2012 Vol. 36, No. 4
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The Daya Bay collaboration has recently reported its first ve→ve oscillation result which points to θ13 8.8°±0.8° (best-fit ±1σ range) or θ13 ≠ 0° at the 5:2σ level. The fact that this smallest neutrino mixing angle is not strongly suppressed motivates us to look into the underlying structure of lepton avor mixing and CP violation. Two phenomenological strategies are outlined: (1) the lepton avor mixing matrix U consists of a constant leading term U0 and a small perturbation term ΔU; and (2) the mixing angles of U are associated with the lepton mass ratios. Some typical patterns of U0 are reexamined by constraining their respective perturbations with current experimental data. We illustrate a few possible ways to minimally correct U0 in order to fit the observed values of three mixing angles. We point out that the structure of U may exhibit an approximate μ-τ permutation symmetry in modulus, and reiterate the geometrical description of CP violation in terms of the leptonic unitarity triangles. The salient features of nine distinct parametrizations of U are summarized, and its Wolfenstein-like expansion is presented by taking U0 to be the democratic mixing pattern.
A new general parameterization with eight mixing parameters among Z, γ and an extra neutral gauge boson Z’ is proposed and subjected to phenomenological analysis. We show that in addition to the conventional Weinberg angle θW, there are seven other phenomenological parameters, G’,ξ,η,θl,θr, r and l, for the most general Z-γ-Z’ mixings, in which parameter G’ arises due to the presence of an extra Stueckelberg- type mass coupling. Combined with the conventional Z-Z’ mass mixing angle θ’, the remaining six parameters, ξ,η,θl-θ’,θr-θ’, r and l, are caused by general kinetic mixing. In all eight phenomenological parameters, θW, G’,ξ,η,θl,θr, r and l, we can determine the Z-Z’ mass mixing angle θ’ and the mass ratio MZ=MZ’ . The Z-γ-Z’ mixing that we discuss are based on the model-independent description of the extended electroweak chiral Lagrangian (EWCL) previously proposed by us. In addition, we show that there are eight corresponding independent theoretical coe cients in our EWCL, which are fully xed by our eight phenomenological mixing parameters. We further nd that the experimental measurability of these eight parameters does not rely on the extended neutral current for Z’, but depends on the Z-Z’ mass ratio.
We study the dynamics of entanglement and teleportation in Bell-diagonal states. Using the concepts of concurrence and fidelity, the analytical expressions of the entanglement, the output entanglement and the average fidelity with decoherence are obtained for this model. We discover a class of initial states in which the output entanglement and the average fidelity are destroyed by decoherence. The quality of teleportation depends on the system parameters and time.
We study the entanglement property in matrix product spin-ring systems systemically by von Neumann entropy. We nd that: (i) the Hilbert space dimension of one spin determines the upper limit of the maximal value of the entanglement entropy of one spin, while for multiparticle entanglement entropy, the upper limit of the maximal value depends on the dimension of the representation matrices. Based on the theory, we can realize the maximum of the entanglement entropy of any spin block by choosing the appropriate control parameter values. (ii) When the entanglement entropy of one spin takes its maximal value, the entanglement entropy of an asymptotically large spin block, i.e. the renormalization group fixed point, is not likely to take its maximal value, and so only the entanglement entropy Sn of a spin block that varies with size n can fully characterize the spin-ring entanglement feature. Finally, we give the entanglement dynamics, i.e. the Hamiltonian of the matrix product system.
Decon nement phase transition is studied in the FL model at finite temperature and chemical potential. At MFT approximation, phase transition can only be first order in the whole μ-T phase plane. Using a Landau expansion, we further study the phase transition order and the possible phase diagram of decon nement. We discuss the possibilities of second order phase transitions in the FL model. From our analysis, if the cubic term in the Landau expansion could be cancelled by the higher order fluctuations, second order phase transition may occur. By an ansatz of the Landau parameters, we obtain a possible phase diagram with both the first and second order phase transitions, including the tri-critical point which is similar to that of the chiral phase transition.
The prompt fission neutron spectra for the neutron-induced fission of 235U at En < 5 MeV are calculated using nuclear evaporation theory with a semi-empirical model, in which the nonconstant and constant temperatures related to the Fermi gas model are taken into account. The calculated prompt fission neutron spectra reproduce the experimental data well. For the n(thermal)+235U reaction, the average nuclear temperature of the fission fragment, and the probability distribution of the nuclear temperature, are discussed and compared with the Los Alamos model. The energy carried away by γ rays emitted from each fragment is also obtained and the results are in good agreement with the existing experimental data.
Based on the first ionization chamber (IC) prototype, the structure, working gas component and electrode material of the IC are improved. The test of the improved IC shows that the plateau length is about 2000 V, the plateau slope is less than 0.2%/100 V, the sensitivity is 19.6 pA/rad·h-1, the up-limitation of the linearity can be up to 3.6×105 rad/h, and the applied voltage can be operated to 3500 V. The test results show that the performance of the improved IC meets the requirements of the beam loss monitor.
The calibration detector of a gamma ray monitor (GRM) is designed to detect alpha particles from 241Am and to send out the coincidence signal to the GRM X-ray detector. The silicon photomultiplier (SiPM), as a novel photon device, is a good candidate to convert alpha-exciting fluorescent photons into electric signals. Three types of SiPMs from SSPM and MPPC, each having an active area of 3 mm×3 mm, were compared in the matter of the spectra from low-intensity light, dark count, crosstalk probability and I-V curve. The temperature coe cient of SSPM-0710G9MM was also characterized. The application of a SiPM on the GRM has been proved to be feasible.
In a TPC, ion feedback from the readout detector can cause a space-charge effect and distort the electrical field in the drift region. Gating is one of the effective methods to solve this problem, which can block ions at the expense of losing a certain amount of primary electrons. Compared with the traditional design with a wire structure, gating based on GEM foil is more attractive because of its simplicity. In this paper, the factors in uencing the electron transmission e ciency are studied with simulations and experiments. After optimizing all these parameters, an electron transmission e ciency over 80% is obtained.
This work is concerned with ion beam dynamics and compares the emittance to aberration ratios of two-and three-electrode extraction systems. The study is conducted with the aid of Version 7 of SIMION 3D ray-tracing software. The beam dependence on various parameters of the extraction systems is studied and the numerical results lead to qualitative conclusions.
Ion beam characteristics using diode and triode extraction systems are investigated with the aid of the computer code SIMION 3 D, Version 7.0. The diode (two electrode extraction system) and triode (three-electrode extraction, acceleration-deceleration system) extraction systems are designed and optimized with di erent geometric parameters of the electrode system, voltage applied to the extraction electrode, and plasma parameters inside the ion source chamber, as well as by the ion beam space charge. This work attempts to describe the importance of the acceleration-deceleration extraction system. It shows that besides an increase of the beam energy, the ion beam has lower emittance than the two-electrode extraction system. Ion beams of the highest quality are extracted whenever the half-angular divergence is minimum for which the perveance current intensity and the extraction gap have optimum value. Knowing the electron temperature of the plasma is necessary to determine plasma potential and the exact beam energy.
Two types of dielectric wall accelerator (DWA) structures, a bi-polar Blumlein line and zero integral pulse line (ZIP) structures were investigated. The high gradient insulator simulated by the particle in cell code con rms that it has little in uence on the axial electric field. The results of simulations using CST microwave studio indicate how the axial electric field is formed, and the electric eld waveforms agree with the theoretical one very well. The in uence of layer-to-layer coupling in a ZIP structure is much smaller and the electric eld waveform is much better. The axial of the Blumlein structure's electric field has better axial stability. From both of the above, it found that for a shorter pulse width, the axial electric field is much higher and the pulse stability and delity are much better. The CST simulation is very helpful for designing DWA structures.
A flexible high power RF test stand has been designed and constructed at IHEP to test a variety of 500 MHz superconducting RF components for the upgrade project of the Beijing Electron Positron Collider (BEPCⅡ), such as the input coupler, the higher order modes (HOMs) absorber and so on. A high power input coupler has been conditioned and tested with the RF power up to 250 kW in continuous wave (CW), traveling wave (TW) mode and 150 kW CW in standing wave (SW) mode. A prototype of the HOMs absorber has been tested to absorb power of 4.4 kW. An introduction of the test stand design, construction and high power tests is presented in this paper.
Since the tapered quarter wave resonator (QWR) cavity is proven to have a much lower peak surface magnetic field in the short plate and a lower peak surface electric field near the beam tube compared with the straight outer conductor QWR, it has been recommended for the separated sector cyclotron linac injector system in the heavy ion research facility in Lanzhou. This paper is focused on the multipacting (MP) analysis for the tapered QWR with a frequency of 80.5 MHz and beta of 0.085. Using the Analyst program, MP bands can be simulated and analyzed with the Particle Tracking module to identify potential problems in the cavity design. This paper will present the simulation results of MP for the tapered QWR cavity.
Accelerator based positron sources are widely used due to their high intensity. Most of these accelerators are RF accelerators. An LIA (linear induction accelerator) is a kind of high current pulsed accelerator used for radiography. A conceptual design of an intense pulsed positron source based on an LIA is presented in the paper. One advantage of an LIA is its pulsed power being higher than conventional accelerators, which means a higher amount of primary electrons for positron generations per pulse. Another advantage of an LIA is that it is very suitable to decelerate the positron bunch generated by bremsstrahlung pair process due to its ability to adjustably shape the voltage pulse. By implementing LIA cavities to decelerate the positron bunch before it is moderated, the positron yield could be greatly increased. These features may make the LIA based positron source become a high intensity pulsed positron source.
A Penning surface H- ion source test stand has been developed for the China Spallation Neutron Source (CSNS) at the Institute of High Energy Physics (IHEP). H- beams with a current up to 50 mA and a pulse length up to 520μs at a repetition rate of 25 Hz are obtained at present. In order to improve the extraction system based on both the results of the emittance measurements in experiment and the simulation results of the effect of the extraction system on the beam emittance, three dimensional electromagnetic finite element analysis and particle tracking are undertaken. First, the magnetic field of the deflecting magnet is studied in detail, and then the effect of the extraction geometry on the beam transport is also investigated.
The design and optimization procedure of a pulse compressor is presented. A C-band (5712 MHz) pulse compressor using a TE0;1;15 mode cylindrical cavity with dual side-wall coupling irises has been designed. Also the coupling coeffcient, position of the short plane and size of the bottom groove have been optimized by using HFSS.
A LaBr3:Ce scintillator has a high light output (~60000 p.e/MeV) and a short decay constant (<25 ns), which makes it good for time spectrometry. Compared with a BaF2 scintillator, it can bear a much higher count rate, and can be coupled to photomultipliers without using a quartz window. In this work, a positron annihilation lifetime spectrometer (PALS) consisting of two bulks of φ25 mm×25 mm LaBr3:Ce scintillator coupled to two XP20D0 photomultipliers, respectively, was built. A time resolution of FWHM=206 ps was measured for the PALS with a 60Co source at the energy window for 22Na. With this spectrometer, a reasonable lifetime value τ=221±4 ps in a pure Si sample is obtained, which means that the utilization of LaBr3:Ce as the detector for a PALS is feasible.
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