Laser enrichment of Gd-157 based on polarization selection rules

在Gd原子的吸收光谱中，与奇同位素元素的超精细结构宽度相比，同位素的位移非常小。因此，基于极化选择规则的选择性光电离方案似乎适合于分离Gd-157。在该方案中，使用三个线偏振激光脉冲来制备排列态。偶同位素的电离受到选择规则的抑制，而奇同位素的超精细相互作用则允许选择性电离。测量了Gd在3步光电离方案中适用于第1步和第2步跃迁的吸收光谱，并确定了同位素位移和超精细结构常数。通过J=2*2*1*0的激发途径，Gd-157的富集率可达80%。然而，这种方法的选择性会因外加磁场而丧失，因为中间态排列会被磁场破坏，最终导致均匀同位素电离。分析了磁场对同位素选择性的影响，讨论了高选择性的条件。实验计算了选择性光离Gd离子与Gd原子之间的电荷转移碰撞截面。讨论了原子蒸汽密度对保持高选择性的影响。

In the absorption spectrum of atomic Gd, the isotope shifts are very small compared with the widths of hyperfine structures of the odd isotopic elements. Therefore the selective photoionization scheme based on the polarization selection rules seems to be suitable to separate Gd-157. In this scheme three linearly polarized laser pulses are used to prepare aligned states. Ionization of the even isotopes in inhibited by the selection rules, while the hyperfine interactions in the odd isotopes allow selective ionization.The absorption spectrum of Gd for the transitions applicable to the 1st and 2nd steps in the 3-step photoionization scheme were measured and the isotope shifts and the hyperfine structure constants were determined. By using the excitation pathway with J=2*2*1*0, the enrichment of Gd-157 up to 80% was demonstrated. The selectivity of this method, however, will be lost by an external magnetic field because the intermediate-state alignment is destroyed by the magnetic field that finally leads to ionization of the even isotopes. The effects of magnetic field on isotope selectively was analyzed and the condition for high selectivity was discussed.The charge transfer collision cross section between the selectivity photoionized Gd ions and Gd atoms were experimentally evaluated. The atomic vapor density to maintain high selectivity was also discussed.