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The β-asymmetry parameter A for the Gamow-Teller decay of 60 Co was measured by polarizing the radioactive nuclei with the brute force low-temperature nuclear-orientation method. The 60 Co activity was cooled down to milliKelvin temperatures in a 3 He-4 He dilution refrigerator in an external 13 T magnetic field. The β particles were observed by a 500 µm thick Si PIN diode operating at a temperature of about 10 K in a magnetic field of 0.6 T. Extensive GEANT4 Monte-Carlo simulations were performed to gain control over the systematic effects. Our result, A = −1.014 (12)stat (16)syst, is in agreement with the Standard-Model value of −0.987(9), which includes recoil-order corrections that were addressed for the first time for this isotope. Further, it enables limits to be placed on possible tensor-type charged weak currents as well as other physics beyond the Standard Model.

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We have measured the hyperfine splitting of the 7P 1/2 state at the 100 ppm level in Fr isotopes ( 206g,206m,207,209,213,221 Fr) near the closed neutron shell (N = 126 in 213 Fr). The measurements in five isotopes and a nuclear isomeric state of francium, combined with previous determinations of the 7S 1/2 splittings, reveal the spatial distribution of the nuclear magnetization, i.e. the Bohr-Weisskopf effect. We compare our results with a simple shell model consisting of unpaired single valence nucleons orbiting a spherical nucleus, and find good agreement over a range of neutron-deficient isotopes ( 207−213 Fr). Also, we find near-constant proton anomalies for several even-N isotopes. This identifies a set of Fr isotopes whose nuclear structure can be understood well enough for the extraction of weak interaction parameters from parity non-conservation studies.PACS numbers: 21.10. Gv,27.80.+w,32.10.Fn Weak interaction studies in heavy atoms require for their interpretation precise knowledge of the atomic and nuclear wavefunctions. To extract nucleon-nucleon weak interaction couplings from the weak interaction induced parityviolating anapole moment [1], nuclei with simple and regular magnetic properties are desirable [2-4]. The nuclear magnetic moment is used to benchmark nuclear structure theories for calculating the anapole moment [3], which is a contact field effect produced inside the finite extent of the nucleus. Here we explore the regularity of the magnetic properties of a chain of Fr isotopes and find that 207−213 Fr in the vicinity of the neutron shell closure mark a range where the nuclear structure is sufficiently tractable for standard model tests and constraints on new physics [5].To lowest order, the atomic hyperfine interaction can be described using a point-like nucleus characterized by the magnetic dipole moment. Deviations from the pointlike approximation of the nucleus, referred to as hyperfine anomalies, come from considering how finite magnetic and charge distributions affect the interaction between the magnetization of the nucleus and the magnetic field created by the electrons. The magnetic contribution is known as the Bohr-Weisskopf (BW) effect [6,7]. The difference in the nuclear charge distribution (Breit-Rosenthal (BR) effect [23][24][25]) produces very small variations between isotopes, leaving the BW effect dominant [26,27]. As a new generation of proposed parity violation experiments in atoms (including Fr) and molecules starts [8][9][10][11][12][13][14], it is important to understand the limiting factors due to the nuclear structure, e.g. the nuclear magnetization, for the interpretation Fr with up to 6 neutron holes, we find near-constant magnetic hyperfine anomalies for the odd-Z, even-N isotopes [15]. The neutron rich odd-even isotope 221 Fr shows a different behavior due to the deformation of the nucleus. The odd-Z, odd-N isotopes have anomaly contributions from both the proton and the valence neutron.BW effect measurements usually require precise knowledge of both, hyper...