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.

The measurement of the β asymmetry parameter in nuclear β decay is a potentially very sensitive tool to search for non V − A components in the charge-changing weak interaction. To reach the required precision (percent level) all effects that modify the emission pattern of the β radiation, i.e. the geometry of the setup, the effect of the magnetic field on the trajectories of β particles as well as (back)scattering in the source, on the sample holder and on the detector, have to be correctly taken into account in the analysis of the data. A thorough study of these effects and a new method based on detailed GEANT4 Monte-Carlo simulations that was developed for this purpose is presented here. The code was developed for β asymmetry measurements by means of the Low Temperature Nuclear Orientation (LTNO) method, but can in principle be generalized to other experimental setups using other polarization techniques.

An experimental set-up is described for the precise measurement of the recoil energy spectrum of the daughter ions from nuclear beta decay. The experiment is called WITCH, short for Weak Interaction Trap for CHarged particles, and is set up at the ISOLDE facility at CERN. The principle of the experiment and its realization are explained as well as the main physics goal. A cloud of radioactive ions stored in a Penning trap serves as the source for the WITCH experiment, leading to the minimization of scattering and energy loss of the decay products. The energy spectrum of the recoiling daughter ions from the ¬-decays in this ion cloud will be measured with a retardation spectrometer. The principal aim of the WITCH experiment is to study the electroweak interaction by determining the beta-neutrino angular correlation in nuclear ¬-decay from the shape of this recoil energy spectrum. This will be the first time that the recoil energy spectrum of the daughter ions from ¬-decay can be measured for a wide variety of isotopes, independent of their specific properties. IntroductionThe standard model of the electroweak interaction is very successful in describing the interaction both qualitatively and quantitatively. However, it contains many free parameters and ad hoc assumptions. One of these is that from the five possible types of weak interactionsvector (V), axial-vector (A), scalar (S), tensor (T) and pseudoscalar interaction (P) -just V and A interactions are present at a fundamental level. Together with maximal parity violation this has led to the well known V A structure of the weak interaction. Most experimental limits for the S and T coupling constants in the charged current sector are rather weak, though [1,2,3,4],

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We measured the half-life of the superallowed 0 + → 0 + β + emitter 26 Si to be 2245.3(7) ms. We used pure sources of 26 Si and employed a high-efficiency gas counter, which was sensitive to positrons from both this nuclide and its daughter 26 Al m . The data were analyzed as a linked parent-daughter decay. To contribute meaningfully to any test of the unitarity of the Cabibbo-Kobayashi-Maskawa (CKM) matrix, the f t value of a superallowed transition must be determined to a precision of 0.1% or better. With a precision of 0.03%, the present result is more than sufficient to be compatible with that requirement. Only the branching ratio now remains to be measured precisely before a ±0.1% f t value can be obtained for the superallowed transition from 26 Si.