5:45 PM - 5:48 PM
★ [SGC51-P01] Present status for in-situ noble gas analysis by Sputtered Neutral Mass Spectrometry with tunneling ionization
3-min talk in an oral session
Keywords:noble gas, in-situ analysis, Sputtered Neutral Mass Spectrometry
LIMAS is mainly composed Ga focused ion beam (FIB) for primary ion, femtosecond (fs) laser for tunnel ionization of sputtered particles, and time-of-flight mass spectrometer. Depth profiling procedure is as follows. The FIB system is carried out high spatial resolution with large primary current. The smallest beam size was 8 nm at 3 pA, and the largest size was 600 nm at 30 nA. The fs laser can ionize the all element even He because of high energy density. The energy density is 8E15 W/cm2 because the pulse energy and width was 6.3 W and 30 fs, respectively, and the laser pulse was focused on a volume of 50 x 50 μm2. This laser pulse induces tunneling ionization (Delone and Krainov, 1998). Helium depth profile method in a few micrometer has been developed with LIMAS and atomic force microscope for crater depth measurement.
LIMAS can detect tens ppma He from sub-micrometer area on solid surface. In presence useful yield of He is 0.02% which should be up to 1% by optimize laser focusing and irradiation position. The yield of 0.02% corresponds to 4000 He atoms for single He ion count. The back ground of He is much less than that of the NG-MS because LIMAS is used by dynamic operation under the ultra-high vacuum.
LIMAS can measure locally concentrated noble gas because the sputter rate of the pulsed FIB is low, which indicates that this measurement is almost non-destructive isotope analysis. This feature can play a role for analysis of tiny and precious samples such as the Itokawa particles. U,Th-He and K-Ar dating for single grain can be applied by the in-situ noble gas analysis. The in-situ dating may obtain a new insight of thermal history of igneous rocks by comparing conventional in-situ dating method such as U-Pb age.