Hyperfine splitting in highly charged B-like ions

Previous high-precision measurements of the groundstate hyperfine splitting in heavy H-like ions [1-5] intended to probe QED in the strong external electromagnetic field generated by a heavy nucleus. However, accurate calculations (see Ref. [6] and references therein) later revealed that the uncertainty of the predicted splittings, which mainly originates from the nuclear magnetization distribution correction (Bohr-Weisskopf effect), is comparable in magnitude with the QED correction. Accordlingly, a direct identification of QED effects on the hyperfine splitting in heavy H-like ions appeared to be unfeasible. It was shown instead, that this uncertainty can be significantly reduced in a specific difference of the hyperfine splitting values of Hand Li-like ions with the same nucleus [7]. High-precision measurements of the hyperfine splitting in heavy Li-like ions are presently in preparation [8]. The motivation for accurate calculations of the hyperfine splitting in B-like ions is twofold. From one side, highprecision prediction of the hyperfine splitting of B-like Fe may be important for astronomical search [9]. From the other side, the study of the hyperfine splitting in heavy Blike ions can be used to reduce the uncertainty associated with the Bohr-Weisskopf effect in some specific difference of the hyperfine splitting values for Band Li-like ions or Band H-like ions. The origin of this reduction is essentially the same as for the related g-factor values [10]. In a recent work [11] we have calculated the groundstate hyperfine splitting of B-like ions. The interelectronicinteraction correction of first order in 1/Z is evaluated within a rigorous QED approach. The higher-order terms are calculated employing the large-scale configurationinteraction Dirac-Fock-Sturm method. Table 1 presents the individual contributions and the total theoretical results for the ground-state hyperfine splitting in heavy Blike ions of particular interest. It can be seen that the uncertainties of the total theoretical values are completely determined by the Bohr-Weisskopf effect. These uncertainties can be strongly reduced employing the experimental values for the hyperfine splitting in the corresponding Hlike ions. We use the experimental values for the ground state hyperfine splitting, ∆E exp = 1.2159(2) eV for Hlike Pb [4] and ∆E exp = 5.0840(8) eV for Hlike Bi [1], to extract the Bohr-Weisskopf corrections for the 1s state employing the theoretical values for all other contributions from Ref. [6]. Considering different models for the nuclear magnetization distribution, we have found that the ratio of the Bohr-Weisskopf correc-