Small Angle Neutron Scattering SANS (Neutron scattering) by Samuel Ghebru

Outline What is Neutron Scattering ~ SANS Neutron Nuclear Interaction & Magnetic Interaction SANS Instrumentation Physics Behind SANS Experimental Examples SANS Application Disadvantages of SANS Sources

Small Angle Neutron Scattering The technique of small-angle neutron scattering (SANS) is used for studying the structure of a material on length scale of 10 to 1000 Å. In particular, SANS is used to study the shapes and sizes of the particles dispersed in homogeneous medium. It involves scattering of a monochromatic beam of neutrons from the sample and measuring the scattered neutron intensity as a function of the scattering angle. Neutrons are not electrically charged with magnetic spin ½. Typical wavelength = 2 angstrom and Energy available at most reactors is 0.1 – 500meV. Small scattering angles (~ 0.5–10°). Neutrons can penetrate a sample far better than other scattering techniques. This is b/c neutrons are electrically neutral. Two types of neutron scattering: Scattering due to nuclear interaction and a magnetic interaction due to unpaired electron.

Neutron Resources In nuclear reactors centers using Nuclear fission. Ex. Laue-Langevin (ILL) in France. This is done using 235-Uranium fissionwww.ill.fr Spallation sources by using proton accelatrators. Ex. ISIS in Great Britain. There are some 37 neutron sources in 21 countries and of these 23 are in Europe, 10 in North America (including Canada), 2 in Japan and 1 in each of Australia & India.

SANS Instrumentation

Monochromator (filters neutrons at different wavelenghts), Collimation(slit used to sample beam size), Sample table (10X10cm), detector (helium used)

SANS Method Physics Behind SANS

In SANS one measures the intensity of neutron scattered through small angles, usually less than 1 degree. Using Van Hove scattering equation b = scattering length density, r = is the location of individual scatterer Is a function of neutron energy and neutron momentum. The Intensity is basically a measure of the number of neutrons scattered per second by the sample into a unit solid angle in a given direction, Having final energies E that lie between Ef and Ef + dEf

The advanced version of the neutron scattering equation is Fermi’s. calculates the partial differentional cross section that gives you the amount of neutron scattered per unit area per unit time. where ν and ν' are initial and final states of the scatterer,Pv is the population factor of the initial states. V is either Nuclear or magentic potential.

Experimental Results Unheated and heated samples of Silicon Carbide SiC/SiCf is considered in this presentation to show how the Intensity varies with varying temperature. Sample Silicon Carbide N3-1 and N4-1 is considered. Is a ceramic used for polishing materials (Abrasive material). Both samples are heated to 1750 and 1900 K for 60min revealing different Intensity curves on low Q region. The next slide is the Intensity curve for N3-1 sample that is unheated and see there is an increasing reduction in intensity in low Q region for increasing thickness.

Intensity Vs Momentum Vector

The results of small angle neutron scattering experiments on on this compounds shows, that heat treatment of the N3-1 and N4-1 samples showed no effect up to 1200°C. At higher temperatures (1750°C and 1900°C) dramatic changes of the scattering patterns indicate structural changes. This is confirmed in detail by the results of the data analysis. The changes in the internal structure are more extensive for the Nicalon N3-1 type than for the Hi-Nicalon N4-1 type of specimen. This experiment is based on nuclear interaction and no magnetic interaction is being considered. Next slide we will see example how the magnetic spin of materials are studies in the Low Q region as in the previous example.

Magnetic Phase transition using varying temperature. For doped CoS1.9Se0.1 system, the Curie temperature is lowered to TC = 90 K, Abrupt ferromagentic transition for T<Tc = 90k.

SANS Application I found the following site has it all about Neutron scattering and specifically SANS. URL = Biological microsolutions: protiens, nucleic acids, membranes & membrane components. DNA Studying polymers, Mechanism of molecular assembly in solutions, Engineering, Geophysics, Mateiral sceince, Forensic science. Identification of atomic structure and magnetic nature of Metallic alloy.

Disadvantages of SANS SANS is a routine technique available at neutron-scattering facilities associated with research nuclear reactors. No home version of this technique. Neutron sources are very expensive to build and to maintain. It costs millions of dollars annually to operate a nuclear research reactor and it costs that much in electrical bills alone to run a pulsed neutron source. Another problem with this technique is that neutron flux is very low. Presently, the neutron flux available at these reactors is equivalent to the X-ray flux available in the 1940s. This makes for long measurement times and increased demand from researchers to use the facilities. In addition, the interaction of neutrons with matter is weak. Therefore, large samples are required. Although small angle scattering techniques provide beneficial data, these limitations exclude their use in this work.

Sources National Institute Of Standards and Technology Institute Laue-Langevin, France Wikipedia Los Alamos National Laboratory Oak Ridge National Laboratory

30m NIST SANS