Single Crystal Diffractometer - Science

TOPAZ is designed and constructed as a versatile and flexible environment SCD for neutron scattering. It will be the first Laue diffractometer to implement options to polarize the neutron beam routinely for magnetic scattering experiments. Due to interchangeable sample environment TOPAZ will be able to collect diffraction intensities at cryogenic and elevated temperatures. Polarized incident neutron beam and magnetic field option on the sample will be valuable tools to decipher complex and directional magnetism and magnetic transitions in single crystals. TOPAZ will also assist in studies of therapeutics and medical compounds, as was shown for aspirin and paracetamol, to reveal functional dependent differences in hydrogen locations and bonding to comprehend a compound’s individual effectiveness.

Detector array to cover more than 2.5 sr of real space. Each dark brown square is the front face of a 2-D energy dispersive Anger camera detector.

A global goal for TOPAZ will be to collect data on small crystals in a matter of minutes rather than days. A highly efficient beam transport system using m=3.6 or higher super mirror coated guide is incorporated along the full length (14.5 m) of the incident beam path and focuses the neutrons on the sample position with little losses. The last 2 m of the focusing guide are interchangeable to match the beam divergence with the sample characteristics equivalent with optimizing the intensity for each data collection. Where high resolution is desired a constant 10 mrad beam divergence over the incident wavelength band of 0.5 – 4 Å can be chosen. Energy dispersive area detectors will record Bragg intensities and diffuse scattering highly efficiently by mapping over 2.5 steradians of real space. A full diffraction pattern will be collected using 3 – 4 crystal orientations.

Neutron Beam Polarization with 3He SEOP Spin Filter Setup

A 3He spin filter using the Spin Exchange Optical Pumping (SEOP) method will be implemented to polarize the incident neutron beam. The spin filter setup v.1 for TOPAZ was designed and constructed at Hamilton College by Prof.s G. Jones and B. Collett as part of the graduation thesis of F. Dias and J. Steinberg. It was tested and delivered to the polarization laboratory in October 2006. It will continued to be tested and optimized on the neutron beam at the SCD instrument at IPNS. The picture below shows the portability and ease of installation of the system at the IPNS diffractometer. Dr. G. Jones and T. Tong are placing the prepared polarizer unit in the instrument. The 3He spin filter cell is inside a magnetic holding filed inside a solenoid (black outline in the picture) to ensure a stable polarization. The neutron beam enters and exits through the front and back window (hole in solenoid cover on front face). The unit is optically pumped in situ which allows constant and stable 3He polarization to in turn constantly polarize the neutron beam.

Polarized 3He can be used as a neutron spin filter to polarize a broad energy spectrum of neutrons. As a prototype for use on the single crystal diffractometer (SCD) at the Spallation Neutron Source (SNS), we have built a compact system to continuously polarize a 3He spin filter by spin-exchange optical pumping. Polarizing the 3He in the neutron beam provides a constant neutron polarization and reduces the sensitivity to relaxation mechanisms. The compact polarizer, 28 cm in diameter and 31 cm long, includes NMR coils to quickly flip the 3He polarization with 0.01% loss.  The achieved 67% 3He polarization corresponds to Pn = 91% with Tn = 20%, at 0.3 nm, confirmed with the CoFe crystal analyzer.

Advantages of 3He spin filters are that a broad energy range of neutrons (cold – epithermal) can be polarized simultaneously. They have a minimal effect on beam divergence but a wide angular acceptance.

A polarized 3He neutron spin filter has a highly spin dependent absorption cross-section which means that neutrons with anti-aligned spin “see” a thick absorption target whereas neutron with aligned spins see a thin target. Therefore neutrons are filtered very efficiently due to their spin orientation.

"Compact 3He Neutron Polarizer with Online Optical Pumping and Fast-Switching of Spin-Up/Spin-Down Polarizing Mode", G.L. Jones, F. Dias, B. Collett, W.C. Chen, T. Gentile, P.M.B. Piccoli, M.E. Miller, A.J. Schultz, H.Y. Yan, X. Tong, M. Snow, W.T. Lee, C. Hoffmann, J. Thomison, Proceedings of the Seventeenth Meeting of the International Collaboration on Advance Neutron Source (ICANS-XVII, April 24-29, 2005, Santa Fe, New Mexico, USA), Vol. III, 838-843 (2006).

Applications

TOPAZ can address problems and greatly expand the range of materials explored in chemistry, earth sciences, materials science and engineering, solid-state physics, and biology. It will be uniquely helpful in studies of therapeutics and medical compounds, such as aspirin and paracetamol, to show differences in functional hydrogen locations and bonding to decipher the compound’s “modus operandi”.

TOPAZ will address problems and greatly expand the range of materials explored in chemistry, earth sciences, materials science and engineering, solid-state physics, and biology. Single crystal sample sizes that will reach current X-ray samples are not feasible on current neutron diffraction instruments but will be suitable for experiments at TOPAZ. It will be possible to look at large molecules i.e. of the late-transition metal oxo complex family K7Na9[O=PtIV(H2O)L2], L = [PW9O34]9- in detail, rare earth low temperature magnetic phases (i.e. RMnO3 type perovskite phases), and molecular magnet compounds to study both, hydrogen positions and magnetic scattering, with one data collection. The hydrogen bonding information will reflect the molecular arrangement and response to external conditions of temperature & pressure and gas environment; magnetic scattering provides information on global and molecule specific magnetic moments.