This week, ISOLDE, CERN’s nuclear physics facility, is celebrating 50 years of physics. But after half a century of studying radioactive isotopes, the facility is on the brink of a new phase in its history, as its upgrade, HIE-ISOLDE, nears completion.

“ISOLDE makes sure that it is always improving,” says Razvan Lica, a CERN PhD student working at the ISOLDE facility.

Watch the fourth part in our documentary series about ISOLDE to find out more about the HIE-ISOLDE upgrade and the people building it. (Video: Christoph Madsen/CERN)

The HIE-ISOLDE upgrade, which will allow ISOLDE to collide beams of isotopes into targets at higher energies, is a chance for the facility to reinvent itself. The HIE stands for High Intensity and Energy, and physicists hope that it will guarantee ISOLDE another ten to fifteen years at the forefront of this area of research.

Currently, to produce radioactive isotopes, ISOLDE takes proton beams from one of CERN’s accelerators, the Proton Synchrotron Booster (PSB) and fires them into a target. The target then sends out many radioactive isotopes, which can be directed down beamlines to various experiments. HIE-ISOLDE uses a new, unique, linear accelerator (linac) to take these beams and accelerate them again, before sending them on to secondary targets, where nuclear reactions occur. 

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The new linac had to fit into just 16 m of space. “We had to develop a very compact linac. That’s what makes it unique. In other facilities, every cavity has its own cryostat but if we had to do that it would be far too long, so we had to squeeze all of them into one cryomodule. We had to have the solenoids fitted too, they’re almost the same length as a cavity, so we had to do lots of design, research and development. The biggest challenge was to design in spaces with clearances of just 1 mm,” explains Yacine Kadi, project leader for HIE-ISOLDE. (Image: Maximilien Brice/CERN)

“When we talk about elements we use their proton number. A heavy element is one with a higher proton number, but you can have many different isotopes of the same element. These have the same proton number but a different number of neutrons,” explains Liam Gaffney, who works on the Miniball set-up, attached to one of the HIE-ISOLDE beamlines.

Physicists like Liam use these isotopes to research a range of topics, from astrophysics, by recreating reactions that happen in the stars, to the internal structure and shape of exotic nuclei, giving us an insight into the building blocks of the world around us.

“Previously we couldn’t do as many of the reactions as we wanted to with radioactive isotopes, as the beam energy wasn’t high enough. To study the shape of the nuclei of the heaviest elements we need higher energies to overcome an increase in the nuclei charge. More protons means a higher positive charge, and since two positive nuclei repel each other, it means a higher energy is needed to collide them,” he continues.

“Higher energy opens a new field. We had a stepping stone with the REX upgrade, when ISOLDE first introduced the possibility of reaccelerating isotopes, in 2001, but with the higher energies from HIE-ISOLDE, it’s a new realm,” says Karl Johnston, ISOLDE’s physics coordinator, who hopes the upgrade will mean even more applications are found for ISOLDE’s research.


The energy upgrade means that ISOLDE can now collect information about the properties of nuclei that were previously not accessible. Eventually, researchers will also be able to study isotopes with even more or even fewer neutrons, which are less stable and harder to produce in a laboratory. So far these isotopes have been out of the reach of physicists.

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There are currently three spaces for experiments to be attached to HIE-ISOLDE, with the hope that seven or more will eventually run each year. There is one permanent station attached to the linac, called Miniball, seen here, which can be set up to run multiple different experiments (Image: Julien Ordan/CERN)

“Higher energy gives us the chance to study many different things. We focus on fundamental questions concerning the structure of nuclei,” explains Liam. “Studying reactions inside the stars to learn more about how the different elements are produced. Asking questions like: why are there so many heavy elements, like uranium, on the planet?”

“HIE-ISOLDE is a major breakthrough and is the result of almost eight years of research and development, of prototyping and design. It’s a huge adventure and what makes us most proud isn’t even that we managed to build the machine but that from the start we have seen new physics and new, enthusiastic users,” enthuses Maria Borge, who led the ISOLDE group from 2012 to 2017.

Challenge accepted

“Engineers told me it was mission impossible”
- Yacine Kadi, leader of the HIE-ISOLDE project

But building a machine of this scope hasn’t been easy. Yacine Kadi, who leads the HIE-ISOLDE project, starts to laugh as he spends minutes listing the challenges the project faced.

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Each cryomodule contains more than 10 000 parts, which need to be carefully cleaned, calibrated and installed. (Image: Maximilien Brice/CERN)

With scarce resources, the design and development phase of the project relied on early-career researchers to carry out the majority of the work. It was a risk that paid off: “We didn’t have the resources to hire virtually any staff, but we made sure we only took the absolute best – we couldn’t afford not to – and they did a fantastic job. But then they left before the project was finished!” he explains.

With his fair share of challenges, Yacine had to rethink construction materials when the metal niobium proved too costly, and amend original plans to avoid a building crossing the border between Switzerland and France.

“Engineers told me it was mission impossible,” exclaims Yacine. “It was a big, complex project and the choices we made weren’t things we had much experience of at CERN. This meant we had to develop novel ideas and at the same time profit from technological breakthroughs made at CERN, for the Lepton Positron Collider (LEP). In the end it was just a question of the imagination of our physicists and technical staff.”

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The inflatable T-Rex at HIE-ISOLDE is the mascot of the REX experiment, which was an earlier post-accelerator at ISOLDE. (Image: Julien Ordan/CERN)

HIE-ISOLDE is unique in its design because it had to fit a lot of accelerating power into a very compact space. Linear accelerators use radiofrequency cavities to accelerate a beam. Normally an accelerator will house each one of these cavities in its own cryostat – a vacuum chamber that supercools the cavity so that the helium needed for the superconductors to work stays liquid – but HIE-ISOLDE didn’t have the room for each cavity to have its own cryostat. Instead, one way the engineers kept the system compact was to build cryomodules that each contain five cavities but require only one cryogenic system.

“Yes, HIE-ISOLDE was a challenge from a technical point of view, but it was a major human adventure for me. You increase your field of knowledge, and you work in different domains, so you meet many different people. I met people I wouldn’t ever have met even after spending forty years at CERN,” he continues.

Currently, HIE-ISOLDE is nearing the completion of its energy upgrade and has already had two successful running periods with more than 15 experiments. The last of the four superconducting cryomodules is due to be installed over the winter shutdown in 2018, and will allow the machine to accelerate the radioactive beams to energies of 10MeV/u.

“Others might get to that energy but no other facility in the world can accelerate very heavy nuclei. We can do that,” says Maria, emphasising the importance of this new upgrade, which cements ISOLDE’s role at the forefront of nuclear physics for the foreseeable future. 


This week, ISOLDE, CERN’s nuclear facility, is celebrating 50 years of physics with a series of articles and a short documentary series that takes a closer look at the facility and the people that work there. See the rest of the series here.