arXiv subjects

F. Montes

Publications and source records attributed to F. Montes.

At least 19 recordsLinked to original sources

arXiv2026

$\beta$-decay Measurements Near the $N=40$ Island of Inversion to Quantify Cooling of Accreted Neutron Star Crusts

Understanding the thermal structure of the outer crust of accreting neutron stars is important to interpret astronomical X-ray observations. Ground-state to ground-state $\beta$-decay transitions of neutron-rich nuclei comprising the crust enable Urca neutrino cooling processes that affect this thermal structure. Here we constrain the ground-state to ground-state transition strengths for the decays of $^{57}$Sc, $^{57}$Ti, and $^{59}$Ti based on experimental data. The data were obtained by combining total absorption $\gamma$-spectroscopy data from the SuN detection system with $\beta$-delayed neutron emission data from the NERO detection system at Michigan State University's National Superconducting Cyclotron Laboratory. We find $\log ft=$5.8$^{+0.3}_{-0.2}$ and $\log ft=$5.34$^{+0.08}_{-0.24}$ for the decays of $^{57}$Ti and $^{59}$Ti, respectively, and find no evidence for ground-state feeding in the decay of $^{57}$Sc. The results indicate weaker transitions than predicted by theory and indicated by previous measurements, resulting in reduced efficiency of neutrino cooling in accreted neutron star crusts in systems that exhibit X-ray superbursts.

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arXiv2026

Direct Measurement of the $^{59}$Cu$(p,\alpha)^{56}$Ni Excitation Function to Constrain the Ni--Cu Cycle Strength and Its Impact on Explosive Nucleosynthesis

A new direct measurement of the 59Cu(p,a)56Ni excitation function from 2.43-5.88 MeV in the center-of-mass was performed in inverse kinematics using the high-efficiency MUSIC active-target detector at FRIB. This reaction plays a critical role in constraining the strength of the NiCu cycle in different explosive astrophysical scenarios such as Type I X-ray bursts and the nu-p process in neutrino-driven winds after a core-collapse supernova. The newly derived stellar rate is systematically lower than previous estimates, suppressing NiCu cycle recycling in X-ray bursts to below 3% and enhancing nu-p process efficiency throughout the relevant temperature range, potentially extending its effective operation to temperatures as high as T9 = 3.94 (+0.99/-0.85).

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arXiv2026

Detailed Study of the $^{59}$Cu(p,$\alpha)^{56}$Ni Reaction and Constraints on Its Astrophysical Reaction Rate

The $^{59}$Cu$(p,\alpha)^{56}$Ni reaction plays an important role in explosive astrophysical scenarios such as Type I X-ray bursts and the $\nu p$-process in neutrino-driven winds following a core-collapse supernova, where it regulates the flow of nucleosynthesis through the NiCu cycle and the synthesis of heavier nuclei. We present a direct measurement of the $^{59}\mathrm{Cu}(p,\alpha)^{56}\mathrm{Ni}$ excitation function from 2.43--5.88~MeV in the center-of-mass frame, performed in inverse kinematics with the high-efficiency MUSIC active-target detector at FRIB. The angle- and energy-integrated cross sections extend direct measurements to lower energies than previously reported and remove the angular-integration model dependence of earlier work. To extrapolate the rate to astrophysical energies, we constrain the statistical-model description through a systematic optimization of the DEM-3 $\alpha$-optical model potential geometry, and quantify the model-selection uncertainty with a Bayesian model averaging analysis over 96 TALYS combinations. The resulting stellar rate carries a temperature-dependent uncertainty factor of 1.26--1.63 over $T_9 = 0.2$--10 and is systematically lower than the REACLIB evaluation, remaining below the competing $(p,\gamma)$ rate for $T_9 \lesssim 3.94$. These results substantially weaken the inferred NiCu cycle strength and establish the $^{59}$Cu$(p,\gamma)^{60}$Zn rate as the dominant remaining uncertainty.

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arXiv2025

Constraining the Synthesis of the Lightest p Nucleus 74Se

We provide the first experimental cross section of the $^{73}\text{As}(p,\gamma)^{74}\text{Se}$ reaction to constrain one of the main destruction mechanisms of the p nucleus $^{74}\text{Se}$ in explosive stellar environments. The measurement was done using a radioactive $^{73}\text{As}$ beam at effective center-of-mass energies of 2.9 and 2.3 MeV/nucleon. Along with the total cross-section measurement, statistical properties of the $^{74}\text{Se}$ compound nucleus were extracted, constraining the reaction cross section in the upper Gamow window of the $\gamma$ process. The impact of the experimentally constrained reaction rate on $^{74}\text{Se}$ production in Type II supernovae was investigated through Monte Carlo one-zone network simulations. The results indicate that the overproduction of $^{74}$Se by Type II supernova models cannot be resolved by nuclear physics alone and point toward the need for a more detailed understanding of the astrophysical conditions of relevance for the $\gamma$ process.

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arXiv2025

Constraining r-process nucleosynthesis with multi-objective Galactic chemical evolution models

The astrophysical site(s) of the r-process are uncertain, with candidates such as neutron star mergers and magneto-rotational supernovae predicting different event rates, delay times, and heavy-element yields. Galactic chemical evolution models constrain these properties by comparing model predictions with observed abundances. We explore, in a systematic and data-driven way, the astrophysical conditions under which r-process enrichment can reproduce the observed trends of multiple neutron-capture elements in the Milky Way. Rather than assuming a fixed site, we adopt a flexible, parametric approach to test whether a common set of r-process parameters can explain the chemical evolution of several heavy elements. We compute a grid of one-infall, homogeneous models varying: Eu yield per event, r-process event rate, enrichment delay time, and progenitor mass range. For each of the $\sim 1.5 \times 10^5$ models, we predict [X/Fe] vs. [Fe/H] trends by scaling Eu yields with the solar r-process pattern. A multi-objective optimisation based on Pareto fronts identifies models that best reproduce the abundance trends. Best-fitting models favour short delay times ($\leq 30\ \rm Myr$), low-mass progenitors ($\sim 20-25\ \rm M_\odot$), and an effective Eu injection of $\sim 2 \times 10^{-7}\ \rm M_\odot$ per event. Stars more massive than $\sim 80\ \rm M_\odot$ are too rare to dominate the enrichment. While heavy elements can be reproduced, lighter ones show stronger conflicts with Eu, reflecting that the solar r-process scaling relation becomes less valid toward lighter elements. No single class of r-process events, under solar-scaled yields, can explain light and heavy neutron-capture elements; at least two components are required: a main r-process consistent with solar and r-rich stars, and a weaker component producing enhanced light r-process elements, similar to that observed in r-poor stars.

astro-ph.GA↗

arXiv2025

New approach for the quantification of uncertainties in reaction modeling via data-driven multi-objective optimization

We introduce a new multi-objective optimization approach to determine uncertainty-quantified nuclear reaction parameters in the Hauser-Feshbach framework. By simultaneously accounting for all available data across multiple reaction channels we capture parameter correlations and estimate data-driven uncertainties. We implement in the Ni-Ge region yielding uncertainty-quantified model parameters for both stable and unstable isotopes. We estimate resonance spacings for nuclei beyond experimental reach and validate our method by calculating a known cross-section outside our optimization region.

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arXiv2025

Impact of newly measured $\beta$\nobreakdash-delayed neutron emitters around \myisoSimp{78}{Ni} on light element nucleosynthesis in the neutrino-wind following a neutron star merger

Neutron emission probabilities and half-lives of 37 beta-delayed neutron emitters from 75Ni to 92Br were measured at the RIKEN Nishina Center in Japan, including 11 one-neutron and 13 two-neutron emission probabilities and 6 half-lives measured for the first time, which supersede theoretical estimates. These nuclei lie in the path of the weak r-process occurring in neutrino-driven winds from the accretion disk formed after the merger of two neutron stars, synthesizing elements in the A~80 abundance peak. The presence of such elements dominates the accompanying kilonova emission over the first few days and has been identified in the AT2017gfo event, associated with the gravitational wave detection GW170817. Abundance calculations based on over 17000 simulated trajectories describing the evolution of matter properties in the merger outflows show that the new data lead to an increase of 50-70 percent in the abundance of Y, Zr, Nb, and Mo. This enhancement is large compared to the scatter of relative abundances observed in old very metal-poor stars and is therefore significant in the comparison with other possible astrophysical processes contributing to light-element production. These results underline the importance of including experimental decay data for very neutron-rich beta-delayed neutron emitters into r-process models.

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arXiv2025

First measurement of 87Rb({\alpha}, xn) cross sections at weak r-process energies in supernova {\nu}-driven ejecta to investigate elemental abundances in low-metallicity stars

Observed abundances of Z ~ 40 elements in metal-poor stars vary from star to star, indicating that the rapid and slow neutron capture processes may not contribute alone to the synthesis of elements beyond iron. The weak r-process was proposed to produce Z ~ 40 elements in a subset of old stars. Thought to occur in the {\nu}-driven ejecta of a core-collapse supernova, ({\alpha}, xn) reactions would drive the nuclear flow toward heavier masses at T = 2-5 GK. However, current comparisons between modelled and observed yields do not bring satisfactory insights into the stellar environment, mainly due to the uncertainties of the nuclear physics inputs where the dispersion in a given reaction rate often exceeds one order of magnitude. Involved rates are calculated with the statistical model where the choice of an {\alpha}-optical-model potential ({\alpha}OMP) leads to such a poor precision. The first experiment on 87Rb({\alpha}, xn) reactions at weak r-process energies is reported here. Total inclusive cross sections were assessed at Ec.m. = 8.1 - 13 MeV (3.7 - 7.6 GK) with the active target MUlti-Sampling Ionization Chamber (MUSIC). With a N = 50 seed nucleus, the measured values agree with statistical model estimates using the {\alpha}OMP Atomki-V2. A re-evaluated reaction rate was incorporated into new nucleosynthesis calculations, focusing on {\nu}-driven ejecta conditions known to be sensitive to this specific rate. These conditions were found to fail to reproduce the lighter-heavy element abundances in metal-poor stars.

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arXiv2025

Object Detection with Deep Learning for Rare Event Search in the GADGET II TPC

In the pursuit of identifying rare two-particle events within the GADGET II Time Projection Chamber (TPC), this paper presents a comprehensive approach for leveraging Convolutional Neural Networks (CNNs) and various data processing methods. To address the inherent complexities of 3D TPC track reconstructions, the data is expressed in 2D projections and 1D quantities. This approach capitalizes on the diverse data modalities of the TPC, allowing for the efficient representation of the distinct features of the 3D events, with no loss in topology uniqueness. Additionally, it leverages the computational efficiency of 2D CNNs and benefits from the extensive availability of pre-trained models. Given the scarcity of real training data for the rare events of interest, simulated events are used to train the models to detect real events. To account for potential distribution shifts when predominantly depending on simulations, significant perturbations are embedded within the simulations. This produces a broad parameter space that works to account for potential physics parameter and detector response variations and uncertainties. These parameter-varied simulations are used to train sensitive 2D CNN object detectors. When combined with 1D histogram peak detection algorithms, this multi-modal detection framework is highly adept at identifying rare, two-particle events in data taken during experiment 21072 at the Facility for Rare Isotope Beams (FRIB), demonstrating a 100% recall for events of interest. We present the methods and outcomes of our investigation and discuss the potential future applications of these techniques.

physics.ins-det↗

arXiv2024

Study of the $in ^{34}$Ar($\alpha,p$)$^{37}$K reaction rate via proton scattering on $^{37}$K, and its impact on properties of modeled X-Ray bursts

Background: Type I X-Ray bursts (XRBs) are energetic stellar explosions that occur on the surface of a neutron star in an accreting binary system with a low-mass H/He-rich companion. The rate of the $^{34}$Ar($\alpha,p$)$^{37}$K reaction may influence features of the light curve that results from the underlying thermonuclear runaway, as shown in recent XRB stellar modelling studies. Purpose: In order to reduce the uncertainty of the rate of this reaction, properties of resonances in the compound nucleus $^{38}$Ca, such as resonance energies, spins, and particle widths, must be well constrained. Method: This work discusses a study of resonances in the $^{38}$Ca compound nucleus produced in the $^{34}$Ar($\alpha,p$) reaction. The experiment was performed at the National Superconducting Cyclotron Laboratory, with the ReA3 facility by measuring proton scattering using an unstable $^{37}$K beam. The kinematics were designed specifically to identify and characterize resonances in the Gamow energy window for the temperature regime relevant to XRBs. Results: The spins and proton widths of newly identified and previously known states in $^{38}$Ca in the energy region of interest for the $^{34}$Ar($\alpha,p$)$^{37}$K reaction have been constrained through an R-Matrix analysis of the scattering data. Conclusions: Using these constraints, a newly estimated rate is applied to an XRB model built using Modules for Experiments in Stellar Astrophysics (MESA), to examine its impact on observables, including the light curve. It is found that the newly determined reaction rate does not substantially affect the features of the light curve.

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arXiv2024

Machine-Learning-Enabled Measurements of Astrophysical (p,n) Reactions with the SECAR Recoil Separator

The synthesis of heavy elements in supernovae is affected by low-energy (n,p) and (p,n) reactions on unstable nuclei, yet experimental data on such reaction rates are scarce. The SECAR (SEparator for CApture Reactions) recoil separator at FRIB (Facility for Rare Isotope Beams) was originally designed to measure astrophysical reactions that change the mass of a nucleus significantly. We used a novel approach that integrates machine learning with ion-optical simulations to find an ion-optical solution for the separator that enables the measurement of (p,n) reactions, despite the reaction leaving the mass of the nucleus nearly unchanged. A new measurement of the $^{58}$Fe(p,n)$^{58}$Co reaction in inverse kinematics with a 3.66$\pm$0.12 MeV/nucleon $^{58}$Fe beam (corresponding to 3.69$\pm$0.12 MeV proton energy in normal kinematics) yielded a cross-section of 20.3$\pm$6.3 mb and served as a benchmark for the new technique demonstrating its effectiveness in achieving the required performance criteria. This novel approach marks a significant advancement in experimental nuclear astrophysics, as it paves the way for studying astrophysically important (p,n) reactions on unstable nuclei produced at FRIB.

physics.ins-det↗

arXiv2024

Direct cross-section measurement of the weak r-process 88Sr({\alpha},n)91Zr reaction in {\nu}-driven winds of core collapse supernovae

About half of the heavy elements beyond iron are known to be produced by the rapid neutron capture process, known as r-process. However, the astrophysical site producing the r-process is still uncertain. Chemical abundances observed in several cosmic sites indicate that different mechanisms should be at play. For instance, the abundances around silver measured in a subset of metal-poor stars indicate the presence of a weak r-process. This process may be active in neutrino-driven winds of core collapse supernovae where (${\alpha}$,n) reactions dominate the synthesis of Z ~ 40 elements in the expelled materials. Scarcely measured, the rates of (${\alpha}$,n) reactions are determined from statistical Hauser-Feshbach calculations with ${\alpha}$-optical-model potentials, which are still poorly constrained. The uncertainties of the (${\alpha}$,n) reaction rates therefore make a significant contribution to the uncertainties of the abundances determined from stellar modeling. In this work, the $^{88}$Sr(${\alpha}$,n)$^{91}$Zr reaction which impacts the weak r-process abundances has been probed at astrophysics energy for the first time; directly measuring the total cross sections at astrophysical energies of 8.37 - 13.09 MeV in the center of mass (3.8 - 7.5 GK). Two measurements were performed at ATLAS with the electrically-segmented ionization chamber MUSIC, in inverse kinematics, while following the active target technique. The cross sections of this ${\alpha}$-induced reaction on $^{88}$Sr, located at the shell closure N = 50, have been found to be lower than expected, by a factor of 3, despite recent statistical calculations validated by measurements on neighboring nuclei. This result encourages more experimental investigations of (${\alpha}$,n) reactions, at N = 50 and towards the neutron-rich side, to further test the predictive power and reliability of such calculations.

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arXiv2023

Neutrino-Driven Outflows and the Elemental Abundance Patterns of Very Metal-Poor Stars

The elemental abundances between strontium and silver ($Z = 38-47$) observed in the atmospheres of very metal-poor stars (VMP) in the Galaxy may contain the fingerprint of the weak $r$-process and $\nu p$-process occurring in early core-collapse supernovae explosions. In this work, we combine various astrophysical conditions based on a steady-state model to cover the richness of the supernova ejecta in terms of entropy, expansion timescale, and electron fraction. The calculated abundances based on different combinations of conditions are compared with stellar observations with the aim of constraining supernova ejecta conditions. We find that some conditions of the neutrino-driven outflows consistently reproduce the observed abundances of our sample. In addition, from the successful combinations, the neutron-rich trajectories better reproduce the observed abundances of Sr-Zr ($Z= 38-40$), while the proton-rich ones, Mo-Pd ($Z= 42-47$).

astro-ph.HE↗

arXiv2023

Measurement of Charge State Distributions using a Scintillation Screen

Absolute cross sections measured using electromagnetic devices to separate and detect heavy recoiling ions need to be corrected for charge state fractions. Accurate prediction of charge state distributions using theoretical models is not always a possibility, especially in energy and mass regions where data is sparse. As such, it is often necessary to measure charge state fractions directly. In this paper we present a novel method of using a scintillation screen along with a CMOS camera to image the charge dispersed beam after a set of magnetic dipoles. A measurement of the charge state distribution for 88Sr passing through a natural carbon foil is performed. Using a Bayesian model to extract statistically meaningful uncertainties from these images, we find agreement between the new method and a more traditional method using Faraday cups. Future work is need to better understand systematic uncertainties. Our technique offers a viable method to measure charge state distributions.

physics.ins-det↗

arXiv2023

First direct measurement constraining the $^{34}$Ar($\alpha$,p)$^{37}$K reaction cross section for mixed hydrogen and helium burning in accreting neutron stars

The rate of the final step in the astrophysical $\alpha$p-process, the $^{34}$Ar($\alpha$,\textit{p})$^{37}$K reaction, suffers from large uncertainties due to lack of experimental data, despite having a considerable impact on the observable light curves of x-ray bursts and the composition of the ashes of hydrogen and helium burning on accreting neutron stars. We present the first direct measurement constraining the $^{34}$Ar($\alpha$,p)$^{37}$K reaction cross section, using the Jet Experiments in Nuclear Structure and Astrophysics (JENSA) gas jet target. The combined cross section for the $^{34}$Ar,Cl($\alpha$,p)$^{37}$K,Ar reaction is found to agree well with Hauser-Feshbach predictions. The $^{34}$Ar($\alpha$,2p)$^{36}$Ar cross section, which can be exclusively attributed to the $^{34}$Ar beam component, also agrees to within the typical uncertainties quoted for statistical models. This indicates the applicability of the statistical model for predicting astrophysical ($\alpha$,p) reaction rates in this part of the $\alpha$p process, in contrast to earlier findings from indirect reaction studies indicating orders-of-magnitude discrepancies. This removes a significant uncertainty in models of hydrogen and helium burning on accreting neutron stars.

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arXiv2022

Horizons: Nuclear Astrophysics in the 2020s and Beyond

Nuclear Astrophysics is a field at the intersection of nuclear physics and astrophysics, which seeks to understand the nuclear engines of astronomical objects and the origin of the chemical elements. This white paper summarizes progress and status of the field, the new open questions that have emerged, and the tremendous scientific opportunities that have opened up with major advances in capabilities across an ever growing number of disciplines and subfields that need to be integrated. We take a holistic view of the field discussing the unique challenges and opportunities in nuclear astrophysics in regards to science, diversity, education, and the interdisciplinarity and breadth of the field. Clearly nuclear astrophysics is a dynamic field with a bright future that is entering a new era of discovery opportunities.

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arXiv2022

Constraining nucleosynthesis in neutrino-driven winds: observations, simulations and nuclear physics

A promising astrophysical site to produce the lighter heavy elements of the first $r$-process peak ($Z = 38-47$) is the moderately neutron rich ($0.4 < Y_e < 0.5$) neutrino-driven ejecta of explosive environments, such as core-collapse supernovae and neutron star mergers, where the weak $r$-process operates. This nucleosynthesis exhibits uncertainties from the absence of experimental data from $(\alpha,xn)$ reactions on neutron-rich nuclei, which are currently based on statistical model estimates. In this work, we report on a new study of the nuclear reaction impact using a Monte Carlo approach and improved $(\alpha,xn)$ rates based on the Atomki-V2 $\alpha$ Optical Model Potential ($\alpha$OMP). We compare our results with observations from an up-to-date list of metal-poor stars with [Fe/H] $<$ -1.5 to find conditions of the neutrino-driven wind where the lighter heavy elements can be synthesized. We identified a list of $(\alpha,xn)$ reaction rates that affect key elemental ratios in different astrophysical conditions. Our study aims on motivating more nuclear physics experiments on $(\alpha, xn)$ reactions using current and the new generation of radioactive beam facilities and also more observational studies of metal-poor stars.

astro-ph.HE↗

arXiv2022

$^{57}$Zn $\beta$-delayed proton emission establishes the $^{56}$Ni $rp$-process waiting point bypass

We measured the $^{57}$Zn $\beta$-delayed proton ($\beta$p) and $\gamma$ emission at the National Superconducting Cyclotron Laboratory. We find a $^{57}$Zn half-life of 43.6 $\pm$ 0.2 ms, $\beta$p branching ratio of (84.7 $\pm$ 1.4)%, and identify four transitions corresponding to the exotic $\beta$-$\gamma$-$p$ decay mode, the second such identification in the $f p$-shell. The $p/\gamma$ ratio was used to correct for isospin mixing while determining the $^{57}$Zn mass via the isobaric multiplet mass equation. Previously, it was uncertain as to whether the rp-process flow could bypass the textbook waiting point $^{56}$Ni for astrophysical conditions relevant to Type-I X-ray bursts. Our results definitively establish the existence of the $^{56}$Ni bypass, with 14-17% of the $rp$-process flow taking this route.

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