Session GE: Nuclear Astrophysics III

Lasers for the Observation of Multiple Order Nuclear Reactions

Nuclear reaction rates become nonlinear with respect to flux (1/cm²/s) in extreme environments such as those found during stellar nucleosynthesis and terrestrial nuclear detonations. To observe these effects directly in the laboratory, extremely high particle fluences (1/cm²) are necessary but not sufficient. Reactor-based neutron sources, such as the Institut Laue-Langevin’s high-flux neutron reactor, were the closest to meeting this challenge, albeit over ∼hour time scales. In contrast, multiple rapid reactions occur on a picosecond time scale in ultra-high flux environments, preventing nuclei from returning to their ground states between reactions. Data on the cross- sections of excited nuclear states, which differ significantly from those of ground states due to spin/parity effects, is needed in order to accurately model high-flux environments. In order to replicate these effects in the laboratory, short high-fluence pulses on the order of the lifetime of a typical nuclear excited state (generally <1 ns) are required. Particle beams generated by petawatt lasers are uniquely positioned to meet this need with the potential to produce fluences of 10¹⁷ protons per cm² and 10²² neutrons per cm² over a few pico-seconds or less. In addition to providing a quantitative analysis of the rates of multiple rapid reactions in general, this talk examines a number of laser-based experiments that could be conducted in the near future to observe multiple rapid reactions for laboratory-based astrophysics and the measurement of exotic cross-sections.   

Constraining modeling of the 3He+3He six-nucleon system using data from high energy density plasma experiments at the OMEGA laser

High-Energy-Density (HED) plasma experiments are used to study the ³He+³He reaction in conditions closely mimicking an astrophysical environment at the National Ignition Facility (NIF) and OMEGA lasers.^1,2,3 These efforts provide a complement to more traditional accelerator experiments. Recently, new diagnostic technology has allowed the ³He³He proton spectrum from implosions at the OMEGA laser to be measured down to 2.5 MeV proton energy for the first time, at a center-of-mass energy E_cm~165 keV. The new spectra are contrasted to R-matrix models of the ³He³He proton spectrum³ constrained using data from the T+T reaction, and found to favor a model constrained by T+T data obtained at E_cm=160 keV⁴ over a model constrained by T+T data obtained at E_cm=16 keV⁵. The new broad-band proton spectra will be used to constrain state-of-the-art modeling of the ³He+³He six-nucleon system.

[1] Gatu Johnson et al., Phys. Plasmas 24, 041407 (2017).

[2] Gatu Johnson et al., Phys. Plasmas 25, 056303 (2018).

[3] Zylstra et al., Phys. Rev. Lett. 119, 222701 (2017).

[4] Wong et al., Nucl. Phys. 71, 106 (1965).

[5] Sayre et al., Phys. Rev. Lett. 111, 052501 (2013).

    

New measurement of β-decay properties of neutron-rich Se to Y isotopes relevant to r-process nucleosynthesis

We have measured the β-decay half-lives and β-delayed neutron emission probabilities of neutron-rich ^92-96Se, ^94-99Br, ^97-102Kr, ^99-104Rb, ^101-106Sr, and ^104-108Y isotopes with the BRIKEN detector setup at the Radioactive Isotope Beam Factory (RIBF). Our results include a first measurement of β-delayed one-neutron emission probabilities for 23 isotopes and two-neutron emission probabilities for 16 cases. The data is important to test theoretical models of mid-shell nuclei and astrophysics models of nucleosynthesis during the weak r-process. In particular, we will discuss the importance for the calculation of the decay properties measured in our experiment of including the neutron-γ competition during the de-excitation from intermediate states populated in the β-decay process. We also present the impact of the new data for models of the r-process in high-entropy winds of core-collapse supernovae.   

Constraining stellar electron-capture rates.

Electron-capture (EC) rates play a key role in various astrophysical phenomena, such as core-collapse supernovae (CCSN), cooling of the neutron star crust, and nucleosynthesis in thermonuclear supernovae. The stellar conditions cannot be reproduced in the laboratory and to estimate the EC rates at extreme thermodynamic conditions one has to rely on theoretical models. Previous studies show the importance of temperature-dependent effects for stellar EC calculations on few nuclei near N=50. The effects of the temperature on EC rates have been further investigated recently, based on shell model and QRPA calculations, for nuclei that play an important role during the collapse phase of (CCSN) (N≈50, Z≳28). In addition, to quantify the impact of the new temperature-dependent calculations on the dynamics of the collapse, numerical simulations of CCSN were performed with the spherically-symmetric GR1D simulation code.

Besides, the theoretical models must be benchmarked with experimental data where available, i.e. primarily from the ground state of the parent nucleus. Over the past decades, great progress has been made to constrain electron-capture rates on stable nuclei by using reactions in forward kinematics. However, the unstable neutron-rich nuclei capturing the most during, for example, the core-collapse supernovae, remained inaccessible. The use of the (d,²He) charge-exchange reaction in inverse kinematics with the Active-Target Time-Projection Chamber and the S800 Spectrograph was developed at NSCL/FRIB, for extracting Gamow-Teller strengths in the β+ direction on unstable nuclei. This makes it possible, for the first time, to constrain electron-capture rates on neutron-rich nuclei.

Recent results of the temperature-dependent EC rates study on N=50 nuclei and of the pilot ¹⁴O(d,²He) experiment will be presented.   

ICF plasma based measurements of the T + 4He cross section

Inertial Confinement Fusion (ICF) implosions provide a relatively new platform for studying nuclear astrophysics. Unlike beam-target accelerator measurements, nuclear reactions occurring in these implosion experiments are a direct surrogate for astrophysical systems as the reactions occur at comparable plasma conditions. The tritium (T) + 4He reaction is important for big-bang nucleosynthesis (BBN) production of 7Li which has a notable abundance anomaly. Observations of metal-poor halo stars [1] show approximately one third of the 7Li abundance from predictions that match the abundance inferred from the cosmic microwave background. [2] The important energy range for BBN is a center of mass energy (Ecm) between 60-160 keV [3], corresponding to a range in temperature of 13 − 50 keV. The lower end of this range can be easily studied with implosions conducted at ICF facilities. The T + 4He reaction produces 7Li and a 2.4 MeV gamma, the latter of which can be measured using gamma detectors based on the Cherenkov mechanism. Experiments conducted at the OMEGA laser facility are described along with preliminary results. This research can potentially impact the BBN modeling community by improving confidence in the reaction rate at relevant energies.

[1] F. Spite and M. Spite, Abundance of lithium in unevolved halo stars and old disk stars-interpretation and consequences, Astron. Astrophys. 115, 357 (1982).

[2] M. Tegmark et al. (SDSS Collaboration), Cosmological parameters from sdss and wmap, Phys. Rev. D, 69, 103501 (2004).

[3] S. Burles et al., Sharpening the predictions of big-bang nucleosynthesis, Phys. Rev. Lett., 82, 4176 (1999).   

Probing Nuclear Uncertainties in Kilonova Modeling: Beta Decay Rates

The rapid neutron capture (r-process) is one of the main mechanisms whereby elements heavier than iron are synthesized, and is responsible for the creation of the heaviest isotopes of the actinides. Kilonova emissions are modeled as being largely powered by the radioactive decay of species synthesized via the r-process and in principle, observations of these offer insight into nucleosynthetic processes that occur in the merger. Given that the r-process occurs far from nuclear stability, nucleosynthesis calculations are subject to large uncertainties from unmeasured quantities. We highlight the impact of theoretical global beta decay calculations in contributing to these uncertainties. We also incorporate a variety of different theoretical nuclear physics inputs, including mass models, decay rates, and fission yields, into nucleosynthesis calculations. We show the range of uncertainty these can generate and show the impact on key isotope production for nuclear heating, light curve evolution and nuclear cosmochronometry.   

Constraining the Astrophysical γ Process: Cross Section Measurement of the 82Kr(p,γ)83Rb Reaction in Inverse Kinematics

The astrophysical γ process is considered to be the main production mechanism of a small group of proton-rich isotopes, the p nuclei. This mechanism consists of the “burning” of preexisting r- and s-process seeds, through a series of photodisintegration reactions. The astrophysical site where such high temperature conditions are fulfilled is not yet clearly identified. Networks of nuclear reactions are simulated under appropriate astrophysical conditions in order to reproduce the p-nuclei abundances observed in nature. However, as experimental cross sections of γ-process reactions are largely unknown, the related reaction rates are based entirely on HF theoretical calculations and thus carry large uncertainties. For this purpose the total cross section of the ⁸²Kr(p,γ)⁸³Rb reaction has been measured at incident energies between 3.1 and 3.7 MeV. The experiment took place at the NSCL at MSU, where the ⁸²Kr beam was directed onto a hydrogen gas cell in the center of the Summing NaI(Tl) detector. Results on the spectra obtained using the γ-summing technique and the extracted cross section will be presented along with its comparison to theoretical calculations using the NON-SMOKER and TALYS codes for various inputs of nuclear level density and γ-ray strength function.   

Model Independent Measurements of α-ANCs for the 12C(α,γ)16O Reaction

The ¹²C(α,γ)¹⁶O reaction is often considered the “Holy Grail” of nuclear astrophysics. It determines the absolute abundance of ¹²C and ¹⁶O in our universe and plays a fundamental role in the late stages of stellar evolution. However, direct measurement of this reaction is not possible with current experimental methods. This is because the Gamow peak at 300 keV is far below the Coulomb barrier where the cross section is on the order of 10⁻¹⁷ b. Low-energy extrapolations from higher energy measurements have proven challenging for this reaction and thus the reaction rate is not known to the desired uncertainty of 10%. One way to reduce uncertainties related to low-energy extrapolations is an indirect technique that measures Asymptotic Normalization Coefficients (ANCs) of bound states extracted from sub-Coulomb α-transfer reactions. This approach provides a valuable tool for studying astrophysically important reaction rates since the results are nearly model independent. One of the remaining sources of uncertainty for extrapolation is the α-ANC of the ground state of ¹⁶O. The preliminary results of an experiment performed at the Texas A&M University Cyclotron Institute using the TexPPAC detector will be presented for the sub-Coulomb α-transfer reaction of ¹²C(²⁰Ne,¹⁶O)¹⁶O.