CERN: 30 years of ISOLDE at the Proton Synchrotron Booster

ISOLDE (Isotope Separator On-Line), CERN’s radioactive beam facility, has reached a milestone of 30 years of world-class science using protons from the Proton Synchrotron Booster (PSB). On 26 May 1992, a ceremony was held to celebrate the experiment facility’s relocation from the CERN Synchrocyclotron (SC) to the PSB. Guests were welcomed by then CERN Director-General, Carlo Rubbia, and talks were given by ISOLDE Committee Chair Björn Jonson (Chalmers University), who discussed new physics potential, and Claude Détraz (Director of IN2P3), who described ISOLDE’s importance to European nuclear physics. The ceremony concluded with Rubbia pushing the button to deliver “first beam”. Experiments at the new facility then started in earnest in June 1992.

Since 1967, ISOLDE had operated with 600-MeV protons from the SC with great success. Its initial objectives in nuclear physics had broadened to include atomic measurements – introducing optical methods, followed by lasers, to probe hyperfine structures of radioactive species – and condensed-matter physics, with the realisation that implanted radioactive nuclei could probe the solid-state environment.

By the 1980s, as the SC was reaching the end of its lifetime, the impact and importance of ISOLDE physics convinced the CERN Directorate to mandate the construction of a new version of the machine, within the main CERN accelerator complex, connected to the PSB. This scientific enterprise would benefit from increased proton energies from the PSB (initially 1 GeV and later 1.4 GeV), which would produce higher radioactive yields, improving and broadening scientific measurements. For the ISOLDE collaboration, beyond the higher production rate of nuclei, the facility’s integration into the main CERN accelerator chain ushered in a longer and open future.

Since 1992, a plethora of scientific riches have been generated through ISOLDE’s rebirth as a major CERN facility at the PSB, with higher proton energies and radioactive yields, pulsed beams and more space. This new potential was first tapped with pioneering studies of halo nuclei, but new innovations soon complemented the programme, including precision measurements of nuclear masses using novel ion-trapping techniques.

Over the decades, ISOLDE has maintained its position at the forefront of science by continuously updating its facility and beamlines. Significant progress has been achieved in the domain of production targets and ion sources and, especially, on laser ionisation, which has improved the elemental purity of the beams. These developments have increased the variety of isotopes that can be produced, thus widening the scientific scope of the facility. ISOLDE now encompasses nuclear structure, reactions and astrophysics; fundamental interactions; atomic and molecular physics; material science; and aspects of life sciences and radio-medicine.

A major innovation was the acceleration of radioisotopes to initiate reactions – further increasing ISOLDE’s scientific reach and fostering the development of new techniques. From 2001, in an extension to the original hall, the REX-ISOLDE post-accelerator delivered beams at 2.2 MeV/u, allowing electromagnetic excitation of radioactive nuclei (Coulomb excitation). The associated γ decay measured in a bespoke detector array, MiniBall, revealed many surprises concerning the shapes of exotic isotopes (here is one example). These successes motivated an upgrade to higher energies, to surmount the Coulomb barrier and allow nuclear reactions of exotic nuclei to be studied. This upgrade took the shape of a new superconducting linear accelerator named HIE-ISOLDE, which has been delivering beam since 2015 and reached 10 MeV/u in 2018. Among the new techniques sparked by the availability of higher energy beams is a novel solenoidal spectrometer, first used shortly before LS2 to probe the evolution of nuclear shell structure and reactions of interest for astrophysics.

As the facility’s scientific capabilities increased, new users flocked to the facility. The strong scientific collaboration of 300 users and eight member countries praised by Rubbia in 1992 has now grown to more than 900 users, and the number of countries that have signed the memorandum of understanding has doubled since then.

ISOLDE has some exciting medium-term ambitions, including the desire to further increase   its yield of radioactive nuclei, which the delivery of 2-GeV protons would make possible. In the long run, the collaboration seeks to improve both the scientific capability of the facility and its capacity, in order to diversify and increase its scientific output. With agile and innovative improvements, the future of ISOLDE will be as illustrious as its past.

This article was first published on 8 June by CERN.