Fig. 1-10 Bender and Si single crystal (250 mm x 40 mm x 5 mm) used in the neutron diffractometer for biological macromolecules (BIX-1)

Table 1-1

Comparison of the specifications for the new and the old monochromators

Fig. 1-11 Reflected intensity as a function of the bending curvature. The abscissa is the curvature, i.e. reciprocal of the bending radius R. The ordinate is the peak intensity of the rocking curve. The intensity increases with the curvature to the fracture point of the crystal (the right end of the curve).

We have already constructed, at JRR-3M, several pieces of apparatus dedicated to neutron experiments such as neutron diffraction etc. One of the most fundamental components of the apparatus is the monochromator which diffracts monochromatized neutron beams. In conventional neutron experiments, HOPG (Highly Oriented Pyrolytic Graphite) crystals have been widely used for the neutron monochromator. For the structure analysis of biological macromolecules such as proteins, however, we needed to devise a new type of monochromator with much higher performance than the HOPG monochromator. After repeated trials and careful examination, we adopted a design in which an elastically bent silicon perfect crystal was used to obtain parallel-aligned and high-intensity neutron beams. As a result, we succeeded in the first practical use of this new type of monochro-mator. Although the basic idea itself of the elastically bent silicon had been discussed for some years, it had been difficult to reach practical use. In this neutron monochromator, a Si single crystal plate is readily bent like a bow by increasing the tension in the piano wire attached to the bending device (Fig. 1-10). Therefore, we can adjust, within the elastic limit of the plate, beam characteristics such as the reflected intensity, beam divergence, etc. Change in the reflected intensity as a function of the bending curvature is shown in Fig. 1-11. The specifications of the new and the old monochromators are compared in Table 1-1. Due to its compactness and high versatility, this new type of monochromator has already been used in the neutron experiment apparatus at JRR-3M. Currently, there are 7 pieces of equipment in total, operating or under construction.

Reference I. Tanaka et al., An Elastically Bent Silicon Monochromator for a Neutron Diffractometer, J. Appl. Crystallogr., 32, 525 (1999).

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