Session JE: Nuclear Astrophysics IV

The 12C + 12C Reaction Rate and the Explodability of Core Collapse Supernovae

Reaction rates have a very strong impact on the evolution of stars. Therefore, they can also change the thermodynamic structure of massive stars before the onset of collapse. We present the results of supernovae simulations for stars with masses from 12 to 28 solar masses evolved using the reaction rate for ¹²C + ¹²C fusion from Caughlan & Fowler (1988) and the one from Tumino (2018). We show the impact of this rate on the final explodability as a function of progenitor mass. In addition to that, we show that the explosion of the supernova is only sensitive to the entropy and density profile before collapse. Therefore, if a change in the reaction rate changes the density and entropy profiles before collapse, it can also affect the explodability as a function of progenitor mass.   

20Ne(α,p)23Na studied to constrain supernova type Ia nucleosynthesis

Uncertainties in the ²⁰Ne(α,p)²³Na reaction rate have been found to show a great deal of variability when predicting the final nuclei abundances produced in type Ia supernovae [1]. Around temperatures of 5 GK, the Gamow window ranges from 2.4 to 5.4 MeV. Previous studies have measured the ground state cross section for this reaction at higher, less astrophysically relevant energies using time reversal reaction experiments [2,3]. Furthermore, those studies do not consider excited state contributions, and therefore, act as only approximations of the total reaction rate. Probing both the ground and first excited states, a cross section measurement was conducted for the ²⁰Ne(α,p)²³Na reaction measuring 142 energy steps between 2.9 and 5 MeV center of mass energies. This work was completed using the Rhinoceros windowless gas target setup and the 5U accelerator at the University of Notre Dame Nuclear Science Lab. The experimental data will be shown and preliminary analysis discussed.

[1] A. Parikh, J. José, I. R. Seitenzahl, and F. K. Röpke, Astronomy & Astrophysics 557, A3 (2013).

[2] C. Bingham, K. Van Der Borg, R. De Meijer, A. Van Der Woude, Nuclear Physics A323, 26 (1979).

[3] R. Spear and I. Wright, Aust. J. Phys. 21, 307 (1968).   

The 17F(α, p)20Ne Reaction Rate in Type 1 X-Ray Bursts from the Inverse Reaction

The 17F(α, p)20Ne reaction has been identified as an alternate pathway for

breakout from the hot-CNO cycle that can be important in some X-ray burst

scenarios. We have measured the 20Ne(p, α)17F cross section at low energies

ranging from 4.80 to 7.60 MeV using a proton beam on a neon gas cell with an

activation method and NaI detectors at the Fox Accelerator Laboratory at

Florida State University. Later, we measured the same reaction through

inverse kinematics with a 20Ne beam on a methylene target at the ATLAS

facility at Argonne National Laboratory. We used an annular Si strip detector for alpha

detection with recoiling heavy ions detected in the Enge spectrograph by the

MONICA focal plane detector. Studying the inverse kinematics reaction

allows us to distinguish alpha particles being emitted either to the ground or

first excited state of 17F. Cross sections will be compared to previous work,

and preliminary results from an R-matrix analysis constraining properties of

states in 21Na will be presented.   

Constraining the 30P(p,γ)31S reaction rate in ONe nova nucleosynthesis via 31Cl β-delayed proton decay using GADGET

Classical novae occur in stellar binaries involving a white dwarf and a hydrogen-rich companion star. Accreted material from the donor star is compressed, heated, and mixed with the outer layers of the underlying white dwarf until it ignites in a thermonuclear runaway. For the most massive oxygen-neon (ONe) white dwarfs, the ³⁰P(p,γ)³¹S reaction rate has been identified as the largest remaining nuclear uncertainty for modeling A>29 nucleosynthesis. The β⁺ decay of ³¹Cl strongly populates a 260-keV resonance which dominates the total rate for proton capture on ³⁰P. The Gaseous Detector with Germanium Tagging (GADGET) was built to measure the proton branching ratio of this resonance via ³¹Cl(βp)³⁰P. Here we present the results of an experiment at the National Superconducting Cyclotron Laboratory, in which we measured the weakest β-delayed proton intensity ever for resonances below 400 keV. With this result, shell model calculations for the lifetime, and past work on other resonances, we computed the thermonuclear rate for ³⁰P(p,γ)³¹S. Simulations were performed using this newly constrained rate to predict the chemical and isotopic abundances of ONe nova ejecta. We compare these expected ratios with presolar grain data and astronomical observations to calibrate nuclear thermometers.   

58​Ni(3​​He,t)5​8​Cu*(γ) Measurements with GODDESS to Constrain Astrophysical Rate of 5​7​Ni(p,γ)5​8​Cu

Models of nucleosynthesis in supernovae can be validated by comparing the predicted abundance of specific radioisotopes with that inferred from the observation of gamma-rays emitted in supernovae remnants. The predicted abundance of one such isotope, 44Ti, is especially sensitive to the reaction rate of 57Ni(p,γ)58Cu.  Thus, in order to effectively validate these models, it is crucial to precisely determine the rate of 57Ni(p,γ)58Cu. Despite its importance, no experimental rate exists for this reaction, and theoretical rates differ by up to a factor of five in the temperature region of interest.  To experimentally constrain this rate, structure properties of 58Cu were measured via the 58Ni(3He,t)58Cu*(γ) reaction using GODDESS (GRETINA ORRUBA Dual Detectors for Experimental Structure Studies) at Argonne National Laboratory’s ATLAS facility.  The coupling of these two highly efficient arrays allows for precise determination of 58Cu level energies via the detection of gamma-rays in coincidence with charged particles.  Because the reaction rate depends exponentially on these level energies, this precision is critical. Experimental data and preliminary results will be presented.

Measuring excitations in 83Se via the (d,pγ) reaction with 82Se beams to inform weak r-process nucleosynthesis

The mass-80 peak of the r-process abundance pattern is believed to be the product of a possible weak r-process that could occur in hot (~1 GK) environments such as core-collapse supernovae. Surman et al. assessed that neutron capture rates on unstable ^81,83Se isotopes had a significant effect on the calculated weak r-process abundance pattern [1]. However, for stable ⁸²Se there are limited (n,γ) data and the evaluated cross sections are inconsistent. To inform the contributions to the (n,γ) cross section from both direct and compound-nucleus processes, the (d,pγ) reaction with 8.4 MeV/u ⁸²Se beams and a 320 μg/cm² CD₂ target was measured at ATLAS. GODDESS was deployed to detect gamma rays in GRETINA in coincidence with reaction protons in ORRUBA. Beam-like recoils were analyzed in an ionization chamber. Spectroscopic factors have been extracted for the 1/2⁺ (540 keV) and 5/2⁺ (582 keV) exited states in ⁸³Se, which were then used to deduce the direct neutron-capture cross section. The resolving power of coincident charged particle and gamma-ray spectroscopy facilitated extending the spectroscopy of excited states in ⁸³Se.  Preliminary results for ⁸³Se excitations and the deduced (n,γ) cross sections will be presented. 

[1] R. Surman et al., AIP Advances 4, 041008 (2014)   

Upgrading ANASEN for 18Ne(α,p)21Na direct measurement

The Array for Nuclear Astrophysics and Structure with Exotic Nuclei (ANASEN) is a gas target and charged particle detector designed for studying (α,p) reactions. Previous measurements used a position-sensitive proportional counter along the beam axis with a barrel of thick silicon detectors far from the beam axis to track protons from these reactions. We have developed a new approach, replacing the proportional counter with a hexagonal barrel of thin silicon detectors. This allows pure helium gas to be used and improves the tracking resolution, though at a cost of overall efficiency. The performance in stable beam test measurements will be presented with a preliminary report on a direct measurement at TRIUMF of the ¹⁸Ne(α,p)²¹Na excitation function.    

Progress Towards Low Energy Measurement of 14N/15N(α, γ) Reactions

The St. George recoil mass separator at the University of Notre Dame was designed to study alpha capture reactions relevant for stellar burning. In one of the commissioning experiments, the measurement suffered from ¹⁸O background in the beam. A Wien filter was installed upstream of the jet gas target last year to reduce the beam contamination. The preliminary results from the recent ¹⁴N (α, γ)¹⁸F measurements, used to demonstrate 18O background reduction and the progress towards low energy measurement of ¹⁵N(α, γ)¹⁹F will be presented.   

Improvements to the Study of 20Ne(α,γ)24Mg at Astrophysical Energies Using St. George

Recoils from the ²⁰Ne(α,γ)²⁴ reaction have been successfully measured in inverse kinematics on the St. George recoil mass separator system at astrophysically relevant energies.

However, the first of these measurements suffered from significant background, setting an upper limit on the strength of resonances that could be studied.

Recently, a Wien Filter was added to the beam line in order to enhance the purity of the beam being sent to the HIPPO helium gas-jet target.

I will discuss the improvements to the ²⁰Ne(α,γ)²⁴ reaction study as a result of this addition.   

The spin distribution of the level density

The level density is necessary for nuclear reaction calculations, such as neutron capture reaction calculations for the determination of the abundance of  elements. We implement a methodology based on the shell model and methods of statistical spectroscopy for the calculation of spin- and parity- dependent level densities. We present calculations of the spin distribution of the level density for nuclei whose valence nucleons occupy the sd and pf shells. A staggering pattern of the spin distribution of the level densities is observed for even-even nuclei. This behavior of the spin distribution for even-even nuclei deviates from the widely used form of the spin distribution and we present various calculations of its possible form for even-even nuclei.