Very cold neutron diffraction and interferometry

At the high flux reactor of the Institute Laue Langevin in Grenoble a new interferometer for very cold neutrons has been installed. The interferometer employs three diffraction gratings, mounted on precision rotation tables and translation stages which allow rotation around all three axis in space and a translation perpendicular to the incident beam and to the lines of the grating. They are arranged parallel to each other have a periodicity of 2μm and a grating area of about 10 cm2. At the first version of the interferometer the distance from the first to the third grating was 0.5 m. For intensity reasons it was decided to use phase gratings in transmission mode, where the stripes of the grating have rectangular cross-section. grating. In the process of our neutron diffraction measurments we saw that the ratio of the width of steps and gaps, the stripes height and the cross-section of the stripes of the latter gratings made of quartz-glass are better suited for a satisfactory performance than those of the first two series. We measured the intensities and the ratio of the intensities of the zero and first order diffraction and calculated the intensity of the exit beam of the interferometer, which depends on the above mentioned ratio. As a result of this one can see that the step height of the first and the third grating should produce a phase shift of π/2 and that of the second grating a phase shift of π to avoid zero order diffraction which is not used at the second grating. The velocity of neutrons having a wavelength of 100A is40m/s. This small value causes severe problems concerning background vibration, which disturbs the operation of the interferometer. Therefore it was necessary to mount the gratings on an optical table with dimensions 6 x 1.2 x 0.4m3 and a weight of 1.8 t, which is supported by pneumatic isolation legs. In addition an active vibration isolation system has been developed to isolate the table against vibrations with frequencies below two Herz. (Author, shortened by G.Q.)