Fine-tuning a particle accelerator

Some experiments at Sanford Underground Research Facility (Sanford Lab), like the Majorana Demonstrator or LUX-ZEPLIN, expect to wait years before detecting the telltale signature for which they are searching. The Compact Accelerator System for Performing Astrophysical Research (CASPAR), however, runs a new experiment every few months.

This relatively quick turn-around allows CASPAR to study individual stellar processes, one at a time. Each experiment produces a new puzzle piece. Over time, these pieces add up, and theorists can put together a larger, more complete picture of the universe.

Every reaction the CASPAR team investigates requires two elements to interact—a projectile and a target. And every time CASPAR shifts its focus to a new puzzle piece, researchers must make numerous, infinitesimal adjustments to the accelerator's beamline and target to produce the desired reaction.

“This may sound a bit funny, but running a nuclear particle accelerator is actually way harder than I expected it to be,” said Mark Hanhardt, support scientist for Sanford Lab. “While I’ve run accelerators before, they only required the flip of a switch and a bit of typing. This is much more complex. All the magnets serve different functions, and there are 27 fields in the computer—each field minutely impacting the beam.”

Aiming the beamline

To get the right reaction, then, you must first insure the projectile is composed of the right element.

“When we accelerate particles, it accelerates everything in the area with a charge, both the particles we want and many we don’t want,” Hanhardt explained.  

The key to funneling only the desired particles—particles with the right charge, mass, and speed—is a 25 degree bend in the beamline. If you were able to look at the beam before it reached the bend, you’d see many particles that are heavier, or lighter, too slow or too fast for the experiment. After that bend, however, the beamline only consists of particles with uniform charge, mass, and speed—with .01% accuracy.

“It’s called a monoenergetic beam,” said Hanhardt. “That’s incredibly important, because without it we’d be throwing all kinds of particles at the beam, like a shotgun spraying the target, which is as messy as it sounds.”

How does a simple bend in the accelerator accomplish so accurate a feat? At this bend lies a very important blue magnet, called an analyzing magnet, which determines what is allowed to continue along the beamline.

“Under the direction of this magnet, particles with the wrong characteristics will either turn too sharply or too wide, falling away from the beamline,” Hanhardt said. “Only the particles with exactly the properties we want will turn exactly 25 degrees.”

Creating the target

Another important adjustment lies at the end of the beamline: the target.

“Our targets are not just individual atoms, but the nuclei of those atoms,” said Hanhardt. “We just want to hit each atom right at the center.”

To aim at such small targets, researchers must gather thousands of these atoms in a confined space. The target material varies according to the interaction they want to study and could include anything from nitrogen and carbon up to magnesium.

Solid Targets

Researchers must make sure their solid targets consist of a thin layer of uniform atoms. For stability, these elements are usually stored on a heavier backing material, such as stainless steel or tantalum, which are kept cool by water circulating behind the backing. The solid targets must also be replaced every couple of days, as their composition will change during the experiment.

“The particles in the beam have a specific energy,” said Hanhardt. “If the target is too thick, then the particles will lose too much energy as they penetrate, which changes the variables we are working with.”

An upcoming CASPAR experiment involves a Boron 11 target. To prepare their target, researchers will evaporate and spray a thin layer of Boron 11 on a tantum backing. This method ensures that the covering is not too thick, and that beam particles will not lose energy passing through multiple layers of Boron 11.

Mark Hanhardt holds a Boron 11 target.

Photo by Erin Broberg

Gaseous Targets

Sometimes an experiment requires an element in its gaseous form. But this type of target is a bit trickier than solid ones.

“One CASPAR experiment will require Neon 22. Because it’s a noble gas and won’t bond to anything, it cannot be plated out,” explained Hanhardt. “The difficulty is that the beamline itself is a vacuum, but a gaseous target is…well, not a vacuum. That means that we have to take a vacuum beamline and put it next to a chamber with gas, without letting them mix. It’s not easy.”

Some researchers approach this problem by simply fixing a clear window between the beamline and the chamber holding the gaseous target. They call this system, sensibly, a “window gas target.” However, sending the beamline through a glass window causes a slight change in energy, and thus in the accuracy of data. This is a change that CASPAR’s researchers are not willing to afford.

So, the researchers at CASPAR will utilize a system called a “windowless gas target.”

The system works like this. The gas is sent into a small section of the beamline. Six giant pumps allow the gas to pass through a short way, before drawing it out at the end of that section. To reach this portion of the line, in which the gaseous target is circulating, the particle beam passes through a tiny pinhole. This pinhole allows the particle beam to travel through just fine. But if any gas tries to get through, the pumps will suck it away.

“This allows us to have a vacuum on one side of the pinhole and a gaseous target on the other—without interacting as long as the pumps are constantly running,” Hanhardt said. “It’s much more complex than a window gas target, but our increased accuracy is worth it.”

Photo of an ion beam passing through the gas of a gas target. The beam enters through the aperture from the left, travels through the gas and excites the atoms of the gas, which emit light during de-excitation.

Photo courtesy CASPAR

Turning on the beam

Once the adjustments are made, the team bombards the target with either a proton beam or alpha beam generated in the accelerator. The power the beam dissipates in the target is up to 100 watts, “which is the same power as a good light bulb,” said Frank Strieder, associate professor of physics at SD Mines and principal investigator for CASPAR.

This type of experiment allows researchers to record the number of reactions that occur over time in different conditions.

“Unlike other underground experiments, we look at many different interactions and are not focused on discovering just one event,” said Dan Robertson, research associate professor at the University of Notre Dame. “All of these details give us a better understanding of the life of a star and what material is kicked out into the Universe during explosive stellar events.”