BYLINE: Chris Patrick
Newswise — In this International Year of Quantum Science and Technology, physicists worldwide are celebrating a century since the establishment of quantum mechanics. The celebration has brought to the forefront the concepts of how tiny particles that make up the ubiquitous atom give rise to our visible universe. It is also highlighting that while scientists now have a far greater understanding of how things work in the quantum realm, there are still some mysteries to be solved.
One of these mysteries is how particles “spin.”
In the quirky world of quantum mechanics, all fundamental particles like electrons, quarks, and gluons have an intrinsic angular momentum, a “spin.” Particle spin in the quantum realm is a key concept being explored by nuclear physicists at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility.
One focus of this research is on discovering the origins of proton and neutron spins. Scientists have known for a long time that simply adding the spin of the quarks inside these nuclear particles isn’t enough. The gluon spins also contribute, as does the chaotic, swirling motion of the quarks. Now, nuclear physicists at Jefferson Lab are measuring how all the pieces inside the proton or neutron make it spin.
“In most materials, the nuclei are all spinning in random directions. If you want to understand this property, it really helps to get them all spinning in the same direction. And that’s what a polarized target does,” said Christopher Keith, group leader for the Target Group at Jefferson Lab.
To enable these experiments, the Target Group was tasked with designing and building a polarized target specifically for use in the CEBAF Large Acceptance Spectrometer for 12 GeV (CLAS12). This detector is installed in Experimental Hall B, one of the four experimental halls in Jefferson Lab’s Continuous Electron Beam Accelerator Facility, a DOE Office of Science user facility that supports the research of more than 1,650 nuclear physicists worldwide.
Some polarized targets at Jefferson Lab are made of helium-3 gas, which acts as a source of neutrons for the beam electrons to interact with. But CLAS12’s polarized target uses solid materials, which offer a more compact source of either protons or neutrons.
Starting up the spin
Keith says building the target presented a unique set of challenges.
“Nuclear spins are very difficult to polarize directly, so we take an indirect approach. We begin by adding extra electrons to the target material and polarize their spins using a combination of very cold temperature and a very strong magnetic field. We then apply microwaves at a very specific frequency, and the electron polarization gets transferred to the nuclei,” he said. “This is a process called Dynamic Nuclear Polarization, and we’ve been doing it at Jefferson Lab for more than two decades. But making all this happen inside CLAS12 required extensive R&D.”
In this case, the magnetic field was produced by CLAS12’s strong, superconducting solenoid magnet, which is mainly used to shield the sensitive CLAS12 detectors from unwanted particles. But it was also designed for polarized targets, because scientists knew spin experiments would be a big part of the CLAS12 physics program.
Unfortunately for the Target Group, those same scientists packed the inside of the solenoid with detectors and electronics, leaving little room for a polarized target. Keith said designing the target to fit within these geometric constraints was the team’s biggest challenge.
“It was a very, very tight fit,” he said.
Cold constraints
To reach the highest levels of polarization, the team cools the target samples to just one degree above absolute zero, or -458 F.
“We do this by placing the target material, which is frozen ammonia, in a bath of ultracold superfluid helium,” said James Brock, the Target Group engineer who designed the helium refrigerator.
While the extreme cold improves the polarization of the extra electrons, it also slows the rate at which the nuclear spins lose their polarization. For the target to work, solid samples of frozen ammonia or deuterated ammonia, which are only 5 centimeters long, must be suspended within the center of CLAS12’s solenoid magnet coil, a full 4 meters from the detector’s narrow opening.
As a result, the entire target system is shaped a bit like a four-meter-long syringe, with the quasi-needle containing the small, 5-cm long samples material buried deep within CLAS12 detectors.
Once the target’s electrons are polarized in the center of CLAS12, the target material is illuminated with microwaves at a very specific frequency. This necessary step transfers the polarization from those electrons to the nearby nuclei.
“There are actually two frequencies that work,” said Keith. “One polarizes the nuclei spins in the same direction as the magnetic field, and the second polarizes them in the opposite direction. So, we can compare the scattering of electrons from the nuclei when the only thing that has changed is the direction of the spin.”
The Target Group also developed smaller superconducting magnets that sit inside the refrigerator and improve the uniformity of CLAS12’s magnetic field right at the target, as this enhances the polarization transfer caused by the microwaves. The group also built new electronic instrumentation to measure its polarization. The existing electronics, called Q-meters, were more than 30 years old, and replacement parts were impossible to find. James Maxwell, a physicist in the Target Group, worked with Hai Dong from Jefferson Lab’s Fast Electronics Group to produce modern, up-to-date Q-meters to measure the polarization of the target.
In total, the target system took five years to create, with more than a year of testing after design and construction. But thanks to Brock’s clever designs, the entire system could be transported to Hall B and placed on the beamline in only a week.
So far, the target has been used in one set of experiments, with more to come. Though physicists are still analyzing these data, the target performed reliably and perhaps broke a polarization record.
According to Keith, “The deuteron is hard to polarize, and in electron experiments like this, we’re usually limited to a maximum value of about 45%. But we think we might have hit 55% on one occasion, which could be a record for experiments in an electron beam.”
The proton polarization, which is typically higher than deuterons, averaged between 80 and 85%.
A time-saving trolley
This polarization, however, doesn’t last forever. The radiation generated as the electron beam strikes the target eventually takes a toll. The Target Group had to replace the samples three to four times a week, a feat easier said than done.
Originally, the team thought that a target sample change would require taking down several meters of the beamline, a pipe that the electron beam travels through, to reach the samples. Considering reassembly, this effort would take up to eight hours in total.
“It was going to be very painful,” Keith said. “But James Brock came up with this really clever idea where we don’t actually have to take any of the beamline down.”
Brock’s idea was to add wheels inside the target’s refrigerator. To cool the sample to such ultralow temperatures, it’s placed in a small container, or bath, that is filled with superfluid helium. Brock added wheels to the superfluid bath, so it could be pulled back from the center of the CLAS12 magnet to a small opening that could accessed without taking down the beamline. They nicknamed this “the trolley” because the superfluid bath moves on rails.
“We spent a lot of effort learning how to do this, and it works fantastically,” Keith said. “We did it 75 times during the CLAS12 experiment, and it never failed.”
Because this method only takes 30 minutes, it saved the team about 18 days of work and experimental run time in changing out targets. That meant that the experiment took much more data than it would have otherwise. Brock said his idea was “born out of necessity.”
This method’s efficiency also yielded a colorful bonus.
The ammonia, which was provided by the University of Virginia, is clear when it’s first frozen. In the electron beam though, it turns purple.
“We’ve always seen the ammonia become discolored when it’s in the beam,” said Brock. “So, we were expecting it. What we weren’t expecting was an intense flash of blue light. It only lasts a few seconds and happens somewhere between the temperatures of liquid helium and liquid nitrogen. In our other targets, the ammonia was already too warm by the time you pulled it out of the refrigerator.”
This process, called thermoluminescence, has been observed in other irradiated materials, but the atomic or molecular source of the light emanating from solid ammonia is not known. The team hopes to measure its exact wavelength during the next experiment with the target.
“Right now, it’s just a curiosity. But who knows? It might tell us something interesting about the damage that the electron beam causes to the target sample,” Keith said.
Once the scheduled experimental run concluded, the target system was removed and packed off carefully to storage. It’s standing by for the next series of approved experiments that will require its unique geometry and capabilities.
Teamwork makes the dream work
The entire Target Group, which is made up of about 12 people, participated in various aspects of the Hall B polarized target’s design and construction. Brock said the support of the technicians, who combined have more than 250 years of experience and include several who have been with the group for its entire 30 years of existence, was essential to the team’s success.
“We have some of the best technicians at Jefferson Lab, and their experience is decades long,” he said.
Two former Old Dominion University (ODU) graduate students, Victoria Lagerquist (now at the University of Bonn, Germany) and Pushpa Pandey (now at MIT), also earned their Ph.D.s building and operating the target.
The Target Group also worked with the CLAS collaboration to ensure the target would meet their needs. The target was supported by the DOE Office of Science, Office of Nuclear Physics and partially funded through a National Science Foundation grant awarded to Christopher Newport University, ODU, and the University of Virginia.
“It was a big collaborative group effort to put this thing on the floor,” said Keith, who hopes the team will publish three papers about the overall target system, its new electronics, and the sample-replacement method. “We knew it was going to be a really challenging project, and everybody was just really happy with the way it turned out.”
Further Reading
Quarks Spin in a Surprising Way
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Jefferson Science Associates, LLC, manages and operates the Thomas Jefferson National Accelerator Facility, or Jefferson Lab, for the U.S. Department of Energy’s Office of Science. JSA is a wholly owned subsidiary of the Southeastern Universities Research Association, Inc. (SURA).
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, visit https://energy.gov/science
