Francium is a candidate for atomic parity non-conservation (PNC) experiments. Its simple atomic structure has been the subject of extensive experimental research facilitated by the ability to trap and cool significant numbers of atoms. The studies include the location of energy levels, their hyperfine splittings and their lifetime. All of these levels are close to the ground state. The results show a remarkable agreement with calculated ab initio properties to a degree that is comparable with other stable alkali atoms. The quantitative understanding of francium has made possible the exploration of avenues for a PNC measurement in the optical and the microwave regimes. These precision experiments have the potential to enhance our understanding of the weak coupling constants between electrons and nucleons, as well as between nucleons.

We have measured the hyperfine structure of the 7P 1͞2 level for 2082212 Fr to a precision of 300 ppm. These measurements along with previous ground state hyperfine structure measurements reveal a hyperfine anomaly. The hyperfine anomaly exhibits a strong sensitivity to the radial distribution of the neutron magnetization, providing a good way to probe this quantity. We use neutron radial distributions from recent theories to qualitatively explain the measurements. PACS numbers: 21.10.Gv, 27.80. + w, 32.10.Fn One of the properties of nuclei that can be probed with precise measurements of hyperfine structure is the nuclear magnetization distribution. The Bohr-Weisskopf effect [1,2] has been known for many years, but experimental and theoretical advances have now allowed more broadly based and detailed investigations [3][4][5][6]. There is much interest in obtaining the structural details of heavy nuclei, as these nuclei are involved in understanding quantum electrodynamic (QED) effects in heavy atoms [7], atomic parity nonconservation (PNC), time reversal violation, and nuclear anapole moments [8]. We have measured five different Fr isotopes. Comparison of adjacent isotopes allows extraction of the nuclear magnetization distribution of the last neutron, a quantity that is, in general, very difficult to study [9].Bohr-Weisskopf effect measurements usually require detailed knowledge of both hyperfine structure constants and magnetic moments. We show in this Letter that precision measurements of the hyperfine structure in atomic states with different radial distributions can give information on the hyperfine anomaly [10] and be sensitive to the nuclear magnetization distribution. Laser trapped radioactive atoms, cooled to mK temperatures, are an ideal sample for high precision Doppler-free laser spectroscopy [8]. High precision allows searching for higher order effects in the hyperfine structure.Francium is an excellent element for understanding the atom-nucleus hyperfine interactions, and eventually weak interactions. First, because of the large Z, hyperfine effects proportional to Z 3 are larger than in lighter atoms. Second, the simple atomic structure allows ab initio calculations of its properties [11][12][13]] that have been experimentally tested [14]. Third, Fr has a large number of isotopes spanning almost 30 neutrons with lifetimes greater than 1 s that cover a wide range of nuclear structure. Fourth, because of its proximity to Pb, where the charge radii are known extremely well from many techniques [15], we can determine the charge radii of the light Fr isotopes with some confidence.Coc et al. [16,17] measured the 7S 1͞2 ground state hyperfine constants for 16 Fr isotopes, but only one magnetic moment has been measured [18]. We have focused on extracting hyperfine anomaly information using the available data in the literature and our new precision spectroscopy of the 7P 1͞2 hyperfine structure on five francium isotopes. Previous measurements of the 7P 1͞2 [17] were not of sufficient precision to observe...

Weak interactions within a nucleus generate a nuclear spin dependent, parity violating electromagnetic moment, the anapole moment. We analyze a method to measure the nuclear anapole moment through the electric dipole transition it induces between hyperfine states of the ground level. The method requires tight confinement of the atoms to position them at the anti-node of a standing wave Fabry Perot cavity driving the anapole-induced micro-wave E1 transition. We explore the necessary limits in the number of atoms, excitation fields, trap type, interrogation method, and systematic tests necessary for such measurements in francium, the heaviest alkali.

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