Argonne Tandem Linac Accelerator System (ATLAS)
Description
The ATLAS1 facility, at Argonne National Laboratory (ANL), is a premier superconducting linear accelerator for studies of nuclear structure and nuclear astrophysics in the vicinity of the Coulomb barrier – the energy barrier between two nuclei due to electrostatic repulsion of their positive charge that must be overcome for them to undergo a nuclear reaction. Heavy-ion beams ranging over all possible elements, from hydrogen to uranium, can be accelerated to energies as high as 17 MeV per nucleon and delivered to one of three target areas. Beam species available at ATLAS are produced using two techniques. Beams of proton and neutron rich nuclei with masses up to atomic weight (A) ~ 60 are produced by the in-flight method, reaccelerating fragments are selected by their charge/mass ratio following the interaction of primary beam incident on a production target. Rare isotopes with A ~ 80-160 are provided by the recently commissioned Californium Rare Isotope Breeder Upgrade (CARIBU). In this technique, fission fragments from a ~ 1 Curie Californium-252 source are stopped in a gas catcher for subsequent selection and reacceleration. The start of CARIBU operation is especially noteworthy as it provides access, for the first time, to neutron rich rare isotopes necessary to advance understanding of the location and path of the rapid neutron capture (or “r”) process.
Science
The scientific focus at ATLAS is to provide experimental observations of important properties for key nuclei to underpin the development and testing of a comprehensive theory of nuclei and their interactions that has predictive power and quantified uncertainties. These efforts will answer some of the deepest questions about the evolution of the cosmos and the structure of matter. A specific goal is to significantly advance our understanding of r-process nucleosynthesis. Most of the light elements are produced in the cores of stars by fusion reactions. One possible scenario for creation of heavier elements such as silver, gold, and uranium is that they are produced in the rapid neutron capture r-process when massive stars explode as supernovae. Creation of these elements takes place within a few seconds through reactions involving thousands of different short-lived isotopes. An understanding of the elemental abundances resulting from supernovae requires the generation of models, which in turn requires a large number of physics inputs, such as the masses of some of the isotopes. CARIBU provides isotopes that lie near the r-process nuclei by capturing fragments from the fission of Californium for research and reacceleration. Since the commissioning of CARIBU, the (heretofore unknown) masses of 33 neutron-rich nuclei provided by CARIBU have been precisely measured at the level of 10-100 parts per billion. First-ever simulations that incorporate information from the new mass measurements at CARIBU indicate there is a significant increase in “waiting time” for reactions for the elements Tin and Antimony compared to calculations with commonly used mass models. These results, combined with those from reaction studies with stable beams, will elucidate presently unknown details of the nuclear processes which fuel stars, determine stellar evolution, and are responsible for the origin of the heaviest elements in nature.
