Researchers challenge conventional wisdom concerning how heavy elements are formed in exploding stars
Tin prevents steel from corroding in cans of food and provides the ringing sound in church bells. As recently discovered at Oak Ridge National Laboratory, tin also plays a pivotal role in the creation of hundreds of heavy elements in catastrophic stellar explosions. The finding debunks a long-held belief that the formation of any one of these elements has little effect on the creation of others.
To understand how heavy elements evolved billions of years ago into the ancient stardust that created and sustains life on Earth, ORNL theoretical astrophysicist W. Raphael Hix and his collaborators build sophisticated computer models that track how heavy subatomic nuclei, such as tin, antimony and lead, become heavier by capturing neutrons. The models need, as input, the probabilities that thousands of different types of heavy nuclei will capture free neutrons under the conditions in which stars become supernovae. The conditions are indeed extreme—temperatures of a few billion degrees, with relatively few heavy nuclei in a bath of a billion trillion free neutrons per cubic centimeter.
In this hot stellar gas, photons—energetic particles of invisible and visible light—not only carry the temperature of the gas but also "boil off" neutrons (photodisintegration) that have just been captured by heavy nuclei. When the neutron capture rates and the boiling-off rates balance out, the system is in equilibrium. Calculations of the number of nuclei of any one isotope at this moment are relatively straightforward, relying on properties such as nuclear masses rather than the rate at which fast heavy nuclei capture free neutrons.
As the system cools, however, this balance is broken. Scientists previously thought that some neutron capture rates might have an effect—but only to influence a small number of nuclei. As hundreds of heavy nuclei are formed in the explosion, too many nuclei and not enough time (less than seconds) are available for a global impact to occur. This is one reason that theoretical estimates were thought sufficient to use the neutron capture rates as input to the models.
Necessity was another reason that estimates were used. The nuclei responsible for heavy element formation are radioactive isotopes that do not occur naturally on Earth. Therefore, measuring their neutron capture rates before they spontaneously change into other nuclei is a daunting task.
"ORNL has the only facility in the world that can produce and accelerate a beam of radioactive tin nuclei to a high enough energy that enables an indirect experimental determination of the rate of the neutron capture reaction," says Michael S. Smith, experimental physicist at ORNL's Holifield Radioactive Ion Beam Facility. "We have conducted such experiments on five different nuclei, with a goal of providing an empirical foundation for the simulations.
"To improve the explosion models, we hope to determine experimentally neutron capture rates of select isotopes with our accelerator," he adds. "Raph and his collaborators will put our new rates in the model, run a new simulation of an exploding star and get new predictions of the rapid creation of heavy isotopes." The researchers did not, however, expect any single new rate to change the predictions dramatically.
One of the five ORNL experiments was led by Ray Kozub of Tennessee Technological University. Kozub accelerated tin-130 nuclei and aimed the beam at a polyethylene target containing deuterium—heavy hydrogen with one proton and one neutron. As the beam interacts with the target, many neutrons leave the deuterium nuclei in the target and attach to passerby tin-130 nuclei in a so-called "transfer reaction," while the protons from the broken deuterium nuclei are ejected from the target. Sophisticated detector systems measure the products of the reactions—protons and heavy tin-131 nuclei.
"This indirect approach is one of the few ways by which we can determine how many times per second a neutron attaches itself to a tin nucleus in an exploding star," Smith says. "This capability is one of the major motivations for building facilities that produce radioactive beams."
The view that measuring certain individual neutron capture rates may actually be important evolved during simulations performed by Josh Beun, then a Ph.D. graduate student at North Carolina State University. In collaboration with his adviser Gail McLaughlin, Hix and Rebecca Surman of Union College in upstate New York, Beun was simulating the rapid neutron capture process, or the r-process, which produces about half of the elements present in stars and on Earth. These nuclei captured neutrons in milliseconds to seconds. "Astrophysicists are not sure where the r-process happens," Hix says, "but our latest finding should help point us in the right direction."
When Beun changed the neutron capture rates for four of the nuclei studied in ORNL experiments, he found the expected result. "These increased capture rates would make a little difference but only locally—that is, only for nuclei with similar masses," says Hix. However, he adds, "For tin-130 we saw a large effect that really puzzled us. We needed more than two years to figure out how tin-130 can make a huge impact on the abundances of other isotopes.
"What we found is that a particular isotope's neutron capture rate can matter on a global scale, affecting the rate of production of 100 heavier isotopes. As the gas cooled, tin-130 nuclei more likely held onto captured neutrons, making them unavailable for other captures."
Tin-130 is not the only nucleus that has this special role. "We found by happenstance that a fraction of these neutron capture reactions actually matter early enough to make a global change," says Hix.
The team found that tin-130 has three important characteristics that make it special: First, tin-130 is abundant at the point when the r-process is fervently operating because of the presence of at least 100 free neutrons for every heavy nucleus. Second, tin-130 has a slow radioactive decay rate, enabling the isotope to hold its neutrons for a longer time.
A third characteristic is that the mass difference between tin-130 and tin-131 is considerably larger than differences found in other neighboring pairs of nuclei. As the temperature in the explosion drops and fewer photons are present, neutron capture rates matter for a longer time.
"The earlier a nucleus breaks out of the balance between neutron capture reactions and boiling-off reactions, the better the chance of effecting global change," Hix says. "The heavier nuclei have more time to form." The breaking of this balance depends on the mass difference between the nucleus and its nearest neighbors.
The group is currently surveying thousands of nuclei to see which others satisfy these criteria and therefore can have a global impact on element formation in stellar explosions. Experimental nuclear physicists are very excited about this change in the conventional wisdom, primarily because it suggests that measurements of other neutron capture rates must be carried out to predict accurately heavy element formation in supernovae. Perhaps more significant, the finding also provides an additional reason for the construction of a $600 million, next-generation facility in the United States, called the Facility for Rare Isotope Beams, that would produce intense beams of radioactive nuclei.—Carolyn Krause
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