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],

The presence of isospin mixing in the T = 5/2 ground state of 71 As was studied via anisotropic positron emission from oriented nuclei. A small isospin-forbidden Fermi component in the predominantly Gamow-Teller β decay was established, corresponding to an isospin mixing probability of (13 ± 4) × 10 −6 . The sign of the magnetic moment of 71 As was determined to be positive.

The nuclear magnetic moment of the nucleus 59 Cu, with one proton and two neutrons outside the closed N = Z = 28 shells, was measured in an on-line experiment combining ␤-NMR with low temperature nuclear orientation and with particle detectors operating at a temperature of about 10 K. From the data the center frequency ͑B ext =0͒ = 209.51͑22͒ MHz was derived. Using the hyperfine field of Cu in host iron from the literature the result for the moment is ͓ 59 Cu͔ = + 1.891͑9͒ N , which reveals a large deviation from the proton p 3/2 single-particle value. This provides strong experimental evidence for a massive shell breaking at 56 Ni.

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