The origin of the elements heavier than Iron - the r-process

The rapid neutron-capture process (r-process) is believed to account for roughly half the abundance of elements above Z ~ 30. The r-process occurs in high-entropy environments (core-collapse supernovae and neutron-star mergers are leading candidates for the sites) in which extremely high neutron fluxes result in extremely rapid, successive neutron captures, driving the populated isotopic distribution toward very large neutron numbers. These isotopes subsequently β decay back toward stability, contributing to the observable abundance pattern of stable (and very long-lived) nuclei.

Recent precision measurements of elemental distributions of the envelopes of individual low-metallicity halo stars are placing unprecedented constraints on r-process abundance patterns [Sne08]. However, understanding r-process abundances requires a variety of physics input, including neutron densities and temperatures. Additionally, considerable sensitivity has been demonstrated to the properties of neutron-rich nuclei, such as nuclear masses, β-decay lifetimes, neutron-capture rates and fission properties. Presently, due to the unavailablity of experimental neutron-capture cross sections on these very short-lived nuclei, constraints come from a variety of nuclear structure models, which need to be calibrated in the neutron-rich region by a systematic program of spectroscopic measurements of states with single-neutron character. Furthermore, network calculations of late-stage r-process nucleosynthesis indicate that the final abundance pattern is significantly sensitive to neutron-capture cross sections on a particular subset of nuclei around shell closures, such as 81Ni, 76Cu, 78Zn, 80Ga, 86,88As, 131,133,135Cd, 133,135,137Sn, 137Te [Sur09, Sur14], making them prime targets for study. Spectroscopic measurements on such individual nuclei with particular r-process sensitivity are required to more directly constrain calculations of their neutron-capture cross sections.

Single-neutron transfer reactions that selectively populate states of importance for neutron capture can be used to constrain neutron-capture cross sections, and are critical in validating nuclear structure models. Beam intensities at FRIB will allow such reactions to be measured in inverse kinematics with radioactive beams of r-process nuclei to yield excitation energies and spin-parity assignments, along with spectroscopic factors required for calculations of neutron-capture cross sections. However, the nature of such measurements with finite target thicknesses dictate that, in nuclei just a few nucleons away from shell closures, many levels are not resolvable via charged particle detection alone [Pai08]. The γ rays emitted in such reactions can vastly improve sensitivity and information yielded by the measurement, and are critical to surrogate measurements for statistical neutron capture [Hat10], but experiments in inverse kinematics require a precise Doppler correction of the measured γ-ray energies. For beams of ~12 MeV/u, it is necessary to know the interaction point of the γ-ray to 1-2 degrees in order to maintain close to intrinsic Ge-detector resolution. The capabilities of GRETA, when coupled to a high-resolution charged-particle array such as ORRUBA, will be key to enabling such measurements to be performed on r-process nuclei at FRIB.

The 137Te(d,p+γ)138Te reaction is a representative measurement required to constrain the 137Te(n,γ)138Te reaction rate. To date, none of the states likely to contribute significantly to the neutron capture rate have been observed. The projected FRIB beam intensities for 137Te is 3.5x105 ions per second, allowing a high-precision, thin-target measurement at ~12 MeV/u to be performed.

 Benchmarking electron-capture rates - Towards understanding supernovae and processes in neutron stars

Supernovae are critical to our understanding of the Universe. They are the major sources of nucleosynthesis and their shockwaves are considered major drivers of galactic chemical evolution. These energetic and bright explosions are characterized by some of the most extreme conditions encountered anywhere in the Universe and leave behind black holes and neutron stars. For both main types of supernovae, core-collapse (Type II) and thermonuclear (Type Ia), the driving mechanisms are not yet fully understood and nuclear physics input, such as weak-interaction rates, play a crucial role [Lan03]. Neutron stars are among the elusive remnants resulting from gravitational collapse during supernovae. These objects of unrivaled high density are thought to have a very complex, layered structure, with electron-capture (EC) rates being important for the heating of the neutron-star crust [Gup07] and cooling processes [Sch13].

The estimation of EC rates requires detailed knowledge of Gamow-Teller strength transitions in the β+ direction. EC on a large number of nuclei (stable and unstable), primarily with 40 ≤ A ≤ 120, play a role. Moreover, due to the high temperatures in stellar environments, transitions from ground states and excited states are significant. It is impossible to measure even a sizable fraction of all relevant strengths. Therefore, it is important to perform targeted experiments to validate and improve theoretical calculations. Transitions to low-lying excited states are especially critical for electron captures at low stellar temperatures and densities in pre-supernovae stars [Heg01] and for the neutron star crustal processes, and their exact location must be known with high precision. A variety of charge-exchange probes have been used for extracting Gamow-Teller strength distributions from stable nuclei, but the development of high-precision charge-exchange probes for experiments with unstable nuclei proved a challenge. The use of high-resolution γ-ray spectroscopy has emerged as a powerful tool to address this problem, for example, in the use of the (7Li,7Be+γ) reaction in inverse kinematics (see section on “The spin-isospin response of nuclei”). More recently, GRETINA was used in a 46Ti(t,3He+γ) experiment to extract the Gamow-Teller strength to a very weak low-lying transition which dominates the electron-capture rate under most stellar conditions, but which would have been unobservable without the use of GRETINA [Noj14]. Although the experiment was performed in forward kinematics (but with a secondary triton beam), the γ-coincidence technique will be very useful in future charge-exchange experiments performed in inverse kinematics with rare-isotope beams. Achieving high resolution in such experiments; e.g. (p,n) or (7Li,7Be) in inverse kinematics, relies on high-precision Doppler reconstruction since the relevant γ rays are emitted in flight. By combining high-efficiency detection with high-precision γ-ray tracking, GRETA will be a tremendous asset for such experiments. In addition, by placing GRETA at the target station of the planned High-Rigidity Spectrometer (HRS) at FRIB, Gamow-Teller transitions for nuclei with large neutron-to-proton asymmetries can be reached.


[Pai08]  S. D. Pain et al., Proceedings of Nuclei in the Cosmos X, Proceedings of Science 142 (2008).
[Sne08]  C. Sneden, J. J. Cowan and R. Gallino, Ann. Rev. Astro. Astrophys. 46, 241 (2008).
[Sur14]  R. Surman et al., AIP Advances 4, 041009 (2014).