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621.
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Atomic clocks operating at optical frequencies, with much better accuracy compared with microwave atomic clocks, have been assumed to be the next- generation time and frequency standards, Many applications will benefit from this lower frequency un- certainty of optical clocks, such as the re-definition of 'the second', i.e. one of the seven base units of the international system of units (SI), test of the time variation of fundamental physical constants and rel- ativity geodesy. Recently, the neutral atom lattice clock has achieved a lower frequency uncertainty com- pared with the optical ion clock, mainly due to the im- provement of the clock laser frequency stability refer- enced to a long high-finesse ULE cavity and the more accurate evaluation of black-body radiation shift. Strontium is an excellent candidate for the neutral atom optical clock. For the fermionic isotope of stron- tium, it has intrinsically less collision shift and the first order Zeeman shift can be removed by an inter- leaved probing approach.Recently, through the pre-cise measurement of the polarizability of strontium, the black body radiation (BBR) shift of the stron- tium lattice clock, which remains to be the limitation factor of its total frequency uncertainty, is reduced to a lower 10^-18 value.The instability of the strontium lattice clock has reached 3.1 × 10-16/√T, showing the significant advantage over the single ion optical clock. The total systematic uncertainty has reached 6.4 × 10^-18 in fractional frequency, which is the best among all optical clocks until now. 相似文献
624.
625.
Influence of thermal and resonance neutron on fast neutron flux measurement by 239Pu fission chamber
The 239Pu fission chambers are widely used to measure fission spectrum neutron flux due to a flat response to fast neutrons. However, in the meantime the resonance and thermal neutrons can cause a significant influence on the measurement if they are moderated, which could be eliminated by using 10B and Cd covers. At a column enriched uranium fast neutron critical assembly, the fission reaction rates of 239Pu are measured as 1.791× 10-16, 2.350×10-16 and 1.385× 10-15 per second for 15 mm thick 10B cover, 0.5 mm thick Cd cover, and no cover respectively, while the fission reaction rate of 239Pu is rapidly increased to 2.569× 10-14 for a 20 mm thick polythene covering fission chamber. The average 239Pu fission cross-section of thermal and resonance neutrons is calculated to be 500 b and 24.95 b with the assumption of 1/v and 1/E spectra respectively, then thermal, resonance and fast neutron flux are achieved to be 2.30× 106, 2.24× 106 and 1.04× 108 cm-2·-1. 相似文献
626.
采用二次冶炼的方法炼制出K18C镍基高温合金光谱标准样品,其中包含C,Cr,Fe等16种元素。对标准样品的均匀性与稳定性进行了考察。结果表明所研制的K18C镍基高温合金标准样品成分均匀,稳定性优良,5年内各成分量值无变化。委托多家实验室进行协作定值,选择了合适的统计学方法对各协作单位的数据进行检验和计算,获得标准样品的标准值及其扩展不确定度。 相似文献
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628.
We report the experimental realization of a ^88Sr magneto-optical trap (MOT) operating at the wavelength of 461 nm. The MOT is loaded via a 32 cm long spin-flip type Zeeman slower which enhances the MOT population by a factor of 22. The total laser power available in our experiment is about 300mW. We have trapped 1.6 × 10^8 ^88 Sr atoms with a 679nm and 707nm repumping laser. The two repumping lasers enhance the trap population and trap lifetime by factors of 11 and 7, respectively. The ^88 Sr cloud has a temperature of about 2.3 mK, measured by recording the time evolution of the absorption signal. 相似文献
629.
采用位置灵敏探测和散射离子 反冲离子飞行时间测量技术 ,测量了强相互作用区氟离子与氦原子碰撞中的转移电离截面与单电子俘获截面之比 .实验发现 ,该比值随作用强度的增加而减小 .在本工作能区 ,转移电离现象可视作一个电子被俘获、另一个电子被电离的两步过程 ;He原子二重电离的主要机制为两步机制. The cross-section ratios of the transfer ionization to the single electron capture of helium induced by fluorine ions in the strong-interaction region were measured by means of the position-sensitive-detecting and the time-of -flight (TOF) technologies. Is is found that the cross section ratios decrease with the increasing of the interaction-strength κ in this work. In the energy range of this work, the transfer ionization can be considered as a two-step process that one electron is captured by projectile... 相似文献
630.
设计合成了一种用于检测Hg2+的苯并噁唑类荧光探针4-(苯并[d]噁唑-2-基)-2-((二(2-(乙巯基)乙基)氨基)甲基)苯酚(ZY1), 用质谱、核磁氢谱和X射线单晶衍射等方法对其进行了结构表征, 并考察了其光谱性能. 研究结果表明, ZY1对Hg2+有较高的选择性和灵敏度, 它们之间的结合比为1:1, 共存离子及pH值(3.0~9.0)对其测定影响小. ZY1能够可视化检测Hg2+, 并在HeLa活细胞中实现了汞离子的成像, 这对检测生物体内的Hg2+将具有潜在的应用价值. 相似文献