Session MG: Mini-Symposium: Nuclear Physics and the r-Process in the Multi-messenger Era II

Measuring $^{84}$Se(d,p) at 45 MeV/A to reduce uncertainties in spectroscopic factors of states in $^{85}$Se

Neutron-transfer reactions with radioactive ion beams (RIBs) probe the single-neutron components of the wave function of nuclei. This is crucial to our understanding of the direct component of neutron capture. With (d,p) reactions, spectroscopic factors can be extracted through a normalization of the differential cross section calculated using ADWA theory to that observed through experiment. They are, therefore, heavily dependent on the parameters chosen to model the final bound state nucleus. A combined method using high and low energy RIBs allows for both a peripheral and more central probe of the nucleus, thereby constraining the bound state parameters and reducing the uncertainties on the extracted spectroscopic factors. Using this method for the first time with heavy, neutron rich RIBs, the spectroscopic factors of low lying states in $^{85}$Se are being studied through $^{84}$Se(d,p). With the low-energy measurement at 4.5 MeV/A already completed [1], the high-energy measurement at 45 MeV/A was performed at the NSCL. ORRUBA and SIDAR were used to detect reaction protons in coincidence with heavy ion recoils using the S800 spectrograph. Preliminary results, including spectroscopic factors will be presented. [1] J.S. Thomas et al., Phys. Rev. C 76, 044302 (2007)   

Developments and prospects for GODDESS at FRIB

GODDESS is a coupling of an upgraded version of the highly-segmented silicon detector array ORRUBA (Oak Ridge Rutgers University Barrel Array) to the large HPGe detector arrays currently in operation the US. The commissioning campaign in 2015 employed Gammasphere for a series of stable-beam experiments; in early 2019, the system has been deployed with GRETINA for a campaign of measurements using stable, re-accelerated and in-flight radioactive beams at ATLAS at ANL for understanding nova and r-process nucleosynthesis. For this campaign, the instrumentation for GODDESS has been upgraded, along with a set of new beam detectors. These provide event-by-event particle identification and tracking of beam-like particles, critical for both the analysis of the experiments as well as real-time diagnostics for tuning radioactive beams. In parallel to these developments, ORRUBA has recently been coupled to the S800 at the NSCL for fast beam experiments with r-process nuclei. A brief summary of the upgrades, and prospects for deployment with GRETA for experiments at FRIB will be presented.   

Level structure of $^{\mathrm{96}}$Mo from the (d,p$\gamma )$ measurement with $^{\mathrm{95}}$Mo beams and GODDESS

When used in conjunction with radioactive ion beams in inverse kinematics, the surrogate reactions method (SRM) can inform neutron capture cross sections on isotopes of interest to the r process. The use of SRM requires a modern nuclear reaction model of the (d,p) reaction that includes deuteron break-up and a level scheme that is as detailed as possible. Recently, the (d, p) reaction was confirmed as a surrogate for (n, $\gamma )$ reactions in normal kinematics using a $^{\mathrm{95}}$Mo target.$^{\mathrm{1}}$ To extend the benchmarking of the surrogate reactions method to inverse kinematics, a (d, p$\gamma )$ measurement with $^{\mathrm{95}}$Mo beams was performed using GODDESS (Gammasphere ORRUBA: Dual Detectors for Experimental Structure Studies) at ATLAS. This GODDESS experiment is the first (d, p$\gamma )$ measurement on $^{\mathrm{95}}$Mo to populate two-neutron configurations of states below 4 MeV. This (d, p$\gamma )$ study is an important step in refining the reaction theory and current level scheme of $^{\mathrm{96}}$Mo for use in the surrogate reaction analysis. Preliminary results detailing the differential cross sections of low-lying states in $^{\mathrm{96}}$Mo and extension of the $^{\mathrm{96}}$Mo level scheme will be presented. [1] A. Ratkiewicz et al, Phys. Rev. Let., \textbf{122} 052502 (2019).   

Measuring the $^{134}{Te}(d,p\gamma)^{135}{Te}$ Reaction with GODDESS to Deduce the Single-Particle Structure of $^{135}{Te}$ and Inform Neutron Capture

$^{134}$Te ($t_{1/2}=42$ minutes), a beta-decay precursor to stable $^{134}$Xe, can be destroyed in an r-process environment by neutron capture. Constraint of the $^{134}$Te$(n,\gamma)^{135}$Te cross section is key to explaining an overabundance of $^{134,136}$Xe observed in pre-solar grains\footnote{U. Ott, Astrophys. J. \textbf{463}, 344 (1996).}. Due to its proximity to the $Z=50$ and $N=82$ closed shells, neutron capture on $^{134}$Te is expected to largely occur via direct capture into low-lying states in $^{135}$Te ($Z=52$, $N=83$), which can be constrained via the measurement of level energies, spin-parities, and spectroscopic factors using neutron transfer reactions such as $(d,p)$. Previous studies of $^{135}$Te revealed a fragmented level structure that cannot be resolved by charged particles alone\footnote{J.M. Allmond, \textit{et. al.}, Phys. Rev. C \textbf{86}, 031307 (2012).}. However, level energies can be constrained via the detection of gamma rays emitted by $^{135}$Te in coincidence with charged ejectiles. The $^{134}$Te$(d,p\gamma)^{135}$Te reaction was thus measured with the coupled GODDESS (GRETINA-ORRUBA: Dual Detectors for Experimental Structure Studies) detectors at the ATLAS facility at Argonne National Laboratory. Preliminary results will be presented.   

Cross-section measurement of the $^{85}$Br($\alpha$,xn) and it’s implication in the weak r-process.

The fast-expanding neutron rich neutrino-driven winds in the core-collapse supernovae is one of the most favorable scenario for the nucleosynthesis of the Z = 38-47 elements. Charge particle reactions, especially ($\alpha$,xn) on heavy nuclei of the range $80 < A < 90$ create seeds for the weak r-process populating abundances of near stable isotopes for the Sr-Cd range. These abundances are significantly sensitive to the ($\alpha$,n) and ($\alpha$,2n)reaction rates. Only very few of these ($\alpha$,xn) reactions had been measured in the energy range relevant for weak r-process astrophysical conditions. Sensitivity studies of such scenario show that $^{85}$Br($\alpha$,xn) is one of the most significant reaction to impact the abundances of the seeds to the weak r-process. To measure the cross-section of $^{85}$Br($\alpha$,n) and ($\alpha$,2n), the HABANERO detector is used, which is a neutron counter system that includes either BF$_3$ or $^3$He gas-filled proportional counter tubes embedded in the matrix of polyethylene, designed to achieve somewhat constant and energy independent efficiency for neutrons up to 20 MeV. Preliminary results from the RIB experiment $^{85}$Br($\alpha$,xn) along with brief details of the experimental setup will be presented.   

Properties of neutron-rich $^{\mathrm{71,72,73}}$Ni

The rapid neutron capture process (r-process) is responsible for the synthesis of approximately half of the abundance of the heavy elements. The recent LIGO and Virgo gravitational-wave detection from two colliding neutron stars combined with the wealth of electromagnetic follow-up measurements across the electromagnetic spectrum demonstrated the production of heavy nuclei in an r-process. Despite knowing at least one location for the r-process, many open questions remain. The uncertainties in the nuclear physics inputs present a large barrier to accurately model the abundance distributions in large-scale nucleosynthesis calculations. In particular, neutron-capture rates are the most uncertain theoretical input and the most diffcult to measure directly. The $\beta $-Oslo method is one indirect approach for constraining the neutron-capture cross section. The $\beta $ decay of a short-lived nucleus is used to populate the high-energy states in a daughter nucleus and the subsequent photon deexcitation is monitored and used to infer the nuclear level density (NLD) and the $\gamma $-ray strength function ($\gamma $SF). The NLD and $\gamma $SF are then input into a Hauser-Feshbach model to constrain the neutron capture cross section. A series of experiments have been performed at the National Superconducting Cyclotron Laboratory along the Ni elemental chain. The preliminary results obtained for $^{\mathrm{71,72,73}}$Ni will be presented.   

$\beta$-decay studies of Co isotopes using total absorption spectroscopy

Approximately half of the elements heavier than iron are thought to be produced in the r process. Recent insights into the astrophysical site of this critical process highlight the need for experimental data on short-lived neutron-rich nuclei. R-process nucleosynthesis sensitivity studies show that the final abundance distributions of r-process nuclei are greatly impacted by $\beta$-decay properties, such as half-lives and $\beta$-delayed neutron-emission probabilities. To inform the global models used to calculate these properties in calculations, we have measured the $\beta$-decay strengths for several neutron-rich Co isotopes using the technique of total absorption spectroscopy with the Summing NaI (SuN) detector at the NSCL. The resultant $\beta$-decay intensities and deduced Gamow-Teller strengths are compared to QRPA calculations. Results for $^{66,68}$Co decay will be presented along with a look at the systematics for this region.   

Constraining the cross section of 82Se(n,g)83Se to validate the beta-Oslo method

The r-process is believed to be one of the major sources of heavy elements. In order to better understand the r-process, neutron-capture cross sections are needed. Neutron-capture cross sections of many r-process nuclei are poorly known due to short half-lives. This has led to the development of techniques such as the beta-Oslo method, which uses beta decay to populate highly excited states of a nucleus. The resulting de-excitation via the emission of gamma rays is used to extract the NLD and gSF of the daughter nucleus. These nuclear properties are used to experimentally constrain the neutron-capture cross section. A validation will be performed with the $^{82}$Se(n,gamma)$^{83}$Se reaction. The beta decay of $^{83}$As to $^{83}$Se has been studied at the NSCL. The NLD and gSF of $^{83}$Se has been extracted using the beta-Oslo method and fed into TALYS to constrain a neutron-capture cross section. The constrained cross section will be compared to a direct measurement.   

Mass Measurements of r-process Nuclei Using the TOF-B$\rho $ Technique at the NSCL

The r-process plays a key role in the nucleosynthesis of more than half of the nuclei heavier than iron. Mass is one of the most fundamental nuclear data for r-process models because it is essential to calculate other nuclear properties such as Q-values for $\beta$-decays, neutron capture rates and equilibrium abundance distributions. The trends of masses along the isotopic chains towards N=82 can help us to test the models calculating the masses in the r-process path which is out of reach for current facilities. At the NSCL, we have conducted a mass-measurement experiment using time-of-flight--magnetic-rigidity (TOF-B$\rho$) technique for neutron-rich isotopes from Zr to Ru around N=70 produced by the projectile fragmentation of $^{124}$Sn. I will present the preliminary results of this experiment.