Neutron scattering studies of structural transformations and vibrational spectra of ice after high pressure treatment

Abstract

The transitions of the recovered high-pressure phase ice VIII first to high-density amorphous (hda) and low-density amorphous ices, and finally to cubic Ic, and hexagonal Ih ice were observed at heating using real-time neutron diffraction. Inelastic incoherent neutron scattering measurements on the hdu ice, ice Ih and high-pressure phase ice VI revealed similarity between the amorphous phase and crystalline ice VI and led to the new proposition that hda ice consists of two interpenetrating hydrogen-bounded networks with no hydrogen bonds between “sublattices”.     

Crossover between the thermodynamic and nonequilibrium scenarios of structural transformations of H<Subscript>2</Subscript>O I<Emphasis Type="Italic">h</Emphasis> ice during compression

A detailed investigation of different scenarios of structural transformations of H₂O Ih ice during compression to a pressure of 2 GPa in the temperature range from 77 to 200 K is performed. In the range of temperatures and pressures being treated, detailed data are obtained for the density and the shear modulus for different phases of ice including the hda, IX, and XII phases. The experimentally obtained correlations for the density and ultrasonic velocities, with due regard for the available data of structural investigations, are used to identify the transformation sequences Ih→hda (below 135 K), Ih→II→VI (above 165 K), and Ih→IX→VI (from 155 to 180 K). In the vicinity of the crystallization temperature of amorphous ice, i.e., at about 140 K, an anomalous transformation pattern is observed, which is interpreted as predominantly the Ih→XII phase transition. The temperature crossover is discussed between the mode of solid-phase amorphization (Ih→hda) and crystal-crystal transitions, as well as the metastable nature of IX ice and the mechanism of solid-phase amorphization.     

Vibration properties of low-fraction hydrogen in deuterium ices

Inelastic incoherent neutron scattering spectra of D2O high-density amorphous （hda） ice, ice-Ⅷ and ice-Ⅱ mixed with small amount of H2O （（5%） have been measured recently on high-energy transfer spectrometer at Rutherford Appleton Laboratory （UK）. The hydrogen atom on D2O ice lattices has three distinguished vibrational modes, two bending at low frequencies and one stretching at high frequencies, and their frequencies are slightly different for different phases of ice. It was found that the lower one of the bending modes is located at -95 meⅤ for hda-ice, at -95 meⅤ for ice-Ⅷ and at -96 meⅤ for ice-Ⅱ and they are all lower than the value of 104 meⅤ for ice-Ih. It was also measured that the O-D and O-H covalent bond stretching modes of ice-Ⅷ are at -315 and -425 meⅤ, ice-Ⅱ at 307 and -415 meⅤ, hda-ice at 312 and -418 meⅤ, respectively. They are significantly higher than the values of ice-Ih at -299 and -406 meⅤ, respectively.     

Hydrogen mean force and anharmonicity in polycrystalline and amorphous ice

The hydrogen mean force from experimental neutron Compton profiles is derived using deep inelastic neutron scattering on amorphous and polycrystalline ice. The formalism of mean force is extended to probe its sensitivity to anharmonicity in the hydrogen-nucleus effective potential. The shape of the mean force for amorphous and polycrystalline ice is primarily determined by the anisotropy of the underlying quasi-harmonic effective potential. The data from amorphous ice show an additional curvature reflecting the more pronounced anharmonicity of the effective potential with respect to that of ice Ih.     

The correlation of molecular orientation in liquid water by neutron and X-ray scattering

The neutron diffraction structure factor for heavy water at 22°c has been measured for momentum transfers, ?Q, up to a Q of 13 å ^-1. The results have been interpreted by abstracting the molecular structure effects as far as possible and using models for the correlation of orientation of pairs of molecules ranging from completely uncorrelated orientation to that for adjacent molecules in ice I. Neither these nor any of the popular models for the structure of water, nor a non-hydrogen bonded model, fit the neutron diffraction data over the whole range of Q. Nevertheless, information about the molecular-centres structure factor is obtained, especially for Q up to about 3 å ^-1.

The method of analysis is generalized and applied to x-ray diffraction data and in particular to x-ray data for liquid water. It is shown that this method of analysis has some advantages over the conventional one. The effect of correlation of molecular orientation is less than that of molecular structure on x-ray scattering by water and rather more extensive information is obtained for the molecular-centres structure factor than from neutron data—if the conventional assumption of atomic electron clouds is accepted.

A comparison of the x-ray and neutron scattering data for liquid heavy water shows that the two sets of data are reasonably consistent for any of the usual models for water, except for the region of Q between 3 and 5 å ^-1. The molecular-centres structure factor of water has a weak double humped first maximum with peaks at 2·0 å ^-1 and 2·9å ^-1, probably followed by a damped oscillation. This contrasts with atomic liquids but is similar to that found in liquid carbon tetrachloride. The structure of liquid water is not, therefore, necessarily unique or unusual.     

Ground tests of nuclear planetology instruments at the JINR experimental facility

In this work the results of ground tests with active neutron spectrometer DAN (Dynamic Albedo of Neutrons) are presented, which have performed at the Joint Institute for Nuclear Research to simulate space experiments on martian or lunar surfaces and to test ability of active neutron methods to detect layers of subsurface water ice or water at depths in range 0–40 cm. For this experiment we assembled thick models of soil (20–30 metric tons) from a dry material similar in the elemental composition with martian and lunar regolith. Polyethylene buried inside the target at different depths was used as a simulant of thin water/water ice layer.     

Prompt fission neutron spectra of <Superscript>238 </Superscript>U(n,f) above emissive fission threshold

Model calculations were performed to interpret prompt fission neutron spectra (PFNS) of the ²³⁸ U(n, f) reaction for incident neutron energies -18 MeV. Pre-fission (pre-saddle) reaction neutron spectra were calculated with Hauser-Feshbach statistical model, ²³⁸ U fission and reaction cross-section data being described consistently. The increase of the cut-off energy of (n, nf) reaction neutron spectra with excitation energy of fissioning nucleus is described. For -9 MeV the low-energy PFNS component, which is due to the contribution of pre-fission (n, nf) neutrons, is compatible with measured data. Average energy of prefission (n, nf) neutrons is shown to be rather dependent on . For -18 MeV, a decrease of measured PFNS average neutron energies is interpreted. Spectra of neutrons, evaporated from fission fragments, were approximated as a sum of two Watt distributions. The reduced fission fragment velocity is assumed for the neutron emission during fragment acceleration. Several interpretations of observed soft neutron excess are investigated, i.e., possible uncertainties of emissive fission contributions and additional neutron source. We claim the soft neutron excess cannot be attributed to the pre-saddle neutrons contribution.Received: 3 February 2003, Revised: 24 April 2003, Published online: 9 October 2003PACS: 25.85.Ec Neutron-induced fission     

D-T中子与平板型聚乙烯材料作用的次级中子角度谱实验研究

基于反冲质子法建立了一种测量D-T中子与平板型宏观样品作用的次级中子角度谱的实验方法.为保证探测器的能量线性并在较低的中子有效测量下阈(0.5 MeV)情况下获得好的中子-伽马射线甄别性能,采用高、低能段分别测量的方法.采用事件记录法,同时记录了次级中子和伴随伽马射线的脉冲形状甄别和脉冲幅度二维信息,利用基于ROOT数据分析平台编写的离线数据分析程序,完成了伴随伽马射线的挑选和扣除,以及高、低两能段反冲质子谱的拼接,并成功的将神经网络技术应用于中子能谱的解谱,获得了D-T中子与9和18 cm厚平板型聚乙烯材料作用的0.5-15 MeV的次级中子角度谱实验结果.实验模型的MC模拟由MCNP5完成,数据库采用ENDF-VI,实验结果和MC计算结果在实验不确定度范围内一致. 关键词： D-T中子 积分中子学 反冲质子法 次级中子能谱     

Dirac quasinormal modes of two-dimensional charged dilatonic black holes

Following some recent unexpected hints of neutron production in high-voltage atmospheric discharges, we present a measurement of the neutron flux in plasma discharges in electrolytic cells. We use two different types of neutron detectors, polyallyl diglycol carbonate (PADC, aka CR-39) tracers and indium disks. At 95 % C.L. we provide an upper limit of 1.5 neutrons cm \(^{-2}\)  s \(^{-1}\) for the thermal neutron flux at \({\approx } 5\) cm from the center of the cell. Allowing for a higher energy neutron component, the largest allowed flux is 64 neutrons cm \(^{-2}\)  s \(^{-1}\) . This upper limit is two orders of magnitude smaller than the signal previously claimed in an electrolytic cell plasma discharge experiment. Furthermore the behavior of the CR-39 is discussed to point out possible sources of spurious signals.     

Continuous-wave operation of a broadly tunable thermoelectrically cooled external cavity quantum-cascade laser

Continuous-wave operation of an external cavity quantum-cascade laser on a thermoelectric cooler is reported. The active region of the gain element was based on a bound-to-continuum design emitting near 5.15 microm. The external cavity setup was arranged in a Littrow configuration. The front facet of the gain chip was antireflection coated. The laser could be tuned over more than 170 cm(-1) from 4.94 to 5.4 microm and was single mode over more than 140 cm(-1). The output power was in excess of 10 mW over approximately 100 cm(-1) and in excess of 5 mW over approximately 130 cm(-1) at -30 degrees C.     

Freezing of glycerol-water mixtures under pressure

We investigated freezing of pure glycerol as well as glycerol-water (GW) mixtures with 3:1 and 3:2 volume fractions as a function of pressure in the 0-10?GPa range by ruby fluorescence spectroscopy and neutron scattering. We find that the glass transition pressure increases from 5.5?GPa for pure glycerol to 6.5?GPa for the 3:1?GW mixture, with unusually small pressure gradients above. For higher water concentrations close to 3:2, phase separation occurs above 2?GPa where most of the water is expelled in the form of ice VII. The results suggest that glycerol is able to effectively hydrogen bond not more than ≈2.5 H(2)O molecules per glycerol, which seems to support conclusions from molecular dynamics simulations. The data indicate that these fluids could become important as pressure transmitting media for neutron scattering in the 0-7?GPa range, including at low temperatures.     

浦项中子装置上209Bi中子全截面测量(英文)

在浦项中子装置(PNF) 的100 MeV电子直线加速器上利用透射法和飞行时间法测量了209Bi 的0.1~100 eV的中子全截面。文中对于实验装置特点、用吸收片法拟合本底、拟合中子飞行距离和时间零点的处理过程做了较为详细的描述。测量结果与以前发表的实验数据以及评价数据ENDF/B-VII.1 做了比较,比较结果符合得较好。     

Temperature dependence of structure and density for D?O confined in MCM-41-S

Using neutron diffraction, we have tracked the temperature dependence of structural properties for heavy water confined in the nanoporous silica matrix MCM-41-S. By observing the correlation peak corresponding to the pore-pore distance, which is determined by the scattering contrast between the silica and the water, we monitored the density of the confined water. Concurrently, we studied the prominent first diffraction peak of D(2)O at ≈ 1.8 ?(-1), which furnishes information on the microscopic arrangement of the water molecules. The data show the presence of a density maximum at ≈ 275 K (± 10 K), a property similar to bulk water, and the occurrence of a density minimum at ≈ 180 K (± 10 K). The prominent diffraction peak of D(2)O is found to shift and sharpen over a wide T range from 200 to 270 K, reflecting structural changes that are strongly correlated with the changes in density. We also observe the continuous formation of external ice, arising from water expelled from the pores while expansion takes place within the pores. An efficient method for monitoring the density of the confined D(2)O using a triple-axis spectrometer is demonstrated.     

Improved experimental limit on the electric dipole moment of the neutron

An experimental search for an electric dipole moment (EDM) of the neutron has been carried out at the Institut Laue-Langevin, Grenoble. Spurious signals from magnetic-field fluctuations were reduced to insignificance by the use of a cohabiting atomic-mercury magnetometer. Systematic uncertainties, including geometric-phase-induced false EDMs, have been carefully studied. The results may be interpreted as an upper limit on the neutron EDM of |d(n)|< 2.9 x 10(-26)e cm (90% C.L.).     

Neutron dosimetric measurements in shuttle and MIR

Detector packages consisting of thermoluminescence detectors (TLD), nuclear emulsions and plastic track detectors were exposed at identical positions inside MIR space station and on shuttle flights inside Spacelab and Spacehab during different phases of the solar cycle. The objectives of the investigations are to provide data on charge and energy spectra of heavy ions, and the contribution of events with low-energy deposit (protons, electrons, gamma, etc.) to the dose, as well as the contribution of secondaries, such as nuclear disintegration stars and neutrons. For neutron dosimetry 6LiF (TLD600) and 7LiF (TLD700) chips were used both of which have almost the same response to gamma rays but different response to neutrons. Neutrons in space are produced mainly in evaporation and knock-on processes with energies mainly of 1-10 MeV and up to several 100 MeV, respectively. The energy spectrum undergoes continuous changes toward greater depth in the attenuating material until an equilibrium is reached. In equilibrium, the spectrum is a wide continuum extending down to thermal energies to which the 6LiF is sensitive. Based on the difference of absorbed doses in the 6LiF and 7LiF chips, thermal neutron fluxes from 1 to 2.3 cm-2 s-1 are calculated using the assumption that the maximum induced dose in TLD600 for 1 neutron cm-2 is 1.6 x 10(-10) Gy (Horowitz and Freeman, Nucl. Instr. and Meth. 157 (1978) 393). It is assumed that the flux of high-energy neutrons is at least of that quantity. Tissue doses were calculated taking as a mean ambient absorbed dose per neutron 6 x10(-12) Gy cm2 (for a10 MeV neutron). The neutron equivalent doses for the above-mentioned fluxes are 52 micro Gy d-1 and 120 micro Gy d-1. In recent experiments, a personal neutron dosimeter was integrated into the dosimeter packages. First results of this dosimeter which is based on nuclear track detectors with converter foils are reported. For future measurements, a scintillator counter with anticoincidence logic is under development.     

Ice XII in its second regime of metastability

We present neutron powder diffraction results which give unambiguous evidence for the formation of the recently identified new crystalline ice phase [2], labeled ice XII, in completely different conditions. Ice XII is produced here by compressing hexagonal ice I(h) at T = 77, 100, 140, and 160 K up to 1.8 GPa. It can be maintained at ambient pressure in the temperature range 1.5相似文献   

Evidence for TeV gamma-ray emission from a region of the galactic plane

Gamma-ray emission from a narrow band at the galactic equator has previously been detected up to 30 GeV. We report evidence for a TeV gamma-ray signal from a region of the galactic plane by Milagro, a large-field-of-view water Cherenkov detector for extensive air showers. An excess with a significance of 4.5 standard deviations has been observed from the region of galactic longitude l E (40 degrees, 100 degrees) and latitude /b/ < 5 degrees. Under the assumption of a simple power law spectrum, with no cutoff in the EGRET-Milagro energy range, the measured integral flux is phi gamma(>3.5 TeV) = (6.4 +/- 1.4 +/- 2.1) x 10(-11) cm(-2) s(-1) sr(-1). This flux is consistent with an extrapolation of the EGRET spectrum between 1 and 30 GeV in this galactic region.     

NaCl-CaCl_2盐水低温拉曼光谱特征及在包裹体分析中的应用

准确判断流体包裹体的含盐类型及盐度的定量计算一直是流体包裹体研究的重要内容。传统测试包裹体盐度的方法主要是冷冻法,为了克服冷冻相变不易观测等缺点,作者利用激光拉曼光谱在低温(-180℃)下测试了NaCl-H2O,CaCl2-H2O及NaCl-CaCl2-H2O体系溶液图谱,发现在低温(-180℃)下NaCl.2H2O和CaCl2.6H2O两水合物在3 420和3 432 cm-1处峰值与冰晶的3 092 cm-1峰值比和盐度之间有很好的线性关系,建立了流体包裹体盐度的工作曲线。该方法用人工合成碳酸盐岩含烃盐水包裹体进行了验证,并在东营凹陷丰深6井石英盐水包裹体进行了实例分析。结果表明,低温拉曼光谱技术不仅可识别盐的类型,也可确定盐度,应用效果良好。     

Ab initio study of the structure and dynamical properties of crystalline ice

We investigated the structural and dynamical properties of a tetrahedrally coordinated crystalline ice from first principles based on density functional theory within the generalized gradient approximation with the projected augmented wave method. First, we report the structural behaviour of ice at finite temperatures based on the analysis of radial distribution functions obtained by molecular dynamics simulations. The results show how the ordering of the hydrogen bonding breaks down in the tetrahedral network of ice with entropy increase, in agreement with the neutron diffraction data. We also calculated the phonon spectra of ice in a 3× 1× 1 supercell using the direct method. So far, due to the direct method used in this calculation, the phonon spectra are obtained without taking into account the effect of polarization arising from dipole–dipole interactions of water molecules, which is expected to yield the splitting of longitudinal and transverse optic modes at the Γ point. The calculated longitudinal acoustic velocities from the initial slopes of the acoustic mode are in reasonable agreement with the neutron scattering data. Analysis of the vibrational density of states shows the existence of a boson peak at low energy of the translational region, a characteristic common to amorphous systems.     

Structure of a new dense amorphous ice

The detailed structure of a new dense amorphous ice, VHDA, is determined by isotope substitution neutron diffraction. Its structure is characterized by a doubled occupancy of the stabilizing interstitial location that was found in high density amorphous ice, HDA. As would be expected for a thermally activated unlocking of the stabilizing "interstitial," the transition from VHDA to LDA (low-density amorphous ice) is very sharp. Although its higher density makes VHDA a better candidate than HDA for a physical manifestation of the second putative liquid phase of water, as for the HDA case, the VHDA to LDA transition also appears to be kinetically controlled.