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Structural information on hydrogen atoms and hydration water molecules obtained by neutron protein crystallography is expected to contribute to the elucidation and improvement of protein function. However, many proteins, especially membrane proteins and protein complexes, have large molecular weights and the unit cells of their crystals have large volumes, which are out of the range of unit-cell volumes measurable by conventional diffractometers because a large unit-cell volume causes difficulty in separating Bragg peaks close to each other in the spatial and time dimensions in diffraction images. Therefore, a new diffractometer has been designed at the Japan Accelerator Research Complex (J-PARC), which can measure crystals with a large unit-cell volume. The proposed diffractometer uses a large camera distance (L2 = 800 mm) and more than 40 novel large-area detectors (larger than 320 × 320 mm). In addition, a decoupled hydrogen moderator, which has a narrow pulse width, is selected as the neutron source. This diffractometer is estimated to be able to measure crystals with a lattice length of 250 Å along each axis at dmin = 2.0 Å. Ellipsoidal and curved shapes were introduced in the vertical and horizontal guide designs, respectively, providing an estimated neutron flux of 6 × 105 n s−1 mm−2 in the wavelength range 1.5–5.5 Å.

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