Meet ISOLDE: Targeting new discoveries

The target lab at ISOLDE (Image: Christoph Madsen/CERN)

For Richard Catherall, the age-old alchemist’s dream of changing one element into another is a simple reality of his working day.

Targets are vital for any experiment at ISOLDE, as it is within this component that the radioactive isotopes are produced. In part two of our series we look at the backbone of any ISOLDE experiment, the target production.(Video: Christoph Madsen/CERN)

Thirty-four years ago he began his career at CERN, and today he is one of just a handful of people here capable of building the targets – the crucial components in the production of exotic isotopes that are used in the low and high energy beams necessary for an ISOLDE experiment to run.

ISOLDE,Experiments and Tracks
Richard Catherall (seen here on the far left) arrived at CERN in 1983. Now he is one of just a handful of people at ISOLDE capable of producing the targets that give the experiments the ability to change one element into another. (Image: CERN)

As ISOLDE celebrates 50 years of cutting-edge physics, we delve deeper into what goes into building these vital elements of CERN’s longest-running facility.

Exploring exotic new realms

Each target is tailor-made for each of ISOLDE’s experiments. They are each built from different materials, to produce the required isotopes when the high-intensity proton beam from the Proton Synchrotron Booster (PSB) is directed into it. There are more than 100 combinations of materials and ion sources, which can be put together in a variety of ways to build targets that produce the different isotopes. It is here that protons from the bottle of hydrogen at the start of CERN’s accelerator chain produce the 1300 different isotopes being studied at ISOLDE.

“I’m just there to make sure it works,” Richard says modestly.

“I do find it challenging,” he explains. “Depending on the approved scientific proposal, we design and build the appropriate target for the requested nuclei. The challenge comes when we have to design a target and ion source combination to produce beams of nuclei that have never been produced before. If we look to the nuclear chart, the new and exciting physics often comes from the nuclei far from stability, where production rates of short-lived isotopes are extremely low and sometimes unknown. The exciting part is being able to produce pure beams of nuclei at the extremity of the nuclear chart.”

This is a target at ISOLDE for producing tantalum-232, after it has been irradiated. Once a target is irradiated it is handled by robots. (Image: Maximilien Brice/CERN)

Richard’s enthusiasm for his role is infectious. I find myself captivated as he goes into detail describing a new technique the team have developed, to build a target that produces the rare isotope astatine, which involves reversing the polarity of the entire machine. It’s estimated that less than 30 grams of this unstable element are available on Earth at any one time, and it is incredibly difficult to reproduce in a laboratory as it decays so quickly, so the achievement is clear.

“We build about thirty targets a year, on demand. Each target has to produce enough isotopes for the experiment to be successful, but this is hard to test before actually bombarding it with protons. We do a quality check with stable beams beforehand, but the quantity of radioactive nuclei is something we can only verify just before the experiment starts,” says Richard.

But Richard and his colleagues can be proud, since their targets (along with the ion source) are able to produce the largest selection of isotope beams, and the most pure, of any ISOL physics laboratory in the world.

From car construction to nuclear discovery

The targets are handled by robots (two of many robots used at CERN for physics research). This is because once the targets are placed on the target station and hit with a beam of protons, they become radioactive. So, for safety reasons, they can only be handled by specially adapted robotic arms.

The robot arms in ISOLDE’s target area allow the radioactive targets to be handled safely (Image: Maximilien Brice/CERN)

These robots may look familiar if you’ve ever seen a TV advert of a car being built. They were originally designed to do just that, but in ISOLDE they have been adapted and made radiation-resistant to move the targets without human intervention.

Increasing intensity, increasing rarity

The targets, and the team that produce them, are a vital part of what makes ISOLDE such a unique facility at CERN. Soon the team will be pushed to produce more reliable targets, and harder-to-produce isotopes, as the arrival of the new Linac4 will increase the intensity and energy of the beam provided by the PSB.

For Richard and his team, this just adds to the excitement of their daily work. As we look back over fifty years of physics at ISOLDE, we can also look forward to the bright future ahead. “There’s still lots of opportunity for new isotopes to be discovered,” he concludes.


Find out more about ISOLDE, read the rest of the Meet ISOLDE series here.