Before Mu2e, there was MECO.
Scientists, researchers, and engineers were ecstatic. They had been trying to carry out the Muon to Electron COnversion experiment for nearly two decades on two continents, and now it would finally be built at Brookhaven National Laboratory.
Undeterred when the project was pulled in 2005, they adjusted their plans and designs to run this groundbreaking experiment at Fermi National Accelerator Laboratory in Batavia, Illinois.
The muon enigma
Mu2e aims to solve a mystery that has puzzled experimentalists and theorists alike since the discovery of the muon in 1936: Scientists have never observed a muon transform into its lighter cousin, the electron, without also emitting other particles.
Observation of direct muon-to-electron conversion “would provide unmistakable evidence of physics beyond the Standard Model,” said experiment co-spokesperson Jim Miller, a scientist at Boston University.
Electrons, muons and taus are flavors of particles called leptons. Just like strawberry, chocolate and vanilla ice cream cannot transform into one another, despite being flavors of Neapolitan ice cream, muons seemingly are prevented from converting directly into electrons.
Measuring any such conversion process once, let alone several times over the course of an experiment, is no easy feat. To observe the muon-to-electron conversion signal, Mu2e will be about 10,000 times more powerful than the SINDRUM II experiment, which finished collecting data in 2000 and was the last experiment to search for direct muon-to-electron conversion.
If only one in 100 million billion (1017) muons transforms into an electron, Mu2e will see it.
Producing pions: A target’s tale
Before this can happen, physicists need pions.
The production target, a carefully shaped material that intercepts a particle beam, assumes that critical but difficult job. When a beam of protons hits the fixed production target, pions come out in all directions and decay almost immediately into muons, which spiral down through other components of the experiment to a detector, from which they (hopefully) emerge as electrons.
Researchers developed many iterations of the Mu2e production target, some of which are shown here. The first and simplest design (far left) is a tungsten rod, while the penultimate design (far right) has fins, segments, and rings to optimize temperature control and minimize sag. Image: Dave Pushka
Mu2e components are made with as little material as possible because particle interactions increase with the amount of material in the experiment, interfering with the signal researchers hope to observe. This presented unique challenges to the production target design team.
Residing in a vacuum chamber inside a superconducting cylindrical magnet, the production target is subject to extreme conditions. A proton beam beats against the target every second, causing its temperature to increase to around 1,700 degrees Celsius (3,092 degrees Fahrenheit), the temperature experienced by the hottest parts of a NASA space shuttle re-entering Earth’s atmosphere.
Researchers soon realized that their initial design, a relic of the MECO experiment, was expensive. Too expensive. A gold rod encased in a titanium jacket, this target needed to be cooled with circulating water via an elaborate system of water pumps, nozzles and other infrastructure.
“That was when some of our colleagues pointed out that we may not need to actively cool the target at all,” said Steve Werkema, accelerator upgrade manager for Mu2e.
Two modifications – switching to a target that releases heat on its own, called a radiatively cooled target, and reducing beam power from 25 kilowatts to 8 kilowatts – not only saved money and simplified infrastructure but also reduced safety concerns.
Now, researchers needed a new production target. For the target material, they turned to a section of the periodic table known as the refractory metals. Refractory metals are advantageous in experiments like Mu2e because they have high melting points and are averse to corrosion even under high temperatures.
The researchers ultimately chose tungsten, a heavy, dense metal that can withstand high temperatures and brutal pounding by bunches of proton beam. This decided, it was back to the drawing board, literally.
Revisit, revise and repeat
The first Mu2e tungsten target looked like a thick, long pencil. Six millimeters (around 0.25 inches) in diameter and 160 millimeters (a little over 6 inches) long, the tungsten rod produced lots of pions.
The problem? There was no way to support this structure in vacuum.
To solve this conundrum, researchers attached parts that look like megaphones to both ends of the rod. Spaghetti-like spokes suspended these components in a bicycle ring structure that secures the target and assists a robotic arm with target removal and disposal.
The Mu2e production target is suspended by spokes within a bicycle wheel structure. Image: Dave Pushka
“That’s when we started discovering problems that we had to overcome, one by one,” Werkema said.
The first problem they encountered was corrosion.
Ordinarily, tungsten is corrosion-proof, but studies showed that even the tiniest bit of oxygen in the vacuum chamber causes problems at Mu2e temperatures and pressures.
“Think about it like your car. Fenders rust and you get these big pieces of rust that fall off, and pretty soon you have no fender left,” said Dave Pushka, lead production target engineer at Fermilab.
The production target would corrode so fast that it wouldn’t last a year. Researchers improved the vacuum chamber to mitigate this effect. While they still anticipate some tungsten-oxide formation, it shouldn’t be enough to cause the target to fail quickly.
The researchers then wondered: How long could a proton beam bombard the target before it failed due to stress and fatigue? In an important milestone, Rutherford Appleton Laboratory researchers in England developed a prototype target and hit it with an electrical pulse until it failed. They concluded that, at least from that failure mode, the target would last more than a year.
The third challenge was temperature. Researchers were concerned that the target might deform like a stick of butter at a July picnic before reaching its prescribed lifetime (about 43 weeks of beam time).
As protons hit the production target, kinetic energy turns into heat, causing the target to expand outwardly and sag in the middle. This instability causes yet more sagging, as the spokes that support the target pull its ends together, putting more force on both ends and making the target sag further.
People on both sides of the Atlantic worked in friendly competition to develop the best target model. Ultimately, several design elements, such as springs connecting the spokes to the bicycle ring, were introduced to combat fatigue and target sag.
In its current version, the target still looks very much like an unsharpened pencil. It is gray, relatively heavy, and 200 millimeters (nearly 8 inches) long, with cylindrical rings on each end, fins that dissipate heat from the target core and brace against sag and empty space separating segments of the central rod.
The fins, which make the target look like a star from the ends, require finesse to function as intended under the harsh proton beam.
“As you add more fins, the surface of a fin doesn’t see cooler temperatures. It instead sees another fin at the same hot temperature. This means that there are some diminishing returns in terms of fin structure and number and heat dissipation,” Pushka said.
Mu2e Project Manager Ron Ray of Fermilab suggested that segmenting the target’s core could ameliorate this temperature-dependent issue. The researchers have found that introducing spaces between short, cylindrical segments of tungsten allows them to fine-tune temperatures along the target.
The gatekeepers of optimal target design
Meanwhile, three teams of engineers work to eliminate as many surprises as possible at beam-on.
“The production target team wants to know what happens with each change to the target or beam,” said Kevin Lynch, professor of physics at York College of the City University of New York and Mu2e production target design team member. “Our models track everything from pion production to muon-to-electron conversions to the way energy accumulates in components throughout the experiment.”
These independent calculations, performed by Lynch’s team at York College and Bob Bernstein’s team at Fermilab, are what senior engineer Ingrid Fang works with.
Fang, who has worked at Fermilab for over two decades, applies Lynch’s calculations to the geometry provided by Pushka, sets up the model, and solves for stress and temperature at every point in the target. The simulations are so complex it takes a supercomputer three or more days to solve the millions of equations.
“We have to find that sweet spot between temperature and muon yield,” Fang said.
It is Fang’s result that is studied by scientists, researchers and engineers. It is Fang’s result that makes or breaks decisions. It is Fang’s result that ultimately determines whether the production target moves on to construction or goes back to design.
“Now, it’s the grand finale,” Fang says of the current design. “We combined the target with its supporting structure and put all the loads, including beam pulsing, radiation load, gravity, and pre-tension on the bolts that secure the system, into the model, and the results look very promising.”
The current version of the tungsten production target has circular rings at the ends and its core is finned and segmented to allow for sag minimization and temperature control. Image: Dave Pushka
Building a target 101
Researchers know that life in the lab is full of ups and downs, tinkering and revision. What originally started as a gold, water-cooled rod has evolved into a segmented and finned, radiatively cooled, tungsten apparatus that meets project goals. Scientists, researchers, engineers and analysts have looked at more than 35 target designs over the years.
The researchers remain undaunted as they now face the latest challenge – actually building the production target.
“Tungsten is difficult to machine. You can’t cut it with a lathe. You can’t saw it. It has to be ground or electrode discharge machined,” Pushka said. He notes that there are at least three or four contractors in the Chicagoland area, and more beyond, who can perform this intricate work.
Werkema and Pushka estimate that the target will take 12 weeks to manufacture and another 12 weeks to assemble and align with the beam. Then, after Mu2e construction finishes in 2022, there’s another year of setup, measurements and calibration required before the experiment starts running in 2023.
“That seems like a long time away, but it seems like no time at all when you consider the first designs were made in the late 1990s. Right now, it feels like we’re finishing up because we overcame all of these engineering and design challenges, and now new stuff shows up and gets installed every week. You can actually see the progress,” Werkema says.
“I’ve worked on a lot of experiments at Fermilab,” Pushka said. “Mu2e is the hardest, most difficult experiment I’ve ever worked on and, I think, that we have ever tried to pull off. It’s extremely difficult from the science and engineering point of view.”
It seems that the researchers, for now, are on target to discover new physics through Mu2e.
This work is supported by the U.S. Department of Energy Office of Science.
Below is a press release issued by Lawrence Berkeley National Laboratory on the LUX-ZEPLIN project. A Fermilab group led by Eric Dahl, a joint appointee and associate professor at Northwestern University, is responsible for implementing key parts of the critical systems that handle the xenon in the LUX-ZEPLIN detector. These include the heat exchange system that allows the entire 10-ton xenon target to be purified every 2.3 days, as well as the controls system that safeguards the greater than $10 million xenon payload in emergency scenarios. View the original Berkeley Lab press release on their news site.
Q: How do you get a 5,000-pound, 9-foot-tall particle detector, designed to hunt for dark matter, nearly a mile underground?
A: Very carefully.
Last week, crews at the Sanford Underground Research Facility (SURF) in South Dakota strapped the central component of LUX-ZEPLIN (LZ) – the largest direct-detection dark matter experiment in the U.S. – below an elevator and s-l-o-w-l-y lowered it 4,850 feet down a shaft formerly used in gold-mining operations.
This final journey of LZ’s central detector on Oct. 21 to its resting place in a custom-built research cavern required extensive planning and involved two test moves of a “dummy” detector to ensure its safe delivery.
Jake Davis, a SURF mechanical engineer who worked on the cryostat move, said, “Between the size of the device, the confines of the space, and the multiple groups involved in the move, the entire process required rigorous attention to both the design and the scheduling. Prior to rigging the detector under the cage, we did testing with other cranes to see how it would react when suspended. We also completed analysis and testing to ensure it would remain nice and straight in the shaft.”
He added, “The ride was slow, right around 100 feet per minute. The ride to the 4,850-foot level typically takes 13-15 minutes. Today, it took close to 45 minutes. I rode in the cage, watching it through an inspection port in the floor. There was a huge sigh of relief after the move, but there’s still a lot of work ahead to finish LZ.”
The LUX-ZEPLIN central detector makes its way to Yates Shaft at Sanford Underground Research Facility, which can be seen in the background, for its nearly mile-long descent down an elevator shaft. The move was successfully completed last week. Photo: Nick Hubbard/Sanford Underground Research Facility
Theresa Fruth, a postdoctoral research fellow at University College London who works on LZ’s central detector, said that keeping LZ well-sealed from any contaminants during its journey was a high priority – even the slightest traces of dust and dirt could ultimately affect its measurements.
“From a science perspective, we wanted the detector to come down exactly as it was on the surface,” she said. “The structural integrity is incredibly important, but so is the cleanliness, because we’ve been building this detector for 10 months in a cleanroom. Before the move, the detector was bagged twice, then inserted in the transporter structure. Then, the transporter was wrapped with another layer of strong plastic. We also need to move all our equipment underground so we can do the rest of the installation work underground.”
The central detector, known as the LZ cryostat and time projection chamber, will ultimately be filled with 10 tons of liquid xenon that will be chilled to minus 148 degrees Fahrenheit. Scientists hope to see telltale signals that are produced when dark matter particles interact with the heavy xenon atoms in this cryostat.
The liquid form of xenon, a very rare element, is so dense that a chunk of granite can float atop its surface. It is this density, owing to the heavy atomic weight of xenon, that makes it a good candidate for capturing particle interactions.
The cryostat is a large tank, assembled from ultrapure titanium, is about five-and-a-half feet in diameter. It contains systems with a total of 625 photomultiplier tubes that are positioned at its top and bottom (see a related article). These tubes are designed to capture flashes of light produced in particle interactions.
Pawel Majewski of the Rutherford Appleton Laboratory in the UK, who led the design, fabrication, cleaning and delivery of LZ’s inner cryostat vessel for the U.K. Science and Technology Facilities Council, said, “Now it is extremely gratifying to see it … holding the heart of the experiment and resting in its final place in the Davis Campus, one mile underground.”
Crews at the Sanford Underground Research Laboratory in Lead, South Dakota, begin to lower the LUX-ZEPLIN central detector. Its nearly mile-long descent down an elevator shaft, and its delivery to a research cavern where it will hunt for dark matter, were successfully carried out last week. Photo: Nick Hubbard/Sanford Underground Research Facility
LZ is designed to hunt for theorized dark matter particles called WIMPs, or weakly interacting massive particles. Dark matter makes up about 27 percent of the universe, though we don’t yet know what it’s made of and have only detected it through its gravitational effects on normal matter.
LZ is 100 times more sensitive than its predecessor experiment, called LUX, which operated in the same underground space. Placing LZ deep underground serves to shield it from much of the steady bombardment of particles that are present at Earth’s surface.
LZ’s cryostat will be surrounded by a tank filled with a liquid known as a scintillator that will also be outfitted with an array of photomultiplier tubes and is designed to help weed out false signals of unwanted particle “noise.” And the cryostat and scintillator tank will be embedded within a large water tank that provides a further buffer layer from unwanted particle signals.
While LUX’s main detector was small enough to fit in the SURF elevator, LZ’s cryostat narrowly fit in the elevator shaft.
It was first moved outside of a cleanroom at the surface level, picked up with a forklift and carried into position below the elevator cage. It was then attached to the underside of the cage with slings and straps, where it was slowly moved down to the level of the Davis Cavern, its final resting place.
Once detached from the elevator cage, it was moved using air skates on a temporarily assembled surface – akin to how an air hockey puck moves across the table’s surface. Because of the cryostat’s size, crews had to first temporarily remove underground duct work to allow the move.
Jeff Cherwinka, the LZ chief engineer from the University of Wisconsin, led the planning effort for the move along with SURF engineers and other support.
“This was the most challenging move of a detector system that I have ever done in decades of working on experiments,” Cherwinka said.
Murdock “Gil” Gilchriese, LZ project director and a physicist at Lawrence Berkeley National Laboratory (Berkeley Lab), said, “Next, the cryostat will be wrapped with multiple layers of insulation, and a few other exterior components will be installed.”
Berkeley Lab is the lead institution for the LZ project.
“Then it will get lowered into the outer cryostat vessel,” he added. “It will take months to hook up and check out all of the cables and make everything vacuum-tight.”
Most of the LZ work is now concentrated underground, he said, with multiple work shifts scheduled to complete LZ assembly and installation.
There are plans to begin testing the process of liquefying xenon gas for LZ in November using a mock cryostat, and to fill the actual cryostat with xenon in spring 2020. Project completion could come as soon as July 2020, Gilchriese said.
Erin Broberg of the Sanford Underground Research Facility contributed to this press release.
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Founded in 1931 on the belief that the biggest scientific challenges are best addressed by teams, Lawrence Berkeley National Laboratory and its scientists have been recognized with 13 Nobel Prizes. Today, Berkeley Lab researchers develop sustainable energy and environmental solutions, create useful new materials, advance the frontiers of computing, and probe the mysteries of life, matter, and the universe. Scientists from around the world rely on the Lab’s facilities for their own discovery science. Berkeley Lab is a multiprogram national laboratory, managed by the University of California for the U.S. Department of Energy’s Office of Science.
DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit energy.gov/science.
The Sanford Underground Research Facility’s mission is to enable compelling underground, interdisciplinary research in a safe work environment and to inspire our next generation through science, technology, engineering, and math education. For more information, please visit the Sanford Lab website at http://www.sanfordlab.org.
If you have seen a photograph of Fermilab in the past three decades, odds are good that Reidar Hahn was behind the lens. You name it, and he’s probably taken a picture of it: a neutrino detector the size of a hockey rink, the experiments that co-discovered the top quark, a 570-megapixel camera peering into space. He’s captured underground groundbreakings, orchestrated photo shoots with one-of-a-kind magnets, delivered images of baby bison. Hahn has documented the birth and conclusion of experiments across the lab – and in the process, perhaps explored more of Fermilab’s nooks and crannies than anyone else.
Fermilab’s staff photographer of 32 years will retire this December. As he leaves, his work will be on display in Fermilab’s art gallery. Hahn’s show, “Collections,” runs from Nov. 6, 2019, through Jan. 3, 2020, with a free artist’s reception on Nov. 8 from 5-7 p.m.
Hahn’s photos of science at Fermilab are omnipresent at the laboratory. They populate the Fermilab photo archive, grace brochures and webpages and even hang as enormous banners in Wilson Hall’s soaring atrium – appropriately enough, right above the art gallery. But rather than displaying more of the same, “Collections” features photographs from Hahn’s life beyond the lab.
“I’m looking forward to showing some of my personal work instead of the science photos that people know me by,” he said.
An avid outdoorsman (with a forestry degree to boot), Hahn has spent much of his free time camping, canoeing and hiking his way through all 50 states and photographing both the people and the nature therein. “Collections” will group some of his favorite photos by themes, including winter, flowers, trees, water, and farming communities.
“He’s a true artist,” said Georgia Schwender, Fermilab’s art gallery curator. “I think it’s an honor to be able to see his photographs in this setting – and that people will be able to see another side of Reidar’s photography that is more his personal vision of the world.”
Schwender wanted to highlight the artistry that few people get to see, but she also organized the exhibit as a way to say farewell and recognize Hahn for his profound impact on the lab.
“There are 2,000 people at this place, and I’ve never met anybody on site that didn’t know Reidar,” said Marty Murphy, an accelerator operations specialist at Fermilab who turned to Hahn for photography advice two decades ago. “He’s one of the most kind and generous people I’ve met, and one of the most talented and capable people I’ve ever met. There are a lot of talented people at Fermilab, but none of them are really like Reidar.”
After stints as a hot-tar roofer and seven years as a newspaper photographer, Hahn joined Fermilab as a photographer in 1987. He has since worked for five of Fermilab’s six directors and knew and photographed the sixth – founding director, Robert Wilson.
“Mrs. Wilson once told me that I had captured the best picture of Bob that anyone had ever taken – and that was when Wilson was in his eighties,” Hahn said. “That was nice of her. She didn’t have to say that to me.”
Since then, Hahn has seen a lot of changes at the lab, including the growth of Fermilab’s computing sector, the lifespan of the Tevatron and the discovery of new particles, and the move from collider physics to a focus on neutrinos. Experiments moved from film in bubble chambers to the now common digital readouts, paralleling the biggest change in Hahn’s photographic career: the switch from film to digital cameras.
“Reidar’s photos have been an absolutely essential record of the lab’s activities for the last 32 years and capture important elements of the lab’s history that aren’t preserved in any other type of record,” said Valerie Higgins, Fermilab’s archivist. “His images make the wonder that many of the lab’s scientists see in the experiments they perform or the questions they explore accessible to a wider audience.”
For Hahn, who also has a degree in technical communication, it’s been a 32-year education in physics and engineering – one that he’s shared through photography but also in conversations with visitors to the lab.
“It’s really impressive how well he grasps the science of what we do,” Murphy said. “He has a great understanding of it and he explains it well – and he’s a very easy-going person. He’d be equally comfortable talking with Leon Lederman or talking with an intern.”
Hahn says he’d often couldn’t help asking questions about the research whenever he showed up to take a photo.
“People have been great to share what they’re working on with me, and I have a much greater understanding of how the universe works, and a real appreciation for all the tough, hard work people do at a frontier-science laboratory,” Hahn says. “I learned all kinds of things that I wouldn’t have if they hadn’t shared that with me.”
Hahn plans to expand his photography collections in retirement from his new home base in Idaho. After providing 32 years of consistent visual style as well as guidance and a helping hand, he’ll undoubtedly be missed at his second home.
“Reidar Hahn is one-of-a-kind,” said Fermilab Director Nigel Lockyer. “He’s smart, skilled, always working to get things done, always helping people from all across the lab. He embodies the best of the Fermilab spirit, and the lab has been lucky to have him.”