Fernando Montes

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 (,n) reactions dominate the synthesis of Z ~ 40 elements in the expelled materials. Scarcely measured, the rates of (,n) reactions are determined from statistical Hauser-Feshbach calculations with -optical-model potentials, which are still poorly constrained. The uncertainties of the (,n) reaction rates therefore make a significant contribution to the uncertainties of the abundances determined from stellar modeling. In this work, the Sr(,n)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 -induced reaction on 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 …

NUCLEAR REACTIONS 3 He (32 S, α) 31 S, E= 128 MeV; measured reaction products, Eγ, Iγ; deduced γ-ray energies, levels, T 1/2, resonances. Comparison with the shell-model code NuShellX calculations. Modern Markov chain Monte Carlo-based Bayesian statistical techniques. The Doppler Shift Lifetimes (DSL2) facility at the TRIUMF Isotope Separator and Accelerator (ISAC-II) facility.

The elemental abundances between strontium and silver (Z= 38–47) observed in the atmospheres of very metal-poor stars in the Galaxy may contain the fingerprint of the weak r-process and νp-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 …

Nuclei around the N= 4 0 “island of inversion” exhibit interesting structure features that have been the focus of several experimental and theoretical studies. The present work presents the first complete study of the β-decay feeding intensity distribution and Gamow-Teller distribution for the β decay of Mn 64 to Fe 64 up to≈ 10 MeV. The β-decay intensity function was extracted from total absorption spectroscopy measurements made at the National Superconducting Cyclotron Laboratory with the Summing NaI (Tl)(SuN) detector. The experimental results are compared to shell model calculations with and without the inclusion of the ν g 9/2 orbital. From this comparison it is clear that the ν g 9/2 orbital is essential for the accurate description of the Fe 64 β-decay strength above≈ 3 MeV, emphasizing once again the transitional nature of this nucleus into the N= 40 island of inversion.

Nucleosynthesis in the νp-process occurs in regions of slightly proton-rich nuclei in the neutrino-driven wind of core-collapse supernovae. The process proceeds via a sequence of (p,γ) and (n,p) reactions, and depending on the conditions, may produce elements between Ni and Sn. Recent studies show that a few key (n,p) reactions regulate the efficiency of the neutrino-p process (νp-process). We performed a study of one of such (n,p) reactions via the measurement of the reverse (p,n) in inverse kinematics with SECAR at NSCL/FRIB.Such proton-induced reaction measurements are particularly challenging, as the recoils and the unreacted projectiles have nearly identical masses. An appropriate separation level can be achieved with SECAR, and along with the incoincidence detection of neutrons these measurements become attainable. The preparation of the SECAR system for accommodating its first (p,n …

The rate of the final step in the astrophysical α p process, the Ar 34 (α, p) K 37 reaction, suffers from large uncertainties due to a 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 Ar 34 (α, p) K 37 reaction cross section, using the Jet Experiments in Nuclear Structure and Astrophysics gas jet target. The combined cross section for the Ar 34, Cl (α, p) K 37, Ar reaction is found to agree well with Hauser-Feshbach predictions. The Ar 34 (α, 2 p) Ar 36 cross section, which can be exclusively attributed to the Ar 34 beam component, also agrees to within the typical uncertainties quoted for statistical models. This indicates the applicability of the statistical model for predicting astrophysical (α, p) reaction rates in this part …

Transient Low Mass X-ray Binaries (LMXBs) in their quiescent phase are observed to be cooling over timescales of decades. Studying this cooling behavior reveals a great deal of information about the properties of neutron stars. The β-decay/e-capture cycles lead to an appreciable cooling of the crust but the strength of this Urca cooling depends primarily on the ground-state to ground-state β-decay transition strengths. A= 33 mass chain is supposedly the strongest cooling agent for crusts composed of X-ray burst ashes which relies in part on the strong ground state branch in 33 Mg-33 Al decay measured with high resolution β-delayed γ-ray spectroscopy. However, 33 Mg has been recently confirmed to have a negative parity ground state making 33 Mg-33 Al a first-forbidden decay. This leads to a discrepancy with theoretical predictions and the 33 Mg decay experiment results are questioned in the literature, citing …

The lighter heavy elements of the first r-process peak, between strontium and silver, can be synthesized in the moderately neutron-rich neutrino–driven ejecta of either core–collapse supernovae or neutron star mergers via the weak r–process. This nucleosynthesis scenario exhibits uncertainties from the absence of experimental data from (α, xn) reactions on neutron–rich nuclei, which are currently based on statistical model estimates. We have performed a new impact study to identify the most important (α, xn) reactions that can affect the production of the lighter heavy elements under different astrophysical conditions using new, constrained (α, xn) reaction rates based on the Atomki-V2 αOMP. Our results show how when reducing the nuclear physics uncertainties, we can use abundance ratios to constrain the astrophysical conditions/environment. This can be achieved in the near future, when the key (α, xn …

β-delayed γ-neutron spectroscopy has been performed on the decay of A= 84 to 87 gallium isotopes at the RI-beam Factory at the RIKEN Nishina Center using a high-efficiency array of He 3 neutron counters (BRIKEN). β− 2 n− γ events were measured in the decays of all of the four isotopes for the first time, which is direct evidence for populating the excited states of two-neutron daughter nuclei. Detailed decay schemes with the γ branching ratios were obtained for these isotopes, and the neutron emission probabilities (P x n) were updated from the previous study. Hauser-Feshbach statistical model calculations were performed to understand the experimental branching ratios. We found that the P 1 n and P 2 n values are sensitive to the nuclear level densities of 1 n daughter nuclei and showed that the statistical model reproduced the P 2 n/P 1 n ratio better when experimental levels plus shell-model level densities …

The formation of nuclei in slightly proton-rich regions of the neutrino-driven wind of core-collapse supernovae could be attributed to the neutrino-p process (νp-process). As it proceeds via a sequence of (p,γ) and (n,p) reactions, it may produce elements in the range of Ni and Sn, considering adequate conditions. Recent studies identify a number of decisive (n,p) reactions that control the efficiency of the νp-process. The study of one such (n,p) reaction via the measurement of the reverse (p,n) in inverse kinematics was performed with SECAR at NSCL/FRIB. Proton-induced reaction measurements, especially at the mass region of interest, are notably difficult since the recoils have nearly identical masses as the unreacted projectiles. Such measurements are feasible with the adequate separation level achieved with SECAR, and the in-coincidence neutron detection. Adjustments of the SECAR system for the first (p,n …

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 …

Type I X-Ray bursts (XRBs) are extremely 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. If accretion reaches a critical fraction of the Eddington luminosity, a thin-shell instability causes a He-flash in the surface nucleosynthesis, resulting in thermonuclear runaway. This is accompanied by a burst of photons in the X-ray spectrum, referred to as a''light curve'', which has a steep rise preceding the peak, followed by an exponential decay. The rate of the reaction 34Ar (α, p) 37K, as one of the last reactions in the so-called (α, p) process that characterize the burst rise, may influence the shape of the light curve, as shown in the most recent XRB sensitivity studies. This work discusses an experiment to study resonances of the 38K compound nucleus of the (α, p) reaction on the 34Ar nucleus via proton scattering on 37K. The …

The lighter heavy elements of the first r-process peak, between strontium and silver, can be synthesized in the moderately neutron-rich neutrino–driven ejecta of either core–collapse supernovae or neutron star mergers via the weak r–process. This nucleosynthesis scenario exhibits uncertainties from the absence of experimental data from (α, xn) reactions on neutron–rich nuclei, which are currently based on statistical model estimates. We have performed a new impact study to identify the most important (α, xn) reactions that can affect the production of the lighter heavy elements under different astrophysical conditions and using new, constrained (α, xn) reaction rates based on the Atomki-V2 αOMP. We have identified a list of relevant reactions that affect elemental abundance ratios that can be compared to abundances from metal-poor stars. Our results show how when reducing the nuclear physics uncertainties, we …

Low mass X-ray binaries that transiently accrete matter onto their neutron stars are excellent laboratories for studying dense matter physics. These systems alternate between outburst and quiescence phases and monitoring the surface temperatures of neutron stars in quiescence reveals a great deal of information about their structure and composition. But infering these properties requires a complete understanding of different nuclear reactions that heat or cool the crust. Urca cooling is a source of neutrino cooling in the crust that strongly depends on the ground-state to ground-state beta decay transition strengths. The 33Mg-33Al transition is predicted to be the strongest Urca cooling agent for crusts composed of X-ray burst ashes. This relies partly on the strong ground state branch measured in high-resolution beta-delayed gamma-spectroscopy of 33Mg. However, recent measurements of a negative parity ground …

Proton-and alpha-capture reactions on unstable proton-rich nuclei power astrophysical explosions like novae and X-ray bursts. Direct measurements of these reactions are crucial for understanding the mechanisms behind these explosions and the nucleosynthesis at such sites. The recoil mass separator, SECAR (SEparator for CApture Reactions) at the National Superconducting Cyclotron Laboratory (NSCL) and the Facility for Rare Isotope Beams (FRIB), has been designed with the required sensitivity to study (p,γ) and (α,γ) reactions, directly at astrophysical energies in inverse kinematics, with radioactive beams of masses up to about A = 65. The complete SECAR system, including two Wien Filters for high mass resolution, has been installed at Michigan State University and is currently being commissioned. The present article introduces the SECAR concept, its scientific goals, and provides an update of the …

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

The 26Al (a, p) reaction impacts final abundances in lower-temperature X-ray burst models. We have performed a direct thin-target measurement of this reaction at several energies approaching the Gamow window, by impinging a batch-mode beam of 10^ 6 26Al ions/s from the ReA3 accelerator on the JENSA gas-jet target. Ejectiles were detected using the ORRUBA silicon detector array and recoils were detected using the GODDESS position-sensitive fast ionization chamber. This approach enables determination of partial and total reaction cross sections, providing stringent constraints on Hauser-Feshbach predictions. An overview of the experiment and some preliminary results will be presented. Work supported in part by the US Department of Energy (Office of Science and National Nuclear Security Administration) and the National Science Foundation.

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.

“Light” heavy elements (Z = 38 − 47) can be synthesized in the neutrino–driven ejecta of core–collapse supernovae via the weak r–process. This nucleosynthesis scenario exhibits uncertainties from the absence of experimental data from (α, n) reactions on neutron–rich nuclei, and are mostly based on statistical model calculations. We present preliminary results from a recent sensitivity study, using the Atomki–V2 α-nucleus potential to identify the most important (α, n) reactions that can affect the production of “light” heavy elements between strontium and silver under different astrophysical conditions. We also discuss the planning of studies to experimentally determine (α, xn) reaction rates using the MUSIC detector at Argonne National Laboratory and the SECAR recoil separator at FRIB.

The weak r process in neutrino-driven winds following a core-collapse supernova is thought to contribute to the cosmic abundances of the first r-process peak elements between Se and Ag. Sensitivity studies have found that the early nucleosynthesis in the weak r process is primarily driven by (α, x n) reactions due to the high temperatures, and that current nuclear physics uncertainties in the (α, x n) rates result in significant uncertainties of the calculated abundances. The weak r-process path proceeds several nuclei away from stability where (α, x n) reaction cross sections have not yet been measured. In this paper we report the Mo 100 (α, x n) cross section (between 8.9 and 13.2 MeV in the center of mass, corresponding to 3.5–6.8 GK) in inverse kinematics using the Multi-Sampling Ionization Chamber (MUSIC) detector at the Argonne Tandem Linac Accelerator System (ATLAS) facility. With this first measurement …