Precision measurements of the 60Co β-asymmetry parameter in search for tensor currents in weak interactions

The beta-asymmetry parameter (A) over tilde for the Gamow-Teller decay of Co-60 was measured by polarizing the radioactive nuclei with the brute-force low-temperature nuclear-orientation method. The Co-60 activity was cooled down to milliKelvin temperatures in a He-3-He-4 dilution refrigerator in an external 13-T magnetic field. The beta particles were observed by a 500-mu m-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) over tilde = -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.