IMPROVEMENT OF ELECTROSTATIC QUADRUPOLE ZOOM LENS FOR FIXED MULTI-COLLECTOR SYSTEM IN LRI-MS

摘要

The Laser-Resonance-Ionization Mass Spectrometer (LRI-MS) is a type of mass spectrometer with high elemental selectivity and high sensitivity, which is able to overcome the possible isobaric interferences [1]. Although frequently utilized in the reported LRI-MS facilities or instruments [2-5], time-of-flight (TOF) analyzer is not very appropriate for precise isotope ratio measurements due to its inherited shortcomings of the detection system, such as variation of signal detection efficiency [6]. In our laboratory, a LRI-MS system equipped with a magnetic analyzer (MA) has been used to measure the isotope ratios of plutonium [7] and tin [8] with peak-jumping method. Recently, a LRI-MS system equipped with an electrostatic analyzer (ESA), a magnetic analyzer (MA) and a fixed multi-collector system has been developed for precise isotope ratio measurement. In this instrument, the distance between the ion beams of different mass numbers at the focal plane is proportional to the relative mass dispersion. In order to achieve simultaneous detection of ions with different central mass by using the fixed multi-collector system, a zoom lens based on electrostatic quadrupoles has been developed to adjust the distances of the ion beams at the focal plane [9]. However, the zoom effect, that is, the horizontal magnification simulated by SIMION is quite different from that predicted by theoretical model. In this work, the reason why the zoom effect in the simulation is so different from that in theoretical model is carefully analyzed. Based on the analysis, the zoom lens was improved to obtain a more accurate zoom effect. By using a home-made diagnostic tool capable of visualizing the shape of ion beam, the zoom effect of the improved zoom lens was evaluated experimentally.