The HECTOR Detector retires from CASPAR

This month, researchers at Sanford Underground Research Facility (Sanford Lab) returned a detector to the University of Notre Dame (UND). The detector, which has been collecting data for most of 2020, is officially named the High EffiCiency TOtal absoRption spectrometer—but you can call it HECTOR.

“I always get a smile or two at conferences when, with a straight face, I introduce the ‘HECTOR detector.’ It has a good ring to it,” said Orlando Gomez, a doctoral candidate at UND who devoted the majority of his thesis to studying how this detector in particular goes about detecting things.

The HECTOR detector helped researchers with CASPAR (Compact Accelerator System for Performing Astrophysical Research) learn more about stellar nucleosynthesis, a process whereby stars create heavier elements from lighter ones.

According to this process, young stars begin with lightweight elements. Hydrogen, a remnant of the Big Bang, fuses together inside the core of adolescent stars, producing helium. As stars get older and hotter, they begin to fashion heavier elements like carbon, oxygen and neon. In the last throes of life, heavy stars will create many hefty elements from iron to uranium while dying a brilliant supernova death, blasting these elements into space where they will eventually coalesce into planets or reignite into a new generation of stars.

Although it may seem like a tidy theory, this alchemy of turning hydrogen atoms (with just one proton) into atoms like iron (with 26 protons) or even uranium (with a whopping 92 protons) requires eons of messy interactions.

Astrophysicists still have a lot to learn about nucleosynthesis. Using particle accelerators, they can recreate these reactions on Earth. And detectors like HECTOR can help them make sense of what they see.

Taking the stars underground

When Gomez first began using the HECTOR detector with a particle accelerator at UND, he studied a handful of these stellar reactions. His studies pertained to isotopes of palladium and cadmium, heavy and earth-rare elements that are formed during the final eruptions of a dying star.

After those measurements, the research group using the HECTOR detector decided to hit rewind. Rather than studying reactions at the grand finale of a star’s life, they turned their attention to better understanding the earlier stages of a star’s life and different burning scenarios—when stars produced lighter elements, like lithium, neon and oxygen.

These reactions can be problematic to study, because they occur at very low energies. Without high energies forcing particles to meet, many particles will simply scatter away from each other; lower energies mean fewer interactions to study. In space, stars easily get around this issue with eons of time and immense masses on their side. Doctoral students, however, have thesis deadlines and university budgets working against them.

To get around the low-energy problem, these researchers go underground.

On the 4850 Level of Sanford Lab, nearly a mile of rock shields experiments from backgrounds created by our own star, the Sun. This helps researchers focus on the few interactions that do occur at lower energies. Researchers also raise the intensity of the particle accelerator beamline and run the beam for a long time (sometimes weeks at a time)—all to witness more interactions. In January 2020, HECTOR was packed up and shipped to Lead, South Dakota.

Putting HECTOR to work

At Sanford Lab, HECTOR was affixed to CASPAR, a low-energy particle accelerator that allows researchers to send specific particles toward a target, forcing them to interact as they would inside a star. When reactions happen in CASPAR, they give off energy in the form of gamma rays, which HECTOR detects.

HECTOR is an array of 16 individual salt crystals that surround CASPAR’s target chamber. When gamma rays travel through HECTOR’s salt crystals, they deposit energy causing scintillation, or light. This light is picked up by photomultiplier tubes.

“There's a very specific energy that we're looking for, and from the light that's given off, we can infer the energy of the reaction. That's how we track the reaction rate that we're interested in measuring,” Gomez said.

But the next step is where the HECTOR detector really impresses (at least, it impresses doctoral students tasked with analyzing the resulting data sets).

When the HECTOR detector detects scintillation light from gamma rays, it distills that information, making it easier for researchers to access the information they need. Graph courtesy Orlando Gomez 

Displaying the above graphic, Gomez explains: “What you're seeing here is an energy spectrum. Instead of trying to track all of the energy peaks in your spectrum [shown in orange], HECTOR can add up all their energies, giving us this one giant peak [shown in blue].”

Gomez goes on to explain that, whatever reaction HECTOR is observing at the time, it will see a cacophony of gamma rays. But, he says, researchers are most interested in the total energy created by the reactions.

“There are many different ways the gamma rays can come out. We aren’t interested in those

complexities. Since we know the total energy in a reaction is conserved, all those gammas rays should add up to one signal,” Gomez said. “When nuclear reactions get quite complex, HECTOR simplifies the process immensely.”

Building toward discovery

Although the collaboration faced interruptions due to the COVID-19 pandemic, the HECTOR Detector operated for most of 2020, taking data for multiple campaigns. One campaign involving neon-22 could help researchers understand a mysterious abundance of neutrons hurtling through stars. Another experiment was a vital step toward understanding why there seems to be both too much and too little lithium in the Universe.

Mark Hanhardt, a doctoral candidate at South Dakota School of Mines and Technology, will analyze and contextualize data from HECTOR for his own thesis research.

“What does this data mean? How will these measurements change the models that we use to understand, not only individual stars, but the evolution of all the stars in the Universe? My job is to put our data in context of the current field of research,” Hanhardt said. “Still, when I finally type in the parameters of this reaction, it won’t drastically change the way we look at the Universe.”

This is because models of stellar nucleosynthesis are intricately complex. Measurements from the HECTOR detector will provide a few data points for computer models that hinge on a thousand similar parameters. Does this seem like a slow, incremental step toward discovery? It is. But it’s also exactly what the HECTOR detector and the CASPAR experiment were designed to do.

Data from these experiments bolster computer models, which indicate to theorists where new solutions might be hidden. Theorists then outline new ideas, telling researchers where to look next.

Learn more about taking astrophysics underground with CASPAR in this video created by the University of Notre Dame. 

CASPAR is a collaboration between the University of Notre Dame, South Dakota School of Mines and Technology and Colorado School of Mines.