The Oak Ridge Electron Linear Accelerator (ORELA) at ORNL is the only U.S. facility that can provide much of the neutron data needed by computational astrophysicists modeling the nuclear processes by which isotopes are synthesized in red giant stars and supernovae. Measurements from ORELA are being used to improve computer models designed to enhance our understanding of the life and death of stars, the chemical evolution of our galaxy, and the formation of our solar system.
Almost all stable isotopes of the elements found on the earth and even some radioactive isotopes not naturally present on our planet were formed in stars as they burned fuel or exploded. "Almost all elements heavier than iron were made in stellar environments where neutrons play an important, if not dominant, role," says Paul Koehler, a nuclear physicist in ORNL's Physics Division. "Roughly half the isotopes of elements heavier than iron were synthesized in red giant stars. Through a complicated mixing process, newly synthesized elements were carried from inner regions of the star out to its atmosphere, where they are visible to astronomical observations. In fact, the observation of the radioactive element technetium in a red giant star's atmosphere provided the first direct proof that nucleosynthesis occurs in stars."
Intimately linked to the mixing process in a red giant (which our sun will become in a few billion years) is the synthesis of elements heavier than iron through a chain of nuclear reactions known as the slow neutron capture, or s, process. In the s process, free neutrons are produced as a by-product of nuclear reactions that generate the energy that powers the star. Heavier nuclei are synthesized when lighter nuclei capture one of these free neutrons. However, a typical stable nucleus can capture only a few neutrons before it becomes unstable. Through a process known as beta decay (whereby one of the neutrons in the nucleus disintegrates into a proton, an electron, and an antineutrino), the nucleus is transformed into the next heavier element. Starting from iron "seed"” nuclei, this chain of neutron captures and beta decays continues all the way to elements as heavy as lead.
In the red giant stardust model, microscopic grains of refractory materials such as silicon carbide (SiC) form in the cooler outer regions of red giant stars, trapping within them trace amounts of the heavy elements made in the s process. Strong stellar winds from red giants disperse the products of the s process throughout the galaxy. Some of these grains have reached the earth aboard meteorites. The relative abundance of isotopes of many of the trace elements trapped within these stardust grains can be measured with exquisite precision by analyzing the content of stardust recovered from meteorites. These detailed, isotopic signatures provide a rich set of observational data with which to test astrophysical models of stars and of the chemical evolution of our galaxy.
The first, or "classical," model of the s process was proposed in 1957. In this simplified model, it is assumed that the temperature, neutron density, and matter density are constant during the helium-burning pulses in which the s process was thought to occur. In contrast, the s-process environment in real stars is thought to be very dynamic, with large changes in all these properties over relatively short periods of time. Nevertheless, the classical model has been very successful in reproducing the observed abundances of the s-process isotopes in our solar system. However, as the neutron capture data have become more precise, cracks in the classical model have begun to show. In the 1990s, precise new neutron-capture-reaction-rate measurements from ORELA and elsewhere showed that the classical model predicted incorrect abundances of several key s-process isotopes of barium, neodymium, and tin.
Precise, new neutron-capture data have allowed astrophysicists to move beyond the simplistic classical model and test more realistic models of the s process that are closely tied to models of actual stars. The first precise test of the new stellar models of the s process, including the red giant stardust model, was made possible recently by work at ORELA. There, ORNL physicists made the first precise measurements of the neutron-capture cross sections for two isotopes of neodymium, 142Nd and 144Nd. A cross section is a measure of the probability that a nuclear reaction occurs, such as the capture of a neutron by a nucleus.
Stellar s-process model calculations made using previously accepted cross sections for these isotopes were in serious disagreement with stardust data. The problem was that these calculations relied on neutron cross-section measurements made over too limited a range of energies, so they had to be extrapolated down to the lower temperatures predicted by the new stellar models. The new ORELA measurements, which were made with an improved apparatus and over the entire energy range needed by the new stellar models, showed that the old data were in error. With the new ORELA data, the agreement between the stellar model predictions and the measurements of neodymium isotopic abundances in stardust are excellent.
Five more measurements of this type have been made at ORELA through a collaboration led by Koehler, consisting of scientists from ORNL's Physics and Computational Physics and Engineering divisions and scientists from Denison University and Lawrence Livermore National Laboratory. In four out of five cases studied so far, Koehler says the precise ORELA data have demonstrated that extrapolations from previous measurements are in error by 2 to 3 times the estimated uncertainties.
"We are measuring neutron cross sections of isotopes thought to be produced solely by the s process (s-only isotopes) because they are the most important calibration points for testing the stellar models," says Koehler. "If an element's probability of capturing a neutron is very small, it is unlikely that it will be transmuted to another element, so its abundance will be high. If its cross section is very large, it more likely will be destroyed, so its abundance will be low. There are about 30 s-only isotopes. Cross sections at low energies have been measured for only 4 of these isotopes, using ORELA. Precise neutron-capture rates for the 26 other s-only isotopes could also be measured at this neutron source.
"This information also helps us better understand how the s process synthesizes elements, which of the heavier elements were formed first, and how the abundance of elements evolved over the lifetime of the galaxy," Koehler adds. "It allows us to predict how well materials in stars are mixed during the convection process, when heat from the star drives fluid flow. Convection and mixing are thought to play an important role in many astrophysical environments, such as in supernova explosions. But, because of their relative simplicity and because most of the important nuclear physics information can be measured in the lab on the earth, red giant stars offer perhaps the best hope for understanding convection and mixing in astrophysical environments. Precise new neutron-capture cross sections from ORELA and other facilities, as well as new stardust and other astronomical data, are helping theorists unravel the mysteries of complicated mixing processes."
Koehler and his colleagues recently received ORNL seed money to study the formation of proton-rich isotopes of elements heavier than iron during the p process. Some scientists believe that proton-rich isotopes, such as molybdenum-92 and ruthenium-96, are formed when high-energy photons produced late in the lives of massive stars or in supernova explosions knock out neutrons from nuclei. High-energy photons can knock out not only neutrons but also alpha particles, in gamma alpha reactions.
"Cross sections for gamma alpha reactions are next to impossible to measure for these heavier nuclei, but the nuclear statistical model should be able to predict them reliably enough for p-process calculations," Koehler says. "However, current models do a poor job of reproducing the few measurements that have been made. It turns out that by measuring the rate of neutron alpha reactions at ORELA, in which each nucleus captures a neutron and then ejects an alpha particle, we will be able to supply the best constraints for nuclear statistical models."
The neutron alpha measurements are still very difficult because of a background known as the gamma flash. At ORELA neutrons are produced in secondary reactions that occur when an electron beam strikes a tantalum target. The electrons first produce a very intense beam of photons that can blind detectors.
According to Koehler, "With the help of seed money and scientists from Russia and Poland, we were able to scale up a detector that had been pioneered at ORELAcalled a compensated ionization chamberthat makes it possible to count the alpha particles in the presence of the very intense gamma-flash background. Surprisingly, we found that our first proof-of-principle neutron-alpha data, on samarium-147 and neodymium-143, are in reasonably good agreement with the older model from Caltech, but that the rates predicted by two very recent models are roughly a factor of three different from our data, but in opposite directions."
The high neutron flux at DOE's Spallation Neutron Source (SNS), which will be operating in 2006 at ORNL, will allow astrophysics measurements to be made using samples 10,000 times smaller than those currently studied at ORELA. The SNS should be especially useful for measurements of neutron-capture rates of radioactive isotopes, because only tiny amounts will be needed. The SNS could also be used to study very rare stable isotopes that are so expensive that only very small isotopically separated samples are affordable for research.
"The excellent time-of-flight resolution at ORELA makes it the only facility in the United States capable of measuring small, resonance-dominated neutron cross sections," Koehler says. "ORELA would also be an excellent facility for developing detectors for experiments at the SNS. ORELA and spallation sources such as the SNS are complementary facilities, both of which are essential to cover the wide range of measurements needed for nuclear astrophysics."
Related Web sites