Session D09: Nuclear Astrophysics II

26Si + α resonant scattering measurement for the astrophysical 26Si(α, p)29P reaction rate

The ²⁶Si(α, p)²⁹P reaction is considered to have astrophysical importance such as impact on X-ray burst light curves, or on the abundance of ²⁶Al known as a galactic gamma ray source. The cross section of this reaction has, however, never been measured directly, and there was only a study of ³⁰S resonances via limited information from the ²⁸Si(³He, n)³⁰S and ³²S(p, t)³⁰S reactions. We have performed both the ²⁶Si(α, p)²⁹P reaction direct measurement and the ²⁶Si(α, α)²⁶Si resonant scattering measurement in inverse kinematics to determine the reaction cross section at higher energies and to obtain alpha resonance information in the ³⁰S compound system, respectively. The experiment employed the thick-gas-target inverse-kinematics method with a ²⁶Si radioactive isotope (RI) produced at the Center-for-the-Nuclear-Study RI Beam Separator (CRIB) of the University of Tokyo. In the presentation, we will discuss the α resonant scattering result, which covers the excitation energy range of E_x = 12–16 MeV and was analyzed by R-matrix fit, to update the ²⁶Si(α, p)²⁹P thermal reaction rate.   

Direct measurement of the 26Si(α, p)29P reaction for the nucleosynthesis in the X-ray bursts

Nuclear reactions in the αp-process, the sequence of (α, p) and (p, γ) reactions, are important for the nucleosynthesis in the X-ray bursts. The ²⁶Si(α, p)²⁹P reaction is one of the αp-process reactions and considered to have one of the largest effects on the light curve of the X-ray burst. However, there are not sufficient experimental data for this reaction because of the technical difficulties.

 

We performed a direct measurement of this reaction at the CNS RI beam separator (CRIB). The ²⁶Si(α, p)²⁹P reaction was measured in the high temperature region around T = 3 GK using the ⁴He thick gas target method. In spite of the large number of background events, an upper limit on the reaction cross section was obtained, which was about one order of magnitude smaller than that of the NON-SMOKER statistical model.

 

This is the first experimental evaluation of this reaction cross section by a direct measurement. Therefore, the results will be informative for determining absolute value of the cross section for high energy region and constraining the ²⁶Si(α, p)²⁹P reaction rate and the X-ray burst light curve model.

 

The analysis method and the results are discussed in this presentation.   

Study of the 20Ne(p,γ) reaction rate using GRIFFIN γ-ray detectors coupled with the DRAGON recoil spectrometer at TRIUMF

The ²⁰Ne(p,γ) reaction is the reaction in the Ne-Na cycle that limits the production of ²²Na, an important radiotracer (T_1/2=2.6yr), in O-Ne nova explosions (T=0.1-0.4GK). In this temperature range, the ²⁰Ne(p,γ) reaction rate is dominated by the direct capture to the barely-bound 1/2⁺ state in ²¹Na. Located just 7 keV below the one-proton separation energy (S_p), this state is likely a halo state which may enhance the direct capture. A similar enhancement has been observed in the ¹⁶O(p,γ) reaction proceeding via direct capture to the 1/2⁺ state in ¹⁷F located 105keV below S_p. We aim to measure the ²⁰Ne(p,γ) reaction rate near the Gamow window using the DRAGON recoil spectrometer coupled for the first time with six GRIFFIN HPGe clover detectors at TRIUMF. In this talk, the preliminary results from the initial run at 550keV center-of-mass energy will be presented together with the plan for upcoming measurements later this year.   

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

The ²⁰Ne(α,p)²³Na reaction rate is particularly important in predicting final nuclei abundances for type Ia supernovae, especially around temperatures of 5 GK [1]. The corresponding Gamow window for this temperature ranges from 2.4 to 5.4 MeV. Previous studies of this reaction have measured either the ground state, at higher, less astrophysically relevant, energies, or the excited state cross sections, but not both states within the Gamow window [2,3]. Therefore, these act as only estimates of the total cross section for the ²⁰Ne(α,p)²³Na reaction. To address the need for an improved determination of the reaction rate, a cross section measurement was performed for the reaction using the Rhinoceros windowless gas target system and the 5U accelerator at the University of Notre Dame Nuclear Science Lab. The experiment covered 142 energy steps between 2.9 and 5 MeV center of mass energies and probed both ground and first excited states. The experimental data will be shown and preliminary analysis will be 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).   

Total Cross-Section Measurement of 14O(α,p)17F with an Active Target Ionization Chamber

In certain stellar environments such as Type I X-ray Bursts, proton-capture rates exceed β-decay rates which induce burning processes like the hot-CNO cycle, which may ultimately transition to the rp-process. The ¹⁴O(α,p)¹⁷F reaction is thought to be a trigger reaction that induces this transition. Understanding this reaction is thus important to understanding how the rp-process proceeds in X-ray bursts. The Active Target High Efficiency detector for Nuclear Astrophysics (ATHENA) was built and commissioned at the Notre Dame Nuclear Science Lab for the purposes of measuring astrophysically important reactions with low-intensity radioactive beams. ATHENA will be used to measure the total cross section of the ¹⁴O(α,p)¹⁷F reaction with a radioactive ¹⁴O beam produced by the TriSol facility at Notre Dame. Preliminary results, plans, and comparisons to simulations will be discussed.   

Improving 22Ne(a,n) measurements with Liquid 3-He neutron detection

We explore a possibility of a liquid 3-He-based instrument to detect neutrons emitted from 22Ne(a,n) with much higher efficiency than previously realized. Because of the status of the 22Ne(a,n) reaction as a lynchpin of the s-process we hope that this kind of measurement will serve to clarify the low-energy behavior of this and other similar reactions. Our new liquid 3-He concept allows smaller detectors with neutron detection efficiencies as much as a factor of 164 times greater than traditional gaseous 3-He detectors. The expense of this efficiency boost is cryogenic operation around 1K; we will discuss the logistics of deploying this sort of detector in a nuclear beam environment.   

The 17F(α, p)20Ne Reaction Rate in Type I 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 previously measured the ²⁰Ne(p, α)¹⁷F cross section at low

center-of-mass energies ranging from 4.80 to 7.60 MeV via activation

measurements with a proton beam on a ²⁰Ne gas cell at the Fox Accelerator

Laboratory at Florida State University. We subsequently measured the same

reaction through inverse kinematics with a ²⁰Ne beam on a methylene target

at Argonne National Lab’s ATLAS facility. We used an annular Si strip

detector for alpha-particle detection with recoiling heavy ions detected in the

Enge spectrograph by the MONICA focal plane detector. Studying the

reaction in inverse kinematics allows us to distinguish alpha particles emitted

to the first-excited state of ¹⁷F which do not contribute to the reverse (α, p)

reaction on the ¹⁷F ground state. Cross section comparisons and preliminary

results from an R-matrix analysis constraining properties of states in ²¹Na will

be presented.   

Measurement and Analysis of the 17F(p,p') Reaction

X-ray bursts are one of the most frequent transient events observed in the universe. They repeat irregularly with periods from a few hours to months and lead to the synthesis of numerous heavy elements in the rp-process. This process is poorly understood due to the numerous reactions on exotic nuclei. The rp-process is thought to be triggered by the 14O(α,p)17F reaction, and understanding the rate of this reaction will aid in interpreting x-ray burst light curves. Because of the difficulty of studying the forward reaction, recent attempts have been made to study the time reversed reaction, 17F(p,α)14O. Since these studies only probe astrophysical branches to the ground state of 17F, the 17F(p,p') reaction was studied at the Nuclear Science Laboratory at the University of Notre Dame to look for the population of excited states. The data will be presented and preliminary results discussed.   

Development of the Solenoid Spectrometer for Nuclear Astrophysics and Decays (SSNAPD)

The most common explosive astrophysical event is the Type 1 X-Ray Burst (XRB), in which low-Z fuel from a binary companion star incites periodic thermonuclear runaway on the surface of a neutron star. XRBs are a location of heavy element synthesis through the rp-process, and their light curves provide macroscopic information about the system’s neutron star. 

Both ignition of the rp-Process and the shape of the observed light curve are heavily affected by key proton- and α-capture reactions involving Z=6-15, neutron-deficient nuclei. These reactions, such as ¹⁵O(α,γ)¹⁹Ne, are challenging to study directly, even at modern Radioactive Ion Beam facilities. Indirect methods measuring proton- and α-decay branching ratios of excited reaction products provide a feasible way of probing astrophysically-important resonances experimentally.

SSNAPD will be a charged-particle detector array developed to perform such particle-decay experiments at the TriSol facility at the University of Notre Dame. The SSNAPD array will be capable of measuring charged particle branching ratios as low as 10^-5, making it ideally suited for low cross-section reactions that govern XRB behavior. Development status, simulations, testing, and early experimental plans for SSNAPD will be discussed.   

Level Density Studies via Proton Evaporation

Nuclear level densities are of great importance to astrophysical phenomena, which require an understanding of nuclear reaction rates. Existing theoretical Hauser-Feshbach models vary by factors of three or more when used to calculate reaction rates. Thus, additional measurements of nuclear level densities are necessary to improve existing models. The common method of using particle evaporation from compound nuclear reactions is known for its ability to study the nuclear level density. Proton evaporation is then a natural approach to further current understanding. Preliminary results in the study of Al(12C,p) data from the Edwards Accelerator Laboratory will be presented.   

Impact of the 12C + 12C reaction rate on Core Collapse Supernovae

The ¹²C + ¹²C reaction plays an important role in explosive carbon burning in core collapse supernovae.  A new rate has been  measured by [1]  based upon the Trojan Horse Method (THM), which is an indirect method to measure cross-sections at astrophysical energies. The impact of this new rate on initial, pre-collapse models has been studied [2] based upon the Frascati RAphson Newton Evolutionary Code (FRANEC). In this work we study the subsequent impact on core collapse supernovae based upon the an open-source, spherically symmetric, general-relativistic hydrodynamics, CCSN code GR1D [3] modified to include a relativistic version [4] of Supernova Turbulence in Reduced dimensionality (STIR) [5] that incorporates the impact of neutrino heated turbulence on the explosion dynamics.   Explosion dynamics were computed using both the standard rate [6]  for the ¹²C + ¹²C fusion reaction  and the new rate [1].  We find that the new rates significantly alter the explodability of progenitor models.  Among other differences we find that between 13.5 M_⊙ and 15 M_⊙ the  models with the old rate do not explode, whereas the models with the new rate do. Then, between 15.5 M_⊙ and 16.5 M_⊙ the situation is reversed: the old rate models models do not explode and new rate models do. Also, in the region 20.5-22 M_⊙, the models with the old rate do not explode and models with the new rate do. Reasons for these differences are discussed based upon various factors that influence the explodability.

[1]Tumino, A., Spitaleri, C., La Cognata, M., & et al. 2018, Nature, 557, 687–690.

[2] Chieffi, A., Roberti, L., Limongi, M., et al. 2021, ApJ, 916,79.

[3] O’Connor, E. 2015, ApJS, 219, 24.

[4] Boccioli, L., Mathews, G. J., & O’Connor, E. P. 2021, ApJ, 96 912, 29.

[5] Couch, S. M., Warren, M. L., & O’Connor, E. P. 2020,  ApJ, 890, 127.

[6] Caughlan, G. R., & Fowler, W. A. 1988, Atomic Data and Nuclear Data Tables, 40, 283.   

CASPAR Underground Nuclear Astrophysics Laboratory current and future status

The closer accelerator-based experiments approach the burning regime of interest for stellar nucleosynthesis, the lower the reaction probability becomes. With this exponential drop off in cross-section the issue of background interference in signals becomes more problematic even with modern detection techniques. Aboveground experiments have background interactions from cosmic ray interference typically much greater than expected reaction signatures. To eliminate this cosmic interference the CASPAR accelerator laboratory is located a mile underground at the Sanford Underground Research Facility, studying nuclear reactions of astrophysical interest specifically (p,g), (a,g) and (a,n) reactions. The accelerator system has been in a 2-year hibernation and is currently ramping up phase to production mode. This talk will highlight recent measurements at CASPAR including s-process neutron sources, their production chains and neutron poisons, as well as the future timeline for new experimental campaigns.