News Center

Accelerator Operator Stitches Together Storied Career at Bevatron, Cyclotron

Berkeley Lab’s Reba Siero to retire after decades working in patient treatment, high-energy beam experiments.

Photo - Reba Siero, an accelerator operator, in the control room of Berkeley Lab's 88-Inch Cyclotron. (Credit: Paul Mueller/Berkeley Lab)

Reba Siero, an accelerator operator, in the control room of Berkeley Lab’s 88-Inch Cyclotron. (Credit: Paul Mueller/Berkeley Lab)

Catherine “Reba” Siero’s comfort zone is here in the control room, surrounded by walls bristling with a busy mix of modern and time-tested knobs, dials, buttons, glowing lights, switches and screens.

For the past 23 years Siero, who is retiring next month, has served as an accelerator operator at the 88-Inch Cyclotron, a powerful particle-beam machine that started up 54 years ago at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), then managed by the U.S. Atomic Energy Commission.

Her career at the lab stretches back about 37 years, first as a UC Berkeley student conducting biology research at Berkeley Lab. From 1981-93 she ran the control system for particle-beam-based medical treatments at the lab’s Bevatron accelerator, an early version of a machine called a synchrotron.

Siero moved to the 88-Inch Cyclotron when the Bevatron—responsible for pioneering cancer treatments, the Nobel Prize-winning discovery of the antiproton, and the discovery of the antineutron—was decommissioned in 1993.

“This is the world’s largest video game,” Siero says as she begins the methodical process of releasing a powerful beam accelerated by the cyclotron’s 300-ton copper and steel magnet toward a heavily shielded experimental chamber called a “cave.”

The cyclotron can produce intense particle beams of protons, neutrons, or a range of “cocktails” that simulate space radiation and other effects on materials and electronics for satellite companies, government, university and military studies.

The cyclotron has three major machines, known as ion sources, that refine and channel its charged particle beams to experiments, and the cyclotron also drives another machine that conducts research on superheavy manmade elements.

Photo - Reba Siero at the 88-Inch Cyclotron. (Credit: Paul Mueller/Berkeley Lab)

Reba Siero at the 88-Inch Cyclotron. (Credit: Paul Mueller/Berkeley Lab)

Siero monitors the beam intensity at its source inside the cyclotron, and then controls a sequence of magnets to steer the beam through beam lines, visualizing the beam’s profile on green-glowing phosphor screens. She points to a schematic on the wall that shows the mazelike path of the beam to the various caves.

Onsite scientists, who conduct experiments from a small shack outside of the control room, decide when to block the beam or allow it to reach the experiment. They can also tune the beam by enlarging or shrinking its focus. Siero keeps them posted on the beam status and other details.

“The more bends in the beam, the cleaner it gets,” she explains. “You get rid of the ‘junk,’” as magnets draw away more of the unwanted streams of particles at every turn. And experience tells her how to spot a good beam. “You get to know what they ought to look like,” she says.

The “video game” part of her job is learning to precisely tune the magnets to achieve the desired beam. “It’s fun to listen to the folks talk about what they’re doing” in experiments, she says. “What does your beam need to look like? What size spot do you need?”

She announces through the control room’s intercom, “Hello—are you ready for some beam? Alright, coming at you.” On this night the cyclotron delivered a beam of protons, the positively charged particles found at the core of atoms.

There are constant, unexpected challenges at an aging science facility such as the cyclotron, Siero notes, which require troubleshooting skills and keep the job interesting—particularly during the night shift when she can be the lone employee on duty.

“Sometimes you have to go and wrench stuff,” she says. “I came from a mechanically inclined family,” which helps—in her youth she would help her dad fix the family car. “I love a place like this because it is problem-solving. You determine what the problem is, and you go and figure out if you can fix it. This place, the lab in general, is a can-do kind of place—we’ll figure out a way to do it.”

Siero’s career in science was fueled by her science-interested mom and a father who was himself an accelerator operator, serving from 1954-81 at the Bevatron and HILAC.

“My mom was not a scientist yet was curious about science,” Siero says. “She was a birder, an excellent gardener. She was interested in learning about things.”

Siero’s interest in biology led her to a degree in biophysics from UC Berkeley in 1979. While attending UC Berkeley she had a part-time job with a Berkeley Lab researcher who was studying abdominal tumors in mice. That led her to cell culture research, and from there she learned about a new position for a full-time operator to administer cancer treatments using the Bevatron’s high-energy beams.

“Timing was everything,” Siero says. “I interviewed and got on.”

While the Bevatron was designed for particle physics experiments, a medical staff had grown around the part-time use of its beam, which in the ’70s through early ’90s was led by Joseph Castro of UC San Francisco.

Photo - Cancer patients and others were treated for decades with particle beams at Berkeley Lab’s Bevalac. (Credit: Berkeley Lab)

Cancer patients and others were treated for decades with particle beams at Berkeley Lab’s Bevalac. At left is Helen Eisenberg, who helped prepare patients for treatment. (Credit: Berkeley Lab)

From 1954-93, medical teams treated about 3,000 cancer patients and others at the lab’s 184-Inch Cyclotron—a predecessor to the lab’s Bevatron—and the Bevalac, as the Bevatron was called when coupled to another accelerator, the HILAC. The Bevalac proved that beams of neon and carbon were effective against hard-to-treat cancers in the head, neck and eye.

Patients met with doctors and physicists onsite for evaluations and treatments, which typically ran for several weeks. Treatment setups were tested with plastic dummies.

“I set up all the dosimetry equipment, lined it up with the lasers, did the calibration, and then the computer program put in each patient’s dose,” she says. “I really enjoyed being a part of that patient treatment program. It was just kind of a kick in the pants.”

Siero was the first woman to serve as an accelerator operator at Berkeley Lab. She developed a familiarity with the timing clicks and unique beam properties of the Bevatron, and an appreciation for its power—its changing magnetic field would tweak the displays on the CRT screens around the building.

Patients for a time were treated at both the Bevalac and the 184-Inch Cyclotron, and Siero was called to assist in operating that cyclotron’s treatment program. She recalls that its magnetic field was so intense inside the shielding that you could taste your dental fillings, and it could take a typical metal tool “right out of your hand.”

Photo - This historic photo shows two Berkeley Lab physicists standing on the shielding of the Bevatron, which was used in cancer treatments and high-energy physics experiments. (Credit: Berkeley Lab)

This historic photo shows two Berkeley Lab physicists standing on the shielding of the Bevatron, which was used in cancer treatments and high-energy physics experiments. (Credit: Berkeley Lab)

When the Bevatron shut down, Siero says there was a learning curve in moving to the cyclotron, though, “I had the lingo—I understood the language” of accelerator operators. Even so, she says, “Every accelerator is unique. The training is on-the-job. You don’t come at it knowing about it. You learn this by doing it.” During her career Siero has helped to train several cyclotron operators.

Born in Oakland and raised in Walnut Creek in the San Francisco Bay Area, Siero enjoys needlecraft when she’s not working, and her retirement will surely include her lifelong hobby.

“Anything to do with fiber and I’m there,” she says. “I’m a quilter, weaver, spinner, knitter—anything sewing- or fiber-related. I made my first dress when I was in the fourth grade. My grandmother was a seamstress so I must have gotten the genetics.”

She notes that the needlework patterns and techniques “feed into my scientific-type mindset. It’s got a lot of arithmetic in it and patterning.”


Lawrence Berkeley National Laboratory addresses the world’s most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab’s scientific expertise has been recognized with 13 Nobel prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy’s Office of Science. For more, visit

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