Ordered stacking of crystals with adjustable curvatures for hard X- and γ-ray broadband focusing

A stack of plate-like curved crystals is proposed as an optical element for X- and γ-ray focusing. Si mono-crystal plates have been bent by surface grooving and positioned one over the other to form a stack. The relative alignment of the curved diffracting planes in the stack has been tested by hard X-ray diffractometry using a polychromatic and divergent beam. The stack exhibited a single and well defined spot under X-ray diffraction, highlighting that the plates are sufficiently aligned to behave as a single crystal. The curvature of the plates in the stack can be precisely set by tightly controlling the experimental parameters of grooving. Once set, the curvature is self-standing, i.e. it is maintained without any need for an external bender. Thanks to the stacking, it would be possible to realize optical elements with arbitrarily large size. This achievement has important implications for the realization of satellite-borne experiments in astrophysics and instruments for nuclear medicine with superior resolution.