The Effect of the Collector in the Precise Measurement of Argon Isotopic Ratios

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In Ar isotopic measurement by mass spectrometry, the Faraday cup detector for the ⁴⁰Ar signal, and the Daly knob or ion counting system with an electron multiplier for ³⁶Ar and ³⁸Ar signals are used. This is because the signal intensities of the ³⁶Ar and ³⁸Ar are much lower than that of ⁴⁰Ar, and the Daly knob and electron multiplier are more sensitive than the Faraday cup. In our laboratory, we predominantly use the Faraday cup detector for the ⁴⁰Ar signal and the ion counting system with an electron multiplier for the ³⁶Ar and ³⁸Ar signals, for Ar isotopic measurement. However, we found that the discrimination factors for the ⁴⁰Ar/³⁶Ar ratios become constant over the wide range of ³⁶Ar gas amounts and that the ⁴⁰Ar/³⁶Ar ratio can be determined with better precision when the Faraday cup was used to detect both the ⁴⁰Ar and ³⁶Ar signals.