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    Density functional theory study of NO2-sensing mechanisms of pure and Ti-doped WO3 (002) surfaces
    Hu Ming(胡明), Wang Wei-Dan(王巍丹), Zeng Jing(曾晶), and Qin Yu-Xiang(秦玉香)
    Chin. Phys. B, 2011, 20 (10): 102101.   DOI: 10.1088/1674-1056/20/10/102101
    Abstract1362)      PDF (213KB)(1160)      
    Density functional theory (DFT) calculations are employed to explore the NO2-sensing mechanisms of pure and Ti-doped WO3 (002) surfaces. When Ti is doped into the WO3 surface, two substitution models are considered: substitution of Ti for W6c and substitution of Ti for W5c. The results reveal that substitution of Ti for 5-fold W forms a stable doping structure, and doping induces some new electronic states in the band gap, which may lead to changes in the surface properties. Four top adsorption models of NO2 on pure and Ti-doped WO3 (002) surfaces are investigated: adsorptions on 5-fold W (Ti), on 6-fold W, on bridging oxygen, and on plane oxygen. The most stable and likely NO2 adsorption structures are both N-end oriented to the surface bridge oxygen O1c site. By comparing the adsorption energy and the electronic population, it is found that Ti doping can enhance the adsorption of NO2, which theoretically proves the experimental observation that Ti doping can greatly increase the WO3 gas sensor sensitivity to NO2 gas.
    Ab-initio density functional theory study of a WO3 NH3-sensing mechanism
    Hu Ming(胡明), Zhang Jie(张洁), Wang Wei-Dan(王巍丹), and Qin Yu-Xiang(秦玉香)
    Chin. Phys. B, 2011, 20 (8): 082101.   DOI: 10.1088/1674-1056/20/8/082101
    Abstract1386)      PDF (259KB)(1916)      
    WO3 bulk and various surfaces are studied by an ab-initio density functional theory technique. The band structures and electronic density states of WO3 bulk are investigated. The surface energies of different WO3 surfaces are compared and then the (002) surface with minimum energy is computed for its NH3 sensing mechanism which explains the results in the experiments. Three adsorption sites are considered. According to the comparisons of the energy and the charge change between before and after adsorption in the optimal adsorption site O1c, the NH3 sensing mechanism is obtained.
    Spin symmetry in the relativistic modified Rosen–Morse potential with the approximate centrifugal term
    Chen Wen-Li (陈文利), Wei Gao-Feng (卫高峰)
    Chin. Phys. B, 2011, 20 (6): 062101.   DOI: 10.1088/1674-1056/20/6/062101
    Abstract1084)      PDF (179KB)(795)      
    By applying a Pekeris-type approximation to the centrifugal term, we study the spin symmetry of a Dirac nucleon subjected to scalar and vector modified Rosen-Morse potentials. A complicated energy equation and associated two-component spinors with arbitrary spin-orbit coupling quantum number k are presented. The positive-energy bound states are checked numerically in the case of spin symmetry. The relativistic modified Rosen-Morse potential cannot trap a Dirac nucleon in the limiting case $\alpha$→ 0.
    Development of x-ray scintillator functioning also as an analyser grating used in grating-based x-ray differential phase contrast imaging
    Lei Yao-Hu(雷耀虎), Liu Xin(刘鑫), Guo Jin-Chuan(郭金川), Zhao Zhi-Gang(赵志刚), and Niu Han-Ben(牛憨笨)
    Chin. Phys. B, 2011, 20 (4): 042901.   DOI: 10.1088/1674-1056/20/4/042901
    Abstract1564)      PDF (4511KB)(1092)      
    In order to push the grating-based phase contrast imaging system to be used in hospital and laboratories, this paper designs and develops a novel structure of x-ray scintillator functioning also as an analyser grating, which has been proposed for grating-based x-ray differential phase contrast imaging. According to this design, the scintillator should have a periodical structure in one dimension with the pitch equaling the period of self-image of the phase grating at the Talbot distance, where one half of the pitch is pixellated and is made of x-ray sensitive fluorescent material, such as CsI(Tl), and the remaining part of the pitch is made of x-ray insensitive material, such as silicon. To realize the design, a deep pore array with a high aspect ratio and specially designed grating pattern are successfully manufactured on 5 inch silicon wafer by the photo-assisted electrochemical etching method. The related other problems, such as oxidation-caused geometrical distortion, the filling of CsI(Tl) into deep pores and the removal of inside bubbles, have been overcome. Its pixel size, depth and grating pitch are 3 μm×7.5 μm, 150 μm and 3 μm, respectively. The microstructure of the scintillator has been examined microscopically and macroscopically by scanning electron microscope and x-ray resolution chart testing, respectively. The preliminary measurements have shown that the proposed scintillator, also functioning as an analyser grating, has been successfully designed and developed.
    Two-dimensional multiplicity fluctuation analysis of target residues in nuclear collisions
    Zhang Dong-Hai(张东海), Niu Yao-Jie(牛耀婕), Wang Li-Chun(王立春) Yan Wen-Jun(闫文君), Gao Li-Juan(高丽娟), Li Ming-Xing(李明星) Wu Li-Ping(武丽萍), Li Hui-Ling(李惠玲), and Li Jun-Sheng(李俊生)
    Chin. Phys. B, 2010, 19 (7): 072501.   DOI: 10.1088/1674-1056/19/7/072501
    Abstract1178)      PDF (151KB)(576)      
    Multiplicity fluctuation of the target residues emitted in the interactions in a wide range of projectile energies from 500 A MeV to 60 A GeV is investigated in the framework of two-dimensional scaled factorial moment methodology. The evidence of non-statistical multiplicity fluctuation is found in 16O—AgBr collisions at 60 A GeV, but not in 56Fe—AgBr collisions at 500 A MeV, 84Kr—AgBr collisions at 1.7 A GeV, 16O—AgBr collisions at 3.7 A GeV and 197Au—AgBr collisions at 10.7 A GeV.
    The g-factors and magnetic rotation in 82Rb
    Yuan Da-Qing(袁大庆), Zheng Yong-Nan(郑永男), Zuo Yi(左翼), Fan Ping(范平), Zhou Dong-Mei(周冬梅), Wu Xiao-Guang(吴晓光), Zhu Li-Hua(竺礼华), Li Guang-Sheng(李广生), Xu Guo-Ji(许国基), Fan Qi-Wen(樊启文), Zhang Xi-Zhen(张锡珍), and Zhu Sheng-Yun(朱升云)
    Chin. Phys. B, 2010, 19 (6): 062701.   DOI: 10.1088/1674-1056/19/6/062701
    Abstract1676)      PDF (144KB)(634)      
    The g-factors of the intra-band states 12, 13, 14, 15 in a magnetic-rotational band built on the 11 state in 82Rb are measured for the first time by using a transient magnetic field-ion implantation perturbed angular distribution (TMF-IMPAD) method. The magnetic-rotational band in 82Rb is populated by the 60Ni(27Al,4pn)82Rb reaction, and the time-integral Larmor precessions are measured after recoil implantation into a polarized Fe foil. The calculation of g-factors is also carried out in terms of a semi-classical model of independent particle angular momentum coupling on the basis of the four-quasiparticle configuration $\pi ( {g_{9 / 2} } )^2 \otimes \pi (p_{3 / 2} ,f_{5 / 2} ) \otimes \nu ( {g_{9 / 2} } )$. The measured and calculated g-factors are in good agreement with each other. The g-factors and deduced shear angles decrease with the increase of spin along the band. This clearly illustrates the shear effect of a step-by-step alignment of the valence protons and neutrons in magnetic rotation. The semi-classical calculation also shows that the alignment of the valence neutron angular momentum is faster than that of the valence protons, which results in a decrease of g-factors with increasing spin. The present results provide solid evidence of the shear mechanism of magnetic rotation.
    Monte Carlo simulation for bremsstrahlung and photoneutron yields in high-energy x-ray radiography
    Xu Hai-Bo(许海波), Peng Xian-Ke(彭现科), and Chen Chao-Bin(陈朝斌)
    Chin. Phys. B, 2010, 19 (6): 062901.   DOI: 10.1088/1674-1056/19/6/062901
    Abstract1730)      PDF (1073KB)(1431)      
    This paper reports on the results of calculations using a Monte Carlo code (MCNP5) to study the properties of photons, electrons and photoneutrons obtained in the converted target and their transportations in x-ray radiography. A comparison between measurements and calculations for bremsstrahlung and photoneutrons is presented. The radiographic rule and the effect of the collimator on the image are studied with the experimental model. The results provide exact parameters for the optimal design of radiographic layout and shielding systems.
    Characteristics of terahertz coherent transition radiation generated from picosecond ultrashort electron bunches
    Liu Wen-Xin(刘文鑫), Tang Chuan-Xiang(唐传祥), and Huang Wen-Hui(黄文会)
    Chin. Phys. B, 2010, 19 (6): 062902.   DOI: 10.1088/1674-1056/19/6/062902
    Abstract1698)      PDF (1428KB)(779)      
    This paper presents a method of generating terahertz (THz) coherent transition radiation (CTR) from picosecond ultrashort electron bunches including single and train bunches, which are produced by a photocathode radio frequency gun. The radiation characteristics of THz CTR including formation factor and energy spectrum are analysed in detail. With the help of a 2-dimensional particle-in-cell simulation, the radiation characteristics including power, energy and magnetic field are analysed. The results show that the radiation frequency can be adjusted by tuning the repetition frequency of the train bunch and the energy can be enhanced with the train bunches.
    Numerical simulation for space charge dominated beam transport
    Lü Jian-Qin(吕建钦)
    Chin. Phys. B, 2010, 19 (3): 032901.   DOI: 10.1088/1674-1056/19/3/032901
    Abstract1701)      PDF (237KB)(607)      
    To simulate the intense bunched beam transport, a computer program LEADS-3D has been developed. The particle trajectories are analysed with the Lie algebraic method. The third order approximation of the trajectory solutions is made with space charge forces off, and the second order approximation is made with space charge forces on. The particle distribution in the 3D ellipsoid is uniform or Gaussian. Most of the conventional beam optical elements are incorporated in the code. The optimization procedures are provided to fit the beam lines to satisfy the given optical conditions.
    Time-and-space resolved measurements of the emission uniformity of carbon fibre cathode in high-current pulsed discharge
    Liu Lie(刘列), Li Li-Min(李立民), Xu Qi-Fu(徐启福), Cheng Guo-Xin(程国新), and Chang Lei(苌磊)
    Chin. Phys. B, 2010, 19 (3): 032902.   DOI: 10.1088/1674-1056/19/3/032902
    Abstract1709)      PDF (4283KB)(756)      
    The remaining challenges, confronting high-power microwave (HPM) sources and pulsed power generators, stimulate the developments of robust relativistic electron beam sources. This paper presents a carbon fibre cathode which is tested in a single pulsed power generator. The distribution and the development of cathode plasma are observed by time-and-space resolved diagnostics, and the uniformity of electron beam density is checked by taking x-ray images. A quasistationary behaviour of cathode plasma expansion is observed. It is found that the uniformity of the extracted electron beam is satisfactory in spite of individual plasma jets on the cathode surface. These results show that carbon fibre cathodes can provide a positive prospect for developing a high-quality electron beam.
    Pulsed ion beam-assisted carburizing of titanium in methane discharge
    M. Shafiq, M. Hassan, K. Shahzad, A. Qayyum, S. Ahmad, R. S. Rawat, and M. Zakaullah
    Chin. Phys. B, 2010, 19 (1): 012801.   DOI: 10.1088/1674-1056/19/1/012801
    Abstract1408)      PDF (1150KB)(803)      
    The carburizing of titanium (Ti) is accomplished by utilizing energetic ion pulses of a 1.5 kJ Mather type dense plasma focus (DPF) device operated in methane discharge. X-ray diffraction (XRD) analysis confirms the deposition of polycrystalline titanium carbide (TiC). The samples carburized at lower axial and angular positions show an improved texture for a typical (200)TiC plane. The Williamson--Hall method is employed to estimate average crystallite size and microstrains in the carburized Ti surface. Crystallite size is found to vary from ~50 to 100 nm, depending on the deposition parameters. Microstrains vary with the sample position and hence ion flux, and are converted from tensile to compressive by increasing the flux. The carburizing of Ti is confirmed by two major doublets extending from 300 to 390 cm-1 and from 560 to 620 cm-1 corresponding to acoustic and optical active modes in Raman spectra, respectively. Analyses by scanning electron microscopy/energy dispersive x-ray spectroscopy (SEM/EDS) have provided qualitative and quantitative profiles of the carburized surface. The Vickers microhardness of Ti is significantly improved after carburizing.
    Coulomb effects on the formation of proton halo nuclei
    Liang Yu-Jie(梁玉洁), Li Yan-Song(李岩松), Zhu Min(朱民), Liu Zu-Hua(刘祖华), and Zhou Hong-Yu(周宏余)
    Chin. Phys. B, 2009, 18 (12): 5267-5271.   DOI: 10.1088/1674-1056/18/12/025
    Abstract1731)      PDF (445KB)(655)      
    This paper makes some qualitative and quantitative analyses about halo formation rules of some mirror nuclei with the relativistic mean-field (RMF) theory and the Woods--Saxon mean-field model. By analysing two opposite effects of Coulomb interaction on the proton halo formation, it finds that the energy level shift has a larger contribution than that of the Coulomb barrier when the mass number A is small, the hindrance of the Coulomb barrier becomes more obvious with the increase of the mass number A, and the overall effect of the Coulomb interaction almost disappears when ≈ 39 as its two effects counteract with each other.
    Disappearance of isospin effect in projectile fragmentation at intermediate energy
    Ma Chun-Wang(马春旺) Wei Hui-Ling(魏慧玲), Wang Jun-Yang(王俊阳), and Liu Gao-Jie(刘高杰)
    Chin. Phys. B, 2009, 18 (11): 4781-4785.   DOI: 10.1088/1674-1056/18/11/029
    Abstract1552)      PDF (151KB)(797)      
    The 140 MeV/u 40,48Ca+ 9Be and 58,64Ni+9Be reactions are simulated by the statistical abrasion ablation model, and the simulation results are compared to the National Superconducting Cyclotron Laboratory (NSCL) experimental data. By comparing the fragment isotopic distributions of 40,48Ca and 58,64Ni, we study the isospin effect in the projectile fragmentation induced by the neutron-rich nuclei at intermediate energy experimentally and theoretically. It is found that the isospin effect in projectile fragmentation decreases and even disappears as the violence of the collision increases.
    Effects of bulk viscosity on hadron spectra and the Hanbury-Brown Twiss radius by causal viscous hydrodynamics
    Li Jian-Wei(李建伟), Ma Yu-Gang(马余刚), and Ma Guo-Liang(马国亮)
    Chin. Phys. B, 2009, 18 (11): 4786-4790.   DOI: 10.1088/1674-1056/18/11/030
    Abstract1487)      PDF (540KB)(614)      
    The particle spectra and Hanbury-Brown Twiss (HBT) radius of Au+Au collisions at RHIC energy are investigated by a hydrodynamical expanding source with both shear and bulk viscosities ($\zeta$). With a large width of the ratio of $\zeta$ to entropy density s, both the particle transverse momentum spectra and the ratio Rout/Rside of HBT radii in the direction of the total transverse momentum of detected two particles (Rout) and perpendicular to both this direction and the beam direction (Rside) become a little steeper.
    Images of triple gas electron multiplier with pixel-pads
    Dong Jing(董静), Hu Bi-Tao(胡碧涛), Chen Yuan-Bo(陈元柏), and Xie Yi-Gang(谢一冈)
    Chin. Phys. B, 2009, 18 (10): 4229-4233.   DOI: 10.1088/1674-1056/18/10/024
    Abstract1646)      PDF (1801KB)(776)      
    Testing of a triple gas electron multiplier (GEM) with pixel-pads is described. Images by scanning and suspending radioactive sources were obtained by using 96 channels digital data acquisition (DAQ) system which was composed of 96 8× 8 mm2 pads and associated electronics channels.
    $\alpha$ preformation and penetration probability for heavy nuclei
    Zhang Gao-Long(张高龙) and Le Xiao-Yun(乐小云)
    Chin. Phys. B, 2009, 18 (9): 3810-3814.   DOI: 10.1088/1674-1056/18/9/032
    Abstract1854)      PDF (284KB)(683)      
    The $\alpha$ preformation factor and penetration probability have been analyzed for even--even nuclei of Po, Rn, Ra using experimental released energies and α decay half-lives in the frame of the double folding model. It is shown that N=126 is a neutron magic number from $\alpha$ preformation and shell effects play an important role in $\alpha$ preformation. The closer the nucleon number is to the magic number, the more difficult $\alpha$ formation in the parent nucleus is. The preformation factor can supply information on the nuclear structure and the penetration probability mainly determines $\alpha$ decay half-life.
    Calculations of radioactivity and afterheat in the components of the CSNS target station
    Yu Quan-Zhi(于全芝), Liang Tian-Jiao(梁天骄), Yin Wen(殷雯), Yan Qi-Wei(严启伟), Jia Xue-Jun(贾学军), and Wang Fang-Wei(王芳卫)
    Chin. Phys. B, 2009, 18 (9): 3815-3818.   DOI: 10.1088/1674-1056/18/9/033
    Abstract1649)      PDF (755KB)(826)      
    This paper shows the calculations of radioactivity and afterheat in the components of the China Spallation Neutron Source (CSNS) target station, with the Monte Carlo codes LAHET, MCNP4C and the multigroup code CINDER'90. These calculations provide essential data for the detailed design and maintenance of the CSNS target station.
    Monte Carlo simulation of exposure factor
    Zhang Tao(张涛), Liu Yi-Bao(刘义保), Yang Bo(杨波), Wu He-Xi(吴和喜), and Gu Jin-Hu(顾金虎)
    Chin. Phys. B, 2009, 18 (6): 2217-2222.   DOI: 10.1088/1674-1056/18/6/019
    Abstract1650)      PDF (625KB)(763)      
    In this paper, we simulate the exposure factor by a simple model of a free-air ionization chamber with the Monte Carlo programme Geant4. Special emphasis is placed on the discussion of the exposure factor related to parameters of the chamber model. The reason for the variation in exposure factor with incident ray energy is also analysed in terms of reaction cross section for different types of reactions. The obtained results indicate that our simulation is accurate in the calculation of the exposure factor and can serve as a reference in designing air ionization chambers.
    Characteristic analysis of the optical delay in frequency response of resonant cavity enhanced (RCE) photodetectors
    Guo Jian-Chuan(郭剑川), Zuo Yu-Hua(左玉华), Zhang Yun(张云), Ding Wu-Chang(丁武昌), Cheng Bu-Wen(成步文), Yu Jin-Zhong(余金中), and Wang Qi-Ming(王启明)
    Chin. Phys. B, 2009, 18 (6): 2223-2228.   DOI: 10.1088/1674-1056/18/6/020
    Abstract1519)      PDF (605KB)(725)      
    With consideration of the modulation frequency of the input lightwave itself, we present a new model to calculate the quantum efficiency of RCE p-i-n photodetectors (PD) by superimposition of multiple reflected lightwaves. For the first time, the optical delay, another important factor limiting the electrical bandwidth of RCE p-i-n PD excluding the transit time of the carriers and RCd response of the photodetector, is analyzed and discussed in detail. The optical delay dominates the bandwidth of RCE p-i-n PD when its active layer is thinner than several 10 nm. These three limiting factors must be considered exactly for design of ultra-high-speed RCE p-i-n PD.
    Analysis of prompt fission neutron spectrum and multiplicity for 237Np(n, f) in the frame of multi-modal Los Alamos model
    Zheng Na(郑娜), Ding Yi(丁毅), Zhong Chun-Lai(钟春来), Chen Jin-Xiang(陈金象), and Fan Tie-Shuan(樊铁栓)
    Chin. Phys. B, 2009, 18 (4): 1413-1420.   DOI: 10.1088/1674-1056/18/4/022
    Abstract1284)      PDF (269KB)(631)      
    The improved version of Los Alamos model with the multi-modal fission approach is used to analyse the prompt fission neutron spectrum and multiplicity for the neutron-induced fission of 237Np. The spectra of neutrons emitted from fragments for the three most dominant fission modes (standard I, standard II and superlong) are calculated separately and the total spectrum is synthesized. The multi-modal parameters contained in the spectrum model are determined on the basis of experimental data of fission fragment mass distributions. The calculated total prompt fission neutron spectrum and multiplicity are better agreement with the experimental data than those obtained from the conventional treatment of the Los Alamos model.
ISSN 1674-1056   CN 11-5639/O4

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