Instability of the Solid-Liquid Interface of 4He during a Large Transformation

We report an instability of ⁴He crystal surface during a large modification of its shape observed by schlieren photography. ⁴He crystal was nucleated on an ultrasound transducer which was placed in the upper part of the sample cell facing the bottom. The crystal was grown on the transducer and the lowest part of it eventually touched the bottom of the cell. Almost immediately, the crystal started to change shape very quickly to adjust itself to the new boundary condition. During this large deformation, the hexagonal shape of the crystal surrounded by facets became unstable and the surface became corrugated. The corrugation pattern during the deformation was observed by a high-speed camera. The results were compared with Kelvin-Helmholtz type instability which Uwaha and Nozières predicted.      

Cavitation in Liquid Helium Observed in a Flow Due to a Vibrating Quartz Fork

A quartz fork vibrating at high amplitude is used to study cavitation in He I and He II along the saturated vapor pressure (SVP) curve and at slightly elevated pressures. Cavitation is observed as a breakdown of the resonance response at critical velocity when slowly sweeping the frequency of the drive across the resonance and confirmed by visual observation of a bubble occurring in He II in the space between the prongs of the fork. On decreasing the temperature from 4.2 K along the SVP curve the critical velocity slowly increases from about 0.4 m/s to 1 m/s, until a steep increase up to about 2 m/s occurs within about 20 mK just below the superfluid transition. We discuss our results, including the measured dependence of the critical velocity versus overpressure at fixed bath temperature.      

Preliminary Measurements on Grid Turbulence in Liquid 4He

A grid has been pulled through a column of liquid helium at speeds as high as 1 m/s and at temperatures as low as 90 mK. A 300 micrometer Ge thermometer with response time of less than 1 ms measured the temperature rise resulting from the decay of the turbulence generated. It is believed that homogeneous, isotropic quantum turbulence was formed, since mesh Reynolds numbers in excess of 100,000 were created. The rates and power spectra of the energy increases detected in the helium after grid-pulls are determined. The results are compared to other quantum and classical results, and to the theory of the Kelvin wave cascade in a viscosity-free fluid.      

Bose-Einstein Condensation of Magnons in Superfluid 3He

The possibility of Bose-Einstein condensation of excitations has been discussed for a long time. The phenomenon of the phase-coherent precession of magnetization in superfluid ³He and the related effects of spin superfluidity are based on the true Bose-Einstein condensation of magnons. Several different states of coherent precession has been observed in ³He-B: homogeneously precessing domain (HPD); persistent signal formed by Q-balls at very low temperatures; coherent precession with fractional magnetization; and a mode of the coherent precession in compressed aerogel. The coherent precession has been also found in ³He-A in compressed aerogel. Here we demonstrate that all these cases are examples of a Bose-Einstein condensation of magnons, with the magnon interaction term in the Gross-Pitaevskii equation being provided by different types of spin-orbit coupling in the background of the coherent precession.      

He3管检测系统的设计研制

He3管探测器是补偿中子测井仪器中的一个关键部件,它的性能的好坏直接影响着整个仪器的性能。文章通过对3He管探测器在Am-Be中子源下的能谱分析,提出了一套完整可行的He3管检测方案,并给出了实现过程。     

氦原子对镍金属中刃型位错运动行为影响的原子模拟

应用分子动力学(MD)技术和改进分析型嵌入原子法(MAEAM)研究氦原子对镍金属中(a/2)?110?{111}刃型位错迁移行为的影响。首先通过计算晶格的Burgers矢量分布,确定两刃型分位错之间的平衡距离(Ded)约为25.95?,而它们之间的堆垛层错能(Esf)约为108 mJ/m2。然后研究He原子在晶格中不同位置的形成能(Ef),发现He原子在镍金属晶体的张力区域受到晶格的吸引,而其在压缩区域则受到晶格排斥。通过探讨 He 原子与刃型位错之间的相互作用发现,相比于置换He原子而言,间隙He原子对位错迁移行为的影响更强烈。最后,研究表明间隙He原子的迁移在第一个分位错跨过后表现更明显,同时刃型位错也为He原子迁移提供了更快速的扩散路径。     

Migration of As, and H/He ages, in groundwater from West Bengal: Implications for monitoring

From 2002 to 2010 inclusive we monitored concentrations of arsenic (As) and major ions (Ca, Mg, Sr, Na, K, Fe, Mn, Cl, and SO₄) in groundwater from 14 domestic wells and three piezometer nests in a shallow aquifer (<60 m depth), and 3 wells in a deep aquifer (>70 m depth), in southern West Bengal, India. In the deep aquifer, concentrations of As did not change over time despite increases in the concentration of Fe in two wells. The shallow aquifer occurs in two sedimentological settings: palaeo-channel and palaeo-interfluve. At the top of the shallow aquifer of the palaeo-channel, decreases in all constituent concentrations with time, and an ³H/³He age of 1.4 years, proves that the aquifer is beginning to be flushed of pollutants. In As-polluted groundwater (>50 μg/L As) tapped from deeper grey sands of the shallow, palaeo-channel, aquifer, concentrations of As were mostly stable over time, but both increases and decreases occurred with time in response to downward migration of the chemically-stratified water column. In groundwater tapped from Pleistocene brown sands, the concentration of As remained either low and stable (<2 μg/L As), or increased at rates up to 34 μg/L per year. The increases were caused by the flow of As-rich groundwater either downward into brown sand at the base of palaeo-channels, or laterally into a confined, unpolluted, palaeo-interfluvial, aquifer of brown sand that lies regionally beneath a palaeosol.Under the present pumping regime, the prognosis for As-pollution in the shallow aquifer is complex. Wells in brown sand may become polluted over timescales of as little as 2 years, whilst some wells tapping As-polluted groundwater from grey sand will become fit for potable use (<50 μg/L) within a few decades. The evidence of flushing, and of declining As in some of the groundwater from palaeo-channels, which are conduits for recharge of the confined, As-free, palaeo-interfluve aquifer, and probably also the deeper aquifer, offers hopes that the spread of As-pollution will be limited.     

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大牛地气田盒1段气层产能主控因素分析

大牛地气田盒1段砂岩厚度大,非均质性较强,气层主控因素多,有效厚度较大,但产能普遍不高,开发地质研究程度有待提高。本文利用测井、薄片鉴定、压汞实验、试井等测试分析资料,对气层四性关系和产能主控因素进行研究,总结分析盒1段高产气层特征与成因,并在分级评价的基础上进行气层纵横向精细刻画。认为盒1段物性、含气性与有效厚度等参数均影响着气层的产能;含气性较差是气层产能普遍不高的主因,岩相与物性、成藏过程的运移通道以及上覆局部封盖层共同控制着气层最后含气丰度,而在沉积微相控制下的砂体展布和后期成岩作用加强的非均质性影响着含气性分布特征。     

3He Spin Filter for Neutrons

The strongly spin-dependent absorption of neutrons in nuclear spin-polarized ³He opens up the possibility of polarizing neutrons from reactors and spallation sources over the full kinematical range of cold, thermal and hot neutrons. This paper gives a report on the neutron spin filter (NSF) development program at Mainz. The polarization technique is based on direct optical pumping of metastable ³He atoms combined with a polarization preserving mechanical compression of the gas up to a pressure of several bar, necessary to run a NSF. The concept of a remote type of operation using detachable NSF cells is presented which requires long nuclear spin relaxation times of order 100 hours. A short survey of their use under experimental conditions, e.g. large solid-angle polarization analysis, is given. In neutron particle physics NSFs are used in precision measurements to test fundamental symmetry concepts.