||Development of depth profiling for solar wind noble gases implanted in Genesis diamond-like-carbon on silicon substrate targets by using isotope nanoscope
The Sun preserves a chemical composition of the early solar system except for H, He, and Li because of nucleosyntheses. Particles with energy of a few keV per atomic mass unit are continuously released from the Sun to outer space as a solar wind. The solar wind (hereafter SW) composition should contain not only a present solar activity but also a history of the solar system evolution, of which physicochemical property such as elemental and isotopic fractionation in SW particles is one of the important issues to understand a solar activity at a time of the SW irradiation. Airless bodies are exposed to SW at all times (space weathering). The SW particles were incorporated inregolith of airless body andregolith breccia meteorites. Therefore,a mechanism of space weathering can be revealed from the regolith materials, which leads tounderstanding of an evolutionof the asteroid surface.Noble gases are chemically inert and primordial noble gases are depleted in natural solid materials such as planets and asteroid solid materials. Noble gases arevery sensitive to secondary addition into the solid materials. Therefore, noble gases can be utilized as a tracer to investigate the space weathering and the solar activity from the SW irradiated materials. Elemental and isotopic compositions of the Sun have been determined by measurements of primitive meteorites, which are relatively enriched in volatile elements. The volatile elements such as noble gases have been determined by photospheric spectroscopy observations. On the other hand,the isotopic compositions of noble gases were obtained by measurements of the SW irradiated materials such as lunar regolith and artificial targets in the space. Recently, Genesis spacecraft mission by NASA was carried out in order to determine the composition of the Sun and estimate that of the solar nebula. During the Genesis mission, energy, elemental composition of SW had been measured by in-situapparatuses, and the SW was directly collected on various substrates. The data of SW collected samples was used for the comparison with data of previous SW research (i.e., Apollo results), and the estimation on the extent of elemental fractionations of solar composition. Especially, the energy distribution, flux of the SW, and sotopic and elemental composition of the SW noble gases are well determined by insitumeasurements (e.g., Reisenfeld et al., 2013) and laboratory measurements (Grimberg et al., 2008; Heber et al., 2009). Therefore, the SW in the Genesis substrates is the most optimal sample to determine the composition of SW noble gases. In order to discuss the space weathering and the solar activity, an interaction between the implanted SW and the substrate (ion-solid interaction) is an important issue. The SW particles have various energies depends on their acceleration mechanisms. A depth profile of the implanted particlesinto the substrate is correlated with the energy distribution of SW particles. However, depth profiling of noble gases was not established because (1) an implant depth of SW noble gases is much shallower than 1 μm and (2) a conventional depth profiling with secondary ion mass spectrometry (SIMS) was hard to measure noble gases because of their high ionization potentials. Laser ionization mass nanoscope (LIMAS) was developed to measure depth profiles for noble gas isotopes (Bajo et al., 2015), which is a type of secondary neutral mass spectrometer. LIMAS consists of a Ga liquid metal ion source and an aberration corrector for sputtering of nanometer scale area on samples, a femtosecond laser for tunneling-ionization of the sputtered neutrals, and a multi-turn time-of-flight mass spectrometer (TOF-MS) for isotope analysis. A high mass resolution to separate target ion and interference ions and a high sensitivity for target isotopes are required for depth profiling of implanted SW. I have evaluated the performance of a multi-turn TOF-MS (MULTUM II) equipped with the ion injection optics of LIMAS (Section 2). The mass-resolving power of LIMAS increased linearly with increasing the flight path length, and reached 620,000 (FWHM) at 1,000 multi-turn cycles of MULTUM II (flight path length: 1.3 km). The transmittance of LIMAS decreased to 60? 70% after 20 multi-turn cycles of MULTUM II, compared with the linear mode transmittance due to collisions of flight ions with residual gas in mass spectrometer. The transmittance per multi-turn cycle became constant (99.96%) after 20 multi-turn cycles. A useful yield of 3 × 10 - 3 for Si ions was obtained for LIMAS at 30 multi-turn cycles of MULTUM II. From these evaluations, I conclude thatLIMAS transmittance is comparable to those for commercial SIMS measurements. I developed high precision depth profiling for noble gas isotopes (Section 3). To investigate the precise He depth distribution of the Genesis diamond-like-carbon (DOS) sample, three attempts were made as follows:(1) the interfering ions for 4 He were removed with ion gates.(2) A pumping speed for vacuum generation was increased by usingan ion pump with a higher pumping speed for noble gases (Agilent Vaclon Plus 500 “StarCell”).(3) A measurement area was enlarged in order to reduce an crater edge effect. As a result, 4He in the deep region~300 nm from the surface could be determined to 4×10 17atoms cm-3 in the Genesis sample. By these improvements I could measure high energy components of SW-4 He. Moreover, depth profiling of 20 New as determined from the sample for the first time. This depth profiling can be a useful technique to study the evolution of the Sun and the mechanism of the space weathering of solar system objects using SW noble gases.
Hokkaido University（北海道大学）. 博士(理学)