D17

Neutron reflectometer with horizontal scattering geometry

D17 is able to work in a time-of-flight mode, meaning that the reflectivity may be measured over an order of magnitude in scattering vector without moving sample or detector.  This, combined with the high flux of D17 and the flexibility of its resolution, means that kinetic studies of time dependent processes may be performed.

Neutron reflectometry is particularly useful for soft-matter studies because neutrons are strongly scattered by light atoms such as H, C, O and N, of which organic and biological materials are formed.  Moreover, the nuclei of different isotopes of the same element scatter neutrons with different amplitudes while their chemical properties essentially remain unchanged.  An important example is that of 1H (b = —3.74 fm) and 2H ( = 6.67 fm).  There is extremely high contrast between the two isotopes, and neutrons can easily distinguish between the two. The isotope sensitivity gives rise to the method of contrast variation, where the ratio of 1H to 2H in the sample may be varied in an experiment to highlight different parts of the interface.

For biophysics systems, another major advantage of neutron reflectivity over other scattering techniques is that the sample quantity is very small (< 10-6 g), making it financially reasonable to work with expensive or rare macromolecules.

The example below shows a time dependent study of the interaction between a lipid film and the enzyme 'phospholipase A2' from cobra venom in a heavy water solution.  The enzyme attacks the lipid, therefore the reflectivity profile changes as a function of time.  The illustration in the figure shows a possible mechanism.  Each data set, which took 10 minutes to collect, can be analyzed to determine the breakdown of the lipid. The lipid film was completely destroyed in about 8 hours.

The flexibility of D17 means that the time-of-flight instrument resolution, and therefore the neutron flux, can be tailored to the kinetic problem at hand. Fast kinetic experiments have been performed with high quality data sets being measured in less than a minute.

The work below was a collaboration between H. Vacklin, R. K. Thomas, F. Tiberg and G. Fragneto

More details from G. Fragneto

The large multidetector on D17 allows specular and off-specular reflectivity to be simultaneously measured.  An example is shown below, which has the reflectivity measured from a 10 micrometer nickel diffraction grating  The instrument was used in time-of-flight mode, and the total measuring time was around 6 hours.  The intensity on the detector has been integrated vertically, therefore the x-axis corresponds to 2 theta, and the y-axis corresponds to the neutron wavelength (~2 - 20 Å).

The reflected neutrons show a number of off-specular diffraction fringes, Yoneda wings, and diffuse scattering.  Close examination of the specular reflection shows the interference fringes due to the thickness of the grating, which was 900Å.  The picture therefore gives a good estimate of the coherence lengths of the instrument, which can probe films that are many tens of nanometers thick, and lateral structures that are micrometer sized.

The refracted intensity, which also has useful information, was collected in the same scan.

This work was the result of a collaboration between R. Cubitt, F. Ott and A. Menelle

Polarized neutron experiments separate nuclear from magnetic scattering. This is important when the nuclear and magnetic scattering are approximately the same magnitude, as is often the case in reflectometery. The example below shows a measurement of the polarized reflectivity from a [Co/CoO] superlattice. The instrument was used with the polarizing monochromator, but no analyser, and the sample was in a field at 1.5 K. The total measuring time for the two data sets was 8 hours.

In addition to the specular reflection, which can be analysed to determine the magnetic properties of the film, a number of off-specular features are visible in the data. The features are clearly dependent on the polarization of the beam, in particular those features labeled 'correlated roughness', showing that they have a magnetic contribution.

The 'correlated roughness' features are due to lateral correlations along the various Co/CoO interfaces, and the magnetic correlations can have a very different profile to those of the nuclear correlations. Analysis of the polarization dependence of the off-specular reflections enables the separation of the two correlations.

This work was a result of a collaboration between F. Radu, H. Zabel and A. R. Wildes

More details from F. Radu, H. Zabel, and A. R. Wildes