The changes of directed and elliptic flow with incident energy reflect the dynamic competition between mean field and nucleon-nucleon collisions and also between collective rotation and expansion.
The results of Monte-Carlo simulations of extensive air shower are presented to show the difference of hadronic component content at various altitudes with the aim to choose an optimal altitude for the PRISMA-like experiment. CORSIKA program for EAS simulations with QGSJET and GHEISHA models was used to calculate the number of hadrons reaching the observational level inside a circle of 50 m radius around the EAS axis. Then the number of neutrons produced by the hadronic component was calculated using an empirical relationship between the two components. We have tested the results with the ProtoPRISMA array at sea level, and recorded the neutrons which are consistent with the simulation results.
Si wafers with a 220 nm top oxide layer were sequentially implanted at room temperature with 40 keV He and 35 keV H ions at fluence of 5× 1016/cm2 and 1× 1016/cm2, respectively. Techniques of scanning electron microscopy, atomic force microscopy and cross-sectional transmission electron microscopy (XTEM) were used to characterize the thermal evolution of surface damage as well as defect microstructures. Surface blisters as well as the localized exfoliation (～ 0.42 μm in depth) have been observed for samples annealed at temperatures of 500 ℃ and above. XTEM observations reveal a variety of defect microstructures, including cavities, platelets, nanometer or micrometer sized cracks and dislocations. The platelets and cracks are mainly distributed at the depth of about 0.42 μm parallel to the sample surface, which are responsible for the occurrence of the observed surface features. The relations between surface damage and defect microstructures are described in detail.
Three large sized glass resistive plate chambers (RPCs) are built and applied to measure the spatial resolution of the detector. The readout strips are collected to a LC delay-line and the time difference is used to determine the position. Cosmic rays are triggered by a set of two scintillation counters and the coincidently measured positions from the three RPCs are used to deduce the position uncertainty. In average a spatial resolution of 0.90 mm (FWHM) is obtained for a single RPC, with a good uniformity across the detection area. This result suggests that large sized glass RPC operating in the avalanche mode is a promising candidate for muon tomography detection system.
In order to study the possibility of improving the timing performance of the time of flight (TOF) systems, which are made of plastic scintillator counters, and read out by photomultiplier tubes (PMT) with mesh dynodes and conventional electronics, we have conducted a study using faster PMTs and ultra fast waveform digitizers to read out the plastic scintillators. Different waveform analysis methods are used to calculate the time resolution of such a system. Results are compared with the conventional discriminating method based on a threshold and pulse height. Our tests and analysis show that significant timing performance improvements can be achieved by using this new system.
The front-end readout electronics has been developed for silicon strip detectors at our institute. In this system an Application Specific Integrated Circuit (ASIC) ATHED is used to realize multi-channel energy and time measurement. The slow control of ASIC chips is achieved by parallel port and the timing control signals of ASIC chips are implemented with the CPLD. The data acquisition is carried out with a PXI-DAQ card. The software has a user-friendly GUI developed with LabWindows/CVI in Windows XP operating system. The test results show that the energy resolution is about 1.14% for alpha at 5.48 MeV and the maximum channel crosstalk of system is 4.60%. The performance of the system is very reliable and suitable for nuclear physics experiments.
A digital transmission system with 900 MHz bandwidth and over 40 dB linear dynamic range for high intensity pulsed radial detection is designed. The transient signals (with subnanoseconds rising edge) from the detectors could be transmitted to the diagnosing systems kilometers away with high fidelity. The interference and the hostile detecting environment could be adapted with the intelligent functions of this system. With the test for system evaluation, the feasibility and reliability of this system has finally been verified.
In reactor neutrino experiments, the analysis of time correlations between different physical events is an important task. Such analysis can help to understand the physical mechanisms of the signal and background events as well as the details of event selection and background estimation. This study investigates a "sampling and mixing" method used for producing large MC data samples for the Daya Bay reactor neutrino experiment. We designed a simple, generic mixing algorithm and generated large MC data samples for physics analysis from several samples according to their respective event rates. Basic plots based on the mixed data are shown.
The upgrade project of the Hefei Light Source (HLS), named HLS-Ⅱ, is under way, whose storage ring will be reconstructed. The HLS-Ⅱ storage ring has lower emittance and more straight sections available for insertion devices compared with the present HLS storage ring. The scan method is applied to the linear lattice optimization for the HLS-Ⅱ storage ring to get thorough information about the lattice. To reduce the computation amount, several scans with different grid spacing values are conducted. And, the calculation of the chromatic sextupole strength for the achromatic mode is included in the scan, which is useful for nonlinear lattice optimization. To better analyze the obtained solutions in the scan, the lattice properties and the variables of quadrupole strengths are statistically analyzed. And, the process of selecting solutions is described in detail, including the choice of the working point, the settings for the emittance and optical functions, and the restriction of maximum magnet strength. Two obtained lattices, one for the achromatic mode and the other for the non-achromatic mode, are presented, including their optical functions and optimized dynamic apertures.
CBPM is a kind of monitor which is used for the measurement of beam transverse position. And it is more and more popular for its high potential in the resolution performance. In theory, the resolution can reach about 1 nanometer. In this paper, a rectangular CBPM is designed for it has better X-Y isolation than a cylinder one. It has been simulated and measured, and the results agree with each other very well. The procedures and results for the simulation and the cold test will be shown later and it will be proved that this is a reliable method for the CBPM design.
The tuning method of uniform traveling-wave structures based on the non-resonant perturbation field distribution measurement has been widely used in tuning both constant-impedance and constant-gradient structures. In this paper, the method of tuning nonuniform structures is proposed on the basis of the above theory. The internal reflection coefficient of each cell is obtained from analyzing the normalized voltage distribution. A numerical simulation of tuning process according to the coupled cavity chain theory has been done and the result shows each cell is in right phase advance after tuning. The method will be used in the tuning of a disk-loaded traveling-wave structure which is being developed at the Accelerator Laboratory, Tsinghua University.
The storage ring equipped with an electron cooler is an ideal platform for dielectronic recombination (DR) experiments. In order to fulfill the requirement of DR measurements at the main Cooler Storage Ring, a detuning system for the precision control of the relative energy between the ion beam and the electron beam has been installed on the electron cooler device. The test run using 7.0 MeV/u C6+ beam was performed with recording the Schottky spectra and the ion beam currents. The influence of pulse heights and widths of the detuning voltage on the ion beam was analyzed. For the small pulse height, the experimental results from the Schottky spectra were in good agreement with the theoretical results. The frequency shift in the Schottky spectra was significantly reduced for the short pulse width. For the large pulse height, an oscillation phenomenon was observed and some effective ways to reduce the oscillation were pointed out. The detailed description of the phenomenon and the theoretical model based on the plasma oscillation was discussed in this paper. The overall results show that the new detuning system works properly, and could fulfill the requirements of future DR experiment.
The Accelerator Driven Sub-critical System (ADS) is under development aiming at safe disposal of nuclear waste and providing electric power in China. The main accelerator of ADS is composed of two injector sections and one main linear acceleration section. The 650 MHz β=0.82 superconducting cavities will be adopted to accelerate the proton bunches from 360 MeV to 1.5 GeV in the medium energy section. This paper presents the study and design results of this kind of superconducting cavity.
Three ultra-short-period W/B4C multilayers (1.244 nm, 1.235 nm and 1.034 nm) have been fabricated and used for polarization measurement at the 4B7B Beamline of Beijing Synchrotron Radiation Facility (BSRF). By rotating analyzer ellipsometry method, the linear polarization degree of light emerging from this beamline has been measured and the circular polarization evaluated for 700-860 eV. The first soft X-ray magnetic circular dichroism measurements are carried out at BSRF by positioning the beamline aperture out of the plane of the electron storage ring.
The first step of phasing in any de novo protein structure determination using isomorphous replacement (IR) or anomalous scattering (AD) experiment is to find heavy-atom positions. Traditionally, heavy-atom positions can be solved by inspecting the difference Patterson maps. Due to the weak signals in isomorphous or anomalous difference and the noisy background in the Patterson map, the searching of heavy atoms may become difficult. Here, the direct demodulation (DD) method is applied to the difference Patterson maps to reduce the noisy backgrounds and sharpen the signal peaks. The real space Patterson search by using these optimized maps can locate the heavy-atom positions more correctly and accurately. It is anticipated that the direct demodulation method can play an assistant role in the heavy-atom positions determination and facilitate the de novo structure determination of proteins.
Accurate attenuation correction is required in dedicated breast PET imaging systems for image artifact removal and quantitative studies. In this study, a method using only emission data based on consistency conditions is proposed for attenuation correction in breast PET imaging systems. The consistency conditions are exploited to evaluate the accuracy of the estimated attenuation distribution and find the appropriate parameters that yield the most consistent attenuation distribution with the measured emission data. We have proved the validity of the method with the experimental investigations and the single-patient studies using a dedicated breast PET. The results show that the method is capable of accurately estimating the attenuation distribution of a uniform attenuator from the experimental data with various relatively low activities. The results also show that in single-patient studies, the method is robust for the irregular boundary of the breast tissue and provides a distinct improvement in image quality.
A semi-empirical detector response function (DRF) model of Si (PIN) detector is proposed to fit element Kα and Kβ X-ray spectra, which is based on statistical distribution analytic (SDA) method. The model for each single peak contains a step function, a Gaussian function and an exponential tail function. Parameters in the model are obtained by weighted nonlinear least-squares fitting method. In the application, six kinds of elements' characteristic X-ray spectra are obtained by Si (PIN) detector, and fitted out by the established DRF model. Reduced chi-square values are at the interval of 1.11-1.25. Other applications of the method are also discussed.
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