Use of New 1013 Ohm Current Amplifiers in U and Pb Isotope Ratio Analysis by TIMS

Abstract:

For U-Pb dating of small zircon grains as well as for nuclear forensics, sample volume is limited and the ion beam currents in TIMS analyses are typically down to the fA range. For some samples, all ion beams are measured sequentially in a peak jumping experiment using a single collector ion counting detector, i.e. a Daly or a secondary electron multiplier. Such analyses do not take advantage of multicollection and require reasonable ion beam stability. Because of the sequential measurement sample utilization is a concern. With respect to the attainable precision and accuracy, there are at least two major ion counter characteristics to be dealt with: 1) linearity effects related to the dead time of the ion counter and 2) the mass-dependent detection efficiency of the ion counter inducing an instrumental mass bias effect. Both effects need proper calibration and monitoring otherwise could lead to systematic errors. For instance, the accurate measurement of a 1 Mcps signal down to 0.01% would require an accuracy of the dead time correction to <100 ps, which in itself is a challenge. Also proper calibration of detector-induced mass bias effects to the required precision of 0.01% is difficult. For demanding applications like high-precision U-Pb geochronology TIMS measurements, the goal is even to get precision and accuracy down to 100 ppm.

In this paper, we present our latest developments on low noise Faraday cup amplifier technology, that completely avoids the dead time calibration as well as instrumental mass bias effects introduced by ion counters. Faraday cup detectors have proven accuracy down to the ppm range, but have a limitation due to the Johnson noise involved in classical current 10¹¹ Ohm amplifiers. We have developed new current amplifiers using 100 times larger resistor values of 10¹³ Ohm, giving a signal-to-noise improvement of a factor of 10 over the classical 10¹¹Ohm current amplifiers. As such the final precision of very small sample measurements on Faraday cups can potentially be improved by a factor of 10 too.

This presentation will focus on the analysis of small Pb and U using TIMS, for geochronology and nuclear forensics. In our setup, all U and Pb isotopes are measured simultaneously on Faraday cups, except ²⁰⁴Pb which is collected on the SEM since the ²⁰⁴Pb is the least abundant and required for common lead correction.