Reactions Of Astrophysical Interest Research Articles

Reaction dynamics on amorphous solid water surfaces using interatomic machine-learned potentials

Context. Energy redistribution after a chemical reaction is one of the few mechanisms that can explain the diffusion and desorption of molecules which require more energy than the thermal energy available in quiescent molecular clouds (10 K). This energy distribution can be important in phosphorous hydrides, elusive yet fundamental molecules for interstellar prebiotic chemistry. Aims. Our goal with this study is to use state-of-the-art methods to determine the fate of the chemical energy in the simplest phosphorous hydride reaction. Methods. We studied the reaction dynamics of the P + H → PH reaction on amorphous solid water, a reaction of astrophysical interest, using ab initio molecular dynamics with atomic forces evaluated by a neural network interatomic potential. Results. We found that the exact nature of the initial phosphorous binding sites is less relevant for the energy dissipation process because the nascent PH molecule rapidly migrates to sites with higher binding energy after the reaction. Non-thermal diffusion and desorption after reaction were observed and occurred early in the dynamics, essentially decoupled from the dissipation of the chemical reaction energy. From an extensive sampling of on-site reactions, we constrained the average dissipated reaction energy within the simulation time (50 ps) to be between 50 and 70%. Most importantly, the fraction of translational energy acquired by the formed molecule was found to be mostly between 1 and 5%. Conclusions. Including these values, specifically for the test cases of 2% and 5% of translational energy conversion, in astrochemical models, reveals very low gas-phase abundances of PHx molecules and reflects that considering binding energy distributions is paramount to correctly merging microscopic and macroscopic modelling of non-thermal surface astrochemical processes. Finally, we found that PD molecules dissipate more of the reaction energy. This effect can be relevant for the deuterium fractionation and preferential distillation of molecules in the interstellar medium.

A technique for studying (n,p) reactions of astrophysical interest using radioactive beams with SECAR

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) reaction measurement included the development of new ion beam optics, and the installation of the neutron detection system. The aforementioned developments along with a discussion on the preliminary results of the p(58Fe,n)58Co reaction measurement are presented.

Determination of the Zn60 level density from neutron evaporation spectra

Nuclear reactions of interest for astrophysics and applications often rely on statistical model calculations for nuclear reaction rates, particularly for nuclei far from $\beta$-stability. However, statistical model parameters are often poorly constrained, where experimental constraints are particularly sparse for exotic nuclides. For example, our understanding of the breakout from the NiCu cycle in the astrophysical rp-process is currently limited by uncertainties in the statistical properties of the proton-rich nucleus $^{60}$Zn. We have determined the nuclear level density of $^{60}$Zn using neutron evaporation spectra from $^{58}$Ni($^3$He, n) measured at the Edwards Accelerator Laboratory. We compare our results to a number of theoretical predictions, including phenomenological, microscopic, and shell model based approaches. Notably, we find the $^{60}$Zn level density is somewhat lower than expected for excitation energies populated in the $^{59}$Cu(p,$\gamma$)$^{60}$Zn reaction under rp-process conditions. This includes a level density plateau from roughly 5-6 MeV excitation energy, which is counter to the usual expectation of exponential growth and all theoretical predictions that we explore. A determination of the spin-distribution at the relevant excitation energies in $^{60}$Zn is needed to confirm that the Hauser-Feshbach formalism is appropriate for the $^{59}$Cu(p,$\gamma$)$^{60}$Zn reaction rate at X-ray burst temperatures.

The AMS technique as an important tool for the measurement of astrophysical cross sections

Accelerator Mass Spectrometry is a technique commonly used to approach low concentrations of certain long half-life radioisotopes. The most important contribution of the technique is the accurate measure of organic sample ages, by separating masses 12,13 and 14 in the case of carbon allocated in such samples. However, the reach of AMS could cover many other scientific scopes, since it can give us a precise measure of a very small concentration of a radioisotope. On this direction, AMS can be used to approach reactions of interest for astrophysics, if we spot an specific radioisotope which concentration can be measure with AMS. Starting with this, we have selected specific reactions involving 14C, 10Be and 26Al, produced with slow neutrons from a reactor and positive ions at an accelerator. The main idea is to produce a particular reaction and later to measure the radioisotopic concentration using AMS. In this study our first results for 14C and 10Be nuclei produced with neutrons, and the preliminary results for 26Al nuclei produced with deuterium are shown.

Indirect Measurements of n- and p-Induced Reactions of Astrophysical Interest on Oxygen Isotopes

Observations of abundances and isotopic ratio determinations in stars yield powerful constraints on stellar models. In particular, the oxygen isotopic ratios are of particular interest because they are affected not only by nucleosynthesis but also by mixing processes. This review is focused on the measurements via the Trojan Horse Method (THM) that have been carried out to investigate the low-energy cross sections of proton and neutron-induced reactions on 17O as well as the proton-induced reaction on 18O, overcoming extrapolation procedures and enhancement effects due to electron screening. The (p,a) reactions induced on these oxygen isotopes are of paramount importance for the nucleosynthesis in many stellar sites, including red giants (RGs), asymptotic giant branch (AGB) stars, massive stars, and classical novae. The indirect measurement of the 17O(p,a)14N reaction was performed. The strength of the narrow resonance at 65 keV was evaluated and it was used to renormalize the corresponding resonance strength in the 17O+p radiative capture channel. The reaction rate was evaluated for both the 17O(p,a)14N and the 17O(p,g)18F, and a significant difference of 30% and 20% with respect to the literature was found in the temperature range relevant for RG, AGB and massive stars nucleosynthesis. Regarding the 18O(p,a)15N reaction, the strength of the 20 keV resonance was extracted, which is the main contribution to the reaction rate for astrophysics. This approach allowed us to improve the data accuracy of a factor 8.5, as it is based on the measured strength instead of extrapolations or spectroscopic measurements. Finally, the 17O(n,a)14C reaction was studied because of its role during the s-process nucleosynthesis as a possible neutron poison. This study represents the extension of THM to resonant neutron-induced reactions. In this measurement, the subthreshold level centered at -7 keV in the center-of-mass, corresponding to the 8.039 MeV 18O excited level, was observed. Moreover, the THM measurements showed a clear agreement with the available direct measurements because of the additional contribution of the 8.121 MeV 18O level, strongly suppressed in direct measurements because of its l=3 angular momentum. The contributions to the total reaction rate were than evaluated for future astrophysical applications.

PTOLEMEOS: Α 4π γ-detection system for Nuclear Astrophysics

A new 4π detector system, called PTOLEMEOS, especially designed for the study of (p,γ) and (α,γ) reactions of astrophysical interest is presented. Technical data of the target system designed for PTOLEMEOS are also given.

ANC experiments for nuclear astrophysics

Among the indirect methods to determine nuclear cross-section present in literature, the so-called Asymptotic Normalization Coefficient (ANC) has proven to be useful in retrieving the direct part of a radiative capture cross-section in reactions of interest for astrophysics. In this work, the method will be presented, and some results obtained in collaboration between NPI CAS and INFN-LNS will be presented.

Measuring low-energy ([formula omitted]) reaction cross sections using an extended gas target and gas recirculator

Direct measurements of (α,p) reactions of astrophysical interest with radioactive beams presents serious challenges because of the difficult nature of helium targets and the typical low intensities of the beams. To address this, a new technique has been developed for measurements of low-energy (α,p) reactions with heavy ion beams using an extended 4He gas target and a newly developed gas recirculating system. The system was used to measure the 4He(19F, 1H)22Ne reaction as a demonstration. Excitation functions of the 19F(α,p)22Ne and 19F(α,p′)22Ne∗ reactions were successfully measured to show the viability of this technique. Details of the approach and future plans are given.

Proton-Induced Reactions of Astrophysical Interest

A discrepancy exists between the 6Li abundances predicted from big bang nucleosynthesis models and those measured in pre-main sequence stars. To further constrain the predicted abundances of 6Li in these stars, high accuracy measurements are required of reactions destroying 6Li. Namely 6Li(p,γ)7Be and 6Li(p,α) 3He. These have recently been studied at the Laboratory for Underground Nuclear Astrophysics (LUNA) to measure their low energy cross sections. I present both the campaign’s experimental setup and current status of the data analysis.

26Mg target for nuclear astrophysics measurements

Two nuclear reactions of astrophysical interest, 26Mg(3He,d)27Al and 26Mg(d,p)27Mg, were measured for extraction of the Asymptotic Normalization Coefficients. Investigation of the target composition is presented, as well as the effects that showed up during analysis of the in-beam data obtained on CANAM accelerators in the Nuclear Physics Institute of the Czech Academy of Sciences (NPI CAS).

Underground nuclear astrophysics: Why and how

The goal of nuclear astrophysics is to measure cross sections of nuclear physics reactions of interest in astrophysics. At stars temperatures, these cross sections are very low due to the suppression of the Coulomb barrier. Cosmic ray induced background can seriously limit the determination of reaction cross sections at energies relevant to astrophysical processes and experimental setups should be arranged in order to improve the signal-to-noise ratio. Placing experiments in underground sites, however, reduces this background opening the way towards ultra low cross section determination. LUNA (Laboratory for Underground Nuclear Astrophysics) was pioneer in this sense. Two accelerators were mounted at the INFN National Laboratories of Gran Sasso (LNGS) allowing to study nuclear reactions close to stellar energies. A summary of the relevant technology used, including accelerators, target production and characterisation, and background treatment is given.

Indirect techniques for astrophysical reaction rates determinations

Direct measurements of nuclear reactions of astrophysical interest can be challenging. Alternative experimental techniques such as transfer reactions and inelastic scattering reactions offer the possibility to study these reactions by using stable beams. In this context, I will present recent results that were obtained in Orsay using indirect techniques. The examples will concern various astrophysical sites, from the Big-Bang nucleo synthesis to the production of radioisotopes in massive stars.

Using the Trojan Horse Method to Investigate Resonances Above and Below the Threshold in Nuclear Reactions of Astrophysical Interest

Using the Trojan Horse Method to Investigate Resonances Above and Below the Threshold in Nuclear Reactions of Astrophysical Interest

Breakup following interactions with light targets: Investigating new methods to probe nuclear physics input to the cosmological lithium problem.

A well known issue with concordance cosmology is the cosmological lithium problem, where models of Big Bang Nucleosynthesis indicate abundances of 7 Li three to four times larger than values inferred via spectroscopic measurements of metal-poor halo stars [1]. Since the source of this discrepancy remains unclear [2], it is vital to fully understand the nuclear reactions that affect the production of 7 Li during the Big Bang [3]. At the Australian National University, experimental equipment and analysis techniques have been developed for nuclear reaction studies at energies near the fusion barrier, exploiting large solid angle detectors to enable the investigation of breakup without a priori assumption of the breakup kinematics. The extension to reactions of astrophysical interest may help shed light on these reactions. Recent experiments, using these new techniques, have provided a complete picture of the breakup mechanisms of light nuclei in collisions with heavy targets [4]. The present work focuses on obtaining a complete picture of breakup mechanisms of 7 Li following interactions with 27 Al. It has been found that breakup is almost exclusively triggered by nucleon transfer between the colliding partners, to a larger extent than was found for heavier targets. The findings of these experiments, as well as progress towards extensions to astrophysically relevant reactions, such as d + 7 Be [5] will be presented.

Formations of DumbbellC118andC119inside Clusters ofC60Molecules by Collision withαParticles

We report highly selective covalent bond modifications in collisions between keV alpha particles and van der Waals clusters of C(60) fullerenes. Surprisingly, C(119)(+) and C(118)(+) are the dominant molecular fusion products. We use molecular dynamics simulations to show that C(59)(+) and C(58)(+) ions--effectively produced in prompt knockout processes with He(2+)--react rapidly with C(60) to form dumbbell C(119)(+) and C(118)(+). Ion impact on molecular clusters in general is expected to lead to efficient secondary reactions of interest for astrophysics. These reactions are different from those induced by photons.

Developing new methods to investigate nuclear physics input to the cosmological lithium problem

A significant challenge to nuclear astrophysics is the cosmological lithium problem, where models of Big Bang nucleosynthesis indicate abundances of 7 Li two to four times larger than what is inferred via spec- troscopic measurements of metal-poor stars. Recent experimental techniques developed for nuclear reaction studies at energies near the fusion barrier, if extended to reactions of astrophysical interest, may help under- stand nuclear reactions that can affect the production of 7 Li during the Big Bang. Experiments at the ANU, using new experimental techniques, have provided complete pictures of the breakup mechanisms of light nuclei in collisions with heavy targets, such as 208 Pb and 209 Bi (1). These experiments revealed dominant breakup mechanisms which had not even been considered in theoretical models. The study of the breakup of 6 Li and 7 Li following interactions with 58,64 Ni and 27 Al acts as a stepping stone from this previous work towards future ex- perimental studies of breakup reactions of astrophysical relevance. In all cases studied, breakup is dominantly triggered by nucleon transfer between the colliding partners, but the transfer mechanisms are different. The findings of these experiments and experimental considerations for extensions to reactions of light nuclei, such as d + 7 Be, will be presented.

MICROSCOPIC CLUSTER MODELS

General aspect of microscopic cluster models based on the combination of the Generator-Coordinate-Method and of the R-matrix method are presented. The adequacy of such a method to describe cluster states and reactions of astrophysical interest is illustrated by two typical examples.

Recent Developments in Reactions of Astrophysical Interest

We present different models used in nuclear astrophysics. In particular the role of microscopic cluster theories is emphasized. Recent applications on the triple-α process, and on the 12C(α, γ)16O and 3He(α, γ)7Be reactions are discussed.

MOST EFFECTIVE ENERGY IN THERMONUCLEAR REACTIONS OF SOME LIGHT NUCLEI

Gamow peak describes the most effective energy E0 for a nonresonant nuclear reaction to occur. At astrophysical low energies much lower than the Coulomb barrier, the ability of the interacting nuclei to tunnel through the barrier depends on the primitive probability which is proportional to the Gamow factor. The reaction rate will then be determined by the primitive tunneling probability and the astrophysical S-factor. In the literature, tables on the thermonuclear reaction rates are compiled for many reactions of interest in astrophysics by using this low energy approximation. In this paper, we describe a method to obtain E0 by using the exact tunneling probability that is valid for higher energy. We illustrate the method by using three major reactions in the proton-proton chain, 3 He (3 He , 2p)4 He , 3 He (4 He , γ)7 Be and 7 Be (γ, p)8 B . In all cases, E0 starts to divert to lower values than the low energy approximation limit at around T = 109 K and hence generate lower reaction rates. This result might be significantly important for the thermonuclear reactions in the advanced stages of the evolution of massive stars.

The Trojan horse method in nuclear astrophysics: recent results

Difficulties in cross-section measurements at very low energies, when charged particles are involved, led to the development of some indirect methods. The Trojan horse method (THM) allows us to bypass the Coulomb effects and has been successfully applied to several reactions of astrophysical interest. A brief review of the THM applications is reported together with some of the most recent results.