Ne Reaction Research Articles

Direct measurement of the break-out 19F(p, γ)20Ne reaction in the China Jinping underground laboratory (CJPL)

Direct measurement of the break-out 19F(p, γ)20Ne reaction in the China Jinping underground laboratory (CJPL)

Experimental Determination of α Widths of ^{21}Ne Levels in the Region of Astrophysical Interest: New ^{17}O+α Reaction Rates and Impact on the Weak s Process.

The efficiency of the weak s process in low-metallicity rotating massive stars depends strongly on the rates of the competing ^{17}O(α,n)^{20}Ne and ^{17}O(α,γ)^{21}Ne reactions that determine the potency of the ^{16}O neutron poison. Their reaction rates are poorly known in the astrophysical energy range of interest for core helium burning in massive stars because of the lack of spectroscopic information (partial widths, spin parities) for the relevant states in the compound nucleus ^{21}Ne. In this Letter, we report on the first experimental determination of the α-particle spectroscopic factors and partial widths of these states using the ^{17}O(^{7}Li,t)^{21}Ne α-transfer reaction. With these the ^{17}O(α,n)^{20}Ne and ^{17}O(α,γ)^{21}Ne reaction rates were evaluated with uncertainties reduced by a factor more than 3 with respect to previous evaluations and the present ^{17}O(α,n)^{20}Ne reaction rate is more than 20 times larger. The present (α,n)/(α,γ) rate ratio favors neutron recycling and suggests an enhancement of the weak s process in the Zr-Nd region by more than 1.5 dex in metal-poor rotating massive stars.

Intrinsic background of EJ-309 liquid scintillator detectors

In deep-underground experiments, intrinsic detector background can exceed the external one, limiting the detection sensitivity. Intrinsic background in liquid scintillator neutron detectors consists of α particles emitted from radioactive impurities in the detector housing (and possibly in the liquid). We present the results of long background measurements of 12 EJ-309 liquid organic scintillators detectors in the deep-underground Gran Sasso National Laboratory of the Italian Institute of Nuclear Physics. These detectors are a part of the detection array for the SHADES ERC project that aims to measure the very low cross section (down to picobarn) of the astrophysically important reaction 22Ne(α,n)25Mg. With an exposure of ∼35 days we identify an actinide concentration in the detector housing of 8.4(1.8)sta(1.4)sys×10−2ppm and 1.62(0.57)sta(0.03)sys×10−1ppm of 238U and 232Th, resulting in 64 α particle counts per hour. The half-life of 214Po was also determined as 163.7(11)μs.

Measurement of the ^{18}O(α, γ)^{22}Ne Reaction Rate at JUNA and Its Impact on Probing the Origin of SiC Grains.

The ^{18}O(α,γ)^{22}Ne reaction is critical for AGB star nucleosynthesis due to its connection to the abundances of several key isotopes, such as ^{21}Ne and ^{22}Ne. However, the ambiguous resonance energy and spin-parity of the dominant 470keV resonance leads to substantial uncertainty in the ^{18}O(α,γ)^{22}Ne reaction rate for the temperature of interest. We have measured the resonance energies and strengths of the low-energy resonances in ^{18}O(α,γ)^{22}Ne at the Jinping Underground Nuclear Astrophysics experimental facility (JUNA) with improved precision. The key 470keV resonance energy has been measured to be E_{α}=474.0±1.1 keV, with such high precision achieved for the first time. The spin-parity of this resonance state is determined to be 1^{-}, removing discrepancies in the resonance strengths in earlier studies. The results significantly improve the precision of the ^{18}O(α,γ)^{22}Ne reaction rates by up to about 10 times compared with the previous data at typical AGB temperatures of 0.1-0.3GK. We demonstrate that such improvement leads to precise ^{21}Ne abundance predictions, with an impact on probing the origin of meteoritic stardust SiC grains from AGB stars.

Nuclear structure studies in mirror nuclei

The nuclear structure of the A=31 and A=47 mirror couples produced by two fusion evaporation reactions has been elaborated, utilizing the Doppler-shift attenuation method. Excited states in 31P and 31S were populated using the 1p and 1n exit channels, respectively, of the reaction 20Ne + 12C, while in 47Cr and 47V couple excited states were populated based on 28Si + 28Si reaction, as products of 2αn and 2αp exit channels. The A=31 mirror couple was studied utilizing Piave-Alpi accelerator of the Laboratori Nazionali di Legnaro with GASP multidetector array and for A=47 one - with the EUROBALL array using XTU Tandem also in Legnaro. In both cases the lifetime measurements in mirror couples at the same experiment open possibilities for investigations of isospin symmetry. Determined B(E1) strengths in the mirror nuclei 31P and 31S allow to extract the isoscalar component, which can reach up to 24% of the isovector one. The B(E1) values can be modeled by the Equation of motion method. In the case of A=47 mirror couple, the quadrupole moments can be described by shell-model calculations.

Measurement of 19F(p, γ)20Ne reaction suggests CNO breakout in first stars.

Proposed mechanisms for the production of calcium in the first stars (population III stars)-primordial stars that formed out of the matter of the Big Bang-are at odds with observations1. Advanced nuclear burning and supernovae were thought to be the dominant source of the calcium production seen in all stars2. Here we suggest a qualitatively different path to calcium production through breakout from the 'warm' carbon-nitrogen-oxygen (CNO) cycle through a direct experimental measurement of the 19F(p, γ)20Ne breakout reaction down to a very low energy point of 186 kiloelectronvolts, reporting a key resonance at 225 kiloelectronvolts. In the domain of astrophysical interest2, at around 0.1 gigakelvin, this thermonuclear 19F(p, γ)20Ne rate is up to a factor of 7.4 larger than the previous recommended rate3. Our stellar models show a stronger breakout during stellar hydrogen burning than previously thought1,4,5, and may reveal the nature of calcium production in population III stars imprinted on the oldest known ultra-iron-poor star, SMSS0313-67086. Our experimental result was obtained in the China JinPing Underground Laboratory7, which offers an environment with an extremely low cosmic-ray-induced background8. Our rate showcases the effect that faint population III star supernovae can have on the nucleosynthesis observed in the oldest known stars and first galaxies, which are key mission targets of the James Webb Space Telescope9.

Constraints on key O17(α,γ)Ne21 resonances and impact on the weak s process

The efficiency of the slow neutron-capture process in massive stars is strongly influenced by neutron-capture reactions on light elements. At low metallicity, $^{16}$O is an important neutron absorber, but the effectiveness of $^{16}$O as a light-element neutron poison is modified by competition between subsequent $^{17}$O$(\alpha,n)^{20}$Ne and $^{17}$O$(\alpha,\gamma)^{21}$Ne reactions. The strengths of key $^{17}$O$(\alpha,\gamma)^{21}$Ne resonances within the Gamow window for core helium burning in massive stars are not well constrained by experiment. This work presents more precise measurements of resonances in the energy range $E_{c.m.} = 612 - 1319$ keV. We extract resonance strengths of $\omega\gamma_{638} = 4.85\pm0.79$ $\mu$eV, $\omega\gamma_{721} = 13.0^{+3.3}_{-2.4}$ $\mu$eV, $\omega\gamma_{814} = 7.72\pm0.55$ meV and $\omega\gamma_{1318} = 136\pm 13$ meV, for resonances at $E_{c.m.} =$ 638, 721, 814 and 1318 keV, respectively. We also report an upper limit for the 612 keV resonance of $\omega\gamma<140$ neV ($95\%$ c.l.), which effectively rules out any significant contribution from this resonance to the reaction rate. From this work, a new $^{17}$O$(\alpha,\gamma)^{21}$Ne thermonuclear reaction rate is calculated and compared to the literature. The effect of present uncertainties in the $^{17}$O$(\alpha,\gamma)^{21}$Ne reaction rate on weak s-process yields are then explored using post-processing calculations based on a rotating $20M_{\odot}$ low-metallicity massive star. The resulting $^{17}$O$(\alpha,\gamma)^{21}$Ne reaction rate is lower with respect to the pre-existing literature and found to enhance weak s-process yields in rotating massive star models.

The impact of 17O + α reaction rate uncertainties on the s-process in rotating massive stars

ABSTRACT Massive stars are crucial to galactic chemical evolution for elements heavier than iron. Their contribution at early times in the evolution of the Universe, however, is unclear due to poorly constrained nuclear reaction rates. The competing 17O(α, γ)21Ne and 17O(α, n)20Ne reactions strongly impact weak s-process yields from rotating massive stars at low metallicities. Abundant 16O absorbs neutrons, removing flux from the s-process, and producing 17O. The 17O(α, n)20Ne reaction releases neutrons, allowing continued s-process nucleosynthesis, if the 17O(α, γ)21Ne reaction is sufficiently weak. While published rates are available, they are based on limited indirect experimental data for the relevant temperatures and, more importantly, no uncertainties are provided. The available nuclear physics has been evaluated, and combined with data from a new study of astrophysically relevant 21Ne states using the 20Ne(d, p)21Ne reaction. Constraints are placed on the ratio of the (α, n)/(α, γ) reaction rates with uncertainties on the rates provided for the first time. The new rates favour the (α, n) reaction and suggest that the weak s-process in rotating low-metallicity stars is likely to continue up to barium and, within the computed uncertainties, even to lead.

Thep-process in exploding rotating massive stars

Context.Thep-process nucleosynthesis can explain proton-rich isotopes that are heavier than iron, which are observed in the Solar System, but discrepancies still persist (e.g. for the Mo and Rup-isotopes), and some important questions concerning the astrophysical site(s) of thep-process remain unanswered.Aims.We investigate how thep-process operates in exploding rotating massive stars that have experienced an enhanceds-process nucleosynthesis during their life through rotational mixing.Methods.With the Geneva stellar evolution code, we computed 25M⊙stellar models at a metallicity ofZ = 10−3with different initial rotation velocities and rates for the still largely uncertain17O(α,γ)21Ne reaction. The nucleosynthesis calculation, followed with a network of 737 isotopes, was coupled to stellar evolution, and thep-process nucleosynthesis was calculated in post-processing during both the final evolutionary stages and spherical explosions of various energies. The explosions were modelled with a relativistic hydrodynamical code.Results.In our models, thep-nuclides are mainly synthesized during the explosion, but not much during the ultimate hydrostatic burning stages. Thep-process yields mostly depend on the initial number of trans-iron seeds, which in turn depend on the initial rotation rate. We found that the impact of rotation on thep-process is comparable to the impact of rotation on thes-process. From no to fast rotation, thes-process yields of nuclides with mass numberA &lt; 140 increase by 3−4 dex, and so do thep-process yields. Fast rotation with a lower17O(α, γ) rate significantly producess- andp-nuclides withA ≥ 140. The dependence of thep-process yields on the explosion energy is very weak.Conclusions.Our results suggest that the contribution of core-collapse supernovae from massive stars to the solar (and Galactic)p-nuclei has been underestimated in the past, and more specifically, that the contribution from massive stars with sub-solar metallicities may even dominate. A more detailed study including stellar models with a wide range of masses and metallicities remains to be performed, together with a quantitative analysis that is based on the chemical evolution of the Galaxy.

Measurement of Low-Energy Resonance Strengths in the O18(α,γ)Ne22 Reaction

The ^{18}O(α,γ)^{22}Ne reaction is an essential part of a reaction chain that produces the ^{22}Ne(α,n)^{25}Mg neutron source for both the weak and main components of the slow neutron-capture process. At temperatures of stellar helium burning, the astrophysically relevant resonances in the ^{18}O(α,γ)^{22}Ne reaction that dominate the reaction rate occur at α particle energies E_{lab} of 472 and 569 keV. However, previous experiments have shown the strengths of these two resonances to be very weak, and only upper limits or partial resonance strengths could be obtained. This Letter reports the first direct measurement of the total resonance strength for the 472- and 569-keV resonances, 0.26±0.05 and 0.63±0.30 μeV, respectively. New resonance strengths for the resonances at α particle energies of 662.1, 749.9, and 767.6 keV are also provided. These results were achieved in an experiment optimized for background suppression and detection efficiency. The experiment was performed at the Sanford Underground Research Facility, in the 4850-foot underground cavity dedicated to the Compact Accelerator System for Performing Astrophysical Research. The experimental end station used the γ-summing High EffiCiency TOtal absorption spectrometeR. Compared to previous works, the results decrease the stellar reaction rate by as much as ≈46_{-11}^{+6}% in the relevant temperature range of stellar helium burning.

Measurement of the 18O(α, γ)22Ne resonances at JUNA

22Ne(α,n)25Mg is one of the main neutron sources of the s process. 22Ne is produced by the 14N(α, γ)18F(β+)18O(α, γ)22Ne reaction chain in the helium burning, thus, the production rate of 22Ne is dominated by 14N(α,γ)18F and 18O(α,γ)22Ne. At the astrophysical relevant temperatures, the 18O(α,γ)22Ne reaction rates are determined by several low-energy resonances. In this work, the 18O(α,γ)22Ne reaction was measured at the 400 kV accelerator of Jinping Underground Nuclear Astrophysics experiment (JUNA). The γ-ray yields of the resonances between 470 to 770 keV were obtained.

Measuring the 15O(α, γ)19Ne reaction in Type I X-ray bursts using the GADGET II TPC: Hardware

Sensitivity studies have shown that the 15O(α, γ)19Ne reaction is the most important reaction rate uncertainty affecting the shape of light curves from Type I X-ray bursts. This reaction is dominated by the 4.03 MeV resonance in 19Ne. Previous measurements by our group have shown that this state is populated in the decay sequence of 20Mg. A single 20Mg(βp α)15O event through the key 15O(α, γ)19Ne resonance yields a characteristic signature: the emission of a proton and alpha particle. To achieve the granularity necessary for the identification of this signature, we have upgraded the Proton Detector of the Gaseous Detector with Germanium Tagging (GADGET) into a time projection chamber to form the GADGET II detection system. GADGET II has been fully constructed, and is entering the testing phase.

The SHADES neutron detection array

The detection of neutrons in low energy astrophysics poses two main problems: low cross section measurement requires high detection efficiency and ideally, beam-induced and external neutron backgrounds should be identified through an energy sensitive detector. In recent years, capture-gated neutron spectroscopy has found new opportunities to grow through new materials and better availability of modern data acquisition systems. In this work we will present the design, expected capabilities and preliminary characterization of the hybrid neutron detector array SHADES. Its purpose is the first direct measurement of the reaction 22Ne(α, n)25Mg in the Gamow window for s-process nucleosynthesis with LUNA-MV in the deep underground environment of the LNGS.

Transition probabilities in 31P and 31S: A test for isospin symmetry

Excited states in the mirror nuclei 31P and 31S were populated in the 1p and 1n exit channels of the reaction 20Ne + 12C, at a beam energy of 33 MeV. The 20Ne beam was delivered for the first time by the Piave-Alpi accelerator of the Laboratori Nazionali di Legnaro. Angular correlations of coincident γ-rays and Doppler-shift attenuation lifetime measurements were performed using the multi-detector array GASP in conjunction with the EUCLIDES charged particle detector. In the observed B(E1) strengths, the isoscalar component, amounting to 24% of the isovector one, provides strong evidence for breaking of the isospin symmetry in the A=31 mass region. Self-consistent beyond mean field calculations using Equation of Motion method based on a chiral potential and including two- and three-body forces reproduce well the experimental B(E1) strengths, reinforcing our conclusion. Coherent mixing from higher-lying states involving the Giant Isovector Monopole Resonance accounts well for the effect observed. The breaking of the isospin symmetry originates from the violation of the charge symmetry of the two- and three-body parts of the potential, only related to the Coulomb interaction.

Studying the heavy-ion fusion reactions at stellar energies using Time Projection Chamber

Heavy-ion fusion reactions, such as 12C+12C, play crucial roles in stellar evolution and nucleosynthesis in massive stars. However, limited by the low cross section and large background, it is very challenging to study the reactions at stellar energies. We have developed a 1024-channel prototype TPC named MATE (Multi-purpose time projection chamber for nuclear AsTrophysical and Exotic beam experiments) to study these reactions at stellar energies. The design of our prototype TPC and its performance are described, and the preliminary results of a commissioning experiment for measuring the 12C(12C, α0)20Ne reaction at Ecm = 3.0 MeV are presented. Perspectives of future experiments for the 12C+12C fusion cross section measurement are also provided.

New measurement of the Ec.m.=323 keV resonance in the F19(p,γ)Ne20 reaction

At temperatures below 0.1 GK the $^{19}$F$(p,\gamma)^{20}$Ne reaction is the only breakout path out of the CNO cycle. Experimental studies of this reaction are challenging from a technical perspective due to copious $\gamma$-ray background from the far stronger $^{19}$F$(p,\alpha)^{16}$O reaction channel. Here we present the first inverse kinematics study of the $^{19}$F$(p,\gamma)^{20}$Ne reaction, in which we measure the strength of the 323-keV resonance. We find a strength value of $\omega\gamma = 3.3^{+1.1}_{-0.9}$ meV, which is a factor of two larger than the most recent previous study. The discrepancy is likely the result of a direct to ground state transition which previous studies were not sensitive to. We also observe the transition to the first $2^{-}$ state, which has not been observed for this resonance in previous studies. A new thermonuclear reaction rate is calculated and compared with the literature.

Chemi-Ionization Reactions and Basic Stereodynamical Effects in Collisions of Atom-Molecule Reagents.

A new theoretical method, developed by our laboratory to describe the microscopic dynamics of gas-phase elementary chemi-ionization reactions, has been applied recently to study prototype atom–atom processes involving reactions between electronically excited metastable Ne*(3P2,0) and heavier noble gas atoms. Important aspects of electronic rearrangement selectivity have been emphasized that suggested the existence of two fundamental microscopic reaction mechanisms. The distinct mechanisms, which are controlled by intermolecular forces of chemical and noncovalent nature respectively, emerge under different conditions, and their balance depends on the collision energy regime investigated. The present paper provides the first step for the extension of the method to cases involving molecules of increasing complexity, whose chemi-ionization reactions are of relevance in several fields of basic and applied researches. The focus is here on the reactions of Ne* with simple inorganic molecules as Cl2 and NH3, and the application of the method discloses relevant features of the reaction microscopic evolution. In particular, this study shows that the balance of two fundamental reaction mechanisms depends not only on the collision energy and on the relative orientation of reagents but also on the orbital angular momentum of each collision complex. The additional insights so emphasized are of general relevance to assess in detail the stereodynamics of many other elementary processes.

Electronic Rearrangements and Angular Momentum Couplings in Quantum State-to-State Channels of Prototype Oxidation Processes.

An innovative theoretical method to describe the microscopic dynamics of chemi-ionization reactions as prototype oxidation processes driven by selective electronic rearrangements has been recently published. It was developed and applied to reactions of Ne* atoms excited in their metastable 3PJ state, and here, its physical background is extensively described in order to provide a clear description of the microscopic phenomenon underlying the chemical reactivity of the oxidative processes under study. It overcomes theoretical models previously proposed and reproduces experimental results obtained in different laboratories. Two basic reaction mechanisms have been identified: (i) at low collision energies, a weakly bounded transition state is formed which spontaneously ionizes through a radiative physical mechanism (photoionization); (ii) in the hyperthermal regime, an elementary oxidation process occurs. In this paper, the selectivity of the electronic rearrangements triggering the two mechanisms has been related to the angular momentum couplings by Hund’s cases, casting further light on fundamental aspects of the reaction stereodynamics of general interest. The obtained results allow peculiar characteristics and differences of the terrestrial oxidizing chemistry compared to that of astrochemical environments to be highlighted.

Investigation of the low-energy stereodynamics in the Ne(3P2) + N2, CO reactions.

We report on an experimental investigation of the low-energy stereodynamics of the energy transfer reactions Ne(3P2) + X, producing Ne(1S) + X+ and [Ne-X]+ (X = N2 or CO). Collision energies in the range 0.2 K-700 K are obtained by using the merged beam technique. Two kinds of product ions are generated by Penning and associative ionization, respectively. The intermediate product [Ne-X]+ in vibrationally excited states can predissociate into bare ions (X+). The experimental ratio of the NeX+ and X+ product ion yields is similar for both molecules at high collision energies but diverge at collision energies below 100 K. This difference is explained by the first excited electronic state of the product ions, which is accessible in the case of CO but lies too high in energy in the case of N2.

Screening effects on resonant thermonuclear reaction rates

The reaction rate of several astrophysically important nuclear reactions is dominated by the contribution of narrow resonances at the astrophysically most effective energies. In the stellar plasma the partial width of the resonances in the entrance channel is modified due to screening corrections. This effect, so far ignored in stellar reaction network calculations, reduces the conventional Salpeterscreening enhancement of the reaction rate. We derive analytical screening correction factors for the contributions of narrow resonances to the stellar reaction rate and discuss the effects for UC (a, j)lsO, 150(a,7)19 Ne and other reactions of relevance to explosive hydrogen burning