Voltage Divider for Precise Mass Measurement with Two-Stage Double-Focusing Mass Spectrometer at Osaka University

Abstract

At Osaka University, a large two-stage double-focusing mass spectrometer was constructed and its focus-adjustment work is in progress at present. With the new machine, the authors are intending to determine the atomic masses to an accuracy of one part in109or somewhat higher using the peak matching technique. To realize the measurements with such a high accuracy, one of the most important things is the setting up of a high precision voltage divider as the main part of the potential supplying system for the ion-energy selector of the machine. The construction work of this precise voltage divider was finished, and the preliminary resistance calibration of the resistors composing the divider was carried out. The divider consists of three groups; a variable resistance assemblyΔR, a main fixed resistance assembly R and an auxiliary one R'. These assembly, are connected in aπ-shape network as Nier's circuit. Each of the R and R' consists of one hundred 100-kΩresistors to form a total resistance of 10MΩ. All these100-kΩresistors are selected to match with each other to whithin one part in 104. TheΔR consists of a100-kΩresistor and six decades of 10kΩ, 1kΩ, 100Ω, 10Ω, 1Ω, and 0.1Ωper step. The whole system is installed in a box temperature-controlled at30±0.1°C. In this resistance assembly, it is required to know only the ratio values ofΔR/R irrespective of the absolute values of the resistances. All the resistors were calibrated referring to an internal standard with an accuracy of one part per million or less, using the standard bridge technique. For the evaluation of R, the simple relation of the series to parallel resistance ratio was adopted.