Session KE: Nuclear Astrophysics V

Phase Separation of 22Ne and 56Fe in White Dwarfs

Neutron rich nuclei with a small neutron excess, such as ²²Ne and ⁵⁶Fe, may separate in crystallizing WDs and sediment to the core. These separation processes release gravitational potential energy and heat the WD, affecting inferred ages. With a detailed understanding of separation and sedimentation in WDs, we may be able to predict the abundance of neutron rich nuclei in their cores, with implications for the ¹⁴N(α,γ)¹⁸F and ¹⁸O(α,γ)²²Ne reactions that are responsible for converting CNO nuclei into ²²Ne. Recent work on fractionalization processes in WDs finds that ⁵⁶Fe may strongly separate into a nearly pure inner core, which may impact type 1a supernova explosions. This talk will present a brief overview of these problems, the current state of the field, and discuss implications for nuclear astrophysics.    

Investigating Potassium Production in Globular Cluster NGC 2419 with the 39K(3He,d)40Ca Reaction

Globular clusters are characterized by abundance anticorrelations among their low-mass stars. These anticorrelations are a result of unknown mixing processes from the pollution of earlier stellar populations. The globular cluster NGC 2419 was recently discovered to exhibit a Mg-K anticorrelation, with a strong K enrichment observed in about 30% of its red giant stars. The enrichment mechanism stems from hydrogen burning in the polluter stars at temperatures between 100 – 200 MK. However, many reaction rates involved in hydrogen burning at these temperatures have large uncertainties, such as ³⁹K(p, γ)⁴⁰Ca, the key potassium destroying reaction. To constrain this reaction rate, we have measured the ³⁹K(³He, d)⁴⁰Ca transfer reaction using the Split-Pole Spectrograph at the Triangle Universities Nuclear Laboratory. Using Bayesian Markov Chain Monte Carlo techniques, we have extracted spectroscopic factors and excitation energies for states in the astrophysical region of interest with meaningful uncertainties. The implications of these results on the ³⁹K(p, γ)⁴⁰Ca reaction rate will be discussed.   

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

The 1.28 Mev gamma-ray released in the beta-decay of ²²Na may be observed after a nova event due to the 2.6 year half-life of  ²²Na. The slowest reaction of the neon-sodium cycle is the ²⁰Ne(p,γ) reaction, which limits the production of ²²Na in nova explostion. The reaction rate is mainly determined by direct capture to a barely-bound 1/2⁺ state in ²¹Na. Located 7 keV below the ²⁰Ne+p threshold, this state is likely an excited halo state which could enhance the ²⁰Ne(p,γ) reaction rate. Our goal is to measure the reaction rate approaching the Gamow window. Very recently, the ²⁰Ne(p,γ) reaction was measured using the DRAGON recoil separator and six GRIFFIN HPGe detectors at TRIUMF at a center of mass energy of 550 keV. This talk will present results from this first run and plans for a second run early next spring.    

Toward measurements of radioactive molecules for astrophysics

With the advent of detection technology, the list of observed molecules in the Universe has been drastically elongated in the past few decades [McG21]. Although detection of molecules made of stable nuclei is suitable for locating those molecules, they do not provide the temporal information on the molecular production. On the other hand, detection of radioactive isotopes, through their characteristic gamma-rays, probe the time-scale of their production in exchange for the loss of positional resolution. This leads to the use of radioactive molecules as a sensitive tracer of stellar events as it sheds light onto both the temporal and positional information on the nucleosynthesis involved.

Typically, astronomical detections of molecules rely on radiofrequency spectroscopy of rotational transitions. Therefore, precise laboratory measurements of the rotational transitions are essential to facilitate the detection of new molecules. However, such high-precision measurements pose a challenge pertaining to radioactive isotopes because of their low production rate, short lifetime, and high velocity and internal kinetic energy. In this project, we focus on the rotational measurements of silicon monoxide, specifically ³²Si¹⁶O isotopologue (half-life: 157(7) years [ENSDF]). Since its first astronomical detection from Sagittarius B2 [Wil71], SiO has been observed extensively in our galaxy, making it a great candidate for a stellar event tracer. Based on the population of each rotational sublevel, we aim to identify the rotational energy spacing in ³²Si¹⁶O. This information will potentially guide the exploration of novel stellar events and related new physics.

[McG21] Astrophys. J., Suppl. Ser. 259 30 (2022)

[ENSDF] From ENSDF database as of July 6, 2022. Version available at http://www.nndc.bnl.gov/ensarchivals/

[Wil71] Astrophys. J. 167 L97-L100 (1971)   

DAPPER Array Upgrade for Measuring 60Fe Photon Strength Function via Radioactive Beam

A photon strength function (PSF) helps describes the excitation and de-excitation of the nucleus, which aids in the understanding of neutron capture reactions. Accurate neutron capture cross-sections are pertinent for modeling nucleosynthesis. The Detector Array for Photons, Protons, and Exotic Residues (DAPPER) is used to measure the PSFs of iron isotopes, important in the production of heavier elements, via (d,pγ) reactions. Alterations to the current array configuration are in progress in order to measure the PSF of ⁶⁰Fe, a long-lived (t_1/2 = 2.6E6 yr) iron isotope important for dating stellar events. Deriving the excitation energy from the proton in the (d,pγ) reaction requires precise knowledge of the target thickness when back calculating the longitudinal energy loss. CD₂ target thicknesses were characterized using a 228-Th source and a position-sensitive Dual-Axis Duo-Lateral Si detector. Development of a detector that can separate contaminates from a radioactive ⁵⁹Fe beam (t_1/2 = 45 d) on an event-by-event basis is necessary to measure ⁶⁰Fe’s PSF as accurately as possible is in progress.   

Photon Strength Function of 58Fe with DAPPER

The photon strength function (PSF) is important in describing photon emission probabilities and thus it plays a role in radiative neutron capture reactions.  Experiments have shown an enhancement in the PSF at low energy for some nuclei.  This “up-bend” could have a large effect on r-process nucleosynthesis, where many unmeasured nuclei are produced in nature. Experiments have shown an up-bend in both ⁵⁶Fe and ⁵⁷Fe nuclei. A measurement of ⁵⁸Fe’s PSF could see if this trend continues. In addition, doing a measurement of ⁵⁸Fe’s PSF helps to prepare for a future measurement of ⁶⁰Fe’s PSF, which will require a radioactive beam. DAPPER (Detector Array for Photons, Protons, and Exotic Residues) probes PSFs using inverse kinematics (d,p) reactions. DAPPER consists of 128 BaF­₂ detectors, to detect the gamma rays with high efficiency, and one S3 Annular Silicon detector, to detect the proton. A beam experiment was done on August 2^nd 2021 to measure ⁵⁸Fe’s PSF. Preliminary results on the measurement of ⁵⁸Fe’s PSF will be discussed.   

Progress Towards a Single Atom Microscope for Nuclear Astrophysics

The Single Atom Microscope (SAM) project aims to measure rare, low-yield nuclear reactions relevant to nuclear astrophysics. This novel detector technique involves capturing the product atoms in a cryogenically frozen and optically transparent noble gas solid and then counting the embedded atoms via laser-induced fluorescence and optical imaging. Due to the unique absorption and emission wavelengths of the product atoms—enabled by the lattice of noble gas atoms—optical filters can distinguish between them to select the wavelength range of interest, making single-atom sensitivity feasible. Rubidium atoms embedded in solid Krypton are being used for pilot measurements because they are laser-friendly and the system is astrophysically relevant—⁸⁴Kr(p, γ)⁸⁵Rb is a key branching point for determining the reaction flow in the p-process. We will discuss the prospects of single atom detection of neutral Rb in solid Kr.   

Cross section measurements to address the electron screening problem

High-energy-density plasmas are an excellent surrogate for astrophysical conditions under which nucleosynthesis occurs. One important phenomenon which occurs in these plasmas is modified electron screening, which affects stellar rates by tens of percent for some reactions. Accelerator data at low energies must be corrected for bound-electron screening to obtain bare cross sections, before plasma screening models are applied for stellar conditions; in some experiments, notable discrepancies between bound-screening models and the data are observed [M. Aliotta et al., Nucl. Phys. A 690, 790 (2001)]. Here we use inertial fusion implosions at the National Ignition Facility to create low-temperature plasmas with negligible screening to study the D+3He fusion reaction at center-of-mass energies where bound-electron screening, in accelerator measurements, is significant. This methodology can produce results novel for understanding the bound-electron screening, and is a prelude to future experiments studying plasma screening.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

LLNL-ABS-836271   

Understanding globular cluster pollution through nuclear reactions

Globular clusters are known to contain multiple stellar populations. The currently observed stars in globular clusters are contaminated with material from a past generation of stars but the nature of these stars is unclear. Uncertainties in nuclear reaction rates make identification of the polluters difficult. In this paper we report direct and indirect measurement of proton radiative-capture reactions on Si-30 and K-39 to constain the nature of the polluting site in globular clusters.