$r$-Process Nuclear Astrophysics

The study of stellar nucleosynthesis has been of extreme interest to the scientific community for more than 60 years, and as a result, the generation of light nuclei (up to Fe) is well understood. However, the production mechanism of heavy elements still remains a mystery, since the generation of elements heavier than iron is not possible through fusion in the stellar interior [1]. Therefore, one of the most fundamental scientific questions that remains to be answered is where do these heavy elements originate, and how are they created? Nearly half of all the elements heavier than iron are thought to be generated in a very short time through a rapid neutron-capture process ($r$- process). The physical location where this process may occur in nature is not conclusively known, however astrophysical models suggest that it may occur during one or more of the following scenarios: i) core-collapse supernovae [2], ii) two-neutron star mergers [3], iii) black-hole neutron star mergers [4], or iv) jets from supernovae [5].
Inside of these environments, the $r$-process proceeds rapidly through a series of neutron captures, $\beta$ decays, and photodissociations [6]. Since the exact astrophysical scenarios are still under debate, the precise location of the $r$-process path through the nuclear landscape is uncertain, and depends strongly on the model parameters which are used. The classical $r$-process (with an $(n,\gamma)/(\gamma,n)$ equilibrium) results from solving the Saha equation for $T=1.25$ GK, a neutron density of $n_n=10^{22}~$cm$^{-3}$, and isotopes with a neutron separation energy of $S_n=3~$MeV. As a result, these isotopes are very neutron-rich and at the limit of current experimental access using state-of-the-art rare-isotope beams (RIBs) for $A<150$. The current generation of RIB facilities around the world are rapidly expanding the experimental possibilities using neutron-rich beams in this region at moderate-to-high intensity for nuclear-physics experiments. Very recent developments at TRIUMF have allowed for the production of such beams for use in the low-energy experimental hall at ISAC, using proton-induced fission reactions on UCx and UO$_2$ targets. Further advancements, including a neutron-induced fission target (using a proton converter), are also planned for the near future, which will expand the scope of the beam delivery possibilities, and allow for much higher intensity beams in this mass region. Additionally, the Advanced Rare IsotopE Laboratory (ARIEL) will greatly expand TRIUMF's capabilities to produce neutron-rich beams in the predicted $r$-process path, using electron-beam induced photo-fission. Using these new exotic beams, programs of complementary experiments will be possible using the large suite of facilities that currently exist in the ISAC halls. Due to the importance of the $r$-process nuclei for astrophysically relevant studies, several different types of experiments are required to obtain a complete picture of these systems. Some of the most crucial quantities required for the study of nucleosynthesis via the $r$-process are experimental masses to deduce neutron separation energies ($S_n$) and determine the path through the nuclear landscape. The current astrophysical models used for predicting relative elemental abundances are sensitive to $S_n$ values, and thus require precise, accurate experimental data. The neutron separation energy is the binding energy difference between the nucleus of interest, $(Z,A)$, and the neighbouring $(Z,A-1)$ isotope, which can be experimentally accessed with high accuracy and precision by performing mass measurements using Penning traps. Recent work [7], in fact, has demonstrated the power of using Penning-trap mass spectrometry (PTMS) to perform measurement campaigns on a wide range of neutron-rich nuclei using beams from the current/next-generation facilities. The work of our group at the Colorado School of Mines involves a strong collaboration with both the TITAN Penning-trap mass measurement program, and the GRIFFIN decay spectroscopy facility at TRIUMF, where these measurements are performed.
Since mass measurements have not been performed for the vast majority of the proposed $r$-process nuclei, phenomenological mass models are used to predict the masses through extrapolations in regions where no experimental data is available. Of these models, three are commonly used for astrophysically relevant mass predictions: i) the Finite Range Droplet Model (FRDM) [8], ii) Duflo-Zuker (DZ) [9], and iii) the Hartree-Fock-Bogoliubov (HFB-21) [10] approach. The values obtained from each of these mass models are used as input into $r$-process simulation codes, which, in turn, attempt to predict the relative elemental abundances. Due to the closed neutron shells at $N=50,82$, and 126, the $r$-process flow ``slows" across the shell closure and has to wait before the next succession of neutron captures, causing the elemental abundance peaks at $A\approx90,130$, and 195. As a result, the experimental succession of neutron captures, causing the elemental abundance peaks at $A\approx90,130$, and 195. As a result, the experimental determination of $S_n$ values in these regions are crucial for $r$-process modeling, and the highest priority measurements are suggested in the regions around $^{132}$Cd and $^{138}$Sn [6]. Included in this region of high priority are the neutron-rich Indium isotopes, which currently have a lack of experimental mass data across the heavy isotopic chain. Our work at TRIUMF attempts to provide much tighter constraints on these data in hopes of better understanding $r$-process nucleosynthesis as a generator for the heavy elements in the universe.


Citations
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