WITCH experiment
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WITCH (Weak Interation Trap for Charged particles) is a double Penning trap experiment to measure the recoil energy of decaying nuclei. A spectrometer in combination with a position sensitive microchannel plate detector is used to measure ions and their energy. The experiment is located at the ISOLDE Radioactive Ion Beam Facility in CERN. The beam from ISOLDE is bunched by REXTRAP after which it is transferred to the WITCH set-up.
Working principle:
The ion bunch from REXTRAP is slowed down in a pulsed drift cavity in the vertical beamline. After this the bunch is injected into the 9T magnetic field. Here the ion cloud is trapped in the first Penning trap. The purpose of this first Penning trap, the cooler trap, is to cool the ions with buffer gas. In this trap it is possible to apply different excitation modes (dipole, quadropole and octupole). From the cooler trap the ions are transferred to the second Penning trap, the decay trap. In this trap the ions are kept to study their decay. When an ion decays it can escape the decay trap into the spectrometer. The spectrometer consists out of a set of electrodes to create an electrical potential barrier. If the decayed ion's energy is high enough it will be able to overcome this barrier to be detected at the top of the set-up.
Physics goal:
The first goal of the experiment is to measure the so-called beta-neutrino correlation. This is indirectly done by measuring the shape of the spectrum from the recoiling ions. This beta-neutrino correlation can provide, if measured precisely, information about fundamental weak interaction properties. According to the Standard Model the weak interaction has a Vector-Axial vector structure. But there are other mathematical possibilities which have not been detected in nature yet. The WITCH experiment will search for scalar currents in the first place. But it is also possible to look for tensor currents.
Other physics goals are the search for heavy neutrinos, measuring F/GT mixing ratios, measuring electron capture probabilities, etc.
