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 measurements of ψ (2 S )→ ωπ 0 and ψ (2 S )→ π + π − in e + e − experiments are examined. It is found that the non-resonance virtual photon annihilation gives large contributions to the observed cross sections of these two processes. By including this contribution, the form factors and branching fractions of these two decay modes are revised.
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： Chitosan, CS, cross-linked with bivalent palladium has shown enhanced mechanical and thermal properties depending on the transformation of the structure at a microscopic scale. In the present study, CS directly cross-linked by palladium cation membranes (CS-cr-PM) were prepared through solution-casting method. The motion of chitosan chains was great suppressed after crosslinking, making a great reduce of swelling ratio by a water-swelling degree measurement, which led to molecular chain rigidity to be improvement. In order to investigate the chain packing at the molecular level in the ionic cross-linked CS system, the structure of chemically-crosslinked CS is investigated by means of the combined use of wide angle X-ray diffraction (WAXD), Infrared measurements, and a combination of positron annihilation lifetime spectroscopy (PALS) and simultaneous coincidence Doppler broadening (CDB) spectroscopy offers coherent information on both the free-volume related sub-nano level molecular packing and the chemical surrounding of free volume nanoholes in CS-cr-PM as a function of palladium salt loading. The variations in free volume size and size distribution have been determined through the ortho-positroium (o-Ps) lifetime and its lifetime distribution. The studies showed that strong interaction between CS molecules and palladium cations results in the change of crystallinity in formed CS-cr-PM leading to variational chain packing density. Meanwhile, significant inhibition effects on positronium formation due to doping are observed, which could be interpreted in terms of the existence of chlorid ion. Applications of positron annihilation spectroscopy to study the microstructure and correlated positron annihilation characteristics in ionic cross-linked CS system are systematic discussed.
分类： 物理学 >> 凝聚态:结构、力学和热性能 提交时间： 2017-03-26
HU Yuan-Chao
CAO Xing-Zhong
ZHANG Peng
TSUCHIDA Hidetsugu
XU Qiu
JIN Shuo-Xue
LU Er-Yang
LI Yu-Xiao
YU Run-Sheng
WANG Bao-Yi
WEI Long
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： 物理学 >> 凝聚态:结构、力学和热性能 提交时间： 2017-03-26
摘要： Cu precipitates played a key factor in irradiation induced hardening and embrittlement of reactor pressure vessel (RPV).In this work, two kinds of Fe-Cu model alloys with different Cu contents irradiated by Fe ion at 573 K were investigated using positron annihilation techniques and SEM. Doppler Broadening Spectroscopy(DBS) results indicated that with the irradiation dose increased from 0.1 dpa to 1.2 dpa, more vacancy-type defects were induced during Fe ion irradiation, and with different Cu contents, defects condition had few changes. Coincident Doppler Broadening (CDB) results indicated Cu precipitates were formed during the irradiation, which could be verified through the SEM results. The CDB results also showed that the Cu precipitates were the same at 0.1 dpa between the samples with different Cu contents. In Fe0.6Cu samples, more Cu precipitates were formed than that of Fe0.3Cu sample at 1.2 dpa.
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 effect of Cu content on the evolution of defects in Fe-x%Cu alloys (x= 0.15, 0.3, and 0.6) were investigated using Positron Annihilation Lifetime Spectroscopy. The vacancy-type and Cu-vacancy complexes defects were respectively produced by quenching from 1173 K and cold-rolled deformation followed by isochronal annealing. The isochronal annealing results obtained by PALS showed that Cu-vacancy complexes delayed the complete recovery of vacancy-type defects and the complete recovery temperature increased with the content of Cu in 30% deformed Fe-Cu alloys. The increment of Cu content also restrained the migration of vacancies in as-quenched Fe-Cu alloys.
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： Abstract. The microstructural features and the effect of Mo addition in FeCrNi austenitic alloy during incubation period were investigated using positron annihilation technique and micro-Vickers Hardness. The electron irradiation, which could induce vacancy defects in material, was performed at room temperature up to the dose of 1.70-4 and 50-4 dpa, respectively. The defect concentration was estimated about 10-4-10-7 though the standard trapping model. The added Mo atoms could trap vacancies to form Mo-vacancy complexes, which may restrain the migration and growth of vacancy defects during electron irradiation. In addition, the microstructural evolution during electron radiation resulted in hardening, while the added Mo might improve the hardening property of the alloy.
分类： 物理学 >> 核物理学 提交时间： 2016-09-14
摘要： We explore the electron neutrino signals from light dark matter (DM) annihilation in the Sun for the large liquid scintillator detector JUNO. In terms of the spectrum features of three typical DM annihilation channels χχ→νν¯,τ+τ−,bb¯, we take two sets of selection conditions to calculate the expected signals and atmospheric neutrino backgrounds based on the Monte Carlo simulation data. Then the JUNO sensitivities to the spin independent DM-nucleon and spin dependent DM-proton cross sections are presented. It is found that the JUNO projected sensitivities are much better than the current spin dependent direct detection experimental limits for the νν¯ and τ+τ− channels. In the spin independent case, the JUNO will give the better sensitivity to the DM-nucleon cross section than the LUX and CDMSlite limits for the νν¯ channel with the DM mass lighter than 6.5 GeV. If the νν¯ or τ+τ− channel is dominant, the future JUNO results are very helpful for us to understand the tension between the DAMA annual modulation signal and other direct detection exclusions.
分类： 物理学 >> 核物理学 提交时间： 2016-09-13
摘要： The AMS-02 collaboration has just released the cosmic antiproton to proton ratio p¯/p with a high precision up to ∼450 GeV. In this work, we calculate the secondary antiprotons generated by cosmic ray interactions with the interstellar medium taking into account the uncertainties from the cosmic ray propagation. The p¯/p ratio predicted by these processes shows some tension with the AMS-02 data in some regions of propagation parameters, but the excess is not significant. We then try to derive upper bounds on the dark matter annihilation cross section from the p¯/p data or signal regions favored by the data. It is shown that the constraint derived by the AMS-02 data is similar to that from Fermi-LAT observations of dwarf galaxies. The signal region for dark matter is usually required mχ∼O(10) TeV and $\left\sim\mathcal{O}(10^{-23})~\cm^3~\sec^{-1}$.
分类： 核科学技术 >> 辐射物理与技术 提交时间： 2024-08-22
Hong-Xia Xu
Jian-Dang Liu
Bang-Jiao Ye
Zi-Wen Pan
Jun Lin
Jin-Liang Song
Jian-Qing Cao
Chao Yan
Ying-Ping Hao
Jin-Xing Cheng
Qing-Bo Wang
摘要： The stability of matrix graphite under neutron irradiation and in corrosive environments is crucial for thesafe operation of molten salt reactors (MSRs). Raman spectroscopy and a slow positron beam were employedto investigate the effects of He ion irradiation fluences and subsequent annealing on the microstructure anddefects of the matrix graphite. He ions with 500 keV energy and fluences ranging from 1.1 × 1015 ions/cm2to 3.5 × 1017 ions/cm2 were used to simulate neutron irradiation at 300K. The samples with an irradiationfluence of 3.5×1016 ions/cm2 were subjected to isochronal annealing at different temperatures (573K, 873 and1173K) for 3 h. The Raman results revealed that the D peak gradually increased, whereas the intrinsic G peakdecreased with increasing irradiation fluence. At the same irradiation fluence, the D peak gradually decreased,whereas the intrinsic G peak increased with increasing annealing temperature. Slow positron beam analysisdemonstrated that the density or size of irradiation defects (vacancy type) increased with higher irradiationfluence, but decreased rapidly with increasing annealing temperature. The Raman spectral analysis of samplecross sections subjected to high irradiation fluences revealed the emergence of amorphization precisely at thedepth where ion damage was most pronounced, whereas the surface retained its crystalline structure. Ramanand positron annihilation analyses indicated that the matrix graphite exhibited good irradiation resistance to Heions at 300K. However, vacancy-type defects induced by He ion irradiation exhibit poor thermal stability andcan be easily removed during annealing.
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 microstructural features and the effect of Mo addition during incubation period in FeCrNi austenitic alloy were investigated using positron annihilation technique and micro-Vickers Hardness. The electron irradiation, which could induce vacancy defects in material, was performed at room temperature up to the dose of 1.70-4 and 50-4 dpa, respectively. The defect concentration was estimated about 10-4-10-7 though standard trapping model. The added Mo could trap vacancies to form Mo-vacancy complexes, which may suppress the migration and growth of vacancy defects during electron irradiation. In addition, the microstructural evolution during electron radiation resulted in hardening, while the added Mo might to improve the hardening property of the alloy.
分类： 物理学 >> 核物理学 提交时间： 2016-09-10
摘要： We present an extraction of azimuthal correlations between two pairs of charged pions detected in opposite jets from electron-positron annihilation. These correlations may arise from the dependence of the di-pion fragmentation on the polarization of the parent quark in the process e+e−→qq¯. Due to the correlation of the quark polarizations, the cross-section of di-pion pair production, in which the pion pairs are detected in opposite jets in a dijet event, exhibits a modulation in the azimuthal angles of the planes containing the hadron pairs with respect to the production plane. The measurement of this modulation allows access to combinations of fragmentation functions that are sensitive to the quark's transverse polarization and helicity. Within our uncertainties we do not observe a significant signal from the previously unmeasured helicity dependent fragmentation function G⊥1. This measurement uses a dataset of 938~fb−1 collected by the Belle experiment at or near s√≈10.58 GeV.
分类： 物理学 >> 核物理学 提交时间： 2016-08-30
摘要： The gamma-ray observation of dwarf spheroidal satellites (dSphs) is an ideal approach for probing the dark matter (DM) annihilation signature. The latest Fermi-LAT dSph searches have set stringent constraints on the velocity independent annihilation cross section in the small DM mass range, which gives very strong constraints on the scenario to explain the the AMS-02 positron excess by DM annihilation. However, the dSph constraints would change in the velocity dependent annihilation scenarios, because the velocity dispersion in the dSphs varies from that in the Milky Way. In this work, we use a likelihood map method to set constraints on the velocity dependent annihilation cross section from the Fermi-LAT observation of six dSphs. We consider three typical forms of the annihilation cross section, i.e. p-wave annihilation, Sommerfeld enhancement, and Breit-Wigner resonance. For the p-wave annihilation and Sommerfeld-enhancement, the dSph limits would become much weaker and stronger compared with those for the velocity independent annihilation, respectively. For the Breit-Wigner annihilation, the dSph limits would vary depending on the model parameters. We show that the scenario to explain the AMS-02 positron excess by DM annihilation is still viable in the velocity dependent cases
分类： 核科学技术 >> 辐射物理与技术 提交时间： 2024-04-07
摘要： Room-temperature ferromagnetism is observed in the O+-implanted AlN films with O+ doses of 5 × 1016 cm-2 (AlN:O5×1016) and 2 × 1017 cm-2 (AlN:O2×1017). The observed magnetic anisotropy indicate that the ferromagnetism is attributed to the intrinsic properties of O+-implanted AlN films. The out-of-plane saturation magnetization (𝑀S) of the AlN:O5×1016 is about 0.68 emu/g, much higher than that of AlN:O2×1017, 0.09 emu/g, which is due to the excessively high O+ dose made more O+ ions occupy adjacent Al3+ positions in forms of antiferromagnetic coupling. Doppler broadening of positron annihilation radiation measurements demonstrate the existence of Al vacancies in the O+-implanted AlN films. The first-principles calculations suggest that the ferromagnetism originates mainly from the Al vacancies.
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： Effects of deformed dislocation on helium retention in pure iron, including the helium atoms diffusion along the dislocation line and desorption from dislocation trapping sites, were investigated. The dislocation defect was introduced in specimens by cold-rolling, and then 5 keV helium ions were implanted into the deformed specimens. Slow positron beam technology and thermal desorption spectroscopy were used to investigate the evolution of dislocation defects and the desorption behavior of helium atoms under influence of dislocation. The behaviors of S-E, W-E and S-W plots indicate clearly that lots of helium atoms remain in the deformed specimen and helium atoms combining with dislocation change the distribution of electron density. The helium desorption plot indicates that dislocation accelerates helium desorption at 293 K-600 K and facilitates helium dissociation from HenVm (n/m=1.8) cluster.
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： Dislocations would be induced after plastic deformation, which might change the mechanical properties of solids. FeCrNi austenitic model alloy and its Mo-diluted alloy were cold rolled with different degree of thickness reduction. Positrons are sensitive to point defects, which are easily trapped and annihilated around the trapping sites. The mean positron lifetimes have been used to estimate the average dislocation concentration in solids. Meanwhile, the trapping efficiency μ was calculated from the lifetime results. The trapping efficiency value is estimated about 3.31×10-7 cm3s-1 for FeCrNi alloy and 3.31×10-7 cm3s-1 for Mo-diluted alloy, respectively. The increment of the hardness value during plastic deformation is related to the increase of the dislocation density and dislocation pile up in solids.
分类： 物理学 >> 凝聚态:结构、力学和热性能 提交时间： 2017-03-26
Eryang Lu
Xingzhong Cao
Shuoxue Jin
Peng Zhang
Chunxiong Zhang
Jing Yang
Yaru Wu
Liping Guo
Baoyi Wang
摘要： The evolution of microstruture for Fe16.7Cr14.5Ni model alloy and 316 stainless steel irradiated with 140 keV He ions were studied by Positron annihilation spectroscopy. The fluences were 1016 and 5016 He ions/cm2. The irradiation temperature was room temperature and 573 K, respectively. The variation of S parameter-incident positron energy profile indicated that large amount of vacancy-type defects formed after He ion irradiation. Meanwhile, helium atoms deposited in bulk and certain amount of He-vacancy complexes were formed. The vacancy-type defects could be the major defects in track region and He-vacancy complexes would be the main defects in cascade region. The vacancy-type defects could migrate and aggregate to form vacancy clusters and even microvoids at elevated temperature irradiation. The diffusion mechanism of helium atoms might be changed at different irradiation temperature.
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 formation of Cu precipitates in Fe-0.3%Cu binary model alloy after hydrogen ion irradiation at 400 was investigated by positron annihilation spectroscopy based on slow positron beam. In order to study the effect of elevated temperature for the Cu precipitates, the annealing treatment at 400 in Fe-0.3%Cu alloy was also investigated. The S-parameters of specimens increased with the irradiation dose increased, especially in the damage peak region. H+ implantation produced a large number of vacancy-type defects in Fe-0.3%Cu alloy. Compared to the unirradiated samples, the irradiated specimens show an overall major increase in W parameter values. Experimental results indicate that Cu precipitates formed easily under lower irradiation dose at elevated temperature. No obvious Cu precipitates formed when Fe-0.3%Cu alloy was annealed for 2h at 400 .
分类： 物理学 >> 凝聚态:结构、力学和热性能 提交时间： 2017-03-26
M. H. Cui
Z. G. Wang
L. L. Pang
T. L. Shen
C. F. Yao
B. S. Li
J. Y. Li
X. Z. Cao
P. Zhang
J. R. Sun
Y. B. Zhu
Y. F. Li
Y. B. Sheng
摘要： Tungsten has been selected as one of the potential candidate materials to cover some parts of the divertor in the future International Thermonuclear Experimental Reactor (ITER). The accumulation of defects and He induced by neutron irradiation and their impact on the mechanical properties of tungsten are of very importance. In this work, the high pure polycrystalline tungsten samples were implanted by 200 keV He+ with a fluence of 5016 He+/cm2 at temperatures of room temperature(RT), 200, 400 and 800oC. Vacancy-type defects were detected in all implanted samples by means of positron annihilation spectroscopy. Vacancy-type defects produced by He implantation exist in the damaged layer and are decorated by He atoms. With increasing implantation temperature, more He atoms fill in the vacancy-type defects and make for the formation of larger defects. The nano-hardness values were measured by nano-indentation technique. It is observed that implantation hardening occurred for all the implanted samples. With increasing implantation temperature from 200 to 800 oC, the change of the average hardness values which are lower than the value at RT has a tendency of enhancement for the shallower layer and degradation for the deeper layer. The hardness variations are discussed to be the pinning effects of the defects with different density or size.
分类： 物理学 >> 核物理学 提交时间： 2023-06-18 合作期刊: 《Nuclear Science and Techniques》
摘要： The positron annihilation lifetime (PAL) technique has been applied to study the effects of gamma-ray irradiation on polymer materials, such as ethylene propylene diene monomer (EPDM), nitrile butadiene rubber (NBR), and chloroprene rubber (CR), used in various industries. PAL measurements were performed with a conventional fast-fast coincidence system. Polymer materials were irradiated with 25 Gy/h in three conditions (50, 100, 500 Gy) and 5 kGy/h in six conditions (5, 50, 200, 500, 800, and 1 000 kGy) at room temperature with a 60Co gamma-ray source. Afterwards, the irradiated samples were measured by PAL spectroscopy. The delayed lifetime spectra of irradiated and unirradiated samples were decomposed into three lifetime components. The lifetime parameters indicate the free-volume size and concentration. The longest lifetime component represents the free volume size in polymer. Free volume and intensity were multiplied as a measure of relative fraction of free-volume hole. Cross-linking and degradation could occur when a polymer is irradiated. The free volumes of polymers were decreased accordingly. These results were verified through PAL measurement. From the Tao-Eldrup model, the mean radius and free volume fraction of CR, EPDM, and NBR were calculated. We certified the decrease of the fractional free volume. From these results, this study verified the feasibility of PA application on polymers.
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： Solution annealed type 316L austenitic stainless steels were irradiated using 2 MeV Fe ions at room temperature. The implanted fluences were 2012 ions/cm2 and 1013 ions/cm2, respectively. Variable mono-energetic positron beam was performed to characterize the evolution of microstructure and irradiation induced defects. Results show that large amount of vacancy defects formed after heavy ion irradiation. In which, some of mono-vacancies might migrate to form small-sized clusters at room temperature. After irradiation, implanted Fe atoms mainly be interstitials atoms, but some Fe atoms might recombine with vacancies due to their high mobility, which could decrease the defect concentration, effectively.
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