Characterization of Very Low Intensity Ion Beams from the REX/HIE-ISOLDE Linear Accelerator at CERN

The Isotope Separator On-Line Device (ISOLDE) at CERN is one of the world-leading research facilities in the field of nuclear physics. Radioactive Ion Beams (RIBs) are produced when 2.0 GeV protons, driven from the Proton Synchrotron Booster (PSB), impact onto a target. The RIB of interest is extracted and transported to experimental stations either directly or following an acceleration to higher energies. Before meeting the requirements for acceleration, the continuous low-energy RIB is accumulated and cooled into the REX-TRAP Penning trap and transferred toward the REXEBIS charge breeder. The ion beam extracted from the REXEBIS is pulsed and multi-ionized to reach a mass-to-charge ratio (A/q) within the acceptance of the linac. After that, the RIB is accelerated (up to 9.7 MeV/u at A/q = 3.1) through the Radioactive EXperiment normal-conducting linac injector and the recently completed High Intensity and Energy superconducting extension (REX/HIE-ISOLDE) and finally transported to one of the three experimental stations located at the end of High Energy Beam Transfer (HEBT) lines. The manipulation and precise knowledge of the ion beam properties are of primary importance for the experimental apparatus, as well as for the calibration and efficiency of the detection systems. The study of ion beam properties mainly concerns the dynamics over time and space distributions of density, motion and energy, correlated with beam physics, that is to say: the purity of the beam of interest by considering potential sources of contamination, the ion-pulse time-structure, the transversal and longitudinal properties (Courant-Snyder parameters and energy distribution). The core of the thesis aims at defining instrumental methodologies that allow for the determination of RIBs properties from the REX/HIE-ISOLDE linear accelerator, at intensities often too weak to make use of conventional beam-line monitoring detectors (for instance, Faraday cups). We propose new approaches appropriate for very low-intensity ion beams, using a silicon detector as a beam collector. In general, the demonstration of a new measurement concept suitable for sub-femto-Ampere ion beams is corroborated with ordinary measurement techniques at ion beam currents higher than several pico-Amperes. The capability to measure absolute beam intensities over three orders of magnitude is illustrated with a practical use during beam operation, tuning the Radio-Frequency Quadrupole structure. Then, we measure and study the quality of the beam produced from REXEBIS in the scope of evaluating the electron gun's performance and the general charge-breeding conditions. An effort dedicated to optimizing the ion pulse time distribution extracted from an EBIS, called Slow Extraction, led us to use measurements of axial energy distributions further. By justifying the collisional aspects of the plasma, we infer a correlation between the measured axial energy distributions and the temperature of ions. Key indicators of the extracted beam's quality can then be deduced from the ion temperature using Boltzmann distributions, such as the emittance. We illustrate the studies related to the charge-state and the energy dynamics with the case of a multi-charged xenon beam. The question of beam purity is assessed by demonstrating the possibility to map a spectrum of the contamination from residual gas ions on a wide A/q-range. Furthermore, we characterize the beam dynamics in the HEBT lines after acceleration through REX/HIE-ISOLDE linac. The principles behind the identification of the transverse and the longitudinal beam properties are mainly based on multiple acquisitions of the respective trace-space projections. In the case of transverse beam properties, we correlate measurements using the quadrupole-scan method and the double-slit method using a $^{39}$K$^{10+}$ beam of less than one femto-Ampere current, at an energy of 3.8 MeV/u. We show the uncertainties associated with the evaluation of the emittance and Twiss parameters for each method. We then explain the choice made for measuring the beam energy and the analysis of results using RIB. The complete longitudinal beam properties are measured using a superconducting quarter-wave resonator as a buncher, and measuring the beam energy and time-of-flight distributions.