A.L. Washington

Spin echo small angle neutron scattering using a continuously pumped 3He neutron polarisation analyser

We present a new instrument for spin echo small angle neutron scattering (SESANS) developed at the Low Energy Neutron Source at Indiana University. A description of the various instrument components is given along with the performance of these components. At the heart of the instrument are a series of resistive coils to encode the neutron trajectory into the neutron polarisation. These are shown to work well over a broad range of neutron wavelengths. Neutron polarisation analysis is accomplished using a continuously operating neutron spin filter polarised by Rb spin-exchange optical pumping of (3)He. We describe the performance of the analyser along with a study of the (3)He polarisation stability and its implications for SESANS measurements. Scattering from silica Stöber particles is investigated and agrees with samples run on similar instruments.

Spatially modulated structural colour in bird feathers

Eurasian Jay (Garrulus glandarius) feathers display periodic variations in the reflected colour from white through light blue, dark blue and black. We find the structures responsible for the colour are continuous in their size and spatially controlled by the degree of spinodal phase separation in the corresponding region of the feather barb. Blue structures have a well-defined broadband ultra-violet (UV) to blue wavelength distribution; the corresponding nanostructure has characteristic spinodal morphology with a lengthscale of order 150 nm. White regions have a larger 200 nm nanostructure, consistent with a spinodal process that has coarsened further, yielding broader wavelength white reflectance. Our analysis shows that nanostructure in single bird feather barbs can be varied continuously by controlling the time the keratin network is allowed to phase separate before mobility in the system is arrested. Dynamic scaling analysis of the single barb scattering data implies that the phase separation arrest mechanism is rapid and also distinct from the spinodal phase separation mechanism i.e. it is not gelation or intermolecular re-association. Any growing lengthscale using this spinodal phase separation approach must first traverse the UV and blue wavelength regions, growing the structure by coarsening, resulting in a broad distribution of domain sizes.

Spin echo scattering angle measurement at a pulsed neutron source

Two experiments were performed to adapt spin echo scattering angle measurement (SESAME) to pulsed neutron sources. SESAME is an interferometric method that provides enhanced resolution of neutron scattering angles without the loss of neutron intensity that results when collimation is used to improve angular resolution. The method uses the neutron equivalent of optical wave plates to produce a phase difference between the two neutron spin components of a polarized neutron beam. Because the wave plate is inclined to the neutron beam, this phase difference depends sensitively on the trajectory of the neutron. In the absence of a sample, a second wave plate, which is parallel to the first, undoes the phase difference introduced by the first wave plate, producing a polarization identical to that of the incident neutron beam. When a scattering sample is placed between the two neutron wave plates, the cancellation of the phase difference between the neutron spin states is not perfect and the resulting neutron-beam polarization is a measure of the distribution of scattering angles. In the first experiment, thin (30 and 60 µm-thick) magnetized Permalloy films were used as neutron wave plates. In a second experiment, current-carrying solenoids with triangular cross sections were used as birefringent prisms for neutrons. The arrangement of these prisms was such that they mimicked the effect of the neutron wave plates in the first experiment. In both experiments, correlation lengths in the scattering sample of about 1000 A were probed using very simple and inexpensive equipment. These experiments brought to light a number of advantages and disadvantages of implementing SESAME at pulsed neutron sources and provided insights into the relative merits of SESAME and traditional small-angle neutron scattering.

Compact spherical neutron polarimeter using high-Tc YBCO films

We describe a simple, compact device for spherical neutron polarimetry measurements at small neutron scattering angles. The device consists of a sample chamber with very low (<0.01 G) magnetic field flanked by regions within which the neutron polarization can be manipulated in a controlled manner. This allows any selected initial and final polarization direction of the neutrons to be obtained. We have constructed a prototype device using high-T(c) superconducting films and mu-metal to isolate regions with different magnetic fields and tested device performance in transmission geometry. Finite-element methods were used to simulate the devices field profile and these have been verified by experiment using a small solenoid as a test sample. Measurements are reported using both monochromatic and polychromatic neutron sources. The results show that the device is capable of extracting sample information and distinguishing small angular variations of the sample magnetic field. As a more realistic test, we present results on the characterization of a 10 μm thick Permalloy film in zero magnetic field, as well as its response to an external magnetic field.