Implementation of Faraday Cup Amplifiers with 10~(13)Ω Resistors for TIMS measurements at JRC-G.2 in Geel

摘要

This paper describes the installation, testing and implementation of Faraday cup amplifiers with 10~(13)Ω resistors on two TRITON TIMS (Thermal lonization Mass Spectrometer) instruments at the JRC-G.2 in Geel. In order to reduce the signal-to-noise ratio, higher resistances have recently been utilized for Faraday cup current amplifiers. This is particularly important for measuring ion currents at the level of 10~(-16)-10~(-14) A which are in the overlap region between secondary electron multiplier detectors and Faraday cups. Faraday cup amplifiers with 10~(12)Ω resistors have been used successfully for about ten years, whereas 10~(13)Ω resistors have been introduced about 4-5 years ago, and quite recently, upon the initiative of JRC-Geel, a new gain calibration board has been added, which allows the inter-calibration of all Faraday cup amplifiers with 10~(11)Ω, 10~(12)Ω and 10~(13)Ω resistors. The new gain calibration board has been installed recently on two TRITON TIMS instruments at JRC-Geel and thoroughly tested. The performance was found to be close to the theoretical expectations regarding the amplifier noise and satisfactory with respect to the relative gain stability. As part of the testing and method validation, measurements of the gravimetrically prepared reference materials series IRMM-075/2-2 with certified ~(236)U/~(238)U ratios at the order of 10~(-4)-10~(-5) have been performed in order to show the performance for amplifiers with 10~(13)Ω resistors in measurements of low ~(236)U ion currents. Additionally, a uranium material consisting of 99.9989% of ~(238)U with a ~(235)U/~(238)U ratio of about 10~(-5) has been measured using a 10~(13)Ω amplifier resistor for the Faraday cup of ~(235)U. The uncertainty is significantly improved compared to measurements using a secondary electron multiplier detector for ~(235)U. Further examples and applications will be suggested.