2014 Vol. 38, No. 11
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Compared with the charmed baryons, the bottom baryons are not very well known, either experimentally or theoretically. In this paper, we investigate the dipion strong decays of the P-wave and D-wave excited bottom baryons in the framework of the QPC model. We also extend the same analysis to the charmed baryons.
A sizable difference in top quark pair forward backward asymmetry (AFB) is observed at Tevatron. The discrepancy triggers many new physics beyond the standard model (SM) and then constrains the parameter spaces in them. In this article we calculate the AFB of the top-pair production at Tevatron up to next to leading order (NLO) in the little Higgs model (LHM). We find that the contribution of ZH can be large enough to make up the gap between SM prediction and experimental data. Then, the parameter space for the couplings between ZH and quarks are constrained. Thus, this model can fulfill the experimental data, both in AFB and in cross section.
In this comprehensive study the multiplicity characteristics of the backward emitted relativistic hadron (shower particle) through hadron-nucleus and nucleus-nucleus are overviewed in three dimensions. These dimensions are the projectile size, target size, and energy. To confirm the universality in this production system, wide ranges of system size and energy (Elab～ 2.1 A up to 200 A GeV) are used. The multiplicity characteristics of this hadron imply a limiting behavior with respect to the projectile size and energy. The target size is the main effective parameter in this production system. The exponential decay shapes is a characteristic feature of the backward shower particle multiplicity distributions. The decay constant changes with the target size to be nearly 2.02, 1.41, and 1.12 for the interactions with CNO, Em, and AgBr nuclei, respectively, irrespective of the projectile size and energy. While the backward production probability and average multiplicity are constants at different projectile sizes and energies, they can be correlated with the target size in power law relations.
We studied the temperature dependence of the light yield of linear alkyl benzene (LAB)-based and mesitylene-based liquid scintillators. The light yield increases by 23% for both liquid scintillators when the temperature is lowered from 26 ℃ to -40 ℃, correcting for the temperature response of the photomultiplier tube. The measurements help to understand the energy response of liquid scintillator detectors. Especially, the next generation reactor neutrino experiments for neutrino mass hierarchy, such as the Jiangmen Underground Neutrino Observatory (JUNO), require very high energy resolution. As no apparent degradation on the liquid scintillator transparency was observed, lowering the operation temperature of the detector to ～4 ℃ will increase the photoelectron yield of the detector by 13%, combining the light yield increase of the liquid scintillator and the quantum efficiency increase of the photomultiplier tubes.
We are currently investigating the spatial resolution of highly pixelated Cadmium Zinc Telluride (CZT) detector for imaging application. A 20 mm×20 mm×5 mm CZT substrate was fabricated with 600 μm pitch pixels (500 μm anode pixels with 100 μm gap) and coplanar cathode. Charge sharing between two pixels was studied using collimated a 122 keV gamma ray source. Experiments show a resolution of 125 μm FWHM for double-pixel charge sharing events when the 600 μm pixelated and 5 mm thick CZT detector biased at -1000 V. In addition, we analyzed the energy response of the 600um pitch pixelated CZT detector.
CsI film has been one of the most extensively used scintillators for indirect X-ray imaging because of its needle-like micro-structure. The purpose of this paper is to investigate the imaging performance of CsI screen as a function of thickness and radiation quality. Four multilayer scintillation screens with microcolumnar CsI:Tl film (thicknesses of 50 μm, 100 μm, 200 μm and 300 μm) included were prepared and coupled to an optical imaging sensor. The modulation transfer function (MTF), normalized noise power spectrum (NNPS) and detective quantum efficiency (DQE) of these screens were evaluated based on the standard IEC 62220-1, and the results indicated that, in the medium spatial frequency range (1-6 lp/mm), the MTF of CsI screens with the same thickness was lower when the incident X-ray photon energy was higher, possibly owing to scattering and K-fluorescence re-absorption effects. The NNPS in the higher spatial frequency range (above 8 lp/mm) is dominated by stochastic noise while the entrance surface air Kerma (ESAK) decreases. For 100 μm, 200 μm and 300 μm thick CsI screens, the DQE under RQA7 and RQA9 is lower than that under RQA3 and RQA5 due to low absorption efficiency.
The LHAASO project is to be built in south-west China, using an array of 5137 electron detectors for the measurement of incident electrons arriving at the detector plane. For quality control of the large number of electron detectors, a cosmic ray hodoscope with two-dimensional spatial sensitivity and good time resolution has been developed. The first prototype of the electron detector has been tested with the hodoscope and the performance of the detector is found to be consistent with the design.
α from natural radioactivity may interact with a nucleus and emit a neutron. The reaction introduces the background to the liquid scintillator (LS) based neutrino experiments. In the LS detector, α comes from 238U, 232Th, and 210Po decay chains. For Gadolinium-doped LS (Gd-LS) detector, α also comes from 227Ac. The nucleus 13C is a natural component of Carbon which is rich in the LS. The background rate and spectrum should be subtracted carefully from the neutrino candidates. This paper describes the calculation of neutron yield and spectrum with uncertainty estimated. The results are relevant for many existing neutrino experiments and future LS or Gd-LS based experiments.
The Monte Carlo method is used to simulate the beam optics of the WS beam line of RCNP, Osaka University in order to know the effect of collimators on the beam line to control the beam spot. According to the simulation, we do not need to use the collimator to cut the beam and the beam angular resolution can be better than 0.05° in achromatic mode. In the present paper, the actual beam condition during the beam adjustment is listed. The accelerator can provide a 12C beam in achromatic mode and the angular resolution σ=0.7775 mrad ±0.0030 mrad.
The emittance growth induced by Coherent Synchrotron Radiation (CSR) is an important issue when electron bunches with short bunch length and high peak current are transported in a bending magnet. In this paper, a single kick method is introduced that could give the same result as the R-matrix method, but is much easier to use. Then, with this method, an optics design technique is introduced that could minimize the emittance dilution within a single achromatic cell.
An RF power coupler is a key component of the superconducting accelerating system in Chinese ADS proton linac injector I, which is used to transmit 15 kW RF power from the power source to the superconducting HWR cavity. According to the requirement of working frequency, power level, transmission capability and cooling condition, the physics design of coupler has been finished, which includes RF structure optimization, thermal simulation, thermal stress analysis and so on. Based on this design, the prototype of HWR coupler has been fabricated, and it has successfully passed the high power test.
The Rapid Cycling Synchrotron (RCS) is a key component of the China Spallation Neutron Source (CSNS). For this type of high intensity proton synchrotron, the chromaticity, space charge effects, and magnetic field tracking errors between the quadrupoles and the dipoles can induce beta function distortion and tune shift, and induce resonances. In this paper, the combined effects of chromaticity, magnetic field tracking errors and space charge on beam dynamics at CSNS/RCS are studied systemically. 3-D simulations with different magnetic field tracking errors are performed by using the code ORBIT, and the simulation results are compared with the case without tracking errors.
The injector of C-ADS (Chinese Accelerator Driven Sub-critical System) project is a high current, fully super-conducting proton accelerator. Meanwhile, a BLM system is indispensable for this facility, especially in low energy segments. This paper presents some basic simulations for 10 MeV proton by Monte Carlo program FLUKA, as well as the distributions on different secondary particles in three aspects: angular, energy spectrum, and current. These results are beneficial to selecting the detector type and its location and determining its dynamic range matching different requirements for both fast and slow beam loss. Furthermore, in this paper the major impact of the background is also analyzed, such as superconducting cavity X radiation and radiation caused by material activation. This work is meaningful in BLM system research.
A spoke cavity is a TEM-class superconducting resonator with particular advantages: compact structure and high shunt impedance. The 325 MHz β=0.40 (β=v/c, v is the velocity of particle and c is the velocity of light) single spoke cavity (Spoke040) is adopted for the Chinese ADS (Accelerator Driven Sub-critical System) project. The physics and mechanical design has been accomplished, and the fabrication of a prototype is currently in progress. In this paper, the optimization processes for the main radio frequency (RF) and mechanical parameters are analyzed in detail. Two kinds of cavity end-walls (flat and convex) are compared. The convex end-wall is preferred in order to improve mechanical performance of the cavity. Two prototypes of the Spoke040 cavity are in the machining stage, and should be finished in early 2014. Vertical testing is also under preparation.
The China Accelerator Driven Subcritical System (China-ADS) project, which is a strategic plan and aims to design and build an ADS demonstration facility, has been proposed and launched actively in China. Injector Ⅱ as one of the parallel injectors of China-ADS, and is prompted by the Institute of Modern Physics (IMP). In this paper, a new scheme with full period lattice structure for the SC section is proposed. In the new scheme, there are sixteen periods, with one superconducting solenoid and one superconducting cavity included in each period. All of the elements are contained in four cryomodules. The dreadful influence of the mismatch caused by period structural change can be avoided, and the beam quality is favorable. In addition, this new scheme has certain advantages in reducing the project's difficulty and construction risk. The details of the design and beam dynamic simulation for the full period lattice structure are given in this paper.
Since modern accelerators demand excellent stability to magnet power supply (PS), it is necessary to decrease harmonic currents passing magnets. Aiming at depressing the rappel current from the PS in the Beijing electron-positron collider Ⅱ, a wavelet-based active power filter (APF) is proposed in this paper. An APF is an effective device to improve the quality of currents. As a countermeasure to these harmonic currents, the APF circuit generates a harmonic current, countervailing harmonic current from PS. An active power filter based on wavelet transformation is proposed. Discrete wavelet transformation is used to analyze the harmonic components in the supply current, and an active power filter circuit works according to the analysis results. Our simulation and experiment results are given to prove the effect of the APF.
In order to put the Pulse Line Ion Accelerator (PLIA) concept to its practical application, a small experimental platform was built. It was found that the actual axial electric field is smaller than the theoretical calculation, so the accelerated ions will enter into the deceleration zone before leaving the helix, which will seriously affect the acceleration process. Based on the improved parameters, the He+ with 24 keV is accelerated to 55 keV, and the proof-of-principle experiment is completed on this platform.
The transient response analysis of the SLED based on the equivalent circuit is described. Then, a C-band SLED using TE0,1,15 mode cylindrical cavity with TE10-TE01 mode converter has been designed. According to the main RF parameters of the accelerator, the coupling coefficient is optimized to obtain the maximum multiplication factor. The key components of the pulse compressor include a 3 dB directional coupler, a TE10-TE01 mode converter, and a cylindrical cavity, which are simulated and optimized using 3D electromagnetic field simulation software. In addition, the function defining the relation between the coupling factor and aperture size is derived by a mathematical fitting method.
The vibrating wire alignment technique is a method which, by measuring the spatial distribution of a magnetic field, can achieve very high alignment accuracy. The vibrating wire alignment technique can be applied to fiducializing magnets and the alignment of accelerator straight section components, and it is a necessary supplement to conventional alignment methods. This article gives a systematic summary of the vibrating wire alignment technique, including vibrating wire model analysis, system frequency calculation, wire sag calculation, and the relation between wire amplitude and magnetic induction intensity. On the basis of this analysis, this article outlines two existing alignment methods, one based on magnetic field measurement and the other on amplitude and phase measurements. Finally, some basic experimental issues are discussed.
A compact laser plasma accelerator (CLAPA) is being built at Peking University, which is based on an RPA-PSA mechanism or other acceleration mechanisms. The beam produced by this laser accelerator has the characteristics of short duration, high pulse current, large divergence angle, and wide energy spectrum. The beam cannot be produced by a normal ion source and accelerator. The space charge field in the initial is very strong. According to the beam parameters from preparatory experiments and theoretical simulations, a compact beam line is preliminarily designed. The beam line mainly consists of common transport elements to deliver proton beam with the energy of 1-50 MeV, energy spread of 0-± 1% and current of 0-108 proton per pulse to satisfy the requirement of different experiments. The simulation result of a 15 MeV proton beam with an energy spread of ± 1%, current of 400 mA, and final spot radius of 9 mm is presented in this paper.
The radioactivity induced by carbon ions of the Heavy Ion Medical Machine (HIMM) was studied to asses its radiation protection and environmental impact. Radionuclides in the accelerator component, and in the cooling water and air at the target area, which are induced from primary beam and secondary particles, are simulated by FLUKA Monte Carlo code. It is found that radioactivity in the cooling water and air is not very important at the required beam intensity and energy that is needed for treatment, while radionuclides in the accelerator component may cause some problems for maintenance work and, therefore, a suitable cooling time is needed after the machine is shut down.
A design for an efficient monochromatic electron source for Inverse Photoemission Spectroscopy (IPES) apparatus is described. The electron source consists of a BaO cathode, a focus electrostatic lens, a hemispherical deflection monochromator (HDM), and a transfer electrostatic lens. The HDM adopts a "slit-in and slit-out" structure and the degradation of first-order focusing is corrected by two electrodes between the two hemispheres, which has been investigated by both analytical methods and electron-ray tracing simulations using the SIMION program. Through the focus lens, the HDM, and the standard five-element transfer lens, an optimal energy resolution is estimated to be about 53 MeV with a beam flux of 27 μA. Pass energy (P.E.) of 10 eV and 5 eV are discussed, respectively.
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