CASPAR: Studying the Stars from a Mile Underground

28th September 2018

We all have our origin in the stars. The atoms that make up the Earth, the mountains, the oceans, and our human bodies were forged in the hearts of stars. In a very real way, we are connected to the universe through the lineage of our elements. The investigation of that lineage is nuclear astrophysics, the field of study for the Compact Accelerator System for Performing Astrophysical Research (CASPAR) project.

A very abbreviated version of the history of the universe is as follows. After the Big Bang, the universe contained a lot of hydrogen, a little helium, and almost nothing else. Over vast amounts of time, that hydrogen and helium collapsed into the first stars. Inside these stars, hydrogen was converted into more helium by thermonuclear fusion. When the hydrogen in the cores of the most massive of these first stars was used up, fusion of helium began, which created primarily oxygen and carbon. After the helium in the core was exhausted, the carbon began to fuse, creating even heavier elements. Then neon, oxygen, and silicon in turn began to fuse into heavier elements, creating most of the elements up to iron on a period table. At this point, there was no more energy to be gained from further fusion of the elements in the core and these stars collapsed, rebounded, and became supernovae, violently spreading their new elements (as well as unburned hydrogen and helium that never made it to the core of the star) across the cosmos. These ashes, the remnants of a now dead star, mixed with the remnants of other stars in space and then coalesced into new stars, gas giants, and, because of the presence of elements beyond hydrogen and helium, rocky planets. This stellar drama plays out over and over again throughout the universe still today.

Crab Nebula

The Crab Nebula, the result of a supernova explosion.  Image Credit: NASA, ESA, J. Hester, A. Loll (ASU)

But that’s not the full story. The thermonuclear fusion process can only generate elements up to iron, which is the 26th element on a periodic table. There are between 91 and 98 natural elements (depending on who you ask and how you define “natural”). So, how does the universe generate the other 70 or so elements?  About half of those remaining elements can be traced back to the s-process (slow neutron capture process). The s-process takes place in the interior of stars—just as thermonuclear fusion does, but it is an entirely different and more subtle process. We, as nuclear astrophysicists, are trying to understand some of those subtleties using CASPAR.

So, how do we do that? How do we study the stars in a laboratory a mile underground? We can’t just create a star underground to study up close, can we? Well, yes and no. Okay, mostly no. But we can recreate the conditions inside the heart of a star in a laboratory using a particle accelerator. By bombarding various targets with alpha particles (helium nuclei) or protons (hydrogen nuclei) at specific energies, we can recreate those conditions and observe up close how the nuclear reactions occur just as they would inside a star such as our own Sun. (So perhaps you will allow me the shorthand to say we create a star in the lab, because that makes me feel big and important.)

So why do we go underground to do this? The energies to which we need to accelerate the particles to recreate the specific conditions inside of a star are incredibly low, sometimes below 200 keV. (You may think of the conditions inside of a star as fantastically energetic, but the energies of the individual particles are quite low, especially when compared to the TeV energies achieved in the LHC at CERN.) The lower the energy of the particle beam, the lower the energy of the resultant particles we hope to detect. And the background radiation noise on the surface of the Earth is just too much to be able to pick out our signal. On the surface we are constantly inundated with cosmic rays from the Sun and deep space, radiation from cell and radio towers, and natural radiation from the environment around us. Most of the radiation is at a level that has little or no effect on the human body, but it’s high enough to drown out the very quiet signals we can see with the detectors in CASPAR.

By putting a mile of solid rock over our heads, we can block out the noisy radiation. That allows us to find the silence we need to observe the reactions taking place in our little artificial star.

Using CASPAR, we will be able to shed light on the subtle details of the s-process and better understand how the elements in the universe got here and how the universe will evolve in the far future. CASPAR research will extend beyond just understanding the s-process and has already been used to deepen our knowledge of the CNO-cycle, an important set of fusion reactions that take place in stars slightly more massive than our own Sun. All in all, this small accelerator underground is a window into the workings of the universe.

And that, in a nutshell, is the wonderful paradox that is CASPAR: we go underground to study the cosmos and we make a star to better understand how the stars made us.

Sanford Underground Research Facility