A High-Dynamic-Range Counting−Analog Detector System Based on a Secondary Electron Multiplier

Abstract

A high-dynamic-range counting−analog detector system based on a secondary electron multiplier (SEM) is described. The current-to-voltage converter, which is common to both channels, includes a high-speed transimpedance amplifier (TIA) and an automatic bias-correction circuit. This circuit also rejects 1/f noise of the TIA in the 0−2.3 Hz passband of the digital lowpass filter of a 24-bit analog-to-digital converter (ADC) in the electrometric channel. As a result, the noise-free code resolution of the entire channel, which is 18.5 bits, is determined only by the ADC noise. A number of circuit solutions are proposed to minimize the number of circuit components, which have made it possible to place the detector system board in the measuring head of the SEM and thereby reduce the stray capacitances that limit the response speed of the TIA. The maximum counting rate of the pulse-counting channel is 7 ×107 cps and is determined by the time resolution of the SEM. The problems of optimizing the supply voltages of the circuit and the gain of the SEM in the counting−analog mode are considered. The developed system is one of the units of the mass-spectrometry system for the isotope study of noble gases. The SEM-input-referred measurement ranges of the system are as follows: from 1 × 10–16 to 1.2 × 10−11 A at a resolution of 3 × 10−17 A or better for the electrometric channel; from 5 × 10−2 to 5 × 107 ions/s for the pulse-counting channel without counting losses and with the allowance for the SEM dark current; and from 1 × 10–20 to 1.2 × 10−11 A for the entire system. Due to the wide overlap of the measurement ranges, it is possible to perform mutual verification of data obtained in different modes.