Practical Aspects of Planning and Conducting SAS Experiments

Abstract

This chapter begins with tips on writing a successful beam time proposal followed by the guidelines on the optimization of the instrument configuration that allow maximizing output of the high quality experimental data. Methods of the instrument and sample background subtraction are described along with the choice of optimal sample thickness and other parameters that permit minimizing contribution of multiple scattering. Placing SAS data on absolute scale is important as a sensitive indicator of the applicability of the structural model chosen for the SAS data analysis. This can be achieved by using absolute standards or direct beam measurements. Accurate estimation of the effective thickness of powder samples is required for placing SAS data on absolute scale. Several methods of such estimation using different physical parameters are described along with the contrast variation methods using liquid solutions of protonated and deuterium substituted solvents and pressurized gases as contrast matching media. Applying for Beam TimeThe vast majority of neutron and photon sources operate as "user facilities" that provide access to neutron research instruments for researchers from industry, universities, and government agencies. These facilities are operated with different modes of access. At SNS and HFIR, access is available via a general user program. Calls for proposals are usually issued twice a year, and beam time is allocated on a competitive basis after a peer-review process in which proposals are reviewed for scientific merit. A fraction of total available beam time (~25 % at SNS and HFIR) is allocated for scientific research conducted by instrument personnel and for instrument and sample environment upgrades, etc. A limited amount of beam time might be provided via a "rapid access" route for conducting urgent experiments or feasibility measurements. There is no access charge for researchers whose results are made available to the scientific community by being published in the open scientific literature. A fraction of beam time may be used for confidential, proprietary research, which can be performed only under full cost recovery. Most user facilities have financial assistance programs covering travel, room and board, etc. to encourage new users to acquire their first experience in neutron and x-ray scattering.SAS instruments are in great demand and are usually oversubscribed by a factor of 2-3. Because of the strong competition for beam time, developing good proposal writing skills is essential. Some tips that might help first-time users obtain beam time follow.One of the most important factors is choosing the "right" facility for a particular SAS experiment, including the type of radiation (neutrons vs. x-rays), power (brilliance), and the availability of the relevant sample environment. The higher radiation flux provided by synchrotron sources permits better statistics to be obtained in a shorter time, which could be critical for studying weakly scattering samples or for experiments where it is necessary to ...

Technical Aspects of the Pore Structure in Shale Measured by Small-Angle and Ultrasmall-Angle Neutron Scattering: A Mini Review

“…The ratio between the total volume of spheres and the volume of the sample holder is ∼0.52. Therefore, t eff ≈ 0.5 t …”

“…The effect of multiple scattering is strongest in the low- q region, and its probability scales with sample thickness ( t ) and neutron wavelength as t λ 2 . , In one study, the shale samples were thinned down to 0.25 mm, and there were still some multiple scattering effects in the USANS region ( q < 10 –4 Å –1 ) . There is no general method for correcting data with the multiple scattering effect . There are several empirical approaches, such as the MUX routine .…”

“…Use of particulates ensures that SANS is measured from a representative sample of shale, in which structural anisotropy is eliminated. The particle size of shale particulates, however, needs to be around 0.5 mm to minimize scattering from interparticle voids in both the small-angle scattering (SAS) and ultrasmall-angle scattering (USAS) domains . This can be achieved by filtering particulates through a sieve with a mesh size of 0.56 mm and collecting those that do not pass through a second sieve with a mesh size of 0.50 mm.…”

“…After particulates are loaded into a sample cell, the interspacing distance of the cell (nominal thickness) cannot be used to calibrate the data as a result of interparticle voids. An effective thickness can be calculated as t eff = t (ρ bulk /ρ app ), where t is the thickness of the cell, ρ bulk is determined by weighting the sample loaded in the cell with the volume of the cell, and ρ app is defined as the mass of a skeleton normalized by the sample volume (skeleton volume + intraparticle voids) . If the porosity is low, ρ app is replaces with ρ skeleton , which is measured using helium as the displacement gas.…”