Numerical study of multi-step photoionization of gadolinium-160 based on a polarization ionization path developed for odd gadolinium isotopes

Abstract

Abstract The enriched 160 Gd isotope is an important target nucleus for the production of the next-generation β -emitting therapeutic nuclide 161 Tb. To address the limitation that the J = 2 → 2 → 1 → 0 polarization ionization path, although suitable for separating odd Gd isotopes with parallel linearly polarized lasers, cannot be used to enrich even Gd isotopes, we propose an efficient and highly selective ionization scheme for 160 Gd. Based on the density matrix theory, a general four-color, three-step ionization model was established by taking into account magnetic sublevel degeneracy and hyperfine structure. Numerical calculations of photoionization dynamics were performed to optimize laser parameters such as polarization, power density, and frequency detuning, with the aim of enhancing the ionization yield and isotopic abundance of 160 Gd. The results show that, in the absence of Doppler broadening, the ionization yield obtained with orthogonal linear polarization is higher than that obtained with parallel linear polarization in both three-color, three-step and four-color, three-step schemes. To maximize ionization yield, resonant excitation is required in the three-color, three-step scheme, whereas detuned excitation is needed in the four-color, three-step schemes to mitigate the population trapping effect. When Doppler broadening is taken into account, the selective photoionization performance depends on the residual Doppler broadening and on the number of laser beams passing through the atomic vapor. The calculations indicate that the optimal ionization scheme, namely the four-color, three-step scheme with parallel linear polarization, detuned excitation, saturation power density, and a bandwidth of 0.5 GHz, can achieve a 160 Gd abundance exceeding 98% at a residual Doppler broadening of 0.45 GHz, thereby meeting the application requirements.