Stardust and Atom Smashers

Kate Jones, associate professor of physics and astronomy, adapts her Pregame Showcase lecture from September to explain the inner workings of stars and how the atomic nucleus leaves its fingerprints on the chemical composition of the solar system.

By Kate Jones, Associate Professor of Physics and Astronomy

Pumbaa: Hey, Timon, ever wonder what those sparkly dots are up there?

Timon: Pumbaa, I don’t wonder; I know.

Pumbaa: Oh. What are they?

Timon: They’re fireflies. Fireflies that, uh…got stuck up on that big bluish-black thing.

Pumbaa: Oh, gee. I always thought they were balls of gas burning billions of miles away.

Timon: Pumbaa, with you, everything’s gas.

—From The Lion King, Walt Disney Pictures (1994)

Since the dawn of time, man, woman—as well as meerkat and warthog—have all looked to the skies and wondered what those twinkling stars really are. Now we know that Pumbaa was right: Stars are balls of gas—well, actually plasmas—burning billions of miles from Earth.

In order to understand the colossal distances that are the realm of astronomy, it is useful to consider an analogy. If you imagine that our sun is a marble, placed on the 50-yard line at Neyland Stadium, the earth would be the size of a grain of sand one yard away. Most of the solar system, out to about Pluto, would be contained within the distances between end zones.

Using this same scale, the nearest star to the sun, Proxima Centauri, would be another marble located in Nashville. That leaves a lot of pretty much empty space in between.

The sun, Proxima Centauri, and all stars burn by fusing atomic nuclei, and they thereby resist imploding under the gravitational forces of their colossal mass. The atomic nucleus, constructed from neutrons and protons, is less than a thousandth of the size of an atom—yet it provides the source of power for each and every star.

One important aspect of the nature of the atomic nucleus is the underlying shell structure. The periodic table groups elements by their chemical properties. These properties are consequences of atomic shell structure: noble gases, with full shells of electrons, are non-reactive, unlike alkali metals, which have an extra electron to give up in chemical reactions. Similarly, nuclei have “magic numbers” that correspond to full shells of either neutrons or protons (see Fig. 1).

Fig. 1: The electronic shells of the atom result in ordered changes in chemical properties as represented by the Periodic Table. Similarly, nucleonic shells of the atomic nucleus result in sudden changes in properties at these magic numbers. These changes leave an imprint on the abundance of different elements in the universe through their effects on the element synthesis paths. (Adapted from Jones & Nazarewicz, “The Physics Teacher”)

The groupings of the periodic table, which reflect the filling of atomic electron shells, are not useful in nuclear physics. To visualize the different possible combinations of neutrons and protons, relating to all isotopes that exist, it is helpful to reorder the elements by the number of protons and then allow each to have different numbers of neutrons (see video below).

Now we have the chart of the nuclides (Fig. 2), with stable isotopes (black boxes) that make up the vast majority of the matter we and all the things around us are made of, as well as many unstable—or radioactive—nuclei that decay in anything from a tiny fraction of a second to a few million years.

Fig. 2: The black squares show the stable (or very long-lived) isotopes we can find on Earth. The known unstable (or radioactive) isotopes are shown in yellow, and those isotopes that are predicted, but yet to be observed in a laboratory, are shown in green. The nuclear shell closures are shown as blue lines, with the number of neutrons or protons shown in black. The major element synthesis paths are also depicted. (Adapted from Jones & Nazarewicz, The Physics Teacher)

You may wonder, “Where did all these different isotopes come from?” The lightest, most abundant isotopes in the universe—hydrogen and helium—were produced in the big bang, along with trace amounts of other light elements, such as lithium. All the others were produced through nuclear fusion in the core of stars, or in cosmic explosions, such as supernovae.

As a consequence of the stellar nuclear fusion processes that power stars and stop gravitational implosion, heavier elements are created: Hydrogen fuses to helium, helium to carbon, and so on, up to iron. These fusion reactions are exothermic; they release energy causing an outward radiation pressure countering the inward crush of gravity.

However, beyond iron the reactions become endothermic; it costs energy to fuse heavy nuclei. This is because the combined charges from all the positively charged protons in the two nuclei cause repulsion that counteracts the attraction due to the nuclear strong force. Now, the only way heavier elements can be produced is by adding energy, or by capturing uncharged neutrons and then beta decaying. This type of beta decay causes a neutron in the nucleus to change into a proton, resulting in an element of higher atomic number, an upward move on the chart of the nuclei.

These neutron-capture processes are marked on Figure 2 as the s-process (slow neutron capture) and r-process (rapid neutron capture). The r-process occurs in the very neutron-rich part of the chart of the nuclei, passing through many unstable (light-green) isotopes, and is responsible for the creation of approximately half of the elements heavier than iron. The astronomical site of the r-process is unknown, but the prevailing theory is that it occurs in the few seconds taken for a star to explode as a core-collapse supernova. This only happens for the heaviest stars: those that are at least eight times the mass of our own sun.

These cataclysmic explosions are very hot, very dense, and contain many neutrons that can be used in nuclear reactions. This is where the nuclear shell structure comes into play. As more neutrons are captured for a given element, shorter-lived isotopes are produced. It also becomes less favorable to add neutrons, as they become increasingly less bound the higher the number of neutrons in the nucleus. These two effects are exacerbated if a shell closure is crossed. Just as in the atomic case an alkali metal easily gives up its extra electron, a nucleus with one neutron beyond a shell closure will easily give up its extra neutron. The shell closures also affect how much energy a nucleus needs to be excited, and how long it lives. These effects bend the path of the r-process through the chart of the nuclides, resulting in the kinks at the shell closures seen in Figure 2.

It is therefore important to understand how the shell structure of the atomic nucleus changes for exotic, short-lived nuclei. The problem is that exotic nuclei are difficult to measure. Those on the r-process path live for short fractions of a second. They are both difficult to produce in a laboratory and difficult to study in the short period of time before they decay.

To study these fleeting nuclei requires radioactive ion beams; targets of metals, plastics, or gases; and large arrays of detectors. Only the most advanced nuclear physics laboratories in the world can provide the necessary beams. The experiments require the detection of particles, gamma rays (individual photons of light), or sometimes both.

One important example of an investigation of the shell structure close to the r-process path is an experiment led by the UT experimental nuclear physics group with a beam of Tin-132, an isotope of tin with a combined number of neutrons and protons of 132. It should be noted that Tin-132 has eight neutrons beyond the most neutron-rich, stable isotope of tin. Tin-132 has closed shells of both protons (50) and neutrons (82). The states outside this so-called “doubly magic” nucleus were studied by adding a neutron. The states were compared to those outside the well-known doubly magic nucleus Lead-208.

The results, published in the journal Nature, showed that although Tin-132 lives for only forty seconds, it is possibly the best example of a doubly magic nucleus known to date. It also revealed the energy of a previously unknown state in Tin-132, necessary for calculating the neutron capture on heavier isotopes of Tin and neighboring elements that are on the r-process path.

Future studies will simultaneously measure particles and gamma rays, giving increased precision and sensitivity to the experiments. This will be possible in part due to a new, intricate target chamber fabricated in the physics department’s Machine Shop.

The nature of the tiny atomic nucleus, studied in laboratories here on Earth, is shining a light on the inner workings of stars: the way they create elements through fusion in their cores and their explosive deaths.

About the Author
Kate Jones is an associate professor of physics and astronomy. Concerned about the gap between a 1988 text and current research in the field, Jones revised her introductory nuclear physics course so her students could supplement the text by investigating and sharing current ideas and research in the field of nuclear physics. Jones has published more than seventy refereed articles and holds a doctorate in experimental nuclear physics from the University of Surrey in the United Kingdom.