Neutron diffraction from macroscopic objects and transverse coherence of the wavefunction: The Fresnel zone plates

A new high resolution (400 nm) Fresnel Zone Plate (ZP) of 1 mm diameter has been designed and produced in order to accurately measure its spatial resolution and efficiency. This experiment is an extension of the previous tests performed to measure the efficiency of these devices. The measurements were carried out on the diffractometer Morpheus at PSI using a new ZP designed to obtain a small focal length of 0.833 m at 4.7 Å. In this experiment a 0.8 mm edge square neutron source at 4.7 Å was demagnified by the ZP by a factor 2 down to about 0.4 mm at 1.24 m distance from the ZP. With this configuration, using a Gd knife edge, we determined the transverse profile of the image with a spatial accuracy of 0.01 mm. In addition an appropriate configuration was used to measure the efficiency of the ZP, which was equal to 0.28 ± 0.02 , to be compared to 0.325, that is the ideal value at 4.7 Å. The result for the ZP resolution suggests that the image has a broadening in the range of 0.01 mm, corresponding to an effective wave front coherence over a distance greater than 50 μ m , to be compared to some 0.2 μ m as deduced from the indetermination principle, according to the beam divergence. Numerical simulations of the ZP are performed with a partially coherent wave front to show the effect of the phase variation on the intensity at different positions.