Shell model calculation of the beta- and beta+ partial halflifes of 54Mn and other unique second forbidden beta decays

The nucleus 54Mn has been observed in cosmic rays. In astrophysical environments it is fully stripped of its atomic electrons and its decay is dominated by the beta- branch to the 54Fe ground state. Application of 54Mn based chronometer to study the confinement of the iron group cosmic rays requires knowledge of the corresponding halflife, but its measurement is impossible at the present time. However, the branching ratio for the related beta+ decay of 54Mn was determined recently. We use the shell model with only a minimal truncation and calculate both beta+ and beta- decay rates of 54Mn. Good agreement for the beta+ branch suggests that the calculated partial halflife of the beta- decay, (4.94 \pm 0.06) x 10^5 years, should be reliable. However, this halflife is noticeably shorter than the range 1-2 x 10^6 y indicated by the fit based on the 54Mn abundance in cosmic rays. We also evaluate other known unique second forbidden beta decays from the nuclear p and sd shells (10Be, 22Na, and two decay branches of 26Al) and show that the shell model can describe them with reasonable accuracy as well.Comment: 4 pages, RevTeX, 2 figure

Enhancement of the magnetic anisotropy of nanometer-sized Co clusters: influence of the surface and of the inter-particle interactions

We study the magnetic properties of spherical Co clusters with diameters between 0.8 nm and 5.4 nm (25 to 7500$ atoms) prepared by sequential sputtering of Co and Al2O3. The particle size distribution has been determined from the equilibrium susceptibility and magnetization data and it is compared to previous structural characterizations. The distribution of activation energies was independently obtained from a scaling plot of the ac susceptibility. Combining these two distributions we have accurately determined the effective anisotropy constant Keff. We find that Keff is enhanced with respect to the bulk value and that it is dominated by a strong anisotropy induced at the surface of the clusters. Interactions between the magnetic moments of adjacent layers are shown to increase the effective activation energy barrier for the reversal of the magnetic moments. Finally, this reversal is shown to proceed classically down to the lowest temperature investigated (1.8 K).Comment: 13 figures submitted to Phys. Rev.