From Sanford Underground Research Facility: “A ‘game board’ for astrophysicists”

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From Sanford Underground Research Facility

November 19, 2018
Erin Broberg

A nuclides chart is designed to help researchers study the nucleosynthesis of elements—or how they are created.

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Matthew Kapust

Just outside a thick lead door leading to the Compact Accelerator System for Performing Astrophysical Research (CASPAR) in the experiment’s control room, hangs a massive chart. Hundreds of small, colorful blocks identify some of the universe’s smallest units in three vibrant bands that streak across the chart. It is as if an artist took a brush and swiped it across the page. But it isn’t a painting; it’s the chart of nuclides.

“The periodic table is a chart of atoms, but this is a chart of just the nuclei of those atoms—the stable and unstable isotopes of those atoms,” said Mark Hanhardt, support scientist for Sanford Underground Research Facility (Sanford Lab). “Here, we don’t take into account the electrons at all—just the nucleus.” Hanhardt, a Ph.D. candidate in physics at the South Dakota School of Mines and Technology (SD Mines), is focusing on CASPAR.

While the periodic table allows scientists to understand the chemical properties of elements, this chart is specifically designed to help researchers study the nucleosynthesis of elements—or how they are created.

What happens to a nucleus if a neutron is added? If a beta decay occurs? Scientists can locate an element’s nuclei on the chart and visualize the changes that occur at a nuclear level. The numerous details contained in this chart are a bit dizzying. To explain just how this powerful tool is used, Hanhardt has developed a simple analogy.

“If you add a proton, you move one square up. If you add a neutron, you move one over to the right,” said Hanhardt. “Truly, the chart of nuclides is CASPAR’s game board.”

The CASPAR collaboration will use a low-energy accelerator to study the creation of elements inside the heart of stars; using this “game board” helps them explore and track the evolution of elements over time.

The Game Board

This game board has some three very important rules:

Rule 1: Start at the beginning.

The Big Bang created two elements—hydrogen and helium.

“That is where the elements start,” said Frank Strieder, associate professor of physics at SD Mines and principal investigator for CASPAR. “Over time, they build upon each other, moving their way up the board.”

Rule 2: Level up.

From hydrogen and helium, there are multiple ways to “level up” to a heavier element.

The first is through nuclear fusion, which pushes two elements together, creating a heavier element. Other processes include the slow capture of individual neutrons (called the s-Process), the collision of two stars (called the r-Process) or the beta decay of a neutron.

Rule 3: Follow the Valley of Stability.

Isotopes with equal numbers of protons and neutrons are usually more stable than those isotopes with very different numbers. Should a nucleus gain too many of any one particle, it becomes unstable. The thick bands streaking across the chart of nuclides represent what Hanhardt has dubbed the “Valley of Stability.”

“In this band, the isotopes have a relatively equal number of protons and neutrons in each nucleus, so they tend to be more stable,” said Hanhardt. “As isotopes gain too many protons or neutrons, however, they begin to stray from the main path, further from the Valley of Stability, and the more likely it is that a beta decay will occur.”

Playing with the s-process

The rules help researchers better understand how elements can evolve over time. The CASPAR collaboration is most interested in what is called the Slow Neutron Capture Process, or the s-Process. The s-process accounts for the creation of half of all elements heavier than iron.

“Without the s-process, the universe would be very boring, and it probably would not have complex life,” said Strieder.

Here’s how the s-process works, according to Hanhardt.

“Say you start with an element like iron-58. If there is a neutron available, just a free neutron floating around, the iron nucleus can capture it, creating iron-59, another isotope of iron. If that isotope would be stable, it would stick around; however, it is unstable and will undergo beta decay. Beta decay means a neutron is changed into a proton. This will move the nucleus up one and over one to the left on the chart, making it a new element.”

Through this very slow process, you take a jagged path up the chart, building many of the heavier elements. In order for this process to happen, though, there must be a free neutron available. That’s a bit more difficult that it sounds.

“Free neutrons only exist on their own for 10-15 minutes before they decay,” Strieder said. “So, in order to create these elements, there has to be a place in the universe where you have neutrons being created, nuclei that are ready to capture a neutron and a temperature just perfect for these reactions to take place.”

Scientists have a pretty good idea where this happens: in multi-layered stars called thermally pulsing asymptotic giant branch stars (TP-AGB). An example of such a star is “Mira” in the constellation Cetus. What they don’t know, however, is the rate and energy at which the neutrons are produced and captured.

Two upcoming CASPAR experiments aim to discover just how quickly those neutrons are created and how they join other elements over time.

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Defining the Rules

To study these rates, researchers at CASPAR hope to duplicate the reactions they know occur in TP-AGB stars, creating free neutrons. They will be the first people on earth to study these reactions at a low energy—an energy that is the same in the heart of the star.

“The astrophysicists take these numbers we discover and put it into their model of how a star works,” said Strieder. “With this, we can determine how much of the heavier elements were produced per star. Then we can calculate the number of heavier elements that were produced in the entire universe, and check if that is consistent with the number of elements we measure on earth.”

These are big questions to ask of such little reactions. However, it is a fundamental piece in the universal puzzle.

“If we go back to the game board analogy,” said Hanhardt, “we are not so much looking at one specific move on the board, but rather investigating the rules of the game itself. The really fundamental rules—where do these neutrons come from and how fast do they come?”

Bechtel Chart of the Nuclides

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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.
LUX/Dark matter experiment at SURFLUX/Dark matter experiment 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.

Fermilab LBNE
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.

LBNL LZ project at SURF, Lead, SD, USA

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.”