Session F09: Nuclear Astrophysics IV

Study of the 60Ga(β+)60Zn decay of for the Astrophysical rp process

One of the goals of nuclear astrophysics is to understand the various astrophysical events occurring in the cosmos. The most common stellar explosions observed in our galaxy are TypeI X-ray bursts (XRB1). The isotopic abundances obtained from the astrophysical models of XRB1 depend strongly on a number of nuclear reaction rates, occurring both on the surface and inside the crust by the buried ashes. The nuclear burning that creates these ashes is called the rapid proton (rp) capture process. Investigating the rp process enhances our understanding of the dynamics of neutron stars and features of XRB1 spectra. The nuclear reaction flow of the rp process is sensitive to the β⁺ decay properties of the nuclei involved, and the experimental study of such properties is of significant importance. In this study, total absorption spectroscopy (TAS) analysis was performed for the ⁶⁰Ga(β⁺)⁶⁰Zn decay. This experiment was performed at the National Superconducting Cyclotron Laboratory (NSCL). In this presentation, the extracted beta feeding intensity will be discussed, along with a comparison to theoretical shell model and QRPA calculations.   

Sensitivity Study of Type-I X-ray Burst To Nuclear Reaction Rates

Neutron stars in low-mass X-ray binaries, where they can accrete hydrogen-rich or helium-rich materials onto their surfaces, frequently exhibit intense thermonuclear flashes called Type-I X-ray bursts (XRBs). XRBs are powered by the triple-α process, the αp process, and the rapid proton capture process. These different nucleosynthesis processes that drive the event plays a vital role in accurately comparing theoretical models and observations. In our study, we examine

the impact of uncertainties in these nuclear reactions using the ONEZONE model (Cyburt et al., 2016) with different compositions of the accreted material from the companion star. We perform the sensitivity of our X-ray burst model by varying proton and alpha-induced reaction rates in JINA REACLIBV2.2 within representative nuclear physics uncertainties. I will discuss the impact of nuclear reactions on the X-ray burst light curve and ash production.   

Sensitivity studies for the weak r-process: Beta-decay rates

The rapid neutron capture process (r-process) is a mechanism responsible for manufacturing the heaviest elements (A > 70) in very neutron-rich astrophysical environments. Binary neutron star mergers (NSMs) are favorable candidates for r-process production based on recent multimessenger observations, though it is still an open question whether NSMs are the only site of the r-process or one of many. The elemental abundances of metal-poor stars provide an independent probe of the r-process, and these show a robust pattern of heavier elements with greater dispersion in the weak r-process region (70 < A < 120). This suggests the weak r-process offers key evidence to understand the diversity of r-process sources. With the assistance of nuclear facilities such as FRIB, new experimental data can help constrain the conditions and environments associated with theoretical models that best fit the weak r-process region. Here we describe sensitivity studies of unmeasured beta-decay halflives under a variety of astrophysical conditions to understand which species are most crucial to measure in upcoming experimental campaigns.   

Constraining neutron-capture cross section for the i-process for the 151-153Nd(n,γ)152-154Nd reaction via the β Oslo method

Nucleosynthesis of heavy elements has been traditionally attributed to two neutron-

capture processes, namely the s and r processes. Recent astronomical observations

have revealed stars where the abundance distributions cannot be described by the

aforementioned processes and for this reason the astrophysical i process was in-

troduced (i for intermediate between s and r). While we know neutron densities

are between the s and r process, the stellar site where it can occur has not yet

been clearly identified and that is largely because of the nuclear uncertainties. The

i process flow involves isotopes only a few steps from stability, and in this region

the main nuclear physics uncertainty comes from neutron-capture reaction rates.

Specifically neutron capture reactions on Nd isotopes have been identified as impor-

tant for the production of Eu and Sm. With this goal in mind, an experiment was

run at the ATLAS facility using the low-energy beams delivered from CARIBU to

constrain neutron-capture reactions of importance for the i process. β-decays and

their corresponding γ-rays were identified using the SuN detector and the SuNTAN

moving tape system. The β-decay of ^152−154Pr into ^152−154Nd was measured and

the β-Oslo method was used to extract the nuclear level density and γ-ray strength

function of ^152−154Nd; preliminary results from this experiment will be presented

here. From these statistical properties, ^151−153Nd(n,γ)^152−154Nd reaction cross sec-

tions and reaction rates will be constrained and their significance to the i process

will be presented.   

A systematic study of exotic Sr isotopes using the β-Oslo Method: A CARIBU experiment

Understanding of neutron-induced reactions on nuclei far from stability has far-reaching implications for cosmogenic nucleosynthesis and fundamental nuclear physics. Presently, direct measurement of the radiative-capture cross section is experimentally inaccessible for these short-lived nuclei; however, indirect methods such as the β-Oslo method enable the experimental constraint of key nuclear properties that are inputs for reaction-theory calculations.

In particular, reaction rates on neutron-rich Sr isotopes directly in astrophysical abundances through processes that produce the heaviest elements present in the universe. We have performed an experiment at CARIBU at Argonne National Laboratory in order to determine the neutron-capture cross sections of ^93,94,95Sr by constraining the γ-ray strength function (γSF) and nuclear level density (NLD). Low-energy Rb beams were transported to the Summing NaI(Tl) (SuN) detector where coincident β-γ events were measured. The γSF and NLD, properties extracted from the measured γ-ray spectra using the β-Oslo method, contribute the greatest uncertainty in Hauser-Feshbach calculations of neutron-capture reaction rates for short-lived neutron-rich nuclei. The experimental techniques and preliminary results of this work will be presented. Furthermore, the results of this work will shed light on nuclear structure properties for Sr isotopes, leading to significantly improved predictive reaction modeling.   

Preparing GODDESS for the 75Ga(d,pγ)76Ga surrogate measurement to inform i-process nucleosynthesis

A recent study of the metal-poor star HD94028 observed several isotopic abundance ratios (As/Ge, Se/As among others) that were not explained only by summing s- and r-process abundance distributions. A new neutron-capture driven nucleosynthetic process named the i-process has been proposed to explain this observation, taking place at neutron densities intermediate to that observed in the s- and r-processes. A subsequent sensitivity study performed using a one zone simulation to match the observed HD94028 abundances identified the ⁷⁵Ga(n,γ)⁷⁶Ga reaction rate to be the most significant, with variations in this rate showing a strong anti-correlation to the predicted As abundance. Efforts are now underway to use the GODDESS detector system to inform the ⁷⁵Ga(n,γ)⁷⁶Ga reaction rate for the first time at the Facility for Rare Isotope Beams (FRIB) using the ⁷⁵Ga(d,pγ)⁷⁶Ga reaction as a surrogate. A novel surrogate reaction approach that detects protons and heavy charged-particle products in coincidence is proposed to attain improved detection efficiency compared to the traditional approach using only proton-γ coincidences. Simulations, instrumentation, and other developments in preparation of the experiment will be discussed.   

Constraining the 75Zn neutron capture reaction via the β-Oslo method for the weak r-process

Many questions remain about the neutron capture processes responsible for creating the majority of the neutron-rich heavy elements. The i-process and the weak r-process are two lesser understood neutron capture processes whose resulting abundance patterns and required astrophysical environments deviate from those traditionally ascribed to the r-process. 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, the weak r-process 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-section, uncertainties in the reaction rates can be greatly reduced. Here I will present results from the β decay of ⁷⁶Cu in the calculation of the ⁷⁵Zn(n, γ)⁷⁶Zn reaction, in which the uncertainty in the reaction rate has been reduced from over an order of magnitude to a factor of just 2.5. The reaction rate will be presented, as well as its impact on the modeling of weak r-process abundances in the A ∼ 80 region   

β-delayed neutron emission probabilities of neutron-rich Zr to Pd isotopes and their impact on the nuclide abundance left-wing of the second r-process peak

New observational data on the r-process-enhanced metal-poor stars [1] have recently suggested a different source that consistently produces several light r-process elements from Se to Te, distinct from the mechanisms responsible for the heavier elements. These findings also call for new comparisons between nucleosynthesis models and observations for elements around the second r-process peak. However, such comparisons are obstructed by nuclear physics uncertainties, including the β decay properties of neutron-rich progenitors. To address these uncertainties to some extent, β-delayed neutron emission probabilities (P_1n and P_2n values) of ^106-112Zr, ^108-115Nb, ^110-118Mo, ^113-121Tc, ^117-123Ru, ^121-125Rh, and ^124-126Pd isotopes have been measured at the RIBF facility of the RIKEN Nishina Center. The experimental setup combines the silicon detector WAS3ABi with the YSO segmented scintillator surrounded by the BRIKEN ³He counters array [2]. In this talk, we will present the experimental details, the preliminary results in comparison with theoretical β-decay models, and discuss the impacts on the nucleosynthesis calculations of final r-process abundance pattern in this mass region.

[1] I. U. Roederer et al., Astrophys. J. 936, 84 (2022).

[2] A. Tolosa-Delgado et al., Nucl. Instrum. Methods Phys. Res., Sect. A 925, 133 (2019).   

Photon strength function and level density in neutron rich Fe isotopes

In nuclei away from stability, there is a lack of information on nuclear level densities and gamma-ray strength functions. In some stable isotopes, for example, in the Fe-Cd region, an unexpected increase in the gamma-decay probability has been observed. This leads to an up-bend in the gamma-ray strength function (gSF) below ~4 MeV, which has a significant influence on extracted neutron-capture rates. It is unknown how the gSF behaves for neutron-rich nuclei. Furthermore, the nuclear level density (NLD) and gSF are critical components in constraining neutron-capture rates through the indirect beta-Oslo method. Neutron-capture rates are a crucial for models of the r-process, which is presumed responsible for creating over half the isotopes heavier than iron, and measurements are outside the current realm of capability for experimental facilities. At the NSCL, we populated excited states via beta-decay for the neutron-rich nucleus Fe-64, a candidate for the up-bend in the gamma-ray strength function. Gamma rays were recorded with the Summing Na(I) (SuN) segmented total absorption spectrometer, which allows us to simultaneously extract the NLD and gSF. Results will be presented for the $gamma$SF and level density of Fe-64.   

Photon Strength Function of 58Fe with DAPPER Using Forward Analysis

The photon strength function (PSF) describes the average photon emission probabilities of excited states within the continuum.  The PSF thus is important in understanding radiative neutron capture reactions.  Experiments have shown an enhancement in the PSF at low energy for some nuclei.  Low energy enhancement (LEE) theoretically has a large effect on r-process nucleosynthesis, where many nuclei with unmeasured neutron capture cross sections are produced in nature. Previous experiments have shown a LEE in both ⁵⁶Fe and ⁵⁷Fe. A measurement of ⁵⁸Fe’s PSF would determine 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 recoiled proton. Preliminary results on the measurement of ⁵⁸Fe’s PSF using a Forward method as well as the performance of the DAPPER array will be discussed.     

Constraining the Neutron Capture Rate for 90Sr through β-Decay into the Short-Lived 91Sr Nucleus

The slow (s) and rapid (r) neutron capture processes have long been considered to produce nearly the entirety of elements above Fe, but when comparing their yields with spectroscopic data, inconsistencies in abundance arise in the Z=40 region. These differences are expected to be attributable to the intermediate (i) neutron capture process.

Working in weak i-process neutron densities on the order of 10¹³ neutrons/cm³, the ⁹⁰Sr(n,γ)⁹¹Sr capture reaction has a negative correlation to the production of Zr, possibly explaining the discrepancy between the observed and predicted elemental abundances of Zr in i-process environments such as CEMP-i stars.

I will discuss the β-Oslo analysis of ⁹¹Sr to reduce uncertainties in the ⁹⁰Sr(n,γ)⁹¹Sr reaction, measured via the β-decay of ⁹¹Rb into ⁹¹Sr 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 (NLD) and γ-ray Strength Functions (γSF), two critical components in limiting the uncertainty of the neutron capture cross section when it cannot be directly measured. This constrained uncertainty will allow us to better characterize the contribution of ⁹⁰Sr to the i process and make progress in explaining observed abundances in suspected i-process stellar environments.   

Advances in r-process mass measurements at TITAN-TRIUMF

TITAN-TRIUMF has long pursued mass measurements as inputs into nucleosynthesis calculations, first with a Penning trap and now with a Multi-Reflection Time-Of-Flight mass separator. For the latter, recent developments have led to pronounced improvements in resolving power, dynamic range, and sensitivity. They enable TITAN to survey the limits of TRIUMF's RIB production and to approach the r-process path for heavy nuclides. I will provide an overview of the hundreds of species measured, how these mass values are relevant to r- and rp-process, as well as plans for mass and other meausrements to investigate the r- and rp-process.