The Macromolecular Neutron Diffractometer at SNS
Detector array for the MaNDi instrument before installation.
Cutaway view of detector array for the MaNDi instrument. Only half of the detector array is displayed to open the view to the sample position.
MaNDi is optimized for rapid data collection from large structures and will achieve 1.5 Å resolution from crystal volumes between 0.1-1.0 mm3 with lattice repeats in the order of 150 Å. This instrument will use a decoupled hydrogen moderator for optimal resolution and separation of Bragg peaks. The design uses a 30 m flight path and a variable wavelength bandwidth of 2.7 Å to accommodate different types of experiments. This bandwidth variation is achieved by the use of three disc choppers in the incident flight path. With larger crystals (> 1 mm3), it will be possible to obtain useful data in the resolution range of 2.0 to 2.5 Å for unit-cell repeats of up to 300 Å, a revolution in neutron macromolecular crystallography (NMC).
Simulations predict experimental duration times of between one and seven days, which will revolutionize NMC for applications in the field of structural biology, enzymology and computational chemistry.
The design and technologies for MaNDi borrow heavily on work done on the initial suite of SNS instruments, particularly the single-crystal diffractometer TOPAZ. The detector technology used on the two instruments is nearly identical. The MaNDI detectors cover a large solid-angle to record most of the neutrons scattered from a single-crystal sample regardless of the reflection angle. The instrument design accommodates this by situating detectors approximately spherically around the sample.
MaNDi received its first neutrons in May 2012 and is now in commissioning.
The detector design follows a modular approach. A spherical detector mount accommodates the appropriate number of individual modules of two-dimensional, time-sensitive detectors with front face dimensions of 150 × 150 mm, leaving openings for the sample orienter/environment (top) and the incident and exiting direct neutron beam (horizontal plane). The detectors are centered on the sample position and are mounted on a nominal 500 mm radius. This allows enough room to place large-volume samples and maintain sufficient space in the instrument enclosure for the detectors, associated hardware, and detector shielding/collimation.
Schematic layout of the MaNDi instrument. MaNDi uses a 24 m flight path intersected by three bandwidth choppers, which determine the incident neutron wavelength spectra to be used in the experiment. Interchangeable neutron optics are then used to further tailor the incoming neutron beam for each particular experiment. Finally, neutrons diffracted from the crystal are recorded by the spherical detector configuration that surrounds the sample.
The MaNDi Anger camera detectors use a scintillator screen to convert neutrons into photons, which are subsequently enhanced by photomultipliers before being recorded. The spatial resolution of the detector is 1 mm with a minimal sensitivity to gamma rays, hence preserving the signal-to-noise ratio of the Bragg peaks. The detection efficiency of this type of detector using a 1.5 mm thick scintillator is 78% for neutrons with a wavelength of 1 Å. An increase in neutron wavelength is coupled with an increase in the detection efficiency.
Precision crystal mounting is necessary to place the 0.1 mm3 crystals within the neutron beam, and the sample positioning system allows translation and rotation in x, y and z to precisely align the sample. These operations are remotely controlled and motor driven by a user-friendly graphical user interface.
Applications
- Molecular magnets, computational chemistry, and fibers
- Protein studies to provide better drug molecules for the treatment of cancer and HIV
- Studies of enzyme mechanisms to accelerate important industrial reactions
- Mechanisms used by plants to convert light into energy
