分类: 物理学 >> 凝聚态:结构、力学和热性能 提交时间: 2017-03-26
摘要: The contribution of positron source for the results of a positron annihilation lifetime spectrum (PALS) is simulated using Geant4 code. The geometrical structure of PALS measurement system is a sandwich structure: the 22Na radiation source is encapsulated by Kapton films, and the specimens are attached on the outside of the films. The probabilities of a positron being annihilated in the films, annihilated in the targets, and the effect of positrons reflected back from the specimen surface, are simulated. The probability of a positron annihilated in the film is related to the species of targets and the source film thickness. The simulation result is in reasonable agreement with the available experimental data. Thus, modification of the source contribution calculated by Geant4 is viable, and it beneficial for the analysis of the results of PALS.
分类: 物理学 >> 核物理学 提交时间: 2016-09-13
摘要: The precise spectra of Cosmic Ray (CR) electrons and positrons have been published by the measurement of AMS-02. It is reasonable to regard the difference between the electrons and positrons spectra ( △Φ=Φe−−Φe+ ) as being dominated by primary electrons. Noticing that the resulting electron spectrum shows no sign of spectral softening above 20 GeV, which is in contrast with the prediction of standard model. In this work, we generalize the analytic one dimensional two-halo model of diffusion to a three dimensional realistic calculation by implementing a spatial variant diffusion coefficients in DRAGON package. As a result, we can reproduce the spectral hardening of protons observed by several experiments, and predict an excess of high energy primary electrons which agrees with the measurement reasonably well. Unlike the break spectrum obtained for protons, the model calculation predicts a smooth electron excess and thus slightly over predicts the flux from tens of GeV to 100GeV. To understand this issue, further experimental and theoretical studies are necessary.
分类: 核科学技术 >> 核材料与工艺技术 提交时间: 2023-06-18 合作期刊: 《Nuclear Science and Techniques》
摘要: Alterations of mitochondria DNA (mtDNA) 4977 bp common deletion (CD) and mtDNA copy number induced by ionizing radiation were observed in human different cell lines and total body irradiation patients. However, only few experiments have evaluated the levels of the CD and mtDNA copy number in human peripheral blood exposed to ionizing radiation till now. The aim of this study is to analyze the mtDNA alterations in irradiated human peripheral blood from healthy donors as well as to explore their feasibility as biomarkers for constructing new biodosimeter. Peripheral blood samples were collected from six healthy donors, and exposed to 60Co gamma ray with the doses of 0 Gy, 1 Gy, 2 Gy, 3 Gy, 4 Gy and 5 Gy. Levels of the CD and mtDNA copy number in irradiated samples after 2h or 24h incubation were detected using TaqMan real-time PCR, and the CD ratio was calculated. The results showed that the mean of the CD ratio and the CD copy number exhibited a dose-dependent increase 2 h in the dose range from 05 Gy, and of the mtDNA copy number significantly increased 24 h in irradiated groups compared with 0 Gy group after irradiation. It indicates that the parameters in human peripheral blood may be considered as molecular biomarkers to applying construction of new biodosimeter.
分类: 物理学 >> 凝聚态:结构、力学和热性能 提交时间: 2017-03-26
摘要: A series of Sm-doped LDPE-Na2SO4 composites were successfully achieved by melt mixing and hot pressing methods. Their morphology, structure and luminescent properties were characterized by scanning electron microscope (SEM), Energy Dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and photoexcitation (PE), photoluminescence (PL) spectra, respectively. The sizes and concentrations of free volume cavities in composites were studied by positron annihilation lifetime spectroscopy (PALS). SEM micrographs showed that Na2SO4:Sm3+ particles were well-distributed in the polymer matrix. XRD analyses revealed that the addition of Na2SO4:Sm3+ had no effect on the original crystal structure of LDPE. The dominant photoexcitation peak was observed at about 402 nm. The photoluminescence spectra consisted of four main peaks at 563, 598, 644 and 706 nm which could be associated to the transitions 4G5/2→6HJ (J=5/2, 7/2, 9/2 and 11/2, respectively) within the 4f5 electronic configuration of Sm3+. The effect of different doping concentration of Na2SO4:Sm3+ on the luminescent properties of the composites was investigated. The luminescent intensity of the composites increased with phosphor concentration.
分类: 物理学 >> 核物理学 提交时间: 2016-08-31
摘要: The optimization of resistive anode for two dimensional imaging detectors which consists of a series of high resistive square pads surrounding by low resistive strips has been studied by both numerical simulations and experimental tests. It has been found that to obtain good detector performance, the resistance ratio of the pad to the strip should be larger than 5, the nonuniformity of the pad surface resistivity had better be less than 20%, a smaller pad width leads to a smaller spatial resolution and when the pad width is 6mm, the spatial resolution (σ) can reach about 105μm. Based on the study results, a 2-D GEM detector prototype with the optimized resistive anode is constructed and a good imaging performance is achieved.
分类: 物理学 >> 凝聚态:结构、力学和热性能 提交时间: 2017-03-26
摘要: Thermal aging effects on surface of 2.5 MeV Fe ion irradiated Fe-0.6%Cu alloy were investigated using positron annihilation techniques. The samples were irradiated at 573 K to a dose of 0.1 dpa. Their thermal aging was performed at 573 K for 5, 50, and 100 h. From the results of Doppler broadening measurement, an obvious trough could be seen in near surface region from the S parameters and inflection point form at S-W curves. This indicates changes in the annihilation mechanism of positrons in surface region after thermal aging. Coincident Doppler broadening indicates that the density of Cu precipitates in the thermal aged samples decreased, due to recovery of the vacancies. Keywords Fe-Cu alloy; Positron annihilation; Irradiation; Thermal aging
分类: 化学 >> 物理化学 提交时间: 2017-11-05 合作期刊: 《结构化学》
摘要: A d-f heteronuclear metal-organic framework (MOF), {[Tm3Zn6(bipy2)2(mimda)7 (H2O)3]·(H2O)5}n (1, H3mimda = 2-methyl-1-H-imidazole-4,5-dicarboxylic acid, and bipy = 4,4΄-bipyridine), has been synthesized under solvothermal conditions, and structurally characterized by elemental analysis, IR spectra and X-ray single-crystal diffraction. It crystallizes in orthorhombic system, space group Pnma with a = 16.1102(9), b = 33.5805(19), c = 16.8593(10) Å, β = 97.344(11)°, V = 9120.7(9) Å3, Z = 4, F(000) = 5184, the final R = 0.0530 and wR = 0.1306. In complex 1, the Tm(III) ions adopt two types of coordination fashions. Complex 1 shows one-dimensional (1-D) Tm-Zn heteronuclear zigzag chains, and these chains are further linked by H3mimda ligands into Tm-Zn heteronuclear 2-D lattice-like arrays. The 2-D heteronuclear units were connected through [Zn6(mimda)6] rings to give rise to the Tm-Zn heteronuclear cages. Finally, H3mimda ligands connected the cages into a 3-D heterometallic framework by the combination of [TmO7]n and Tm-Zn heteronuclear cages. In addition, the thermal stability and luminescent property have been investigated.
分类: 物理学 >> 凝聚态:结构、力学和热性能 提交时间: 2017-03-23
摘要: A comparative study on the morphology, structure and luminescence modifications of LDPE–Na2SO4:Sm3+ composite induced by gamma irradiation was reported for the first time. The composites were irradiated with irradiation doses of 10, 30, and 50 kGy in air at room temperature. Modifications on structural and luminescence properties of LDPE–Na2SO4:Sm3+ composites were investigated by using several analytical methods. The X-ray diffraction (XRD) patterns revealed that the cross-linking improved the crystallinity of the composites. Scanning electron microscope (SEM) images illustrated the surface modifications of composites induced by irradiation. Results of positron annihilation lifetime spectroscopy (PALS) suggested that gamma irradiation affected the free volumes concentrations in the samples. The photoluminescence (PL) spectra of non-irradiated and irradiated composites consisted of four main peaks at 563, 598, 644 and 706 nm which could be assigned to the 4G5/2 6HJ (J =5/2, 7/2, 9/2 and 11/2, respectively) transitions within the electronic configuration of Sm3+. Meanwhile, we observed an additional peak at about 686 nm in the irradiated composites, owing to the conversion of Sm3+ Sm2+. Concentrations of the Sm2+ centers linearly increased with gamma irradiation doses, which partly accounted for the degradation of PL intensity of Sm3+ peaks.
分类: 物理学 >> 凝聚态:结构、力学和热性能 提交时间: 2017-03-26
摘要: Room-temperature ferromagnetism has been experimentally observed in annealed rutile TiO2 single crystals when magnetic field is applied parallel to the sample plane. By combining X-ray absorption near edge structure spectrum and positron annihilation lifetime spectroscopy, Ti3+-VO defect complexes (or clusters) have been identified in annealed crystals at high vacuum. We elucidate that the unpaired 3d electrons in Ti3+ ions provide the observed room-temperature ferromagnetism. Besides, excess oxygen ions in TiO2 lattice could induce a number of Ti vacancies which increase magnetic moments obviously.
分类: 地球科学 >> 地理学 提交时间: 2021-12-30 合作期刊: 《干旱区科学》
摘要: The Tarim River is the longest inland river in China and is considered as an important river to protect the oasis economy and environment of the Tarim Basin. However, excessive exploitation and over-utilization of natural resources, particularly water resources, have triggered a series of ecological and environmental problems, such as the reduction in the volume of water in the main river, deterioration of water quality, drying up of downstream rivers, degradation of vegetation, and land desertification. In this study, the land use/land cover change (LUCC) responses to ecological water conveyance in the lower reaches of the Tarim River were investigated using ENVI (Environment for Visualizing Images) and GIS (Geographic Information System) data analysis software for the period of 1990–2018. Multi-temporal remote sensing images and ecological water conveyance data from 1990 to 2018 were used. The results indicate that LUCC covered an area of 2644.34 km2 during this period, accounting for 15.79% of the total study area. From 1990 to 2018, wetland, farmland, forestland, and artificial surfaces increased by 533.42 km2 (216.77%), 446.68 km2 (123.66%), 284.55 km2 (5.67%), and 57.51 km2 (217.96%), respectively, whereas areas covered by grassland and other land use/land cover types, such as Gobi, bare soil, and deserts, decreased by 103.34 km2 (14.31%) and 1218.83 km2 (11.75%), respectively. Vegetation area decreased first and then increased, with the order of 2010<2000<1990<2018. LUCC in the overflow and stagnant areas in the lower reaches of the Tarim River was mainly characterized by fragmentation, irregularity, and complexity. By analyzing the LUCC responses to 19 rounds of ecological water conveyance in the lower reaches of the Tarim River from 2000 to the end of 2018, we proposed guidelines for the rational development and utilization of water and soil resources and formulation of strategies for the sustainable development of the lower reaches of the Tarim River. This study provides scientific guidance for optimal scheduling of water resources in the region.
分类: 物理学 >> 核物理学 提交时间: 2017-07-30
摘要: We perform a systematic study of capture excitation functions by using an empirical coupled-channel model. In this model, a barrier distribution is used to take effectively into account the effects of couplings between the relative motion and intrinsic degrees of freedom. The shape of the barrier distribution is of an asymmetric Gaussian form. The effect of neutron transfer channels is also included in the barrier distribution. Based on the interaction potential between the projectile and the target, empirical formulas are proposed to determine the parameters of the barrier distribution. Theoretical estimates for barrier distributions and calculated capture cross sections together with experimental cross sections of 220 reaction systems with 182 ZPZT 1640 are tabulated. The results show that our empirical formulas work quite well in the energy region around the Coulomb barrier. This model can provide prediction of capture cross sections for the synthesis of superheavy nuclei as well as valuable information on capture and fusion dynamics.
分类: 物理学 >> 核物理学 提交时间: 2024-03-19
摘要: The exploration of exotic shapes and properties of atomic nuclei, e.g., α cluster and toroidal shape, is afascinating field in nuclear physics. To study the decay of these nuclei, a novel detector aimed at detectingmultiple alpha-particle events was designed and constructed. The detector comprises two layers of double-sidedsilicon strip detectors (DSSD) and a cesium iodide scintillator array coupled with silicon photomultipliers arrayas light sensors, which has the advantages of their small size, fast response, and large dynamic range. DSSDscouple with cesium iodide crystal arrays are used to distinguish multiple alpha hits. The detector array has acompact and integrated design that can be adapted to different experimental conditions. The detector array wassimulated using Geant4, and the excitation energy spectra of some alpha-clustering nuclei were reconstructedto demonstrate the performance. The simulation results show that the detector array has excellent angularand energy resolutions, enabling effective reconstruction of the nuclear excited state by multiple alpha particleevents. This detector offers a new and powerful tool for nuclear physics experiments and has the potential todiscover interesting physical phenomena related to exotic nuclear structures and their decay mechanisms
分类: 物理学 >> 核物理学 提交时间: 2017-07-30
摘要: A large number of complete fusion excitation functions of reactions including the breakup channel were measured in recent decades, especially in the last few years. It allows us to investigate the systematic behavior of the breakup effects on the complete fusion cross sections. To this end, we perform a systematic study of the breakup effects on the complete fusion cross sections at energies above the Coulomb barrier. The reduced fusion functions F(x) are compared with the universal fusion functions which are used as a uniform standard reference. The complete fusion cross sections at energies above the Coulomb barrier are suppressed by the breakup of projectiles. This suppression effect for reactions induced by the same projectile is independent of the target and mainly determined by the lowest energy breakup channel of the projectile. There holds a good exponential relation between the suppression factor and the energy corresponding to the lowest breakup threshold.
分类: 物理学 >> 核物理学 提交时间: 2017-07-30
摘要: Complete fusion excitation functions of reactions involving breakup are studied by using the em- pirical coupled-channel (ECC) model with breakup effects considered. An exponential function with two parameters is adopted to describe the prompt-breakup probability in the ECC model. These two parameters are fixed by fitting the measured prompt-breakup probability or the complete fusion cross sections. The suppression of complete fusion at energies above the Coulomb barrier is studied by comparing the data with the predictions from the ECC model without the breakup channel considered. The results show that the suppression of complete fusion are roughly independent of the target for the reactions involving the same projectile.
分类: 物理学 >> 核物理学 提交时间: 2024-03-19
摘要: In recent years, the gap between the supply and demand of medical radioisotopes has increased, necessitating new methods for producing medical radioisotopes. Photonuclear reactions based on gamma sources have unique advantages in terms of producing high specific activity and innovative medical radioisotopes. However, the lack of experimental data on reaction cross sections for photonuclear reactions of medical radioisotopes of interest has severely limited the development and production of photonuclear transmutation medical radioisotopes. In this study, the entire process of the generation, decay, and measurement of medical radioisotopes was simulated using online gamma activation and offline gamma measurements combined with a shielding gammaray spectrometer. Based on a quasi-monochromatic gamma beam from the Shanghai Laser Electron Gamma Source (SLEGS), the feasibility of the measurement of production cross section for surveyed medical isotopes was simulated, and specific solutions for measuring medical radioisotopes with low production cross sections were provided. The feasibility of this method for high precision measurements of the reaction cross section of medical radioisotopes was demonstrated.
分类: 物理学 >> 核物理学 提交时间: 2023-12-13
摘要: The reconstruction of the tracks of charged particles with high precision is crucial for HEP experiments toachieve their physics goals. The BESIII drift chamber, which is used as the tracking detector of the BESIIIexperiment, has suffered from aging effects resulting in degraded tracking performance after operation for ap-proximately 15 years. To preserve and enhance the tracking performance of BESIII, one of the proposals is toadd one layer of a thin cylindrical CMOS pixel sensor based on state-of-the-art stitching technology betweenthe beam pipe and the drift chamber. The improvement in the tracking performance of BESIII with such anadditional pixel detector compared to that with only the existing drift chamber was studied using the moderncommon tracking software ACTS , which provides a set of detector-agnostic and highly performant trackingalgorithms that h