Macromolecular Neutron Diffractometer (MaNDi)

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.

The SNS macromolecular diffractometer (MaNDi) will be a state of the art high-resolution macromolecular crystal diffractometer. Optimized for rapid data collection from large structures, MaNDi will achieve 1.5 Å resolution from crystal volumes between 0.1-1.0 mm³ with lattice repeats in the order of 150 Å. The instrument will use a decoupled hydrogen moderator for optimal resolution and separation of Bragg peaks. The design utilizes a 24 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 2.0-2.5  Å for unit-cell repeats of up to 300 Å, a revolution in neutron macromolecular crystallography (NMC).  

Detector array for the MaNDi instrument.

Simulations predict experimental duration times of between 1 and 7 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 instruments, particularly the single crystal diffractometer (TOPAZ). The detector technology used on the two instruments will be nearly identical. The detectors of MaNDI are designed to 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. The detector design follows a modular approach. A spherical detector mount will be constructed to accommodate the appropriate number of individual modules of two-dimensional time-sensitive detectors with front face dimensions of 150 mm × 150 mm, leaving openings for the sample orienter/environment (top) and the incident and exiting direct neutron beam (horizontal plane). The detectors will be centered on the sample position and mounted on a nominal 500-mm radius. This will allow 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 which is 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 which surrounds the sample.

The Anger camera detectors that will be used on MaNDi 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 1mm 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 will be necessary to place the 0.1 mm³ crystals within the neutron beam the sample positioning system will allow translation and rotation in x, y and z to precisely align the sample. These operations will be remotely controlled and motor driven by a user-friendly graphical user interface.

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