A laser ion source based on laser resonant ionization at the outlet of a helium jet was proposed for on-line isotope separator of the Heavy Ion Research Facility in Lanzhou, China a few years ago. An off-line test setup was built at Tsinghua University. The initial experimental results of the setup indicate that carrying neutral Na monoatoms to the outlet of a helium jet and directly ionizing the Na atoms in the helium jet beam by laser resonant ionization is very efficient. It provides an effective method for producing element selected ions with high efficiency, low background, and short transporting time. It is applicable for the separation of various elements, and is particularly useful for refractory elements.

This article is to solve the rate equations of laser resonance ionization for a three energy level system with little simplification. Three cases of utilizing continuous, quasi-continuous and pulsed laser are discussed respectively. The results of typical conditions presently available are given. The variation of the results with different parameters are discussed as well. The calculation method and program can be used for general purpose. As long as the necessary parameters have been determined, the program can be used for quantitative calculation of an experiment.

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