Time-resolved Weak Signal Detection And Its Application In LIBS

The analytical sensitivity of laser-induced breakdown spectroscopy(LIBS) is significantly influenced by the bremsstrahlung emission of the electrons in plasma. Since the time duration of the background emission in LIBS is usually shorter than that of the atomic emission, it’s possible to get high signal to background ratio by using time-resolved signal detection technique.This paper combined with photomultiplier tube and its peripheral hardware circuit to complete the design of gate circuit and amplifier circuit. The main process of this system as follow: the gated photomultiplier completes photoelectric conversion, and uses the high-speed amplifier to complete electrical signal transformation by the digital storage oscilloscope displaying and recording the data. First, this paper respectively describes the design method of photomultiplier tube peripheral hardware circuit from the working principle of the photomultiplier tube and the control circuit to complete photomultiplier tube peripheral circuit. The high gain wide band THS4012 is choosed as amplifier to complete extraction of the photoelectric signal.In this work, a gated photomultiplier tube with a simple and new gating circuit was applied in LIBS signal detection and the copper impurity in aluminum alloy was analyzed with improved analytical sensitivity. The rejection ratio of the background reached 15:1. The limit of detection of copper in aluminum alloy was determined to be 1.02 ppm in single-pulse LIBS, which was much lower than that obtained without using gating technique. This gated photomultiplier tube can be used to reduce influence of the background emission and will be helpful for improving both analytical sensitivity and spatial resolution of LIBS.A new gated amplifier was also applied in this paper. The gated amplifier not only realizes choosing suitable sampling gate from the signal out of photomultiplier tube in order to eliminate the interference of electron bremsstrahlung to atomic radiation signal detection. The gated amplifier but also completes time resolved atomic emission spectrometry detection. Moreover, based on the gated photoelectric tube, it improves switching effect of the previous field effect transistor, and inhibits switching effect. What’s more, the gated amplifier increases the magnification so that the detection sensitivity can be further improved.This paper carries out research and design of practical laser-induced breakdown spectroscopy(LIBS) based on the theory of photonics, and it is supported by the gated photomultiplier tube and low-noise width-band amplifier circuit. The work mainly includes the design and optimization of control circuit, the design of low-noise width-band amplifier circuit, the gated amplifiers and some experimental work. The results of the experiment show that the overall design of the system is feasible, and the measurement accuracy and stability meet the requirements of experiment.