D. Zakoucky

Precision measurements of the

The {beta}-asymmetry parameter A-tilde for the Gamow-Teller decay of {sup 60}Co was measured by polarizing the radioactive nuclei with the brute-force low-temperature nuclear-orientation method. The {sup 60}Co activity was cooled down to milliKelvin temperatures in a {sup 3}He-{sup 4}He 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-tilde=-1.014(12){sub stat}(16){sub 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.

^{60}

The half-life of the {alpha}-decaying nucleus {sup 221}Fr was determined in different environments, that is, embedded in Si at 4 K, and embedded in Au at 4 K and about 20 mK. No differences in half-life for these different conditions were observed within 0.1%. Furthermore, we quote a value for the absolute half-life of {sup 221}Fr of t{sub 1/2}=286.1(10) s that is of comparable precision to the most precise value available in the literature.

Co

It has been claimed recently that half-lives of radioactive nuclei embedded in metals would be significantly affected by electron screening provided by the metal. The effect would further be strengthened at low temperatures. We have determined the half-life-of {sup 253}Es nuclei embedded in iron at temperatures between 4 K and 50 mK. Our results agree with the room temperature literature value within about 2% and show no dependence on temperature over a range of almost two orders of magnitude.

\beta

A new value for the hyperfine magnetic field of copper impurities in iron is obtained by combining resonance frequencies from experiments involving {beta}-NMR on oriented nuclei on {sup 59}Cu, {sup 69}Cu, and {sup 71}Cu with magnetic moment values from collinear laser spectroscopy measurements on these isotopes. The resulting value, i.e., B{sub hf}(CuFe) = -21.794(10) T, is in agreement with the value adopted until now but is an order of magnitude more precise. It is consistent with predictions from ab initio calculations. Comparing the hyperfine field values obtained for the individual isotopes, the hyperfine anomalies in Fe were determined to be {sup 59{Delta}69}=0.15(9)% and {sup 71{Delta}69}=0.07(11)%.

-asymmetry parameter in search for tensor currents in weak interactions

Nuclear Physics Institute, ASCR, 250 68 Rez, Czech Republic CERN, CH-1211 Geneve 23, Switzerland LPC Caen, ENSICAEN, Universite de Caen, CNRS/IN2P3-ENSI, Caen, France Universitat Munster, Institut fur Kernphysik, Wilhelm-Klemm-Str. 9, D-48149 Munster, Germany 4 Instituut voor Kernen Stralingsfysica, KU Leuven, Celestijnenlaan 200D, B-3001, Leuven, Belgium 5 Karlsruher Institut fur Technologie, Institut fur Kernphysik, Postfach 3640, 76021 Karlsruhe, Germany 6 FAIR GmbH, Planckstr. 1, 64291 Darmstadt, Germany 7 TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia V6T2A3, Canada 8 Atomic Physics Laboratory, RIKEN, Wako-shi, Saitama 351-0198, Japan

Half-life of 221Fr in Si and Au at 4 K and at millikelvin temperatures

The nuclear spin-lattice relaxation of {sup 62}Cu in iron has been studied with the low temperature nuclear orientation method. At temperatures ranging from 6.5 mK to about 100 mK and a magnetic field of 0.1 T the relaxation constant for {sup 62}Cu in Fe was found to be C{sub K}[{sup 62}Cu]=4.34(25)sK.