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The long journey to a precise muon measurement

1 June 2021

It isn’t easy transporting a superconducting magnet 5000 kilometers and then reinstalling it seamlessly.

A truck carrying the electromagnet travels along Interstate 88 in Illinois.
The Muon g − 2 electromagnet gets escorted along Interstate 88 in Illinois on 26 July 2013. The precious cargo arrived at Fermilab later that day. Credit: Marty Murphy/Fermilab

In 2001 the Muon g − 2 collaboration at Brookhaven National Laboratory in New York reported an ultraprecise measurement of the muon’s anomalous magnetic moment that was 2.6 standard deviations above the theoretical prediction. The tantalizing but preliminary hint of new physics (see Physics Today, April 2001, page 18) had the particle-physics community eager to see whether the discrepancy would hold with more data.

This past April, the Muon g − 2 collaboration at Fermilab provided a much-anticipated follow-up measurement, which extended the divergence between theory and experiment to 4.2 standard deviations (see Physics Today, June 2021, page 14). Why the two-decade wait for a new result? The answer involves a real estate search, a 5000-kilometer expedition from New York to Chicago, and a lot of troubleshooting.

When the Muon g − 2 researchers announced their first result 20 years ago, they knew that their 14.2-meter-diameter superconducting magnet had done nearly all it could do at Brookhaven. Among other limitations, the intensity of the lab’s muon source constrained the achievable statistics. For the experiment to progress, the team was going to have to either upgrade the accelerator technology at Brookhaven or move the equipment to another facility with a wellspring of high-intensity muons. At one point the researchers discussed moving the magnet overseas to the J-PARC accelerator facility, but the Japanese institution would have had trouble clearing enough space for the ring and accompanying infrastructure.

The Muon g – 2 experiment at Fermilab.
The Muon g − 2 experiment now resides at Fermilab. Credit: Fermilab

As it turned out, the demise of one powerhouse instrument for probing fundamental physics yielded a new lease on life for another. In 2011 Fermilab shut down its flagship particle collider, the Tevatron, which had given way to the higher-energy Large Hadron Collider at CERN. The Illinois laboratory now had the space and accelerator technology to host a project like Muon g – 2. “It opened up $100 million of infrastructure,” including a high-intensity proton source that would generate the muons, says experiment co-spokesperson Chris Polly. The Department of Energy agreed to spend $3 million to transport the magnet to Fermilab and promised another $40 million to fund the experiment if the move went as planned.

Not a typical move

With a prospective new home secured, the challenge turned to transporting a 15 000-kilogram magnetic ring with the same width as a basketball court from central Long Island to suburban Chicago. One of the people tasked with that job was staff scientist Hogan Nguyen. He and his colleagues determined that although the experiment’s supporting steel and iron beams could be dismantled and delivered piece by piece, the magnetic ring had to remain intact. The 1.45-tesla superconducting magnet, the largest in the world when it was meticulously assembled at Brookhaven in the late 1980s and early 1990s, has exquisite field uniformity that was achieved through precise winding, Nguyen says. “People were cringing, thinking about cutting the welded parts and then putting them back together,” Polly adds.

The need to transport the entire ring limited the team’s options. Flying it by helicopter was out due to weight, as was a road trip because of the impracticality of monopolizing hundreds of kilometers of interstate highway. After contracting with the company Emmert International to coordinate the move, the collaboration decided to hire a barge that would tow the instrument south along the Atlantic Seaboard, around Florida, through the Gulf of Mexico, and then up a network of rivers to within 50 kilometers of Fermilab. It was a 3700-kilometer detour. (See the route on the interactive map below, which was created using GPS data taken at two-hour intervals during the move.) Nguyen and colleagues performed calculations to set limits on the bumpiness of the ride so it wouldn’t exceed what the magnet could handle before bending or twisting out of alignment.

On the night of 23 June 2013, a truck loaded with the most precious of cargo pulled out of Brookhaven National Laboratory. The shrink-wrapped ring was covered with accelerometers to gauge the stress on the instrument, and it was held in place with a 27 000-kilogram fire-engine-red steel scaffold that gave the payload the look of a Ferris wheel on its way to a fair. The truck crawled along the William Floyd Parkway en route to Smith Point County Park and the awaiting barge.

The sea voyage included a few tense moments. On day three, fearing choppy seas from a storm ahead, Polly and the transport team that accompanied the instrument decided to make an unplanned stopover at Norfolk, Virginia. Later, as the barge chugged across the Gulf of Mexico, the team anxiously read reports about Tropical Storm Chantal racing westward in the Caribbean Sea. They decided to fork over a $20 000 fuel surcharge to increase the vessel’s maximum speed and ensure safe harbor at the next stop in Mobile, Alabama.

The Muon g - 2 magnet arrives at Fermilab.
The truck carrying the Muon g – 2 magnet sits parked on the Fermilab campus on 26 July 2013, shortly after a celebration to commemorate the experiment’s arrival. Credit: Fermilab

After a slow but steady trip upriver, the barge made its final stop along the Illinois River in Lemont, Illinois. The ring was transferred to another truck, which began a three-day trek to cover the final leg to Fermilab. Authorities briefly closed parts of Interstates 88 and 355; on local roads the transport was led and followed by work crews that removed and then replaced signs and traffic lights that would have impeded the truck’s oversize load. Hundreds of spectators converged on the Costco and nature preserve parking lots where the truck stopped between segments of the trip.

The importance of documentation

On 26 July the magnetic ring finally arrived at the Fermilab complex. It was a triumphant moment, but it didn’t ensure the experiment’s ultimate success. “I realized that I wouldn’t know whether this was going to work for six years,” when the team hoped to have analyzed its first results, Nguyen says. “I couldn’t really celebrate.” After all, prior to the trip the massive electromagnet had been sitting unused in an otherwise empty Brookhaven building for the previous dozen years.

The next two years were spent reassembling the ring, connecting it to Fermilab’s accelerator infrastructure, and building a refrigeration plant to cool the equipment. On 1 July 2015, in the midst of a DOE review for unlocking the $40 million, the team cooled the magnet and turned it on for the first time in 14 years. The magnet received only three-quarters of the expected current. Diagnostics identified a crack in a joint that was creating resistance, and it took months to plan and execute a repair.

Looking for charcoal panels inside the magnet.
Fermilab scientist Karie Badgley threads a borescope into the electromagnet ring in search of a collapsed panel, as Muon g – 2 co-spokesperson Chris Polly looks on. Credit: Reidar Hahn/Fermilab

Even after engineers got the full 5200 amps to course through the electromagnet, it would periodically misbehave. The collaboration was particularly confounded by periodic spikes in pressure that would cause the magnet to quench. After consulting with engineers and physicists who had built the magnet three decades prior, the team came up with a possible explanation: the detachment of one or more charcoal-coated panels, which were designed to trap stray gas molecules that breach the vacuum seal within parts of the ring. The catch was that the panels had been installed at the last minute during construction at Brookhaven, and there was no known documentation detailing what they looked like or where they were placed. “We knew the approximate size, that there were probably three, and that’s about it,” says Fermilab scientist Karie Badgley, who specializes in magnet design.

Detached charcoal.
The removal of these charcoal shards remedied the problem of repeated pressure spikes within the magnetic ring. Credit: Karie Badgley/Fermilab

In December 2016 Badgley began a search for the elusive panels. Focusing on eight locations that allowed easier access to the inside of the magnet, she threaded a borescope into the ring and poked around. After a few attempts, she confirmed that charcoal from all three panels had flaked away from the side of the structure. Badgley and colleagues Kelly Hardin and Cary Kendziora then cut holes in the cryostat and thermal shield, carefully maneuvered around the instrumentation wires, and removed the charcoal piece by piece. In the end, a bunch of charcoal shavings that only a handful of people had known existed proved to put a bigger dent into the hardy electromagnet than did its monthlong journey from New York.

The experiment’s science operations kicked off in 2018, and three years later, the battle-tested Muon g – 2 collaboration was able to announce its results from that first Fermilab run; the experiment is currently in its fourth. Polly says the experiment will remain online for the next few years to do science and calibration. Mu2e, an experiment that will track the rate of muon-to-electron conversions, is scheduled to take over Fermilab’s muon-generation infrastructure in the mid 2020s. Though no decision has been made on the eventual fate of the Muon g – 2 magnet, its traveling days are almost certainly over.

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