Progress at the WITCH experiment towards weak interaction studies

A measurement of the $\beta$–ν angular correlation in nuclear $\beta$- decay is a good probe to search for physics beyond the Standard Model, independent of assumptions like parity, charge and time reversal violation. The WITCH (Weak Interaction Trap for Charged Particles) experiment will measure this correlation with the aim of further constraining the possible existence of scalar currents in the weak interaction or find a positive indication. The setup is located at ISOLDE/CERN and consists of a double Penning trap system combined with a retardation spectrometer to probe the energy of the recoil ions from the $\beta$- decay. The shape of the recoil ion energy spectrum allows to determine the $\beta$–ν angular correlation coefficient, $a$. Past experiments have allowed to measure this parameter with a precision of 0.5–1 %. The aim of the WITCH experiment is to measure $a$ with a precision of about 0.5 %.\\ A first step towards this goal has already been taken in 2006 with the measurement of a recoil ion energy spectrum from the decay of $^{124}$In. The complicated decay scheme of this nucleus, however, did not allow to extract the $\beta$–ν correlation coefficient. Measurements performed as part of this PhD work showed that the requirements for the prime physics candidate, $^{35}$Ar, put even more, stringent technical constraints on the system. To cope with these a major upgrade of the system was found to be necessary. The vacuum has been improved by one order of magnitude (5$\times$10$^{−8}$mbar$\to$ 5$\times$10$^{−9}$mbar in crucial areas), a control system to automate certain processes of the system was developed, parts of the system were improved and made more flexible and a magnetic shield was installed which allows, in combination with an RFQ buncher, to become independent from other experiments and thus provide significantly more time for testing and optimising the system. \\The first trial to measure a recoil ion energy spectrum with $^{35}$Ar in 2009 was hampered by a retardation-dependent ionisation effect – due to a Penning-like electron trap in the spectrometer – that was discovered. The effect was characterised with offline measurements, by simulations and with experience gained at other experiments. The solution to this problem is a conductive wire in the centre of this electron trap, effectively absorbing all electrons before they can cause rest gas atoms to be ionised and disturb the measurement. \\Further, in preparation for the analysis of a recoil ion energy spectrum from $^{35}$Ar decay, systematic effects have been studied in detail. The main effects to be taken into account relate to the properties of an ion cloud in a Penning trap (i.e. the radioactive source for the experiment), the transport of the recoil ions through the spectrometer and their detection. With the current understanding and operation of the system, the focus for the further investigation of systematic effects will have to lie on the determination of the properties of the ion cloud and the characterisation of the efficiency of the detection surface. \\At present, the setup is ready for measuring a good-quality recoil ion energy spectrum from $^{35}$Ar decay from which a first value of $a$ can be extracted. The ionisation effect has been solved and important systematic effects have been identified. The study of systematic effects will continue and this should lead to a good determination of $a$ (δ${a}$ ≈ 1 %) before the CERN shutdown period in 2013.