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 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.
Starting 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 rapid capture of individual neutrons as in the collision of two neutron 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 physicists have 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 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,” Hanhardt said.
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, if it is unstable it could 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,” Hanhardt 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 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.
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 such 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?”