NiV/B₄C Multilayer Reflector For Hard X-Ray Focusing Telescope

X-ray focusing telescope is an important astronomical observation instrument,which only works in space and imaging with a grazing incident geometry.In order to increase the light collecting area,the telescope always uses a nested structure formed by hundreds of mirrors.These mirrors need to be thin and light enough to decrease the weight of the telescope.Compared with the developed countries,the manufacturing technology of X-ray imaging telescopes in China is limited.Our laboratory has already been able to fabricate W/Si and Pt/C supermirrors,whose optical performance is the same as that in the Nu STAR telescope launched by the United States in 2012.But these mirrors cannot meet the reflection requirement for a higher energy range,especially higher than 80ke V.Therefore,the fabrication and performance characterization of NiV/B₄C multilayers adapted to higher energy range are systematically studied in this work.Firstly,we studied the directly hot-slumping of mirror substrate with thin glass.Based on the selection of thin glass plate and mandrel material,we prepared the Pt and BN separation membranes,improved the detecting method of cylindrical mirror substrate,optimized the temperature curve of the hot-forming process,and succeeded to improve the thin glass surface shape,which is typically reduced from more than 200to 60 arc-seconds.Besides,an effective cleaning method for"mildew spot"on the surface of borosilicate thin glass was developed,and the cause of irregular mildew spot on the surface of borosilicate thin glass was explained,we found that this is a kind of alkali phenomenon erosion by water atoms on the surface of glass.Secondly,the influence of background pressure on NiV/B₄C multilayer structure was studied.It is revealed that when the background pressure is higher than 1×10^-4Pa,the B₄C film in the multilayer is oxidized and the thickness changes gradually,resulting in a gradient change in the refractive index of the B₄C film,which leads to the widening of the Bragg reflection peak on the grazing incidence X-ray reflectivity curve.The results indicate that in order to prepare NiV/B₄C multilayer films with high performance,the higher background pressure in the sputtering system is important,with a limit of at least 1×10^-4Pa.Thirdly,the properties of NiV/B₄C multilayers with different NiV film thicknesses were studied.The thickness of B₄C layer was kept at 3 nm,and NiV layers were changed from 1 nm,2 nm,3 nm,4 nm,to 5 nm.The results show that the interface widths of NiV-ON-B₄C and B₄C-ON-NiV,as well as the root mean square of surface roughness of NiV/B₄C multilayers,have no dependence on the thickness of NiV layer.Finally,the annealing effect of NiV/B₄C multilayers with periods of 3 nm and 8nm was studied.The multilayers were first annealed at a series of temperatures with the same annealing time.The results suggest that with the increase of annealing temperature,the d-spacing,and the thickness of NiV layer and B₄C layer are all gradually increased.The strain of the 3nm multilayer gradually changes from compressive to tensile.Then,samples were annealed at 340℃for different constant temperature duration.The results show that the period-spacing,NiV film thickness and B₄C film thickness of both multilayers increase gradually with increasing the annealing time,while the film strain of 3 nm multilayers is always kept at the unchanged tensile state.In this thesis,the fabrication process of thin glass mirror substrate by direct hot-forming was studied in detail,achieving greatly improved performance of surface shape.The fabrication and characterization of NiV/B₄C multilayers were studied,and substantial results related to the structure and properties of NiV/B₄C multilayers were obtained,which provides the technical support for the development of hard X-ray focusing telescopes working at energy range higher than 80ke V.