A new composition in-situ analyzing instrument of planetary rocks and soils

Abstract

It might preserve vital clues about the imprints of life and the past livability of the planet, which are important information for studying the origin and the history of the planet, of planetary rocks or soils, so the measurement of planetary rock or soil composition is a basis of planetary exploration. Most of current composition analysis of rocks or soils, based on alpha particle backscattering technique and spectrum measurement of laser excitation, has disadvantages of low measurement accuracy, large resource demand and long measurement time. So it is urgent to seek different methods to measure the rocks or soils of planetary. A few of merits are included in x-ray active excitation mode: firstly, it has good resolution ability for elements above medium quality; secondly, the detection sensitivity of elements can reduce to 10ppm. As long as anode targets are changed, the detection sensitivity of any element can be matched. JPL is developing a prototype of a new rock composition analysis instrument based on x-ray source in order to satisfy the needs of NASA Mars 2020 exploration. It is irreplaceable as the next generation of planetary rocks or soils composition analysis instrument of x-ray active excitation mode. Therefore, the core content of this paper is to develop a composition analysis instrument, which integrates miniature x-ray source with the silicon drift detector (SDD).As the key technology of x-ray active excitation mode, miniature x-ray source must meet the needs of low power consumption, self-sealing, high intensity and micro-focus spot. For this purpose, a design proposal of the miniature x-ray source is proposed and its theoretical model is established. The size of cathode structure greatly impacts on the size of the whole miniature x-ray tube on account of the simplicity structure of x-ray tubes. So compared with the traditional x-ray source, a new cathode structure is used for the sake of reducing the whole size of the miniature x-ray tube. Not only that, but the size of cathode structure has influence on the structure of filaments, the magnitude of current and the size of focal spot of x-rays. At the same time, mini focal spot is needed to enhance the resolution ratio and intensity of rocks or soils composition analysis instruments. Hence, it is indispensable to optimize the cathode structure in order to realize the miniature size and mini focal spot. A simulation model has been set up based on theoretical calculation and simulated using charged particle optics software COMSOL or SIMION. The shape and the size of x-ray tube’s cathode have been optimized so as to attain the mini focal spot. Additionally, the influence of high voltage loaded on the cathode or the filament to focal spot size has been taken fully into account. Moreover, the SDD detector is also integrated with the miniature x-ray source and the whole volume of the composition analysis instrument decreases which is beneficial to the deep space exploration.