A boost for the ISOLDE beams

The first HIE-ISOLDE cryomodule was commissioned at the end of October. The radioactive ion beams can now be accelerated to 4.3 MeV per nucleon.


The ISOLDE beamline that supplies the Miniball array. The first HIE-ISOLDE cryomodule can be seen in the background, in its light-grey cryostat.

ISOLDE is getting an energy boost. The first cryomodule of the new superconducting linear accelerator HIE-ISOLDE (High Intensity and Energy ISOLDE), located downstream of the REX-ISOLDE accelerator, increases the energy of the radioactive ion beams from 3 to 4.3 MeV per nucleon. It supplies the Miniball array, where an experiment using radioactive zinc ions (see box) began at the end of October.

This is the first stage in the commissioning of HIE-ISOLDE. The facility will ultimately be equipped with four cryomodules that will accelerate the beams to 10 MeV per nucleon. Each cryomodule has five accelerating cavities and a solenoid, which focuses the beam. All of these components are superconducting.

This first beam is the result of eight years of development and manufacturing. One of the major challenges was the construction of the cavities. The HIE-ISOLDE cavities are made of copper coated with a thin layer of niobium, a superconducting material. This technology, which had been used for the LEP and then for the LHC, had to be adapted to the more complex geometry of a quarter-wave cavity. “The production line for this type of superconducting cavity is now fully operational at CERN,” says Walter Venturini Delsolaro, the deputy project leader (BE/RF). To date, ten cavities have been qualified for installation in the accelerator.

Assembling the cryomodule also presented a challenge. Unlike the LHC cryomodules, for example, in which the internal surfaces of the cavities are isolated from the other components, all the elements of the HIE-ISOLDE cryomodule are located in the same vacuum. The cryomodule is therefore more compact, which is essential given the limited space in the ISOLDE building. “Each cryomodule has around 10,000 components,” says Yacine Kadi, the project leader (EN/HDO), “and none of them, not even the smallest screw, can be allowed to compromise the cleanliness of the whole machine.” Materials have been specially chosen to ensure that the components can be perfectly cleaned and that the quality of the vacuum is not degraded while the machine is running. A class ISO5 cleanroom was purpose-built for the assembly of the cryomodules.

Assembly of the first HIE-ISOLDE cryomodule, with its five niobium-on-copper cavities, in the new SM18 cleanroom.

An innovative system for aligning the components in the cryomodule using a laser has been developed by the survey team. “This system enables us to observe the position of the components remotely and, if necessary, to adjust it without opening the cryomodule,” explains Venturini Delsolaro.

After a delicate assembly phase in the cleanroom, the first cryomodule was transported to the ISOLDE hall on 2 May and coupled to the existing REX-ISOLDE accelerator. The hardware commissioning began in the summer, and was followed in September by the first tests with stable beam, culminating in the acceleration of the first radioactive beam on 22 October. HIE-ISOLDE will run for a total of five weeks this year. During ISOLDE’s technical stop between December 2015 and April 2016, another cryomodule will be coupled to the first, increasing the energy to 5.5 MeV per nucleon. Two other cryomodules will be produced from mid-2016 onwards, bringing the final energy to 10 MeV per nucleon for the heaviest nuclei available at ISOLDE.

Higher energies open ISOLDE to a new realm
of physics experiments

A nucleus is a many-body quantum system, with a fixed number of neutrons and protons. Its behaviour, which is influenced by the individual nucleons as well as by the collective contribution of all the protons and neutrons, sheds light on the strong force at play in the nuclear medium.

Until now, beams at ISOLDE were only able to reach just enough energy to explore the collective properties of nuclei, but the higher energies provided by the newly commissioned HIE-ISOLDE will enable physicists to investigate single particle behaviour in a nucleus and the interplay between collective and individual properties. The higher energy will now allow for nucleon transfer reactions on all radioactive nuclei produced at ISOLDE, including the heaviest ones.

Thirty experiments and over six hundred shifts have already been approved and will investigate a range of physics topics from isospin symmetry and collectivity versus single particle aspects, to shapes and shape coexistence. The first of these experiments is already running.


by Corinne Pralavorio