REX-ISOLDE

REX-ISOLDE. The Radioactive beam EXperiment at ISOLDE is a pioneering post-accelerator facility located at CERN in Geneva. It was specifically designed to accelerate the short-lived radioactive ions produced by the ISOLDE separator to energies suitable for nuclear physics experiments. This capability transformed ISOLDE from a facility for low-energy studies into a world-leading laboratory for investigating the structure and reactions of exotic nuclei far from stability.

Overview

Commissioned in 2001, REX-ISOLDE represented a major technical upgrade to the long-standing ISOLDE facility, which had been operational since the 1960s. The project was a cornerstone of the ISOLDE collaboration, involving numerous European universities and research institutes. Its primary purpose was to enable Coulomb excitation and transfer reaction studies by providing beams with energies of 0.8–3.0 MeV per nucleon. This energy range opened a new window into the properties of nuclei with extreme neutron-to-proton ratios, complementing research at other radioactive beam facilities like GANIL in France and RIKEN in Japan.

Technical Design

The technical design of REX-ISOLDE is an ingenious system that accepts low-energy (≈60 keV) ions from the ISOLDE separator and accelerates them through a multi-stage process. Ions are first captured and cooled in a Penning trap (REXTRAP) and then charge-bred to a higher charge state in an electron beam ion source (REXEBIS) to improve acceleration efficiency. The actual acceleration is achieved through a linear accelerator (LINAC) comprising a Radio Frequency Quadrupole (RFQ), an interdigital H-type (IH) structure, and three seven-gap resonators. This design allowed for the efficient acceleration of a wide variety of ion species produced at ISOLDE, from light nuclei to heavy isotopes like lead.

Scientific Programme

The scientific programme of REX-ISOLDE was fundamentally centered on exploring the evolution of nuclear structure away from the valley of stability. Key research themes included the study of shell evolution and magic numbers in exotic nuclei, the investigation of collective phenomena like nuclear deformation and vibrational states, and the measurement of fundamental properties such as nuclear moments and charge radii. The programme heavily utilized the technique of Coulomb excitation in inverse kinematics, where the accelerated radioactive beam impinges on a stable target, such as nickel or gold, allowing precise probing of nuclear excitation spectra.

Key Experiments and Results

Key experiments at REX-ISOLDE have produced landmark results in modern nuclear physics. Studies on neutron-rich isotopes of tin, cadmium, and zinc provided direct evidence for the erosion of the traditional N=50 and N=82 magic numbers far from stability. Pioneering Coulomb excitation measurements on light nuclei like beryllium-11 and boron-12 shed light on halo nuclei and their low-lying excited states. The facility also enabled crucial transfer reaction studies, such as (d,p) reactions, to investigate single-particle properties and spectroscopic factors near the drip line. These experiments were conducted with advanced detection systems like the Miniball gamma-ray spectrometer and the T-REX particle detector array.

Legacy and Impact

The legacy of REX-ISOLDE is profound, having established the technical and scientific foundation for the next-generation facility, HIE-ISOLDE (High Intensity and Energy ISOLDE), which began operation in 2015. It trained a generation of nuclear physicists and demonstrated the feasibility and scientific value of post-accelerated radioactive beams at CERN. Its impact extends beyond nuclear structure, contributing to interdisciplinary research in nuclear astrophysics by providing data on reactions relevant to stellar nucleosynthesis processes. The success of REX-ISOLDE solidified the position of the ISOLDE laboratory as a premier global facility within the NuPECC roadmap for European nuclear science. Category:Particle accelerators Category:CERN Category:Nuclear physics experiments