Behavior of many ions in a Penning trap and results of the WITCH experiment

Precision measurements of the beta−neutrino angular correlation in nuclear beta-decay provide a unique window into the physics beyond the Standard model. The WITCH (Weak Interaction Trap for CHarged particles) experiment aims to measure this correlation, a(beta-nu), in order to impose a more stringent constraint on the exotic scalar current admixture in the beta-decay Hamiltonian. The apparatus is situated at CERN/ISOLDE laboratory and consists of a unique combination of a retardation spectrometer and two Penning traps, with one of them serving as a scattering-free source. This configuration is suited for a precise measurement of the energy spectrum of 35Ar recoiled daughter ions. The shape of the spectrum then allows a determination of a(beta-nu) and consequently of the presence or absence of a scalar current. Radioactive 35Ar ions are created at ISOLDE by impinging 1.2 GeV protons on the target material. After being separated by a magnetic separator and bunched by REXTRAP, a high-capacity Penning trap, they are delivered to the WITCH beam line with an energy of 30 keV per ion. A pulsed drift tube then reduces this energy to about 100 eV, enabling their capture in the cooler trap where they undergo buffer gas cooling and centering processes before being transferred to the decay trap. After decay, the energy spectrum of recoil 35Cl daughter ions is probed in the retardation spectrometer before they are finally counted by an MCP detector. Retardation spectra for various values of a(beta-nu) are simulated by SimWITCH, a Monte Carlo simulation software that tracks the recoil ions from a cloud in the decay trap through the spectrometer. The experimental spectrum is then fitted to simulated spectra and the value of a(beta-nu) can be extracted. The properties of the ion cloud from injection into the cooler trap to transfer into the decay trap are obtained with Simbuca, a simulation software package that exploits native GPU parallelism for fast ab initio simulation of ion cloud dynamics in a Penning trap, with realistic electric and magnetic field maps. Furthermore, as the main focus of this work, systematic effects arising in large ion clouds (of 105 − 106 ions) are studied experimentally with stable 39K ions as well as computationally with Simbuca and comparatively analyzed and presented. Specifically, the influence of space-charge and buffer gas on cyclotron cooling resonances is investigated. Experimental ion cyclotron resonances are compared with ab initio Coulomb simulations and found to be in agreement, showing an increase of central values and FWHMs with increasing space-charge and buffer gas pressure. The ability to accurately simulate the behavior of large ion clouds in specific experimental conditions is of special interest for the design and optimization of high-capacity Penning traps and their operation as mass separators. Another important systematic effect of the WITCH experiment, the magnetron eigenmotion of the ion cloud around the trap center, is experimentally studied under increasing space-charge conditions. In addition, the helium buffer gas pressure in the Penning trap is determined by comparing experimental cooling rates with simulations. In June 2011, an experiment resulting in a first determination of a(beta-nu) with WITCH was performed, albeit with low statistics and with no systematic effects considered. The November 2012 experiment yielded a much larger data set. Combined with a significantly improved data-acquisition system, this enables us to uncover several important systematic effects and account for their influence. These include energy-dependent efficiency of the main MCP, two separate radioactive components stemming from the decay of overshot 35Ar ions implanted into the main MCP detector and from the beta-particles originating in the decay trap, and an effect of the magnetron motion of the entire ion cloud around the trap center. The data is then fitted with a function tailored to account for these systematic effects. However, another background component difficult to account for was found to significantly distort the low energy part of the recoil spectrum and hamper the extraction of a(beta-nu). Using Monte Carlo simulations of the spectrum, it is found that this component is most likely correlated with the amount of radioactive ions in the decay trap and is composed of low-energetic Gaussian-distributed rest gas or buffer gas ions. An overview of measures needed to characterize and reduce this background are given. Finally, constraints related to availability of radioactive beams at ISOLDE leading to the discontinuation of the project are discussed.