Characterization of the NNVT capillary plate collimators

In this paper, we report the results of the characterization campaign of two prototypes of Micro-Channel Plates (MCPs), designed as the X-ray collimators for the Large Area Detector on board the eXTP mission. The devices were developed ad-hoc by North Night Vision Technology Co., Ltd. (Nanjing, China). Measurements involved the study of the angular response (rocking curve) of each device to X-rays of different energies. The study evidenced how the angular response of a collimator changes with the energy of the incoming photons, with the onset of side lobes at high energy, which enlarge the effective field of view of the device, causing a potential contamination of the on-axis signal. Nevertheless, the magnitude of this effect is proven to be acceptable in most situations of astrophysical interest. On the lower hand of the energy spectrum, photons may also modify the angular response due to grazing reflection on the inner walls of the collimator, a phenomenon strongly dependent on the degree of roughness of the surfaces involved. The whole campaign took place at the INAF/IAPS laboratories in Rome.