25Mg Reaction Research Articles

S-processing in AGB Stars Revisited. III. Neutron Captures from MHD Mixing at Different Metallicities and Observational Constraints

Abstract We present post-process neutron-capture computations for Asymptotic Giant Branch (AGB) stars of 1.5–3 M ⊙ and metallicities −1.3 ≤ [Fe/H] ≤ 0.1. The reference stellar models are computed with the FRANEC code, using the Schwarzschild's criterion for convection; our motivations for this choice are outlined. We assume that MHD processes induce the penetration of protons below the convective boundary, when the Third Dredge Up occurs. There, the 13C n-source can subsequently operate, merging its effects with those of the 22Ne(α, n)25Mg reaction, activated at the temperature peaks characterizing AGB stages. This work has three main scopes. (i) We provide a grid of abundance yields, as produced through our MHD mixing scheme, which are uniformly sampled in mass and metallicity. From this, we deduce that the solar s-process distribution, as well as the abundances in recent stellar populations, can be accounted for, without the need of the extra primary-like contributions suggested in the past. (ii) We formulate analytic expressions for the mass of the 13C-pockets generated to allow easy verification of our findings. (iii) We compare our results with observations of evolved stars and with isotopic ratios in presolar SiC grains, also noticing how some flux tubes should survive turbulent disruption, carrying C-rich materials into the winds even when the envelope is O-rich. This wind phase is approximated through the G-component of AGB s-processing. We conclude that MHD-induced mixing is adequate to drive slow n-capture phenomena accounting for observations; our prescriptions should permit its inclusion into current stellar evolutionary codes.

Decay of the key 92-keV resonance in the 25Mg(p,γ) reaction to the ground and isomeric states of the cosmic γ-ray emitter 26Al

Decay of the key 92-keV resonance in the 25Mg(p,γ) reaction to the ground and isomeric states of the cosmic γ-ray emitter 26Al

Nuclear Data Sheets for A=23

Evaluated spectroscopic data and level schemes from radioactive decay and nuclear reaction studies are presented for 23N, 23O, 23F, 23Ne, 23Na, 23Mg, 23Al, and 23Si. This evaluation for A=23 supersedes the earlier one by R. B. Firestone (2007Fi02).Highlights of this evaluation are the following:The recommended absolute γ-ray emission probability (%Pγ) in the 23Ne β-decay was reported with 3.0% systematic uncertainty in 1986BrZQ. In 2007Fi02, the Pγ for the most intense γ ray was recommended with 4% uncertainty. These uncertainties are comparable with the total excited state β feeding uncertainty of 33 l, resulting from 100–67 l (g.s. β feeding) in 23Na (1963Ca06). In this evaluation, notes are added for relative and absolute γ-ray emission probabilities.From new measurements 2020Kw01 constrain the spin of the 7788 keV level, the dominant resonance state for the 22Na(p,γ)22Mg reaction rate at stellar temperature, in 23Mg to (3/2+,5/2+). Earlier assignment was 7/2+.The 23Al ε decay scheme appears to be incomplete. Some of the latest reported data on the 23Al ε decay study in the literature need verification.

Decay of the key 92-keV resonance in the 25Mg(p,γ) reaction to the ground and isomeric states of the cosmic γ-ray emitter 26Al

The 92-keV resonance in the 25Mg(p,γ)26Al reaction plays a key role in the production of 26Al at astrophysical burning temperatures of ≈100 MK in the Mg-Al cycle. However, the state can decay to feed either the ground, 26gAl, or isomeric state, 26mAl. It is the ground state that is critical as the source of cosmic γ rays. It is therefore important to precisely determine the ground-state branching fraction f0 of this resonance. Here we report on the identification of four γ-ray transitions from the 92-keV resonance, and determine the spin of the state and its ground-state branching fraction f0=0.52(2)stat(6)syst. The f0 value is the most precise reported to date, and at the lower end of the range of previously adopted values, implying a lower production rate of 26gAl and its cosmic 1809-keV γ rays.

The Study of the 22Ne(α,γ)26Mg Reaction at LUNA

The 22Ne(α,γ)26Mg reaction is the competitor of the 22Ne(α,γ)25Mg reaction, an effective neutron source for element synthesis through s-process in massive and AGB stars. Currently the ratio between the rates of these two reactions is poorly constrained because of the high uncertainty affecting the 22Ne(α,γ)26Mg reaction rate. Indeed a wide range of values for the 395 keV resonance strength (10−15 - 10−9 eV) is reported in literature, all of them from indirect measurements. The present study represents the first direct measurement which was performed at the ultra-low background LUNA laboratory. An high efficiency detector was installed at the gas target beamline of LUNA 400kV accelerator and the 99% enriched in 22Ne neon gas was irradiated with a 399.9 keV α-beam. No significant signal was detected in the 22Ne(α,γ)26Mg region of interest, thus an upper limit for the 395 keV resonance strength was estimated. A new campaign was completed in August 2019 with an improved setup and some details are reported here.

Constraining the 22Ne(α,γ)26Mg and 22Ne(α,n)25Mg reaction rates using sub-Coulomb α-transfer reactions

The 22Ne(α,γ)26Mg and 22Ne(α,n)25Mg reactions play an important role in astrophysics because they have significant influence on the neutron flux during the weak branch of the s-process. We constrain the astrophysical rates for these reactions by measuring partial α-widths of resonances in 26Mg located in the Gamow window for the Ne22+α capture. These resonances were populated using 22Ne(6Li,d)26Mg and 22Ne(7Li,t)26Mg reactions at energies near the Coulomb barrier. At these low energies α-transfer reactions favor population of low spin states and the extracted partial α-widths for the observed resonances exhibit only minor dependence on the model parameters. The astrophysical rates for both the 22Ne(α,γ)26Mg and the 22Ne(α,n)25Mg reactions are shown to be significantly different than the previously suggested values.

Decay properties of 22Ne + α resonances and their impact on s-process nucleosynthesis

The astrophysical s-process is one of the two main processes forming elements heavier than iron. A key outstanding uncertainty surrounding s-process nucleosynthesis is the neutron flux generated by the Ne22(α,n)25Mg reaction during the He-core and C-shell burning phases of massive stars. This reaction, as well as the competing Ne22(α,γ)26Mg reaction, is not well constrained in the important temperature regime from ∼0.2–0.4 GK, owing to uncertainties in the nuclear properties of resonances lying within the Gamow window. To address these uncertainties, we have performed a new measurement of the Ne22(Li6,d)26Mg reaction in inverse kinematics, detecting the outgoing deuterons and Mg25,26 recoils in coincidence. We have established a new n/γ decay branching ratio of 1.14(26) for the key Ex=11.32 MeV resonance in Mg26, which results in a new (α,n) strength for this resonance of 42(11)μeV when combined with the well-established (α,γ) strength of this resonance. We have also determined new upper limits on the α partial widths of neutron-unbound resonances at Ex=11.112, 11.163, 11.169, and 11.171 MeV. Monte-Carlo calculations of the stellar Ne22(α,n)25Mg and Ne22(α,γ)26Mg rates, which incorporate these results, indicate that both rates are substantially lower than previously thought in the temperature range from ∼0.2–0.4 GK.

Study of the 22Ne(α , γ)26Mg reaction at LUNA

The 22Ne( α , γ)26Mg reaction competes with the 22Ne(α, n)25Mg reac-tion which is the main source of neutrons for the s-process in low-mass Asymptotic Giant Branch (AGB) and massive stars. The 22Ne( α , γ)26Mg reaction rateis affected by a high uncertainty mainly due to the poorly constrained 395 keVresonance which has been studied only indirectly leading to a wide range of pos-sible values for its resonance strength (10-14 - 10-9 eV). The present study represents the direct measurement of the 395 keV resonance of the 22Ne( α , γ)26Mgreaction at LUNA (Laboratory for Underground Nuclear Astrophysics), located at Gran Sasso National Laboratory. Here, the experimental campaigns, setupand some very preliminary results are presented.

Integral measurement of spectrum-averaged cross sections of a few threshold reactions induced by fast neutrons of a TRIGA reactor: comparison with integrated data from excitation functions given in various data libraries

Abstract Integral cross sections of the reactions 24Mg(n,p)24Na, 27Al(n,p)27Mg, 27Al(n,α)24Na, 58Ni(n,d + np)57Co and 60Ni(n,p)60Co were measured for the first time using the fast neutron spectrum of a TRIGA reactor extending from 0.5 to 20 MeV. The values obtained in this work were comparable with the recommended values for a pure 235U prompt fission spectrum. The measured integral value was utilized for integral test of excitation function of each reaction given in some data libraries, namely ENDF/B-VIII.0, TENDL-2017, IRDFF-1.05 and ROSFOND-2010. The integral measurements are generally consistent with the integrated values within 5 %, except for a few cases, e. g. the reaction 60Ni(n,p)60Co, where the data libraries appear to need improvement.

Direct measurements of low-energy resonance strengths of the 23Na(p,γ)24Mg reaction for astrophysics

The NeNa and the MgAl cycles play a fundamental role in the nucleosynthesis of asymptotic giant branch stars undergoing hot bottom burning. The Na23(p,γ)24Mg reaction links these two cycles and a precise determination of its rate is required to correctly estimate the contribution of these stars to the chemical evolution of various isotopes of Na, Mg and Al. At temperatures of 50≲T≲110MK, narrow resonances at Ep=140 and 251keV are the main contributors to the reaction rate, in addition to the direct capture that dominates in the lower part of the temperature range. We present new measurements of the strengths of these resonances at the Laboratory for Underground Nuclear Astrophysics (LUNA). We have used two complementary detection approaches: high efficiency with a 4π BGO detector for the 140keV resonance, and high resolution with a HPGe detector for the 251keV resonance. Thanks to the reduced cosmic ray background of LUNA, we were able to determine the resonance strength of the 251keV resonance as ωγ=482(82)μeV and observed new gamma ray transitions for the decay of the corresponding state in Mg24 at Ex=11931keV. With the highly efficient BGO detector, we observed a signal for the 140keV resonance for the first time in a direct measurement, resulting in a strength of ωγ140=1.46−0.53+0.58neV (68% CL). Our measurement reduces the uncertainty of the Na23(p,γ)24Mg reaction rate in the temperature range from 0.05 to 0.1GK to at most −35%+50% at 0.07GK. Accordingly, our results imply a significant reduction of the uncertainties in the nucleosynthesis calculations.

The s-process enriched star HD 55496: origin from a globular cluster or from the tidal disruption of a dwarf galaxy?

Abstract We present a new abundance analysis of HD 55496, previously known as a metal-poor barium star. We found that HD 55496 has a metallicity [Fe/H] = −1.55 and is s-process enriched. We find that HD 55496 presents four chemical peculiarities: (i) a Na–O abundance anticorrelation; (ii) it is aluminium rich; (iii) it is carbon poor for an s-process enriched star, and (iv) the heavy second s-process peak elements, such as Ba, La, Ce, and Nd, present smaller abundances than the light s-process elements, such as Sr, Y, and Zr, which is not usually observed among the chemically peculiar binary stars at this metallicity. The heavy-element abundance pattern suggests that the main source of the neutrons is the 22Ne(α,n)25Mg reaction. Taking all these abundance evidences together into consideration strongly suggests that HD 55496 is a ‘second-generation of globular cluster stars’ formed from gas already strongly enriched in s-process elements and now is a field halo object. Our dynamical analysis, however, indicates that the past encounter probabilities with the known globular clusters are very low ($\le \!6{{\ \rm per\ cent}}$). This evidence, together with the retrograde motion, points to a halo intruder possibly originated from the tidal disruption of a dwarf galaxy.

Role of the Mechanism of Heavy Cluster Stripping in a 27Al(р, α)24Mg Reaction

The contribution from the mechanism of heavy 23Na cluster stripping from a 27Al(5/2+) initial nucleus to a cross section of the 27Al(р, α)24Mg(0+, 2+) reaction is calculated at Ep = 7.4 MeV and angles of α‑particle emission of 25°–160° (laboratory). Calculations are made with the FRESCO code using the contribution from the 3/2+ ground state of 23Na. Spectroscopic amplitudes at decay vertices of 27Al(5/2+) → 23Na(3/2+) + α and 24Mg(0+, 2+) → 23Na(3/2+) + р reactions are determined from wave functions in the context of the Nilsson model, depending on their deformation. Cross sections calculated for the 0+ and 2+ levels of the 24Mg nucleus are two orders of magnitude lower than those found experimentally. The cross section for the 0+ level grows at θα ∼ 180° in the back semi-sphere, while the one for the 2+ level exhibits a maximum at θα > 140°. The slight contribution from the mechanism of heavy cluster stripping is due to the spectroscopic amplitudes at the decay vertices of the initial and final nuclei owing to a weak overlap of wave functions of 27Al, 23Na, and 24Mg(0+, 2+) nuclei in the Nilsson model.

Study of the differential cross section of the 25Mg(p,p'γ)25Mg reaction for PIGE purposes

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Analysis of high energy resolution data of 26Mg(3He,t)26Al reaction

The Gamow-Teller (GT) transition is a powerful tool to study nuclear structure because of its simple form of the operator στ. The structure of 26Al is studied through Gamow-Teller transitions using nuclear charge-exchange reaction. The reaction 26Mg(3He,t)26Al was performed at an incident energy of 140 MeV/nucleon and scattering angle at and near 0˚. The energy resolution of ΔE = 22 keV allowed us to study many discrete states. Most of the prominent states are suggested that they are excited with ΔL = 0 GT transitions. The GT states were studied up to 18.5 MeV. For the extraction of the B(GT) value, the proportionality between cross section and B(GT) was used. The standard B(GT) values were obtained from the 26Si beta decay, where the mirror symmetry of B(GT) was obtained. The T = 2 GT states are expected in the region Ex ≥ 13.5 MeV. By comparing with the results of 26Mg(t, 3He)26Na reactions, the isospin symmetry of T = 2 GT states is discussed. Due to the high-energy resolution, the decay widths Γ for the states in the Ex > 9 MeV region could be studied. The narrow width of the T = 2 states at 13.592 MeV is explained in terms of isospin selection rules.

Theoretical Discourse on Producing High Temporal Yields of Nuclear Excitations in Cosmogenic 26Al with a PW Laser System: The Pathway to an Astrophysical Earthbound Laboratory

The development of the 10 PW laser system at the Extreme Light Infrastructure is a crucial step towards the realization of an astrophysical Earthbound laboratory. The interaction of high-power laser pulses with matter results in ultrashort (fs-ps) pulses of 10s of MeV ions and radiation that can create plasma and induce nuclear reactions therein. Due to the high fluxes of reaction-driving beam pulses, high yields of radioactive target nuclei in their ground and excited states can be provided in situ on short time scales. Cosmogenic 26Al, which is of pronounced astrophysical interest, is a prime candidate for evaluating these new experimental possibilities. We describe how, for a short duration of Δ t ∼ 200 ps , laser-driven protons with energies above E p ∼ 5 MeV can induce the compound nucleus reaction 26Mg(p, n)26Al leading to high and comparable yields of the three lowest-lying states in 26Al including the short-lived, t 1 / 2 = 1.20 ns state at 417 keV. In the aftermath of the reaction, for a short duration of t ∼ ns , the yield ratios between the ground and the two lowest-lying excited states will resemble those present at thermodynamic equilibrium at high temperatures, thus mimicking high 26Al entropies in cold environments. This can be seen as a possible first step towards an investigation of the interplay between those states in plasma environments. Theory suggests an intricate coupling of the ground state 26Alg.s. and the first excited isomer 26mAl via higher-lying excitations such as the J = 3 + state at 417 keV resulting in a dramatic reduction of the effective lifetime of 26Al which will influence the isotope’s abundance in our Galaxy.

Mapping the demise of collective motion in nuclei at high excitation energy

High energy gamma-rays from the 116Sn + 24Mg reaction at 23A MeV were measured using the MEDEA detector at LNS – INFN Catania. Combining this new data with previous measurements yields a detailed view of the quenching of the Giant Dipole Resonance as a function of excitation energy in nuclei of mass A in the range 120÷132. The transition towards the disappearance of the dipole strength, which occurs around 230 MeV excitation energy, appears to be remarkably sharp. Current phenomenological models give qualitative explanations for the quenching but cannot reproduce its detailed features.

Analysis of cross sections of (n,t) nuclear reaction using different empirical formulae and level density models

This paper aims the calculations of cross section of the 27Al(n,t)25Mg, 50Cr(n,t)48V, 56Fe(n,t)54Mn, 58Ni(n,t)56Co, 64Zn(n,t)62Cu, 70Ge(n,t)68Ga, 92Mo(n,t)90Nb, 106Cd(n,t)104Ag and 114Cd(n,t)112Ag nuclear reactions. These cross section calculations were obtained using the different level density models in ALICE/ASH code. Besides, the cross sections of the investigated reactions at incident energies near 14.6 MeV were predicted by using the empirical formulae suggested in our previous paper and in the literature. Therefore, the predicted nuclear data are compared with the other data taken from the literature.

High-resolution study of levels in the astrophysically important nucleus Mg26 and resulting updated level assignments

The $^{22}$Ne($\alpha,n$)$^{25}$Mg reaction is an important source of neutrons for the s-process. Due to the difficulty in directly measuring the cross section of this reaction and the competing $^{22}$Ne($\alpha,\gamma$)$^{26}$Mg reaction, indirect methods are frequently used to try to instead calculate the relevant reaction rates. Uncertainty, however, persists in the number and properties of the pertinent excited states of $^{26}$Mg. In this paper, the results of proton and deuteron scattering from $^{26}$Mg are presented, giving a list of levels in $^{26}$Mg. In addition, level assignments in $^{26}$Mg are discussed, including some possible changes to the previously accepted assignments in $^{26}$Mg.

Photodisintegration reactions for nuclear astrophysics studies at ELI-NP

Extreme Light Infrastructure - Nuclear Physics facility will come online in Bucharest-Magurele, Romania, in 2018 and will deliver high intensity laser and brilliant gamma beams. We present the physics cases and instruments proposed at ELI-NP to measure capture reactions by means of the inverse photodisintegration reaction. We propose to study the 16O(γ, α)12C reaction using a Time Projection Chamber detector with electronic readout. Several other reactions, such as 24Mg(γ, α)20Ne and reactions on heavy nuclei relevant in the p-process, are central to stellar evolution and will be investigated with a proposed Silicon Strip Detector array and a 4π neutron detector. The status of the experimental facilities and first-day experiments will be presented in detail.

SIMULATED 8 MeV NEUTRON RESPONSE FUNCTIONS OF A THIN SILICON NEUTRON SENSOR.

Neutron response functions of a thin silicon neutron sensor are simulated using PHITS2 and MCNP6 codes for an 8 MeV neutron beam at angles of incidence of 0°, 30° and 60°. The contributions of alpha particles created from the 28Si(n,α)25Mg reaction and the silicon nuclei scattered elastically by neutrons in the silicon sensor have not been well reproduced using the MCNP6 code. The 8 MeV neutron response functions simulated using the PHITS2 code with an accurate event generator mode are in good agreement with experimental results and include the contributions of the alpha particles and silicon nuclei.