Inside an ordinary-looking cupboard in an ordinary-looking office, scientists at CERN store a small glass vial filled with 10 grams of lead. But this is no ordinary metal. The lead in this vial is worth $15,000, and it’s about to go on a journey back to the origins of the universe.
Every fall, the Large Hadron Collider accelerates and collides isotopically pure lead: that is, lead in which every atom has the same number of neutrons. But how do scientists get lead, a heavy metal, into the world’s most powerful particle accelerator?
It starts with a pair of safety gloves.
“Lead is toxic, so we take the necessary precautions,” says CERN scientist Detlef Küchler.
When it’s time to start the lead-ion run, a technician carefully places thin shavings of lead into a crucible. Küchler then slides this crucible inside a ceramic oven that is roughly the same size and shape as an AA battery. This oven—though tiny—can heat the lead to 800 degrees Celsius.
“When you heat ice, it melts into water and then boils into vapor,” says Roderik Bruce, a CERN scientist who oversees the lead ion run. “In the same way, when we heat lead to 800 degrees Celsius, we get a lead vapor.”
But lead atoms aren’t what scientists collide. Lead atoms have an equal number of positively charged protons and negatively charged electrons, making each atom electrically neutral. Neutral particles don’t interact with the magnetic and electric fields used by particle accelerators. And so before scientists can accelerate the lead, they first need to strip off electrons and convert the lead vapor into a cloud of positively charged lead ions.
This happens at the very start of the accelerator chain in something called the source. Once the lead is loaded into the source, the scientists switch it on and inject microwaves, which create a plasma and heat up the electrons. The heated electrons hit the lead atoms and kick out their electrons, turning them into positively charged lead ions. By applying a positive voltage to the source, the scientists move positively charged lead ions toward the next part of the accelerator chain, keeping the electrons inside the plasma.
Because scientists cannot control how many electrons each lead ion loses in the process, they end up with a big mix. “We get a distribution from lead plus 20 to lead plus 35,” Küchler says.
The more electrons a lead ion has lost, the greater the charge it will have. And the greater the charge, the more it will bend in the accelerator’s magnetic field. Because of this, scientists pick the most abundant charge state (lead +29), tune the magnetic fields to this charge, and accept that the trajectories for all the other ions will be either too tight or too loose to keep them within the accelerator.
As these ions start their journey into the interlinking rings of the CERN accelerator chain, their electrons will be stripped off two more times until only lead +82—the electron-free atomic nuclei—remain. When the naked nuclei are finally ready to enter the LHC, scientists are left with only an extremely small portion of the original vapor.
“It’s really inefficient,” Bruce says. “We throw away a lot of lead ions, and we cannot get the same beam intensity as we do for protons.”
To increase the number of lead ions injected into the LHC, scientists recently implemented a process called slip-stacking, which gradually interweaves bunches of lead ions as they travel inside the Super Proton Synchrotron, the last accelerator before the LHC.
“It’s quite special,” says Bruce. “We use the radio-frequency cavities, which we normally use to accelerate the beam, and we adapt the action on the beam so some bunches drift down in energy while others drift up.”
Radio-frequency cavities hold an oscillating electric field that acts like a parent pushing a kid on a swing. Every time the ions pass through the cavity, the electric field gives them a small nudge that incrementally increases their energy.
With slip-stacking, the strength of each nudge is varied between bunches. The faster bunches catch up with the slower bunches, and they gradually interweave, which creates a tighter spacing. “This is new for Run 3,” Bruce says. “It was not possible previously because we needed to update our radio-frequency system.”
But ensuring the beam has enough lead ions is only one of the many challenges related to accelerating (and colliding) lead. “Lead ions have a much higher charge than protons,” Bruce says. “The forces between the ions are much stronger, and they want to push apart.”
Trying to collide two atomic nuclei inside the LHC is like firing two needles at each other across the Atlantic and hoping they meet in the middle. To increase their odds, scientists use magnets to focus the two beams right before they cross at the collision point. These magnets act like lenses and concentrate the particles into streams that are much thinner than a human hair. But the tighter the beam is focused at the collision point, the wider it becomes before and after. If it becomes too wide, some lead ions can hit the inside of the beam pipe and create a precarious situation. “This in turn could cause the magnets to lose their superconducting properties, or, in the worst case scenario, damage them,” Bruce says. “The LHC can absorb only so many stray particles before its components lose functionality.”
To control for this, the scientists limit how much they focus the bunches and have several safety systems that will extract the beam from the LHC if any anomalies are detected.
The lead ion run typically starts a few weeks after the LHC finishes colliding protons, which it does for most of the spring and summer.
When the proton run ends, scientists have four days to prepare the LHC for the lead ion run. “It’s a very busy time for us,” Bruce says.
But as soon as everything is tested and tuned, the entire journey from solid slices of lead to high-energy lead goo is fast: less than 2 hours.
The longest part of the process is filling the LHC with bunches from the SPS, which takes about an hour. Once the two beampipes are packed with 1,240 bunches each, the scientists are ready to accelerate. Over the course of 20 minutes, scientists nudge the lead ions closer and closer to the speed of light. By the time the two beams are ready to collide, the ions have traveled a distance roughly equivalent to a journey from Earth to Saturn. (For reference, it took Voyager 1 more than three years to make this trip, and it was moving at 61,500 kilometers per hour.)
When the ions reach their final energy, the scientists use electric and magnetic fields to gradually drift the crisscrossing beams into a collision course. The amount of energy release per head-on collision is tiny: 1,115 TeV of energy, equivalent to illuminating a single LED Christmas light for 2 milliseconds. But when the energy is packed into an area the size of an atomic nucleus, its density is enormous: equivalent to the state of the universe fractions of a second after the Big Bang.
The lead instantly melts into a soup of its internal components: quarks and gluons. The collisions are so hot and dense that they create a liquid with zero viscosity, meaning it has a perfectly frictionless flow.
The last lead ion run finished in November 2024. Scientists are studying the collisions to better understand fluid dynamics under extreme conditions, and to understand how the quark-gluon plasma that remained after the Big Bang cooled and evolved to form, ultimately, everything in the visible universe.
