A joint Fermilab/SLAC publication

The skinny on the LHC's heavy ions

Detlef Kuchler, a physicist in CERN's Beams department, with the container holding the purified sample of lead used to create heavy ions for the LHC. Photo: M. Brice / CERN

Detlef Kuchler, a physicist in CERN's Beams department, with the container holding the purified sample of lead used to create heavy ions for the LHC. Photo: M. Brice / CERN

Less than 24 hours after the end of the Large Hadron Collider's first high-energy run of proton collisions, beams of lead ions are already circulating in the LHC. If you're like us, you've probably spent the last two years hearing about the importance of proton collisions and learning about how protons are accelerated and collided at the LHC. So now you might be wondering: What are lead ions? Is accelerating them any different than protons? And why dedicate one of the LHC's precious months of operation to lead-ion collisions?

Symmetry is here to help. In this article we'll answer the first two questions and briefly touch on the third. A follow-up article next week will delve more deeply into the physics research that scientists on the ALICE, ATLAS and CMS experiments will do with lead-ion collision data.

What are lead ions?

Lead ions start as lead atoms, which in nature have an atomic nucleus containing 82 protons and between 122 and 126 neutrons, surrounded by a cloud of 82 electrons. An atom of lead becomes an ion of lead when some or all of its electrons are stripped away, leaving the remaining portion of the atom positively charged. The LHC acceleration process gradually strips away all of the lead atoms' electrons, leaving a beam composed only of lead nuclei.

The LHC only accelerates one type, or isotope, of lead that contains 126 neutrons.  Since protons and neutrons have approximately the same mass, an LHC lead ion weighs roughly 208 times more than a proton. It's no wonder that physicists refer to lead ions as "heavy ions."

The first accelerator to collide heavy ions was the appropriately named Relativistic Heavy Ion Collider at Brookhaven National Laboratory in New York, which set the current world record for highest-energy heavy-ion collisions by smashing gold ions at energies of 200 GeV per nucleon. (Physicists prefer to quote heavy-ion beam energies "per nucleon," where protons and neutrons are both nucleons, as this allows for an easier comparison with the energies of beams of protons and other types of ions.)

“Everyone makes the joke that RHIC accelerates gold and we use lead because it’s cheaper,” says John Jowett from CERN's Beams Department. “Actually our lead costs more because it's isotopically pure." In fact the lead used by the LHC costs significantly more: about one dollar per milligram compared to five cents for gold.

The CERN accelerator complex. Image copyright CERN.

The CERN accelerator complex. Image copyright CERN.

How are lead ions accelerated in the LHC?

The LHC's lead-ion beams start with a piece of pure lead 2 centimeters long that weighs 500 milligrams. The lead "sample" is heated to about 500 degrees Celsius to vaporize a small number of atoms. An electrical current is used to remove a few of the electrons from each atom, and then the newly created ions begin the ride of their life.

The ions first travel through a linear accelerator called Linac3, picking up a small amount of energy (.0045 GeV per nucleon) before having more electrons removed. Next, the ions are accumulated and accelerated to .072 GeV per nucleon in the Low Energy Ion Ring, or LEIR. These first three stages in the process - vaporization and acceleration in Linac3 and LEIR - are unique to ions, but as soon as they leave the LEIR they travel the same path as their lighter cousins the protons.

Despite their names, the next two accelerators - the Proton Synchrotron and the Super Proton Synchrotron - also accelerate heavy ions. In the PS the lead ions zoom to 5.9 GeV per nucleon and have the last of their electrons stripped away. The SPS accelerates the beams to 177 GeV per nucleon and finally injects them in two directions into the Large Hadron Collider.

In the LHC the lead-ion beams will be accelerated to 1.38 TeV (1380 GeV) per nucleon and brought into collision in the center of three of the LHC's four major particle detectors - ALICE, ATLAS and CMS. Even with the LHC running at half power, the first collisions of lead ions in the LHC will be more than 13 times more energetic than the previous record set at RHIC.

The coming days will see the first-ever collisions of lead ions in the LHC, but not the first time a lead-ion beam has entered the accelerator. Beams of lead ions were successfully sent through the entire accelerator chain and injected into the LHC in fall 2009, but they only traversed part of the LHC before being stopped. Last night the first lead-ion beams zoomed all the way around the LHC, the first very low intensity collisions will happen soon, and then the LHC accelerator team will quickly increase the intensities of the beams to their maximum for 2010. The goal for this year's lead-ion run is to collide beams containing up to 120 bunches of lead ions, each bunch containing at most 10 billion ions.

"The plan had been to collide up to 62 bunches of lead ions in the first year, but we might go to 120,” says Jowett. “Our time is limited, and we want to squeeze as much as we can out of it."

Compared to the stats for this year's proton run at the LHC—a maximum of 368 bunches, each of which contained more than 100 billion protons—the lead-ion beams might seem incredibly weak. But for the experiments, these relatively low-intensity beams are plenty to work with. Each collision of lead ions produces thousands more particles than a proton collision, which means a huge increase in the amount of data registered in and recorded by the huge particle detectors.

Why collide heavy ions in the LHC?

Physicists from the ALICE, ATLAS and CMS experiments will use collisions of lead ions to study the quark gluon plasma, a state of matter that physicists believe existed millionths of a second after the Big Bang. Probing the quark gluon plasma and its evolution into the matter that makes up today’s universe will shed light on the properties of the force that binds quarks into bigger objects like protons and neutrons.

For more on exactly why the quark gluon plasma is a hot research topic, and how the three experiments will measure it, check back here next week.

Edit: Read the next post on lead-ion collisions in the LHC.

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