From Sanford Underground Research Facility-SURF: “The HECTOR Detector retires from CASPAR”

SURF-Sanford Underground Research Facility, Lead, South Dakota, USA.

From Sanford Underground Research Facility-SURF

Homestake Mining, Lead, South Dakota, USA.

Homestake Mining Company

January 25, 2021
Erin Lorraine Broberg

During its residency at Sanford Lab in 2020, the HECTOR Detector helped researchers better understand how stars form elements.

The HECTOR detector surrounds the target chamber at the end of the CASPAR linear accelerator on the 4850 Level of Sanford Lab. Credit: Orlando Gomez.

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 [below], 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. Credit: 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.

Research at C.A.S.P.A.R.

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About us: The Sanford Underground Research Facility-SURF in Lead, South Dakota, advances our understanding of the universe by providing laboratory space deep underground, where sensitive physics experiments can be shielded from cosmic radiation. Researchers at the Sanford Lab explore some of the most challenging questions facing 21st century physics, such as the origin of matter, the nature of dark matter and the properties of neutrinos. The facility also hosts experiments in other disciplines—including geology, biology and engineering.

The Sanford Lab is located at the former Homestake gold mine, which was a physics landmark long before being converted into a dedicated science facility. Nuclear chemist Ray Davis earned a share of the Nobel Prize for Physics in 2002 for a solar neutrino experiment he installed 4,850 feet underground in the mine.

Homestake closed in 2003, but the company donated the property to South Dakota in 2006 for use as an underground laboratory. That same year, philanthropist T. Denny Sanford donated $70 million to the project. The South Dakota Legislature also created the South Dakota Science and Technology Authority to operate the lab. The state Legislature has committed more than $40 million in state funds to the project, and South Dakota also obtained a $10 million Community Development Block Grant to help rehabilitate the facility.

In 2007, after the National Science Foundation named Homestake as the preferred site for a proposed national Deep Underground Science and Engineering Laboratory (DUSEL), the South Dakota Science and Technology Authority (SDSTA) began reopening the former gold mine.

In December 2010, the National Science Board decided not to fund further design of DUSEL. However, in 2011 the Department of Energy, through the Lawrence Berkeley National Laboratory, agreed to support ongoing science operations at Sanford Lab, while investigating how to use the underground research facility for other longer-term experiments. The SDSTA, which owns Sanford Lab, continues to operate the facility under that agreement with Berkeley Lab.

The first two major physics experiments at the Sanford Lab are 4,850 feet underground in an area called the Davis Campus, named for the late Ray Davis. The Large Underground Xenon (LUX) experiment is housed in the same cavern excavated for Ray Davis’s experiment in the 1960s.

LBNL LZ project at SURF, Lead, SD, USA, will replace LUX at SURF

In October 2013, after an initial run of 80 days, LUX was determined to be the most sensitive detector yet to search for dark matter—a mysterious, yet-to-be-detected substance thought to be the most prevalent matter in the universe. The Majorana Demonstrator experiment, also on the 4850 Level, is searching for a rare phenomenon called “neutrinoless double-beta decay” that could reveal whether subatomic particles called neutrinos can be their own antiparticle. Detection of neutrinoless double-beta decay could help determine why matter prevailed over antimatter. The Majorana Demonstrator experiment is adjacent to the original Davis cavern.

LUX’s mission was to scour the universe for WIMPs, vetoing all other signatures. It would continue to do just that for another three years before it was decommissioned in 2016.

In the midst of the excitement over first results, the LUX collaboration was already casting its gaze forward. Planning for a next-generation dark matter experiment at Sanford Lab was already under way. Named LUX-ZEPLIN (LZ), the next-generation experiment would increase the sensitivity of LUX 100 times.

SLAC physicist Tom Shutt, a previous co-spokesperson for LUX, said one goal of the experiment was to figure out how to build an even larger detector.
“LZ will be a thousand times more sensitive than the LUX detector,” Shutt said. “It will just begin to see an irreducible background of neutrinos that may ultimately set the limit to our ability to measure dark matter.”
We celebrate five years of LUX, and look into the steps being taken toward the much larger and far more sensitive experiment.

Another major experiment, the Long Baseline Neutrino Experiment (LBNE)—a collaboration with Fermi National Accelerator Laboratory (Fermilab) and Sanford Lab, is in the preliminary design stages. The project got a major boost last year when Congress approved and the president signed an Omnibus Appropriations bill that will fund LBNE operations through FY 2014. Called the “next frontier of particle physics,” LBNE will follow neutrinos as they travel 800 miles through the earth, from FermiLab in Batavia, Ill., to Sanford Lab.

FNAL LBNE/DUNE from FNAL to SURF, Lead, South Dakota, USA


SURF DUNE LBNF Caverns at Sanford Lab.

U Washington Majorana Demonstrator Experiment at SURF

The MAJORANA DEMONSTRATOR will contain 40 kg of germanium; up to 30 kg will be enriched to 86% in 76Ge. The DEMONSTRATOR will be deployed deep underground in an ultra-low-background shielded environment in the Sanford Underground Research Facility (SURF) in Lead, SD. The goal of the DEMONSTRATOR is to determine whether a future 1-tonne experiment can achieve a background goal of one count per tonne-year in a 4-keV region of interest around the 76Ge 0νββ Q-value at 2039 keV. MAJORANA plans to collaborate with GERDA for a future tonne-scale 76Ge 0νββ search.


CASPAR experiment target at SURF.

CASPAR is a low-energy particle accelerator that allows researchers to study processes that take place inside collapsing stars.

The scientists are using space in the Sanford Underground Research Facility (SURF) in Lead, South Dakota, to work on a project called the Compact Accelerator System for Performing Astrophysical Research (CASPAR). CASPAR uses a low-energy particle accelerator that will allow researchers to mimic nuclear fusion reactions in stars. If successful, their findings could help complete our picture of how the elements in our universe are built. “Nuclear astrophysics is about what goes on inside the star, not outside of it,” said Dan Robertson, a Notre Dame assistant research professor of astrophysics working on CASPAR. “It is not observational, but experimental. The idea is to reproduce the stellar environment, to reproduce the reactions within a star.”