Compact thermal ionization mass spectrometer for lithium isotope analysis
Lithium isotope measurement plays a significant role in the nuclear fuel cycle.Thermal ionization mass spectrometry(TIMS),known for its high precision,is a commonly used analytical method.Traditionally,large-sized commercial thermal ionization mass spectrometers with a magnetic analyzer deflection radius of 250 mm are employed for lithium isotope analysis,which are bulky and expensive.This study has developed a compact thermal ionization mass spectrometer with a magnetic analyzer deflection radius of only 90 mm,which is small in size and highly suitable for lithium isotope analysis.This compact TIMS features a modular design,consisting of a vacuum system,ion source,ion lens,isolation valve,magnetic mass analyzer,detector,data acquisition and control,and software system.The vacuum system is composed of a mechanical pump,a turbomolecular pump,an ion pump,and a compound vacuum gauge,achieving a vacuum level in the ion source chamber better than 5×10-8 hPa and in the analysis chamber better than 5×10-9 hPa.The ion source consists of an ion source chamber with a cold trap,a 10-position sample turntable,and a floating high-voltage filament power supply with a filament current adjustment range between 0 to 6 A.The ion lens is made up of acceleration voltage electrodes,extraction voltage electrodes,slit voltage electrodes,upper and lower deflection voltage electrodes,left and right deflection voltage electrodes,and an ion lens power supply,with each voltage adjustable between 0 to 6 kV.The magnetic analyzer forms a closed-loop control system with a magnet,Hall probe,and magnet power supply,capable of meeting the mass range requirements between 6 to 45 amu.The detector is composed of three independent Faraday cups and their detection circuits,with the detector output voltage reaching up to 10 V and the background noise of a 4-second integration being less than 30 μV.Data acquisition is realized using a 24-bit ADC data acquisition card.The software is written in C#language combined with a MySQL database,employing the RS485 communication protocol to facilitate data exchange and control with the hardware.The overall instrument achieves an internal precision level for the measurement of natural lithium isotope ratios ranging from 0.01%to 0.03%.
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