D.F.R. Mildner

Characterization of deep weathering and nanoporosity development in shale - a neutron study

Abstract We used small-angle and ultra-small-angle neutron scattering (SANS/USANS) to characterize the evolution of nanoscale features in weathering Rose Hill shale within the Susquehanna/Shale Hills Observatory (SSHO). The SANS/USANS techniques, here referred to as neutron scattering (NS), characterize porosity comprised of features ranging from approximately 3 nm to several micrometers in dimension. NS was used to investigate shale chips sampled by gas-powered drilling (“saprock”) or by hand-augering (“regolith”) at ridgetop. At about 20 m depth, dissolution is inferred to have depleted the bedrock of ankerite and all the chips investigated with NS are from above the ankerite dissolution zone. NS documents that 5-6% of the total ankerite-free rock volume is comprised of isolated, intraparticle pores. At 5 m depth, an abrupt increase in porosity and surface area corresponds with onset of feldspar dissolution in the saprock and is attributed mainly to peri-glacial processes from 15 000 years ago. At tens of centimeters below the saprock-regolith interface, the porosity and surface area increase markedly as chlorite and illite begin to dissolve. These clay reactions contribute to the transformation of saprock to regolith. Throughout the regolith, intraparticle pores in chips connect to form larger interparticle pores and scattering changes from a mass fractal at depth to a surface fractal near the land surface. Pore geometry also changes from anisotropic at depth, perhaps related to pencil cleavage created in the rock by previous tectonic activity, to isotropic at the uppermost surface as clays weather. In the most weathered regolith, kaolinite and Fe-oxyhydroxides precipitate, blocking some connected pores. These precipitates, coupled with exposure of more quartz by clay weathering, contribute to the decreased mineral-pore interfacial area in the uppermost samples. These observations are consistent with conversion of bedrock to saprock to regolith at SSHO due to: (1) transport of reactants (e.g., water, O2) into primary pores and fractures created by tectonic events and peri-glacial effects; (2) mineral-water reactions and particle loss that increase porosity and the access of water into the rock. From deep to shallow, mineral-water reactions may change from largely transport-limited where porosity was set largely by ancient tectonic activity to kinetic-limited where porosity is changing due to climate-driven processes.

Acceptance diagrams for curved neutron guides

Abstract The method of acceptance diagrams is used to obtain analytic expressions for the intensity and spatial and angular distributions of the neutrons transmitted along a one-dimensional curved guide, provided that the length of the guide is greater than the minimum length necessary to eliminate direct radiation. The acceptance area for curved guides is parabolic in shape, and shows the asymmetry in the spatial distribution of the transmitted neutrons. From these formulations the average number of reflections can be determined as a function of wavelength.

Neutron Transmission of Single-Crystal Sapphire

Thermal and cold neutron transmission time-of-flight measurements have been taken on blocks of single-crystal sapphire. The transmission characteristics of single-crystal sapphire are not altered by irradiation for a number of years within a beam port at a reactor. Cooling the filter is only useful for the production of beams with the longest wavelengths. Even for poorer grade sapphire there is no degradation of the neutron transmission characteristics.

Nanoscale constraints on porosity generation and fluid flow during serpentinization

Field samples of olivine-rich rocks are nearly always serpentinized—commonly to completion—but, paradoxically, their intrinsic porosity and permeability are diminishingly low. Serpentinization reactions occur through a coupled process of fluid infiltration, volumetric expansion, and reaction-driven fracturing. Pores and reactive surface area generated during this process are the primary pathways for fluid infiltration into and reaction with serpentinizing rocks, but the size and distribution of these pores and surface area have not yet been described. Here, we utilize neutron scattering techniques to present the first measurements of the evolution of pore size and specific surface area distribution in partially serpentinized rocks. Samples were obtained from the ca. 2 Ma Atlantis Massif oceanic core complex located off-axis of the Mid-Atlantic Ridge and an olivine-rich outcrop of the ca. 1.1 Ga Duluth Complex of the North American Mid-Continent Rift. Our measurements and analyses demonstrate that serpentine and accessory phases form with their own, inherent porosity, which accommodates the bulk of diffusive fluid flow during serpentinization and thereby permits continued serpentinization after voluminous serpentine minerals fill reaction-generated porosity.

Neutron focusing optic for submillimeter materials analysis

A neutron lens constructed with polycapillary glass fibers is used to focus a 50×45 mm2 beam exiting a cold neutron guide onto a spot of 0.53 mm (full width at half maximum) with a current density gain of 80. The characteristics of the lens are presented. This lens is designed to enhance the detection limit and lateral resolution for prompt gamma activation analysis using cold neutron beams.

The neutron transmission of single-crystal sapphire filters

The wavelength dependence of the transmission probability of a beam of neutrons through super optical quality single-crystal sapphire at room temperature has been measured. The measurements extend over the neutron wavelength range 0.05–1.2 nm. Sharp dips in the transmission caused by Bragg reflection are not found. The data below the dip around 0.2 nm in the cross section have been fitted to a function that accounts for multiphonon scattering. Measurements performed on single crystals with increased lattice distortion (or mosaic spread) show an increase in the cross section at long wavelengths. The attenuation of room-temperature Al2O3 is not significantly different from that for liquid-nitrogen-cooled MgO.

Small-angle neutron scattering resolution with refractive optics

The resolution of small-angle neutron scattering instruments is investigated for the case where refractive optics (lenses or prisms) are used. The appropriate equations are derived to describe the position and the spatial variance of the neutron beam at the detector in the horizontal and vertical directions, and the minimum value of the scattering vector. This is given for the spectrometer without any additional optics, and with the insertion of converging lenses or prisms. The addition of the lenses decreases the sample-aperture contribution to the resolution to enable an increase of neutron current at the sample. They also reduce the size of the penumbra of the beam at the detector, thereby lowering the minimum value of the scattering vector. The prisms correct the effect of gravity on the vertical beam position, and make the beam spot less asymmetric.

Prompt gamma activation analysis enhanced by a neutron focusing capillary lens

Abstract A focusing neutron lens using glass polycapillary fibers has been introduced successfully into a prompt gamma activation analysis (PGAA) instrument placed at the exit of a cold neutron guide. The neutron current density gain of the lens is 80, averaged over the focused beam size of 0.53 mm diameter. PGAA measurements have been made on submillimeter particles of gadolinium and cadmium. The results indicate that elemental sensitivities of measurements are increased by ∼ 60, and that particles of sizes smaller than 0.5 mm can be discerned using the focusing lens. The measured gain in prompt gamma signals for these particles is less than anticipated, probably due to alignment difficulties. Gamma ray background associated with the lens is discussed and improvements are suggested.

Macromolecular orientation in hot stretched and injection moulded polystyrene

Abstract Small-angle neutron scattering (SANS) and optical birefringence measurements were used to investigate a wide range of macromolecular orientation in hot stretched and injection moulded polystyrene samples. These two techniques show similar trends even though each is sensitive to different forms of macromolecular orientation. The hot stretched samples are found to deform nonaffinely in relation to the external drawing (for draw ratios higher than 3). The cold condition injection moulded samples showed more orientation than the hot condition injection moulded samples, which were not birefringent even though they did have a residual orientation as seen by SANS. SANS is evidently sensitive to factors controlling orientation at extremes for which birefringence is not.

Correction of optical aberrations in elliptic neutron guides

Modern, nonlinear ballistic neutron guides are an attractive concept in neutron beam delivery and instrumentation because they offer increased performance over straight or linearly tapered guides. However, like other ballistic geometries they have the potential to create significantly non-trivial instrumental resolution functions. We address the source of the most prominent optical aberration, namely coma, and we show that for extended sources the off-axis rays have a different focal length from on-axis rays, leading to multiple reflections in the guide system. We illustrate how the interplay between coma, sources of finite size, and mirrors with non-perfect reflectivity can therefore conspire to produce uneven distributions in the neutron beam divergence, a source of complicated resolution functions. To solve these problems, we propose a hybrid elliptic-parabolic guide geometry. Using this new kind of neutron guide shape, it is possible to condition the neutron beam and remove almost all of the aberrations, whilst providing the same performance in beam current as a standard elliptic neutron guide. We highlight the positive implications for neutron scattering instruments that this new shape can bring. (c) 2012 Elsevier B.V. All rights reserved. (Less)