Towards Neutron Capture on Exotic Nuclei: Demonstrating (d,pγ) as a Surrogate Reaction for (n,γ)

The neutron-capture reaction plays a critical role in the synthesis of the elements in stars and is important for societal applications including nuclear power generation and stockpile-stewardship science. However, it is difficult-if not impossible-to directly measure neutron capture cross sections for the exotic, short-lived nuclei that participate in these processes. In this Letter we demonstrate a new technique which can be used to indirectly determine neutron-capture cross sections for exotic systems. This technique makes use of the (d,p) transfer reaction, which has long been used as a tool to study the structure of nuclei. Recent advances in reaction theory, together with data collected using this reaction, enable the determination of neutron-capture cross sections for short-lived nuclei. A benchmark study of the ^{95}Mo(d,p) reaction is presented, which illustrates the approach and provides guidance for future applications of the method with short-lived isotopes produced at rare isotope accelerators.

Constraining Neutron Capture Cross Sections for Unstable Nuclei with Surrogate Reaction Data and Theory

Obtaining reliable data for nuclear reactions on unstable isotopes remains an extremely important task and a formidable challenge. Neutron capture cross sections-crucial ingredients for models of astrophysical processes, national security applications, and simulations of nuclear energy generation-are particularly elusive, as both projectile and target in the reaction are unstable. We demonstrate a new method for determining cross sections for neutron capture on unstable isotopes, using ^{87}Y(n,γ) as a prototype. To validate the method, a benchmark experiment is carried out to obtain the known ^{90}Zr(n,γ) cross section analogously. Our approach, which employs an indirect ("surrogate") measurement combined with theory, can be generalized to a larger class of nuclear reactions. It can be used both with traditional stable-beam experiments and in inverse kinematics at rare-isotope facilities.

One-nucleon pickup reactions and compound-nuclear decays

One-nucleon transfer reactions, long used as a tool to study the structure of nuclei, are potentially valuable for determining reaction cross sections indirectly. This is significant, as many reactions of interest to astrophysics and other applications involve short-lived isotopes and cannot be measured directly. We describe a procedure for obtaining constraints for calculations of neutron capture cross sections using observables from experiments with transfer reactions. As a first step toward demonstrating the method, we outline the theory developments used to properly describe the production of the compound nucleus 88Y* via the one-nucleon pickup reaction 89Y(p,d)88Y* and test the description with data from a recent experiment. We indicate how this development can be used to extract the unknown 87Y(n,γ) cross section from 89Y(p,dγ) data. The example illustrates a more generally applicable method for determining unknown cross sections via a combination of theory and transfer (or inelastic scattering) experiments.

Simple empirical order parameter for a first-order quantum phase transition in atomic nuclei

A simple, empirical, easy-to-measure effective order parameter of a first-order phase transition in atomic nuclei is presented, namely, the ratio of the energies of the first excited 6+ and 0+ states, distinguishing between first- and second-order transitions, and taking on a special value in the critical region, as data in Nd-Dy show. In the large NB limit of the interacting boson approximation model, a repeating degeneracy between alternate yrast and successive 0+ states is found in the critical region around the line of a first-order phase transition, pointing to a possible underlying symmetry.