Project Sets Focus on Polarized Positrons for Research

  • Illustration of how PEPPo produced positrons from CEBAF's electron beam.

A successful LDRD project proposes coaxing positrons out of the world’s most advanced electron particle accelerator for future nuclear physics experiments

NEWPORT NEWS, VA – Polarized electrons have long been the cornerstone of research at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility — they’re the subatomic particles that physicists use there for cutting-edge nuclear experiments.

But a specialized addition could be coming to the lab — polarized positrons, spinning particles that are anti-particles to electrons — and the tantalizing prospect has captured the attention and support of scientists worldwide.

Joe Grames, an injector physicist at Jefferson Lab, spearheads the Polarized Electrons for Polarized Positrons project (PEPPo). Grames is also the department head for the lab’s Center for Injectors and Sources in the Accelerator Division.

A decade ago, PEPPo’s first phase was proof of principle — noodling out a relatively simple and cost-effective way to produce polarized positrons using the lab’s Continuous Electron Beam Accelerator Facility (CEBAF), a DOE Nuclear Physics User Facility. That phase wrapped up in 2012 with a successful experiment that ran for several weeks.

The second phase, PEPPo-II, began last year as a two-year Laboratory Directed Research and Development (LDRD) project to design an injector that can produce a controlled beam of polarized positrons suitable for research. LDRD projects are typically smaller, more focused research efforts at the national labs. The LDRD program supports small-scale research projects that expand on the lab’s core scientific capabilities.

The first year of the PEPPo-II LDRD project proved so successful, and generated so much positive buzz, that it’s wrapping up early because it grew beyond the confines of DOE’s rules for the LDRD program. In October, the project transitioned to a full research and development project in Jefferson Lab’s accelerator division. This opens up the project to more collaborators, both at the lab and internationally.

“I call this a major success,” Grames said. “We learned during the first year when we were starting to do the work and reaching out to other groups that they were very interested in participating in this. They find it exciting, and they also see the potential for this to really happen at Jefferson Lab.”  

Leigh Harwood, Jefferson Lab’s LDRD program manager, stated that he definitely agrees with Grames’ assessment of it being a great success.

Positrons are a sort of antimatter twin to electrons: The positron has the same mass as the electron, but it has a positive electric charge versus the electron’s negative one. Using both particles in experiments as probes could provide different views and perspectives on the building blocks of matter that make up our universe.

You Spin Me Round

Electrons and positrons both have an inherent “spin,” much like a child’s top. When they’re polarized, they align, all spinning in the same direction. Polarized electrons are a vital feature for CEBAF experiments.

CEBAF’s current job is to shoot a tightly controlled beam of electrons inside vacuum pipes around a large underground racetrack at carefully chosen target materials inside experimental halls, so that researchers can detect and study the shotgun blast of particles that cascade downstream of the collision.

PEPPo showed that the polarization of electrons produced by the accelerator can be transferred to positrons. In the experiment at CEBAF, they shot an electron beam at low energy at a target of tungsten atoms. The electrons brake suddenly, releasing gamma rays, which create electrons and positrons. Magnets then siphoned off the desired positrons, which were found to have kept the polarization of the original electrons.

The results of PEPPo were published in 2016 in the peer-reviewed journal Physical Review Letters.

When PEPPo-II formally kicked off last year as an LDRD project, a postdoctoral physicist was brought onboard as lead scientist to develop the software needed to simulate potential layouts and designs of a high-energy positron source. Originally from Michigan, Mark Stefani was hired after earning a Ph.D. from Old Dominion University in Norfolk.

Now, as PEPPo-II transitions to a division-wide project, there are challenges to devising the right positron source, Stefani said. But none are insurmountable.

“In terms of physics, there’s nothing that’s physically impossible about it,” Stefani said. “It’s just engineering challenges for the conditions that we are hoping to get from this machine.”

Making It So

One challenge, he said, is that the CEBAF has a continuous wave beam as opposed to a pulsed beam. With a pulsed beam, it’s easier to generate and manage the powerful magnetic field needed to control the particles.

Then there are the number of CEBAF magnets — about 2,500 in many varieties — that will need to have their polarity flipped in order to work with positrons, said Grames. Fortunately, many of them can be readily flipped today, but others will require some new engineering in order to function with either electrons or positrons.

Also, the number of positrons that can be produced compared to electrons is staggeringly small, so an especially high-power beam is needed to produce the amount that researchers will want. Luckily, the CEBAF’s capabilities for providing intense polarized electron beams should more than suffice.

Colleagues from Jefferson Lab’s engineering division have already looked at these and other engineering and technical challenges to PEPPo-II.

“And none of them are showstoppers,” Grames said.

Going forward, even more colleagues will be contributing to the project.

“It’ll be a lab-wide effort,” Grames said. “It’ll be the engineering division, the accelerator division — we’ll all work together to realize and overcome the various challenges.”

PEPPo-II is a milestone in a decades-long puzzle: The physics principle behind the technique was initially calculated way back in 1959, and Grames and a French counterpart at the Centre National de la Recherche Scientifique, Eric Voutier, have been working on realizing it since 2007.

‘It’s the scientific users motivating us’

The potential uses for polarized positrons extends beyond particle physics and into real-world arenas like manufacturing and medicine. At lower energies, they could be used to analyze the magnetic properties of materials. And PET scans, or positron emission tomography readouts, are used by physicians to diagnose health conditions and monitor treatment by producing 3-D color images of tissues and organs.

But it will take some years to finesse and finalize a positron injector technique at Jefferson Lab.

Grames said they aim to present a reasonable conceptual design for a positron source to lab leadership by the end of 2022. If that’s approved, the next step would be to build a prototype to demonstrate in an experiment the work that Stefani is still designing and simulating. That could take another two years.

“We would have to show that we can actually collect as many positrons into a good-quality beam as we predicted,” said Grames.

“It’s not tomorrow, but I like to think that, on a five-year timescale, we’re getting to a place where by then we have some direction about whether this would be built at Jefferson Lab.”

Interest in polarized positrons for nuclear experiments is only growing. The lab’s positron working group was established in 2016 and today has more than 250 members from 75 institutions.

“It’s been very rewarding to this point,” Stefani said. “And the fact that there’s so much interest from the scientific users and collaborators — it’s just very motivating.”

The long-term goal, if all goes well, is to build a full positron complex at Jefferson Lab, including a new electron injector to make the positrons, the high-power targets, and the beam line to capture the positrons for CEBAF to accelerate them to 12 GeV.

In 2018, the working group submitted a letter of intent to the lab’s Accelerator Advisory Committee, noting that there were already three years’ worth of experiments that could be done onsite using both polarized and unpolarized positron beams. This compares to about 10 years of electron experiments approved for CEBAF right now.

In 2020, two proposals for positron experiments were submitted to the committee. Both were approved on the condition they be resubmitted in more detail once the injector project is farther along.

A third experiment was proposed this year, but was deferred because of other challenges.

“This is really motivated by our users,” Grames said. “We’re doing this because it’s the scientific users who are motivating us. We’re not pursuing this and saying I hope somebody shows up. We’re doing it the other way around. That’s an important strategy.”

“It’s really hard to predict the future, but this is exactly where I think the project would be in its next stage to being realized — working on the conceptual design seriously is the exact right place we need to be for the future right now.”

Further Reading
Spinning Electrons Yield Positrons for Research
Anti-Electrons Served Up as Lunch Special

By Tamara Dietrich

Contact: Kandice Carter, Jefferson Lab Communications Office, 757-269-7263,


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