Year
2019
Abstract
This paper describes the installation, testing and implementation of Faraday cup amplifiers with 1013Ω resistors on two TRITON TIMS (Thermal Ionization 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 1012 Ω resistors have been used successfully since about ten years, 1013 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 1011 Ω, 1012 Ω and 1013 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 236U/238U ratios at the order of 10-4-10-5 have been performed in order to show the performance for amplifiers with 1013 Ω resistors in measurements of low 236U ion currents. Additionally, a uranium material consisting of 99.9989% of 238U with a 235U/238U ratio of about 10-5 has been measured using a 1013 Ω amplifier resistor for the Faraday cup of 235U. The uncertainty is significantly improved compared to measurements using a secondary electron multiplier detector for 235U. Further examples and applications will be suggested.F1