Session ME: Nuclear Astrophysics VII

Constraining the 75,76Zn neutron capture reactions via the β-Oslo method for the weak r-process

Many questions remain about neutron capture processes occurring far from stability. The r-process in particular is responsible for the synthesis of a large portion of the neutron-rich heavy elements, but because of a lack of nuclear data in this region due to the difficulty in creating both neutron and exotic radioactive ion beams and targets, it is not yet fully understood. To constrain the nuclear properties in this region, we turn to novel techniques. One of these indirect methods is the β-Oslo method, which uses β decay to populate highly-excited nuclear states in the compound nucleus of interest. The decay of these states is then used to extract the nuclear level densities (NLD) and γ-ray strength functions (γSF). By implementing these experimentally-determined statistical properties in the calculation of theoretical neutron-capture cross-sections, uncertainties in the rate can be greatly reduced. Results from the β decay of ^76,77Cu in the calculation of the ^75,76Zn(n,γ)^76,77Zn reaction rates will be presented, as well as their impact on the modeling of the weak r-process abundances in the A~80 region.   

Constraining the Neutron Capture Rate for the Short-Lived 91Sr Nucleus

Recent neutron-star merger observations have provided r-process abundance constraints, which has led to significant attention towards additional neutron-capture processes such as the i-process and n-process. Working between the rates and environmental neutron densities of the r-process and s-process, their reaction pathways and abundance contributions are not yet fully characterized. Operating in densities of 10¹³ - 10²⁰ neutrons/cm³, sensitivity studies have shown these intermediate neutron-capture processes to take reaction pathways through experimentally accessible n-rich nuclei, providing opportunities to constrain the capture rates that define them.

This contribution will review the β-Oslo analysis of ⁹¹Sr, taken with the SuN total absorption spectrometer at the NSCL in 2018. By measuring both γ-ray and excitation energies, a coincidence matrix was produced to perform the Oslo analysis, providing experimental information on the nuclear level density and γ-ray strength functions, two critical components in finding the neutron capture cross section. Since neutron capture rates are historically unconstrained by experimental work, this provides an opportunity to further reduce these uncertainties, better characterizing the contribution of ⁹¹Sr to these exotic capture processes.   

Effect of the 22Mg(α,p)25Al reaction in Type I X-ray bursts

We have performed a direct measurement of the ²²Mg(α,p)²⁵Al reaction cross section in inverse kinematics for a center of mass energy range of 3.5 - 7 MeV using a ²²Mg beam from the Argonne In-Flight Radioactive Ion Separator (RAISOR) and the MUlti-Sampling Ionization Chamber (MUSIC) at Argonne National Laboratory. The dominant (p,γ) nucleosynthesis flow in Type I X-ray bursts (XRBs) is halted at several waiting point nuclei such as ²²Mg, ^24-26Si, ^28-30S and ³⁴Ar due to (p,γ)-(γ,p) equilibrium. It has been suggested that the flow can be bypassed by (α,p) reactions on these waiting point nuclei. However, the present uncertainties in the relevant (α,p) reaction rates at these waiting points hinder the ability to accurately predict the light curve and ash composition of XRBs. Among the reactions on the waiting point nuclei, the ²²Mg(α,p)²⁵Al reaction has been identified as an important reaction bypassing the waiting points for XRB nucleosynthesis. Thus, it is crucial to constrain the reaction rate of the ²²Mg(α,p)²⁵Al reaction at astrophysical energies.   

Studying 11B proton structure via the 10Be(p,n)10B reaction.

The production mechanisms for boron, as well as for beryllium and lithium, are hypothesized

to lay outside well established nucleosynthesis processes. Boron is thought to have been formed

via Core Collapse Supernovae as well as via cosmic ray nucleosynthesis. Furthermore, there is

a possibility that vestiges of boron were produced during the Big Bang. It is an element whose

astrophysical origins facilitate a glimpse into some of the most extreme astrophysical processes

in the Universe. Boron’s stable isotopes, ¹⁰B and ¹¹B, have therefore been studied for some

time. The single proton structure of the ¹¹B isotope, however, is understudied. Understanding

this proton structure would provide useful insight, not only into nucleosynthesis, but into the

overarching knowledge of the isotope’s structure and neutron detection techniques as well. For

the purpose of studying this structure, the ¹⁰Be(p,n)¹⁰B reaction was measured at the Edwards

Accelerator Laboratory using the time of flight method, where a proton beam was incident on

a 90-μg/cm² BeO target. A 0° excitation function was measured in the 2.0 ≤ Ep ≤ 7.0 MeV

energy range, and resonances were observed at Ep = 2.5, 3.5, and 5.7 MeV. Lastly, angular

distributions up to 150° were measured at 2.5 and 5.7 MeV.   

Constraining the νp-process through the first direct measurement of the 56Ni(n,p)56Co reaction cross section at LANSCE

The rate of the ⁵⁶Ni(n,p)⁵⁶Co reaction is a critical input for understanding the synthesis of heavy elements through the νp-process. However, there currently exists no experimental data for the reaction. This is due to the technical difficulties of producing a quality neutron beam at the relevant energies and in the fabrication of a short-lived ⁵⁶Ni target (T_1/2 ≈ 6 days). We are leveraging a world unique capability that currently exists at LANSCE by producing the radioactive sample at the Isotope Production Facility (IPF) then performing the measurement using the well characterized beam of fast neutrons, spanning a broad energy range from 100s of keV to 100s of MeV, available at the WNR facility. Details on the target fabrication process, technical considerations for the experimental setup, and results of the ⁵⁶Ni(n,p)⁵⁶Co measurement will be presented.  With the newly obtained experimental information, the reaction rate of ⁵⁶Ni(n,p) has been updated and compared with other theoretical predictions. The final impact on the νp process will be discussed along with a plan for improving the experimental uncertainty in future measurements using an optimized solenoidal spectrometer.   

(α, n) Reactions in Inverse Kinematics Using SECAR

Observations of metal poor stars in the galactic halo have found significant amounts of star-to-star

scatter in the abundances of elements around Z = 38-47. The nucleosynthesis occurring in the

neutrino driven winds of core collapse supernovae provides a possible explanation for these obser-

vations. In these explosive environments (α, n) reactions close to stability drive heavy element

enrichment. Our knowledge about the nature of this enrichment is limited by the

poorly known nuclear cross sections for these reactions.

Direct measurements of these (α, n) cross sections at astrophysical energies are therefore essential

ingredients for our nuclear and stellar models. In this talk I will discuss a novel

technique for measuring these reactions in inverse kinematics using a recoil separator to detect the

heavy reaction products in coincidence with neutrons. The first measurements of this type have

been carried out using The Separator for Capture Reactions (SECAR) located within NSCL/FRIB.

Principles of the experimental technique will be described along with a discussion of the technical

challenges of using SECAR for this specific purpose.   

Analysis of near-threshold proton resonances in 26Si using the 25Al(d,n)26Si proton transfer reaction.

The synthesis of galactic ²⁶Al is an interesting subject that spans from nuclear astrophysics to observational astronomy. In astrophysical environments, such as Classical Novae and Core-Collapse Supernovae, the ²⁵Al(p,γ)²⁶Si reaction contributes by populating the ²⁶Al isomeric state, which bypasses the observed γ-ray. To understand this impact, the ²⁵Al(d,n)²⁶Si transfer reaction was performed to investigate the near-threshold proton resonances in ²⁶Si using the RESOLUT radioactive beam facility at Florida State University. Here we report the updated proton widths obtained for the lowest l=0 resonance with better precision than in previous measurements. Methods, results and astrophysical implications will be discussed.