Radiative Capture Reactions Research Articles

A New Approach to Determine the 120(α,γ)16O Reaction Rate at Astrophysical Energies

The ∼100% uncertainty in the measured rate of the 12C(α,γ)16O reaction at low energies () is the largest source of uncertainty in any stellar evolution model. With development of new, high-current, energy-recovery linear accelerators (ERLs) and high density gas targets, measurement of the 16O(e, e′α)12C reaction close to threshold using detailed balance opens up a new approach to determine the 12C(α,γ)16O reaction rate with increased precision (< 20%). We have considered the optimal design of an experiment where systematic uncertainties are minimized. Once the new ERLs commence operation near their design specifications, an experiment to validate the new approach we propose should be carried out. Our method has broad applicability to radiative capture reactions in astrophysics.

Analysis of the 3He(α,γ)7Be and 3H(α,γ)7Li astrophysical direct capture reactions in a modified potential-model approach

Astrophysical S factors and reaction rates of the direct radiative capture processes He3(α,γ)7Be and H3(α,γ)7Li, as well as the primordial abundance of the Li7 element, are estimated in the framework of a modified two-body potential model. It is shown that suitable modification of phase-equivalent α−3He potentials in the d waves can improve the description of the astrophysical S factor for the direct He3(α,γ)7Be radiative capture reaction at energies above 0.5 MeV. An estimated Li/H7 abundance ratio of (4.89±0.18)×10−10 is in very good agreement with the recent measurement of (5.0±0.3)×10−10 by the LUNA collaboration.

Beyond the acceptance limit of DRAGON: The case of the [formula omitted]Li([formula omitted])[formula omitted]B reaction

Radiative capture reactions play a pivotal role for our understanding of the origin of the elements in the cosmos. Recoil separators provide an effective way to study these reactions, in inverse kinematics, and take advantage of the use of radioactive ion beams. However, a limiting factor in the study of radiative capture reactions in inverse kinematics is the momentum spread of the product nuclei, which can result in an angular spread larger than the geometric acceptance of the separator. The DRAGON facility at TRIUMF is a versatile recoil separator, designed to study radiative capture reactions relevant to astrophysics in the A∼10–30 region. In this work we present the first attempt to study with DRAGON a reaction, 6Li(α,γ)10B, for which the recoil angular spread exceeds DRAGON’s acceptance. Our result is in good agreement with the literature value, showing that DRAGON can measure resonance strengths of astrophysically important reactions even when not all the recoils enter the separator.

Nuclear Structures of 17O and Time-dependent Sensitivity of the Weak s-process to the 16O(n,γ)17O Rate

Abstract We revisit the radiative neutron capture reaction 16O(n,γ)17O of astrophysical interest, based on the new reevaluated cross-section data. Several potentials are proposed to predict direct capture cross sections. The contributions from single-particle resonances to total capture cross section are quantitatively considered in Breit–Wigner formalism, taking into account the interference term between direct capture and resonant cross sections, which is crucial for the description of the behavior around the resonance energies. A new cross section is achieved based upon χ 2-fittings for optimized resonance parameters using Minuit code, and it has a largely improved agreement with updated experimental data. Statistical errors are also evaluated for the total and Maxwellian-averaged cross sections. It is confirmed that the direct captures dominate the total cross sections; however, resonant contribution also becomes progressively more important as the energy increases to 100 keV. Resonance contribution can increase the reaction rates for energy region 50 keV &lt; E &lt; 100 keV by 5% ± 5%–25% ± 5%, and around 8% ± 5% in comparison with KADoNiS v0.3 rate and the latest data evaluations, respectively. We show a detailed propagation of the uncertainty in the 16O(n,γ) reaction rate to abundances of nuclei, including s-nuclei during the weak s-process with a multi-zone nuclear network calculation. Although an enhanced rate of 16O(n,γ) diminishes the s-process efficiency in the 25 M ☉ stellar model adopted from the Modules for Experiments in Stellar Astrophysics, it can lead to larger abundances of neutrons as well as 22Ne in the late epoch of C burning.

Bayesian analysis of capture reactions $$\varvec{^3}\hbox {He}\varvec{(\alpha ,\gamma )^7}\hbox {Be}$$ and $$\varvec{^3}\hbox {H}\varvec{(\alpha ,\gamma )^7}\hbox {Li}$$

Bayesian analysis of the radiative capture reactions $^3\mathrm{He}(\alpha,\gamma)^7\mathrm{Be}$ and $^3\mathrm{H}(\alpha,\gamma)^7\mathrm{Li}$ are performed to draw inferences about the cross sections at threshold. We do a model comparison of two competing effective field theory power countings for the capture reactions. The two power countings differ in the contribution of two-body electromagnetic currents. In one power counting, two-body currents contribute at leading order, and in the other they contribute at higher orders. The former is favored for $^3\mathrm{He}(\alpha,\gamma)^7\mathrm{Be}$ if elastic scattering data in the incoming channel is considered in the analysis. Without constraints from elastic scattering data, both the power countings are equally favored. For $^3\mathrm{H}(\alpha,\gamma)^7\mathrm{Li}$, the first power counting with two-body current contributions at leading order is favored with or without constraints from elastic scattering data.

Primary γ -ray intensities and γ -strength functions from discrete two-step γ -ray cascades in radiative proton-capture experiments

Background: Reaction rates of radiative capture reactions can play a crucial role in the nucleosynthesis of heavy nuclei in explosive stellar environments. These reaction rates depend strongly on $\ensuremath{\gamma}$-ray decay widths in the reaction products, which are, for nonresonant capture reactions at high excitation energies, derived from the $\ensuremath{\gamma}$-ray strength function and the nuclear level density. Recently, the ratio method was applied to primary $\ensuremath{\gamma}$ rays observed from ($d,p$) reactions and nuclear resonance fluorescence measurements to extract the dipole strength in atomic nuclei and to test the generalized Brink-Axel hypothesis.Purpose: The purpose of this work is to apply the ratio method to primary $\ensuremath{\gamma}$-ray intensities of the $^{63,65}\mathrm{Cu}(p,\ensuremath{\gamma})$ reactions to extract $\ensuremath{\gamma}$-ray strength information on the nuclei $^{64,66}\mathrm{Zn}$. The impact of spin distribution, total $\ensuremath{\gamma}$-ray decay widths, level densities, and width fluctuations on the application of the ratio method will be discussed. Additionally, by comparing the relative $\ensuremath{\gamma}$-ray strength at different excitation energies, conclusions on the validity of the generalized Brink-Axel hypothesis can be made.Method: The radiative proton capture reaction measurements have been performed at the HORUS $\ensuremath{\gamma}$-ray spectrometer of the University of Cologne at one excitation energy for each reaction. Primary $\ensuremath{\gamma}$-ray intensities have been determined by normalizing secondary $\ensuremath{\gamma}$-ray transitions in two-step cascades using their absolute branching ratio. The ratio method was applied to the measured primary $\ensuremath{\gamma}$-ray intensities as well as to previous measurements by Erlandsson $et$ $al.$ at different excitation energies.Results: The relative strength function curve for $^{64}\mathrm{Zn}$ from our measurement shows no significant deviation from the previous measurement at a different excitation energy. The same is true for $^{66}\mathrm{Zn}$ where both measurements were at almost the same excitation energy. Absolute $\ensuremath{\gamma}$-strength function values have been obtained by normalizing the relative curves to quasiparticle random phase approximation calculations because of the absence of experimental data in the respective energy region.Conclusion: The generalized Brink-Axel hypothesis, i.e., the independence of the strength function on the excitation energy, seems to hold in the studied energy region and nuclei. The method to obtain primary $\ensuremath{\gamma}$-ray intensities from two-step cascade spectra was shown to be a valuable and sensitive tool although its uncertainties are connected to the knowledge of the low-energy level scheme of the investigated nucleus. The scaling in the ratio method should be taken with care, because the relative strength is not a simple sum of ${f}_{E1}$ and ${f}_{M1}$ but a somewhat complex linear combination dependent on the excitation energy of the nucleus.

H2(p,γ)He3 cross section measurement using high-energy-density plasmas

An absolute cross section for the radiative capture reaction $^{2}\mathrm{H}(p,\ensuremath{\gamma})^{3}\mathrm{He}$ has been measured at the OMEGA laser facility using inertially confined plasmas. These high-temperature plasmas are created by imploding a fuel-containing capsule using laser ablation, and are advantageous in that they better mimic astrophysical systems. We measure an $S$ factor for this reaction of $0.429\ifmmode\pm\else\textpm\fi{}0.{026}_{\mathrm{stat}}\ifmmode\pm\else\textpm\fi{}0.{072}_{\mathrm{sys}}$ eV b at ${E}_{c.m.}=16.35\ifmmode\pm\else\textpm\fi{}0.40$ keV, which is higher than the adopted evaluations. This reaction is important as a source of nuclear energy in protostars and brown dwarfs. It is also a critical reaction during big-bang nucleosynthesis, and an accurate cross section can be used as a constraint on cosmology.

Nuclear Reactions of Astrophysical Interest

We present different reaction models commonly used in nuclear astrophysics, in particular for the nucleosynthesis of the light elements. Nuclear reactions involved in stellar evolution generally occur at energies much lower than the Coulomb barrier. This property makes the cross sections extremely small, and virtually impossible to be measured in the laboratory. We start with a general discussion of low-energy scattering, and define the various cross sections required for reaction networks (essentially radiative capture and transfer reactions). Then we present specific models. Microscopic theories are based on fundamental principles, such as a nucleon-nucleon interaction, and an exact account of the antisymmetrization between all nucleons. In this context, most calculations performed so far have been done in the cluster approximation, but recent works, referred to as “ab initio” models, go beyond this approximation. Microscopic models can be simplified by neglecting the internal structure of the colliding nuclei, which leads to the potential model, also named the optical model. An alternative approach for the theoretical analysis of the experimental data is based on the phenomenological R-matrix theory, where parameters are fitted to the existing data, and then used to extrapolate the cross sections down to stellar energies. Indirect approaches, such as the Trojan Horse method, are briefly outlined. Finally, we present some typical applications of the different models.

Astrophysical S-factor of the ${d(\alpha ,\gamma )^{6}Li}$ process by Effective Field Theory

Radiative capture reaction $d(\alpha ,\gamma )^{6}Li $ is the only process in the Big-Bang model that produces ${}^{6}Li $. We develop the formalism based on pionless Effective Field Theory (EFT) for deuteron-alpha scattering to derive the cross-section of deuteron-alpha radiative capture. Based on this model, nucleons and alpha particle interact through separable two-body forces. The alpha particle is assumed point particle and the astrophysical S-factor for the $d(\alpha ,\gamma )^{6}Li $ reaction is calculated in the low-energies. The coulomb effects are also considered as a coulomb correction in the cross-section for two first electric multipole transitions, up to Next to Leading Order (NLO). The obtained results are shown a satisfactory agreement between theoretical and experimental results for the calculated observables.

High-Resolution Gamma Spectrometry of a Plutonium Bearing Waste Drum With High-Energy Reaction-Induced Gamma Rays

In the framework of the radioactive waste drum characterization using neutron coincidence counting, the Nuclear Measurement Laboratory of French Alternative Energies and Atomic Energy Commission (CEA) Cadarache is studying plastic scintillators as an alternative to ideal but costly 3He gas proportional counters. Plastic scintillators are at least five times cheaper for the same detection efficiency, and in addition, they detect fast neutrons three orders of magnitude more quickly than 3He detectors. However, they are sensitive to gamma rays with no pulse shape discrimination abilities for large detection volumes, which implies the necessity to identify precisely gamma background sources that may affect the useful signal. This article presents a detailed analysis of the gamma-ray spectrum of a radioactive waste drum containing glove box filters contaminated by plutonium dioxide. Classical gamma emissions following alpha and beta disintegrations are identified, and also those accompanying inelastic scattering (n, $\text{n}^{\prime }$ ) or radiative capture (n, $\gamma$ ) reactions in the whole waste drum, and ( $\alpha $ , n) or ( $\alpha $ , p) reactions in the filtration media, which can lead to neutron-gamma coincidences parasitizing useful coincidences from plutonium spontaneous fissions.

A new approach to monitor $$^{13}\hbox {C}$$-targets degradation in situ for $$^{13}\hbox {C}(\alpha ,\hbox {n})^{16}\hbox {O}$$ cross-section measurements at LUNA

Direct measurements of reaction cross-sections at astrophysical energies often require the use of solid targets able to withstand high ion beam currents for extended periods of time. Thus, monitoring target thickness, isotopic composition, and target stoichiometry during data taking is critical to account for possible target modifications and to reduce uncertainties in the final cross-section results. A common technique used for these purposes is the Nuclear Resonant Reaction Analysis (NRRA), which however requires that a narrow resonance be available inside the dynamic range of the accelerator used. In cases when this is not possible, as for example the 13C(alpha,n)16O reaction recently studied at low energies at the Laboratory for Underground Nuclear Astrophysics (LUNA) in Italy, alternative approaches must be found. Here, we present a new application of the shape analysis of primary gamma rays emitted by the 13C(p,g)14N radiative capture reaction. This approach was used to monitor 13C target degradation {\em in situ} during the 13C(alpha,n)16O data taking campaign. The results obtained are in agreement with evaluations subsequently performed at Atomki (Hungary) using the NRRA method. A preliminary application for the extraction of the 13C(alpha,n)16O reaction cross-section at one beam energy is also reported.

Changes of the abundances of Fe, Ni, Zr isotopes due to neutron radiative capture reactions in heavy water reactors

The abundances of the nuclides in nuclear materials change over time in reactors because of neutron-induced reactions like (n, γ), (n, α), (n, p), etc. Among them, the effect caused by (n, γ) reactions is especially significant in heavy water reactors because they have a large proportion of thermal neutron flux, and the cross sections of (n, γ) reactions for most nuclides are dominant in the thermal region than in regions of higher energies. The changes of the abundances of Fe, Ni, and Zr isotopes in CANDU-6 reactors over 25 years through radiative capture reactions (thermal neutron flux of 2.7 × 1018 n·m−2·s−1, and effects of epithermal, resonance, and fast neutrons are also included) are calculated. Results show that for Fe and Ni isotopes, the changes of abundances are obvious: 57Fe enriches from 2.12% to 12.22%; 58Ni drops from 68.09% to 36.88% and 60Ni increases from 26.23% to 44.35%. For Zr isotopes, the effect is also noticeable, with the abundance of 90Zr and 91Zr decreasing from 51.45% and 11.22% to 50.67% and 9.58%, respectively. . Present results are verified with the ALARA code. Based on these calculations, a new approach to estimating the neutron fluence in heavy water reactors is proposed according to the abundances of several stable isotopes including 56Fe, 57Fe, 58Ni and 60Ni.

Proton Capture Cross-section for 12C at Low Energy

The total cross-section, astrophysical S-factor, and rate of radiative capture reaction 12C(p, γ)13N are calculated using the potential model. In this work, we take into account the capture of protons through s1/2 → p1/2 and d3/2 → p1/2 single-particle transitions, fine-tune the values of the depth and nuclear radius of the Woods-Saxon potential to achieve better comparison with the measured data. The results for the elastic scattering phase shift, astrophysical S-factor, and rates are compared with the previously reported results of the calculations and experimental data.

Relevance of Skyrme forces in the decay dynamics of 28Si⁎ at stellar energies

The decay of 28Si⁎ compound system formed by 12C+16O reaction is analyzed at stellar energies span Ec.m. = 4 - 5 MeV. The conventional SIII Skyrme force is used in conjunction with generalized GSkI and extended eMSL07 Skyrme forces having extended density-dependent terms, that can be potentially useful for the study of finite nuclei and the neutron stars. The dynamical cluster-decay model (DCM) is used to examine the emission of n, p, and α particles in the decay of 28Si⁎ nucleus along with other competing decay channels. The theoretical analysis advocates the emergence of 6Li, 10B, and 14N fragments in the decay of 28Si⁎ nucleus, independent of the choice of Skyrme interactions and the angular momentum effects. The fragile nature of 6Li nucleus may not allow it to survive the stellar conditions and hence the breakup and /or radiative capture processes may appear in. The breakup of L6i is addressed in terms of Kinetic energy (K.E) value of A=6 fragment while the possibility of capture reaction is predicted in terms of positive Q values of radiative capture reactions 6Li(n,γ)7Li, 6Li(p,γ)7Be, and 6Li(α,γ)10B. It has been observed that the structure of fragmentation potential and preformation probability is independent of the choice of Skyrme forces, but their magnitude gets significantly influenced for use of different Skyrme forces. The conventional SIII force could not reproduce the cross sections while eMSL07 and GSkI Skyrme forces give nice agreement with the experimental data.

Background in γ-ray detectors and carbon beam tests in the Felsenkeller shallow-underground accelerator laboratory

The relevant interaction energies for astrophysical radiative capture reactions are very low, much below the repulsive Coulomb barrier. This leads to low cross sections, low counting rates in $\gamma$-ray detectors, and therefore the need to perform such experiments at ion accelerators placed in underground settings, shielded from cosmic rays. Here, the feasibility of such experiments in the new shallow-underground accelerator laboratory in tunnels VIII and IX of the Felsenkeller site in Dresden, Germany, is evaluated. To this end, the no-beam background in three different types of germanium detectors, i.e. a Euroball/Miniball triple cluster and two large monolithic detectors, is measured over periods of 26-66 days. The cosmic-ray induced background is found to be reduced by a factor of 500-2400, by the combined effects of, first, the 140 meters water equivalent overburden attenuating the cosmic muon flux by a factor of 40, and second, scintillation veto detectors gating out most of the remaining muon-induced effects. The new background data are compared to spectra taken with the same detectors at the Earth's surface and at other underground sites. Subsequently, the beam intensity from the cesium sputter ion source installed in Felsenkeller has been studied over periods of several hours. Based on the background and beam intensity data reported here, for the example of the $^{12}$C($\alpha$,$\gamma$)$^{16}$O reaction it is shown that highly sensitive experiments will be possible.

New approach to determining radiative capture reaction rates at astrophysical energies

Radiative capture reactions play a crucial role in stellar nucleosynthesis but have proved challenging to determine experimentally. In particular, the large uncertainty ($\ensuremath{\approx}100%$) in the measured rate of the $^{12}\mathrm{C}(\ensuremath{\alpha},\ensuremath{\gamma})^{16}\mathrm{O}$ reaction is the largest source of uncertainty in any stellar evolution model. With development of high-current energy-recovery linear accelerators (ERLs) and high-density gas targets, measurement of the $^{16}\mathrm{O}(e,{e}^{\ensuremath{'}}\ensuremath{\alpha})^{12}\mathrm{C}$ reaction close to threshold using detailed balance allows a new approach to determine the $^{12}\mathrm{C}(\ensuremath{\alpha},\ensuremath{\gamma})^{16}\mathrm{O}$ reaction rate with significantly increased precision ($&lt;20%$). We present the formalism to relate photo- and electrodisintegration reactions and consider the design of an optimal experiment to deliver increased precision. Once the new ERLs come online, an experiment to validate the approach we propose should be carried out. This approach has broad applicability to radiative capture reactions in astrophysics.

Nuclear dipole response in the finite-temperature relativistic time-blocking approximation

The radiative neutron capture reaction rates of the r-process nucleosynthesis are immensely affected by the microscopic structure of the low-energy spectra of compound nuclei. The relativistic (quasiparticle) time blocking approximation (R(Q)TBA) has successfully provided a good description of the low-energy strength, in particular, the strength associated with pygmy dipole resonance, describing transitions from and to the nuclear ground state. The finite-temperature generalization of this method is designed for thermally excited compound nuclei and has the potential to enrich the fine structure of the dipole strength, especially in the low-energy region. The finite-temperature RTBA equations are derived using the Matsubara Green's function formalism. We show that with the help of a temperature-dependent projection operator on the subspace of the imaginary time it is possible to reduce the Bethe-Salpeter equation for the nuclear response function to a single frequency variable equation also at finite temperatures. The approach is implemented self-consistently in the framework of quantum hadrodynamics and keeps the ability of connecting the high-energy scale of heavy mesons and the low-energy domain of nuclear medium polarization effects in a parameter-free way. The presented calculations of the dipole response within a self-consistent relativistic framework reveal that, although the Landau damping plays the leading role in the temperature evolution of the strength distribution, (i) at moderate temperatures the PVC effects remain almost as strong as at $T=0$ and (ii) at high temperatures they are tremendously reinforced because of the formation of the new collective low-energy modes. In the dipole channel, the latter effect is responsible for the "disappearance" of the high-frequency GDR or, in other words, brings the GDR to the low-energy domain.

Be7 and Li7 nuclei within the no-core shell model with continuum

The production of $^7$Be and $^7$Li nuclei plays an important role in primordial nucleosynthesis, nuclear astrophysics, and fusion energy generation. The $^3\mathrm{He}(\alpha , \gamma) ^7\mathrm{Be}$ and $^3\mathrm{H}(\alpha , \gamma) ^7\mathrm{Li}$ radiative-capture processes are important to determine the $^7$Li abundance in the early universe and to predict the correct fraction of pp-chain branches resulting in $^7$Be versus $^8$B neutrinos. In this work we study the properties of $^7$Be and $^7$Li within the no-core shell model with continuum (NCSMC) method, using chiral nucleon-nucleon interactions as the only input, and analyze all the binary mass partitions involved in the formation of these systems. The NCSMC is an ab initio method applicable to light nuclei that provides a unified description of bound and scattering states and thus is well suited to investigate systems with many resonances and pronounced clustering like $^7$Be and $^7$Li. Our calculations reproduce all the experimentally known states of the two systems and provide predictions for several new resonances of both parities. Some of these new possible resonances are built on the ground states of $^6$Li and $^6$He, and thus represent a robust prediction. We do not find any resonance in the p${+}^6$Li mass partition near the threshold. On the other hand, in the p${+}^6$He mass partition of $^7$Li we observe an $S$-wave resonance near the threshold producing a very pronounced peak in the calculated S factor of the $^6\mathrm{He} (\mathrm{p},\gamma) ^7\mathrm{Li}$ radiative-capture reaction, which could be relevant for astrophysics and its implications should be investigated.

A feasibility study on the determination of 90Sr food-chain transfer using stable strontium as a surrogate and neutron activation analysis

We evaluated the applicability of neutron activation analysis for investigating the potential mobility of 90Sr in a pasture area in Hungary. To measure strontium concentrations, we made use of the 84Sr(n,γ)85Sr radiative capture reaction using neutron activation analysis and performed gamma spectroscopy using Compton-suppression techniques on the resulting 514.0 keV gamma ray. Our values for soil-to-plant transfer factors value of 1.6 (2.2) kg kg−1 were in agreement with recommended values from the ICRP and IAEA, as well as similar studies performed by independent researchers. Our values for plant-to-animal transfer coefficients and concentration ratios varied from suggested values by several orders of magnitude to agreement with other values. Based on our results the utilized transfer of stable strontium could not be regarded as a convenient substitute to help clarify the long-term transfer of radioactive strontium in the environment, because short term dosing was applied. Neutron activation analysis provides a unique niche as a technique with very little chemical processing and short sample analysis times.

A hybrid model description of 13C(p,γ)14N capture reaction

The radiative capture reaction [Formula: see text] is analyzed using a hybrid model approach where the non-resonant component has been constructed employing the potential model with a folded M3Y potential. The one-level Breit–Wigner formula has been used to estimate the cross-sections of the resonant decays of dominant ([Formula: see text]) state of [Formula: see text]N at 8.06[Formula: see text]MeV. The contribution of the broad resonance at 8.77[Formula: see text]MeV ([Formula: see text]) has been dealt with differently. While Breit–Wigner formula has been used where the excitation function data exist, the [Formula: see text]-matrix prediction for the cross-section of decay to a bound state of [Formula: see text]N from the broad resonance has been used where excitation function data are not available. The single particle spectroscopic factors for ground and six excited states of [Formula: see text]N have been obtained from the fits. The resulting astrophysical [Formula: see text]-factor at zero relative energy is [Formula: see text][Formula: see text]keV b. The value is in good agreement with the previously reported [Formula: see text]-matrix result and also consistent within error bars with the published values.