In situ CRNs (Cosmogenic RadioNuclides) play a decisive role in the long term quantifiation of the geomorphological transformation rate, especially the rate of erosion. Among many kinds of CRNs, 10Be and 26Al are most often used for this application. Both nuclides have long half lives (1.36×106 yr, 7.05×105 yr, respectively) and are extremely rarely abundant in the environment. To quantify these nuclides by mass spectrometry, isobar interferences (e.g. 10B against 10Be, 26Mg against 26Al) must be effectively eliminated. Among many methods, only AMS (Accelerator Mass Spectrometry) can achieve this. The fundamental principles for isobar supression in AMS are 1) selectivity in the negative ion generation and 2) isobar identification by the information of energy loss in media. The first characteristic derives from the basic AMS configuration, i.e., the combination of a negative ion source and a tandem type electrostatic accelerator. At the negative ion source, ion species of “negative electron affinity” cannot be ionized. For this reason, 26Mg ions are eliminated in the 26Al-AMS. The second feature can be realized by ion energies in the order of MeV/nucleon. In AMS, ions are accelerated to energies of several MeVs. In this energy range, the amount of energy loss in matter can be measured by a gas ionization chamber or a semiconductor detector. Isobars (like 10Be and 10B) can be identified by the difference of their energy losses. The overall efficiency for 10Be detection in AMS is the order of 10-3. Considering typical production rates of 10Be in the quartz of the crust (3-30 atoms/g-quartz/yr) and a practical time scale of the application (> 1000 yr), ca. 100 g quartz is needed for the AMS measurement within several per cent analytical error. Similarly, in the case of 26Al, the overall detection efficiency (10-4 order), typical production rates (20-200 atoms/g-quartz/yr), and practical exposure age (> 1000 yr) also result in ca. 100 g quartz needed for the detection of 26Al within several per cent analytical error.
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