research papers J. Appl. Cryst. (2019). 52, 599–604 https://doi.org/10.1107/S1600576719004631 599 Received 31 January 2019 Accepted 4 April 2019 Edited by J. M. Garcı ´a-Ruiz, Instituto Andaluz de Ciencias de la Tierra, Granada, Spain Keywords: X-ray astronomy; ammonium dihydrogen phosphate; ADP; crystalline perfection; X-ray topography; residual strain measurement; lattice plane curvature measurement. Characterization of ammonium dihydrogen phosphate crystals for soft X-ray optics of the Beam Expander Testing X-ray facility (BEaTriX) Claudio Ferrari, a * Sara Beretta, a Bianca Salmaso, b Giovanni Pareschi, b Gianpiero Tagliaferri, b Stefano Basso, b Daniele Spiga, b Carlo Pelliciari c and Enrico Giro b a IMEM-CNR Insitute, parco Area delle Scienze 37/A, Parma, PR 43124, Italy, b INAF–Osservatorio Astronomico di Brera, via E. Bianchi 46, Merate, LC 23807, Italy, and c MPE–Department of High-Energy Astrophysics, Max Plank Insitute for Extraterrestrial Physics, Giessenbachstrasse 1, Garching bei Mu ¨ nchen, 85748, Germany. *Correspondence e-mail: claudio.ferrari@imem.cnr.it A new type of X-ray facility, the Beam Expander Testing X-ray facility (BEaTriX), has been designed and is now under construction at INAF– Osservatorio Astronomico di Brera (Merate, Italy) to perform the acceptance tests of the silicon pore optics modules of the ATHENA X-ray telescope. Crystals of high perfection and large dimensions are needed in order to obtain a wide beam (20  6 cm) with an X-ray divergence of <0.5 00 and an X-ray energy purity ÁE/E < 10 5 . To generate X-ray diffracted beams at an X-ray energy of 1.49 keV, ammonium dihydrogen phosphate (ADP) crystals have been considered among other possible choices, because of their reported crystal quality and because they can be grown at sufficiently large size at a reasonable price. In the present paper, the results of the characterization of crystalline quality and lattice planarity of a 20  20  2 mm ADP sample are reported. 1. Introduction ATHENA (Advanced Telescope for High Energy Astro- physics) is an ESA X-ray mission aimed at understanding the evolution of the universe and the key role of black holes. The 2.4 m diameter optics are composed of more than 700 mirror modules (MM) that need to be tested and accepted before integration. To this end, INAF–Osservatorio Astronomico di Brera started in 2012 to design a facility based on the generation of a broad, uniform and low-divergent X-ray beam to illuminate the full aperture of the ATHENA MM with a beam with vertical and horizontal divergence lower than 1.5 00 half energy width (HEW) (Spiga et al., 2012, 2014, 2016; Pelliciari et al., 2015). The Beam Expander Testing X-ray facility (BEaTriX) is now under construction for the 4.51 keV energy, and it is built to host a second beamline at 1.49 keV. The two energies are required to accept the MM, by measuring their point spread function and effective area. For each beamline, the design foresees an X-ray microfocus source, a paraboloidal mirror, a symmetrical channel cut crystal as monochromator and an asymmetrically cut crystal for beam expansion. The vertical divergence of the final beam is limited by the 30 mm dimension of the focal spot of the X-ray source, whereas the horizontal divergence is also defined by the X-ray monochromators and by the asymme- trically cut crystal for the beam expansion (Salmaso et al. , 2018). Four reflections on a monochromation system composed of symmetrically cut crystals are needed to reduce ISSN 1600-5767 # 2019 International Union of Crystallography