DESIGN AND USE OF A DOUBLE-CRYSTAL DIFFRACTOMETER FOR VERY SMALL-ANGLE NEUTRON-SCATTERING AT JRR-3M

The double crystal diffractometer (DCD) for very small angle neutron s cattering (VSANS) in the precise neutron optics (PNO) apparatus on a 3 G beam hole at JRR-3M is described. The DCD consists of a nondispersiv e (1, -1) setting of 111 reflection of silicon single crystals with a sample between two crystals and is set on a vibration isolator table i n a constant temperature chamber. The peak intensity of a rocking curv e without a sample is 2200 cps at a detector position and the signal t o noise ratio is 1.6 x 10(4) in a typical experiment. VSANS measuremen ts for silica powder samples reveal that the q-range is available betw een 2 x 10(-4) and 6 x 10(-2) nm(-1).