Shining aluminum panels hang like heavy curtains on each side of the particle detector MicroBooNE at Fermilab. Thin wires run along the sides of each panel and are bundled together similar to a curtain with a cord.
Even though the heavy panels now block the view of the large detector in its solitary pit with their eye-catching sheen, they are not just decorative. They serve a critical purpose: spying on particles coming from cosmic rays before they hit the actual detector.
MicroBooNE’s shiny new exterior helps scientists identify cosmic rays masquerading as neutrinos. From left: Elena Gramellini, Thomas Mettler. Martin Auger, Mark Shoun, John Voirin. Photo: Reidar Hahn
The signals of cosmic rays
Cosmic rays are a constant rain of particles that are created in our sun or faraway stars and travel through space to our planet.
They’re subjects of many important physics studies, but for MicroBooNE’s research, they simply get in the way. That’s because MicroBooNE scientists are looking for something else — abundant, subtle particles called neutrinos.
Unlocking the secrets neutrinos hold could help us understand the evolution of our universe, but they’re exceedingly difficult to measure. Fleeting neutrinos are rarely captured, even as they sail through detectors built for that purpose.
Add to that the fact that their interactions are potentially drowned in a sea of cosmic rays rushing through the same detector, and you get a sense of the formidable challenge that neutrinos represent.
The MicroBooNE experiment starts with Fermilab’s powerful accelerators, which create neutrino beams that are then propelled through the MicroBooNE detector.
“The neutrino beam here at the lab gives us the right conditions to study neutrinos,” said Elena Gramellini, a Yale University graduate student on the MicroBooNE experiment. “Our challenge is to pick out neutrinos from many cosmic rays passing through the detector.”
Since cosmic rays are made of some of the same particles produced when a neutrino interacts with matter, they leave signals in the MicroBooNE detector that are often similar to the sought-after neutrino signals. Scientists need to be able to sort the cosmic rays in the MicroBooNE data from the neutrino signals.
Tagging and sorting
Even several feet of concrete enclosure would not completely block cosmic rays from hitting a detector such as MicroBooNE, not to mention that such a structure would be inconvenient and expensive. Instead, MicroBooNE uses the aforementioned panels, called a cosmic ray tagger, or CRT. While the panels don’t block cosmic rays, they do detect them.
Each CRT panel has particle-detecting components – strips of scintillator – that lie beneath its shiny aluminum enclosure. Cosmic ray particles can easily pass through aluminum and the scintillator — a clear, plastic-like material — on their way toward the MicroBooNE detector.
The cosmic ray particles deposit energy in the plastic scintillator, which then emits light. An optical fiber buried inside the scintillator captures the emitted light and transmits it to devices that generate the digital information that tells scientists where and when the cosmic ray struck.
“With our current layout of scintillator strips in each panel, we are able to tell precisely where the cosmic ray enters the MicroBooNE detector after it left the panel,” said Igor Kreslo, professor at the University of Bern who designed the CRT panels for MicroBooNE. “Our design effort really paid off and now ensures thorough cosmic ray tracking.“
So why the shiny aluminum shell? It blocks unwanted light from the detector’s immediate surroundings so that only light created by cosmic rays inside a CRT panel reaches the optical fiber and is detected.
Putting up panels
The 49 rectangular CRT panels are the contribution of the University of Bern in Switzerland, one of the 28 institutions collaborating on MicroBooNE worldwide. They produced the panels last winter and shipped them to Fermilab during the spring.
“This was a large project for us, and it took everyone in Bern to finish everything in time,” said Martin Auger, scientist at the University of Bern who planned the arrangement of the CRT panels. “A key moment was the test of the CRT panels after the long journey to Fermilab. All the panels arrived in good shape!”
The installation team overcame a number of challenges —including the tight space in which MicroBooNE stands — to successfully place the panels around the detector.
“The installation crew is a crack team of veteran Fermilab employees,” said John Voirin, who leads experiment installations at the laboratory. “In the end we have a very elegant, safe operating product that is a valuable asset to the experiment.”
Later this year the group will complete the installation by placing the final layer on top of the MicroBooNE detector. Even without it, the CRT already greatly enhances the capabilities of the experiment.
“We started taking data just in time for the first neutrinos delivered to the experiment,” Gramellini said.
This article is dedicated to our dear friend and colleague Gino Bolla.
The Dark Energy Spectroscopic Instrument, called DESI, has an ambitious goal: to scan more than 35 million galaxies in the night sky to track the expansion of our universe and the growth of its large-scale structure over the last 10 billion years. Using DESI — a project led by Lawrence Berkeley National Laboratory — scientists hope to create a 3-D map of a third of the night sky that is more accurate and precise than any other.
A precise map requires that DESI itself be built and assembled with micrometer precision. Fermilab, a Department of Energy national laboratory, is contributing a key piece of the instrument: a large, barrel-shaped device that will hold optical lenses to collect the light from millions of distant galaxies. The smallest deviation in lens alignment could lead to the instrument being permanently out of focus. Every piece of the barrel must be perfectly placed, so the Fermilab team is currently taking every measure to ensure its precise assembly.
The process involves a special machine, meticulous handling and a healthy dose of patience.
This Fermilab team is currently assembling the barrel for the Dark Energy Spectroscopic Instrument, a project being led by Lawrence Berkeley National Laboratory. From left: Jorge Montes, Mike Roman, David Butler, Gaston Gutierrez, Giuseppe Gallo, Otto Alvarez. Photo: Reidar Hahn
Precision assembly
The lens-holding device is a roughly 8-foot-long and 4-foot-wide segmented cylinder — about the size of a small elevator. Once the hulking steel barrel is complete, it will be installed at the Mayall four-meter telescope at the Kitt Peak National Observatory, southwest of Tucson, Arizona.
The lenses will collect the light reflected from the telescope’s mirror and focus it into 5,000 optical fibers, through which the light is transported to special detectors, called spectrographs. With the help of 10 such spectrographs, scientists can measure the distance of the galaxies.
In May, a team of specialists at Fermilab began assembling the barrel’s five segments carefully, checking that each nut and bolt was perfectly situated. But a nuts-and-bolts-level fit isn’t enough. To achieve the precision scientists are aiming for, the DESI barrel and its inner structure must be assembled accurately to within an incredibly tight 20 micrometers. That’s one 10th of the thickness of a sheet of paper.
To achieve the required fit, the team has been making small, critical adjustments to the assembled barrel.
Accurate alignment
The barrel adjustments take place in a vacant area the size of a small bedroom. Four tall pillars – nearly seven feet high – stand at the corners of the space.
Above their heads, a rail, similar to train tracks, connects the tops of the two pillars on one side. A second rail connects the other two. A moveable carriage track spans the gap – like a high bridge spans a river – connecting the two rails. The carriage itself glides along the track.
The team guides the carriage so that it stops just above the barrel. The carriage carries a mechanical arm that points towards the floor. It can rotate in all directions in the space within the pillars. At the end of the arm is a highly sensitive and precise sensor, fixed to an articulating motorized probe.
The arm with the sensor comes to life: It reaches down to the barrel and starts feeling for its surfaces. It searches for specific points on the barrel – a corner, an edge, another significant surface marker. When it finds them, it measures the coordinates in the designated space. Very carefully and with tiny movements, it moves over the whole surface of the barrel, measuring up, down and around the surface. As it does, it records the measurement data and saves it for further analysis. Jorge Montes, one of the team members, strategically places markers on the barrel’s surface to assist their alignment efforts.
After making the measurement, the scientists return the barrel to an outside area. There they disassemble it, realign all the parts, relying on the previously placed markers. They then reassemble it. With great care they bring the once more fully assembled barrel into the empty space and measure anew the precision of their assembly.
Comparing their performance with their previous assembly, they learn which pieces, if any, are misaligned — even slightly — and where they improved the alignment.
The barrel will hold the lenses and optics for DESI, which will map one-third of the night sky. To create an accurate map, the barrel’s pieces must be accurately assembled to within 20 micrometers. Dial Machine of Rockford, Illinois, manufactured the barrel steel components. Its many tons of steel work were machined to the incredible accuracy required. Photo: Reidar Hahn
A magic machine
The precise, slow-moving measuring machine that points out the misalignments is called a coordinate measuring machine, or CMM. The group making these point-by-point measurements, led by Fermilab engineering physicist Michael Roman, uses it to ensure the DESI barrel’s perfect assembly.
With the help of the CMM, they repeat the whole procedure of assembly, measurement and disassembly again and again, always comparing their performance against previous tries. When they reach their alignment within 10 micrometers — about a 10th the width of a human hair — in a certain number of tries, they are satisfied.
“From early on we knew that the barrel needed high-precision measurements for the assembly and that it would be too large for any of the CMMs at Fermilab to perform such measurements,” Roman said.
“In strong support of DESI, Fermilab bought a machine for the dedicated measurements on the barrel,” said scientist Gaston Gutierrez, who is one of the DESI project leads at Fermilab.
Steady and stable
To ensure that the CMM’s measurements are as precise as they need to be, the CMM is set up in an air-conditioned room, where scientists monitor and control the temperature 24 hours a day. Materials expand when they get warm, affecting the accuracy of CMM’s measurements.
So scientists worked out the right control settings for the environmental control system to ensure that the temperature never varied more than one degree from 20 degrees Celsius.
Even the eventual effect of heavy weights on the DESI barrel, including the lenses, can be measured with the new CMM. Scientists place the DESI barrel in the machine and measure it, then add test weights on its sides and remeasure. The team can see how the barrel shrinks or bends, if at all, and determine whether the lenses will hold steady when the telescope is in motion.
The Fermilab team expects to finish all CMM measurements by early 2017. Then they will disassemble the DESI barrel and send it to University College London. In London, their colleagues will install the lenses in the support structures. Once the lenses are installed, the barrel will start its journey to its future home in Arizona.
Measuring the expansion of the universe
Scientists have discovered that our universe is growing bigger and bigger — without any end in sight. Like raisins in a rising loaf of bread, the universe’s galaxies are being pushed apart from each other.
From previous measurements, scientists have a kind of cosmic ruler, a standard length that goes back to the universe’s early beginning. Using this ruler together with the high-precision DESI map, scientists will be able to tell how far galaxies have moved apart and how much our universe has grown throughout its history.
“With the DESI experiment, we want to follow the growing steps of our universe,” Gutierrez said. “We start from today and go backwards in time to measure how much the universe has expanded since its early days.”
The fabrication, assembly and operation of DESI are small but highly important steps toward precisely understanding the universe.
If it’s true that life begins at 50, Fermilab is just getting started.
Next year, the country’s premier particle physics laboratory celebrates 50 years of discovery and innovation. Fermi National Accelerator Laboratory, a U.S. Department of Energy lab, was founded in 1967 under the direction of Robert Wilson and in the ensuing half-century has explored many of the most fascinating mysteries of the universe.
Along the way, scientists working at Fermilab discovered three of the elementary particles that make up our universe, photographed the farthest reaches of space to learn more about dark energy and recently received the Department of Energy’s approval to start construction of the largest physics experiment ever built on (and under) U.S. soil, the Deep Underground Neutrino Experiment.
Today Fermilab launched a new website, 50.fnal.gov, with full details of its 50th anniversary celebrations. Highlights include:
- Saturday, Jan. 21: A public kickoff party for our birthday year, featuring a reception in Wilson Hall at 7 p.m. and a concert by Chicago’s own rock ‘n’ roll marching band, Mucca Pazza. Tickets are on sale now.
- Friday, Jan. 27: Fermilab’s Greatest Hits, a lecture by longtime Fermilab scientist Chris Quigg detailing some of the most important moments from the last 50 years. Tickets are available now.
- Sunday, Feb. 12: The annual Family Open House in Fermilab’s Wilson Hall will celebrate the laboratory’s birthday with hands-on science activities for the entire family. This event is free! Watch our website for more information about registering for tours.
- Thursday, June 1: The Fermilab Art Gallery will host an opening reception for “A Lasting Mark: Artist Angela Gonzales at Fermilab 1967-1998,” an exhibit of work from the artist who helped give Fermilab its visual identity. This event is free!
- Thursday, June 15: The day Fermilab employees first reported to work will be commemorated with a social media celebration. Follow Fermilab on Facebook, Twitter and Instagram for more information.
- Saturday, Sept. 23: Fermilab’s big 50th anniversary open house will welcome the public to tour facilities at the lab that they would normally not get to see. The event will include a science and innovation fair and dozens of activities for families. This event is free as well!
That last event is particularly noteworthy. The open house will be the centerpiece of a slate of events at the lab and online that will look back on half a century of achievements and look forward to a bright future.
More events and activities, including pop-up science events in places around Chicagoland, will take place throughout the year. Watch 50.fnal.gov, where more information will be posted as it becomes available.
Fermilab is America’s premier national laboratory for particle physics and accelerator research. A U.S. Department of Energy Office of Science laboratory, Fermilab is located near Chicago, Illinois, and operated under contract by the Fermi Research Alliance, LLC. Visit Fermilab’s website at www.fnal.gov and follow us on Twitter at @Fermilab.
The DOE 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 science.energy.gov.