From Sanford Underground Research Facility: “Researchers taking inventory of space have found too much and too little”

From Sanford Underground Research Facility

Homestake Mining Company

August 31, 2020
Erin Lorraine Broberg

CASPAR at SURF. Photo by Nick Hubbard.

Have you ever tried counting the stars? If so, you probably gave up long before sunrise. It may surprise you, then, to know that astrophysicists are still tirelessly taking inventory of space.

Astrophysicists use spectra of starlight to determine how much of each element exists throughout the Universe. And their measurements largely confirm what leading theories predict: vast quantities of lightweight hydrogen and helium outweigh smaller sums of hefty elements like iron and uranium. One element, however, has proven stubbornly inconsistent.

In a head-scratching account, researchers have counted both too much and too little lithium in the Universe.

“Lithium is a problem,” said Frank Strieder, principal investigator of the Compact Accelerator System for Performing Astrophysical Research (CASPAR). “In the Universe today, we see an overabundance of the isotope Lithium-6 and an under-abundance of the isotope Lithium-7.”

Frank Strieder, principal investigator for CASPAR and professor of physics at SD Mines, explains how CASPAR’s low-energy beamline can help astrophysicists understand reactions that occur inside stars. Photo by Nick Hubbard.

This unexpected tally points to a possible hidden flaw in cosmological theory. At Sanford Underground Research Facility (Sanford Lab), CASPAR researchers are helping pinpoint exactly where estimates may have gone awry.

Where do elements come from?

The creation of elements, dubbed “nucleosynthesis” by astrophysicists, occurs in two main phases: Big Bang nucleosynthesis and stellar nucleosynthesis.

The first phase lasted only minutes. In the wake of the Big Bang, hydrogen and helium were created en masse, along with trace amounts of lithium and beryllium. After the Big Bang, these lightweight elements were adrift in space, floating in a sea of energy. Gravity gradually drew dense clouds of elements together, ultimately collapsing them into stars.

The second phase, stellar nucleosynthesis, has been churning out elements for eons. Stars are the element factories of the Universe. Young stars fuse hydrogen together to produce helium. As stars get older and hotter, they begin to fashion heavier elements like carbon, oxygen and neon. In the last throes of life, some stars will create hefty elements from iron to uranium before dying a brilliant death, blasting these elements into space where they will eventually coalesce into planets or reignite into a new generation of stars.

SN1987A, a supernova first detected in 1987. NASA/ESA Hubble.

Together, the theories of Big Bang and stellar nucleosynthesis accurately predict the abundance of most elements in the Universe.

“Now comes the caveat,” said Strieder. The inconsistency between predicted abundance of lithium and what is observed in the present Universe cannot be explained away by an inventory miscount. “This discrepancy is significant,” said Strieder. “It’s off by several factors, enough that it cannot be explained by uncertainties in the measurements.”

Theories under scrutiny

Is there a flaw in the otherwise elegant and pragmatic theory of Big Bang nucleosynthesis? Or does the misunderstanding lie in their understanding of stellar nucleosynthesis? Since both processes produce lithium, both are under scrutiny.

Many researchers think the solution lies in incomplete computer models of stellar reactions. To flesh out these models, researchers need more data. How often is lithium created in stellar processes? How quickly is it destroyed? And how are different isotopes of lithium treated differently by the same stellar processes?

Enter CASPAR.

CASPAR is 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. Housed on the 4850 Level of Sanford Lab, nearly a mile of rock shields the experiment from backgrounds created by our own star, the Sun. Using CASPAR, researchers can gain insight into what actually occurs inside the heart of distant stars.

The CASPAR collaboration recently ran an experiment to better understand how lithium is destroyed in stars—as well as what is produced during that reaction.

The experiment was a vital step toward understanding how stars evolve and what role lithium plays in that process, said Mark Hanhardt, a South Dakota School of Mines and Technology (SD Mines) doctoral candidate whose thesis will analyze and contextualize data from the experiment.

Building toward discovery

“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.”

Mark Hanhardt is a South Dakota School of Mines and Technology (SD Mines) doctoral candidate. His thesis will analyze and contextualize data from the recent CASPAR experiment. Photo by Nick Hubbard.

This is because models of stellar nucleosynthesis are intricately complex; they hinge on thousands of similar parameters.

This measurement by CASPAR will provide a few data points for computer models, which could solve a single incongruity in a measurement of one element in the universe. Does this seem like a slow, incremental step toward discovery? It is. But it’s also exactly what CASPAR was designed to do.

“From all the reactions that happen in the stars, we take a number of important reactions and study them case-by-case,” explained Strieder.

A chart of nuclides hangs in the CASPAR control room on the 4850 Level of Sanford Lab. This chart displays the isotopes of elements on the periodic table and helps researchers explain the order in which they are formed in stellar reactions. Photo by Nick Hubbard.

Since the CASPAR collaboration achieved first beam in 2017, the accelerator has tested multiple reactions with a similar goal. 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. On and on the circle goes, spiraling the field toward new discoveries.

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About us.
The Sanford Underground Research Facility 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

LBNE

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 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.”