Higher energies for ISOLDE's radioactive ion beams

Athena Papageorgiou Koufidou
Thursday, September 1, 2016

First published in Accelerating News

On 28 September, the members of the ISOLDE collaboration and major stakeholders came together in a well-deserved celebration. The first phase of the facility’s high energy and intensity upgrade (HIE-ISOLDE) is now complete and a promising future is in sight as experiments started on 9 September.

ISOLDE is the oldest facility still in operation at CERN and one of the most successful. It currently occupies a leading position in the field of radioactive ions research, producing the largest range of isotopes worldwide (over 1300 isotopes of more than 70 elements), which are used in multiple fields of physics: nuclear and atomic physics, astrophysics and fundamental interactions. A key element of ISOLDE’s success is the wealth of technical expertise it has accumulated over the decades, especially in the construction of target‑ion source units. The secret to the facility’s longevity, however, is its vibrant international collaboration and its ability to adapt to the changing physics landscape.

An impressive team is behind HIE-ISOLDE, comprising leading physicists, engineers and other experts in accelerator and beam technologies. Another essential ingredient of the workforce are early stage Marie Curie researchers, who acquire valuable skills in the area of advanced accelerator technologies, reflecting the commitment of ISOLDE on training the next generation of experts.

Taking beam energy and intensity to new heights

The production of radioactive ion beams at ISOLDE begins when a high‑energy proton beam from the PS Booster hits the facility’s target, resulting in a wide variety of reaction products. These are ionised in a surface, plasma or laser ion source and separated according to mass, producing the beam of the preferred element. An RFQ cooler and buncher lowers the temperature of the radioactive beam, thus significantly reducing emittances and energy spreads. The beam is then delivered to the low-energy experimental stations or charge‑bred and post‑accelerated at the REX accelerator.

The energy upgrade of the facility entails the construction of a superconducting linear accelerator (HIE-linac) to increase the energy of radioactive ion beams, a high energy beam transfer line to bring the beam to the experiments, as well as new beam diagnostic tools. The intensity upgrade aims to improve the target and ion source, the mass separators and charge breeder.

HIE-linac takes advantage of many cutting‑edge cryogenics and radiofrequency technologies that were originally developed for the LHC. It is equipped with superconducting radiofrequency cavities made of copper coated with niobium and operating at 101.29 MHz. They are cooled by liquid helium at 4.5 K in ultra‑high vacuum conditions. In the first phase of the energy upgrade, two high‑beta cryomodules, each containing five cavities and one superconducting solenoid magnet, were coupled to REX-linac and commissioned, thus increasing energy to 5.5 MeV per nucleon. Two more cryomodules with the same configuration will be added in the second phase, allowing beams to be accelerated to 10 MeV per nucleon; one is currently in the SM18 clean room, awaiting installation in 2017, and the other is scheduled to be assembled and installed in 2018. In the third and final phase, two low-beta cryomodules, containing six cavities and two solenoids each, will be manufactured and installed in replacement of the 7-gap and 9-gap normal conducting structures of REX, allowing beams to be decelerated to 0.3 MeV per nucleon.

The tunnel at HIE-ISOLDE now contains two cryomodules – a unique set up that marks the end of phase one for the HIE-ISOLDE installation. By Spring 2018 the project will have four cryomodules installed and will be able to reach higher energy up to 10 MeV/u. Image credits: Erwin Siesling/CERN.

After post-acceleration in HIE-linac, radioactive ions enter the high‑energy beam transfer line (HEBT), which is specially designed to preserve emittances. Then, the beam is delivered to the different experimental stations through one of two beam lines that have been in operation since 2015. A third one will be installed in early 2017.

The PS Booster upgrade and the operation of Linac 4 after LS2 are expected to increase the primary proton beam intensity at ISOLDE to 6.7 μA, allowing more exotic isotopes to be produced and more precise measurements to be obtained. However, the new experimental conditions create a set of challenges that necessitate ISOLDE’s intensity upgrade. Higher radiation levels limit the lifetime of the target, thus options for new target materials with a focus on radiation resistance are explored, while materials that are presently used undergo extensive radiation tests. The laser ion source (RILIS) has also been upgraded, improving selectivity and developing new ionisation schemes. Finally, the improvement of the mass separators will reduce isobaric contamination.

HIE-ISOLDE is currently the only next generation radioactive beam facility available in Europe, while SPIRAL-2 and SPES are still under construction,and the most advanced isotope separation on-line (ISOL) facility in the world.

New physics opportunities

HIE-ISOLDE creates a wealth of opportunities for research in many aspects of nuclear physics, astrophysics, as well as solid state physics, because it can produce a wide variety of exotic nuclei at different energies. The upgrade was welcomed by the international nuclear physics community and is in line with the recommendations of the Nuclear Physics European Collaboration Committee. Over thirty experiments have already been approved and are now at the preparation stage.

Nuclear physics

Scientists have been studying the atomic nucleus for more than 100 years, starting with Ernest Rutherford in 1911, yet many open questions remain: What is the nature of nucleonic matter? What happens if we change the energy, momentum, or temperature of the nucleus? Studying radioactive ion beams allows researchers to dig deeper into these questions, as radioactive nuclei often behave differently than stable ones and can reveal certain aspects of nuclear behaviour that their stable counterparts cannot. Accelerating these exotic nuclei to higher energies provides new physics possibilities, matching the innovative theoretical developments of the field. Many of the approved experiments plan to use Coulomb excitation, including studying the physics of super-heavy nuclei, which could reveal the next magic numbers in the very heavy systems. Other experiments will investigate transfer reactions, which may allow physicists to unravel the evolution the structure of the nucleus’s energy levels, also known as its ‘shell structure’.

Nuclear astrophysics

HIE-ISOLDE also paves the way for advances in nuclear astrophysics, a field that explores the abundance of chemical elements in the Universe. Hydrogen and helium, which were produced seconds after the Big Bang, comprise 74% and 24% of ordinary matter in the Universe, while most other elements were created inside stars much later. Astrophysicists have extensively studied how elements up to the iron region are produced, but the processes by which nuclear reactions produced elements with a higher atomic number remain largely a mystery.

Although we know that these heavy elements were created by stellar explosions and nuclear processes in stars, matching specific events to the observed distribution patterns poses a considerable challenge. The higher intensity, reduced emittance and possibility for beam deceleration at HIE-ISOLDE will enable astrophysics experiments to shed light to this problem. Some research teams plan to investigate neutron-rich nuclei that form in the crust of neutron stars, while others will study the proton-capture process that occurs during X-ray bursts or explosions of white dwarves, research the production of chemical elements in the collapsed core of supernovae and address the problem of lithium-7 abundance in the Universe.

Solid state physics

The solid state programme at ISOLDE encompasses materials science, biophysics and biochemistry, complementing nuclear physics research. It would greatly profit from the high purity and intensity ion beams of HIE-ISOLDE, as well as from the modernisation of the facility. Such research can have considerable social benefits as well, because it yields a wide range of applications — from nanomaterials and superconductors to advances in cancer diagnosis and therapy.

A flying start for HIE-ISOLDE

On 9 September, the first exotic beam at HIE-ISOLDE marked the start of operations for the new facility. The experiment investigated charge states of tin isotopes, using transfer reactions and Coulomb excitation of an 110-Sn-26+ beam, post‑accelerated to 4.5 MeV per nucleon. Besides demonstrating the experimental capabilities of the upgraded facility, this successful first run validated the technical choices of the HIE‑ISOLDE team and provided a fitting reward for eight years of rigorous R&D efforts.

Almost half a century after the first ion beams bombarded the ISOLDE target, the facility is thriving and, thanks to the energy and intensity upgrade, continues to create new opportunities for radioactive ions research. The upgrade team and the users are now looking forward to an exciting, intense period.

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