Nuclear Astrophysics at ORELA

One of the primary goals of Nuclear Astrophysics is to obtained a detailed understanding of the synthesis of all the elements which make up our world. Because stellar environments are the only sites with sufficient heat and density to sustain the formation of heavy elements out of lighter elements (outside of the Big Bang), studies of nucleosynthesis involve aspects of both nuclear physics and astrophysics. The synthesis of all heavy elements with A > 60 has been attributed primarily to a combination of two very different reaction processes, the slow (s-) and the rapid (r-) neutron-capture processes. Advances in our understanding of these processes, and of the astrophysical sites where they occur -- red giants and (most likely) supernovae, respectively -- have gone hand-in-hand with advances in laboratory measurements of neutron-capture processes. The overwhelming majority of (n,gamma) cross section measurements, which comprise the crucial input for astrophysical models of heavy element nucleosynthesis, have been made at one facility: the Oak Ridge Electron Linear Accelerator (ORELA) at ORNL. Its long history in the field of (n,gamma) measurements, along with recent technical improvements, make ORELA uniquely suited to enable further advances in our understanding of heavy element nucleosynthesis. Furthermore, measurements at ORELA will be integrated with other ORNL programs in Nuclear Astrophysics, such as the new ORNL initiative in theoretical modeling of supernovae and other astrophysical environments, the explosive hydrogen burning studies with proton-rich radioactive beams at the Holifield Radioactive Ion Beam Facility (HRIBF), and future work at HRIBF with neutron-rich radioactive beams produced by a fission source.

The s- and r-processes are believed to occur under very different astrophysical conditions, and involve very different sequences of neutron captures and subsequent beta decays. The abundances of most heavy elements, however, are the products of both s- and r-process nucleosynthesis (and a smaller contribution from the p-process, involving photodissociation reactions). The goal in modeling these processes is to understand not only the detailed way in which heavy elements are synthesized, but also the conditions of the astrophysical sites where these syntheses occur. For example, previous s-process studies have suggested that a likely site is the He-burning shell of pulsating red giant stars, where the temperature is approximately kT = 30 keV and the neutron density is approximately 10^8 cm^-3. It involves relatively slow neutron captures (capture lifetime > minutes - years), and the reaction sequence consists of successive neutron captures followed by a beta decay, therefore closely following the line of beta stability. This makes the majority of s-process nuclei accessible experimentally by using stable targets. Modeling s-process nucleosynthesis requires hundreds of neutron-capture cross section measurements as input parameters. In the so-called "classical" s-process, the neutron density, temperature, and exposure times are varied to reproduce measured heavy element abundances. Classical s-process studies utilized cross section values averaged over a Maxwellian thermal velocity distribution centered at 30 keV; these studies often assumed that the s-process occurs at a constant stellar temperature. Some work utilizing multiple exposures of different temperatures, neutron densities, and durations did a reasonably good job of reproduce the observed s-process abundances.

Recent work has focussed on more realistic stellar models - for example, low mass asymptotic giant branch (AGB) stars - than the simple phenomenological classical model. A few years ago, reaction network calculations based on a stellar model came close to reproducing the measured s-process abundances for the first time. The new stellar models suggest that a substantial amount of s-process nucleosynthesis occurs at kT = 8 - 10 keV, significantly lower than the 30 keV value used in classical model studies. This is an important motivation for new neutron cross section measurements: many old measurements need to be extended to lower energies (below an old 2 keV cutoff) to enable the reaction rate ( the thermal average of the cross section over the Maxwell-Boltzmann temperature distribution) to be determined to the new lower temperatures. ORELA is uniquely suited to provide these new measurements: recent improvements at ORELA now enable such measurements to be made from 20 eV to 500 keV in a single run, covering all of the relevant range. It is very important to measure such cross sections on isotopes produced only in the s-process (the "s-only isotopes"), especially for those elements with two or more such isotopes (Kr, Sr, Te, Xe, Ba, Sm, Gd, and Os), because they are used to precisely normalize the s-process contributions to all elements, and because the ratios of these isotopic abundances can be compared to extremely precise measurements from meteorites. In fact, the high precision of new abundance measurements of s-only isotopes, primarily in meteorites, is a strong motivation for new, precise (to a few percent) measurements of neutron-capture cross sections. Additionally, detailed analyses of s-process branch points -- where both neutron capture and beta decay occur provide the most sensitive probe of the astrophysical environments (temperatures and neutron densities) where the s-process occurs. These s-process branchings include the groups of isotopes 85Kr, 134-135Cs, 147Nd - 147-148Pm, 151Sm - 152Eu - 153Gd, 163Dy - 163Ho - 164Ho, 169Er - 170Tm, 176Lu, and 185W - 186Re.

s-process studies are also invaluable to our understanding of the r-process. Determining the (still unknown) astrophysical site of the r-process has been one of the long-standing problems in heavy element nucleosynthesis. The r-process consists of fast neutron captures (capture lifetime < 10^-3 s ) followed by beta decay, and the reaction sequence closely follows the neutron drip line (where beta decay lifetimes are very short.) Our understanding of the r-process is hampered by the experimental inaccessibility of the thousands of unstable nuclei near the neutron drip line. Recent theoretical work in supernova modeling, as well as in "phenomenological" nuclear physics studies, suggests that a strong candidate site is in the low-density, high-entropy bubble located just inside of an expanding core-collapse supernova shock wave. This environment has the appropriate heavy-element seeds, as well as the required high temperatures (over kT = 100 keV, or 1 billion degrees kelvin) and neutron densities (approximately 10^20 cm^-3). Although much work remains to be done, this promising new site is a very exciting development in heavy element nucleosynthesis.

Modeling r-process nucleosynthesis requires neutron binding energies and beta decay lifetimes for thousands of nuclei close to the drip line. This necessarily requires a dependence on nuclear models, which are pushed to their limits when predicting nuclear properties near the drip line. In fact, recent work looking at systematics of r-process abundances has been used by some researchers to correct inconsistencies in nuclear models; this use of astrophysical r-process abundances to check nuclear microscopic models may be expanded in the future. However, such work is possible only with very accurate s-process abundances: the intimate link between studies of the s- and the r-process is that the r-process abundances, to which calculated abundances are compared, are determined by subtracting calculated s-process abundances from the observed abundances; these r-process abundances are sometimes referred to as residual abundances. Precise r-process studies therefore require precise s-process studies, and the precision of these studies rely on accurate (n,gamma) cross section measurements primarily made at ORELA. Future work at ORELA will include measurements and calculations to better describe the synthesis of isotopes through the r-process.

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Rev: Monday, 18-Dec-2000