Astrophysical Reaction Rates Research Articles

Re-examining the Lithium abundance problem in Big-Bang nucleosynthesis

One of the three testaments in favor of the big bang theory is the prediction of the primordial elemental abundances in the big-bang nucleosynthesis (BBN). The Standard BBN is a parameter-free theory due to the precise knowledge of the baryon-to-photon ratio of the Universe obtained from studies of the anisotropies of cosmic microwave background radiation. Although the computed abundances of light elements during primordial nucleosynthesis and those determined from observations are in good agreement throughout a range of nine orders of magnitude, there is still a disparity of 7Li abundance overestimated by a factor of ∼2.5 when calculated theoretically. The number of light neutrino flavors, the neutron lifetime and the baryon-to-photon ratio in addition to the astrophysical nuclear reaction rates determine the primordial abundances. We previously looked into the impact of updating baryon-to-photon ratio and neutron lifetime and changing quite a few reaction rates on the yields of light element abundances in BBN. In this work, calculations are performed using new reaction rates for 3H(p, γ)4He, 6Li(p, γ)7Be, 7Be(p, γ)8B, 13N(p, γ)14O, 7Li(n, γ)8Li and 11B(n, γ)12B along with the latest measured value of neutron lifetime. We observe from theoretical calculations that these changes result in improvement by causing further reduction in the abundance of 7Li than calculated earlier.

Vortex photon induced nuclear reaction: Mechanism, model, and application to the studies of giant resonance and astrophysical reaction rate

The vortex photon beam induced nuclear reaction is studied. The interaction formalism of nuclei with vortex photons is developed and incorporated into the statistical reaction model to calculate reaction cross-sections. For 138 nuclei of high nuclear astrophysics and structure interest, the cross-sections of γ-ray emission and neutron production from the decay of the giant resonances (GR) populated by vortex photons, e.g., (γvo,γemit) and (γvo,nprod.), are computed. It is shown that for the (γvo,γemit) and (γvo,nprod.) cross-sections, the GR contribution of an individual L is either enhanced or suppressed depending on the parameters of vortex γ-rays, and the contribution from the GR of a specific L can be identified and deduced. To this end, a novel method to exclusively determine the γ-strength function (γSF) for the GR of a specific L is proposed considering the (γvo,γemit) and (γvo,nprod.) measurements, and the feasibility studies demonstrate that the γSF for the giant quadrupole resonance can be extracted. Furthermore, the astrophysical reaction rates of the vortex photon induced reactions in p-process are investigated. It is indicated that photo-nuclear reactions induced by vortex photons will bring new insights in nuclear physics and astrophysics research.

Indispensability of cross-shell contributions in neutron resonance spacing

Spin- and parity-dependent nuclear level densities (NLDs) are obtained for a configuration interaction shell model using a numerically efficient spectral distribution method. The calculations are performed for 24Na and 25,26,27Mg nuclei using full sd-pf model space that incorporates the cross-shell excitations from the sd to the pf-shell. The obtained NLDs are then employed to determine the s-wave neutron resonance spacing (D0), which is one of the crucial inputs for the predictions of astrophysical reaction rates. Although the considered nuclei are not neutron-rich, the contributions from cross-shell excitations to the pf-shell are indispensable for explaining the experimental data for D0, which otherwise are significantly overestimated.

Astrophysical S-factor and reaction rate for 15N(p,γ)16O within the modified potential cluster model* *Supported by the Ministry of Science and Higher Education of the Republic of Kazakhstan (AP09259174)

We study radiative capture on the ground state of 16O at stellar energies within the framework of a modified potential cluster model (MPCM) with forbidden states, including low-lying resonances. The investigation of the 15N( )16O reaction includes the consideration of resonances due to transitions and the contribution of the scattering wave in the p + 15N channel due to the transition. We calculated the astrophysical low-energy S-factor, and the extrapolated turned out to be within 34.7−40.4 keV·b. The important role of the asymptotic constant (AC) for the 15N( )16O process with interfering (312) and (962) resonances is elucidated. A comparison of our calculation for the S-factor with existing experimental and theoretical data is addressed, and a reasonable agreement is found. The reaction rate is calculated and compared with the existing rates. It has negligible dependence on the variation of AC but shows a strong impact of the interference of (312) and (962) resonances in reference to the CNO Gamow windows, especially at low temperatures. We estimate the contribution of cascade transitions to the reaction rate based on the exclusive experimental data from Phys. Rev. C. 85, 065810 (2012). The reaction rate enhancement due to the cascade transitions is observed from and reaches the maximum factor ~ 1.3 at . We present the Gamow energy window and a comparison of rates for radiative proton capture reactions 12N( )13O, 13N( ) 14O, 14N( )15O, and 15N( )16O obtained in the framework of the MPCM and provide the temperature windows, prevalence, and significance of each process.

Calculation of astrophysical reaction rate and uncertainty for T(d,n)4He using Bayesian statistical approach

Abstract One of the best methods to investigate and calculate a desired quantity using available limited data is the Bayesian statistical method, which has been recently entered the field of nuclear astrophysics and can be used to evaluate the astrophysical S-factors, the cross sections and, as a result, the nuclear reaction rates of Big Bang Nucleosynthesis. This study tries to calculate the astrophysical S-factor and the rate of reaction T(d,n)4He as an important astrophysical reaction with the help of this method in energies lower that electron repulsive barrier, and for this purpose, it uses the R-Software, which leads to improved results in comparison with the non-Bayesian methods for the mentioned reaction rate.

Towards a direct measurement of the E cm = 65 keV resonance strength in 17O(p, γ)18F at LUNA

The 17O(p, γ)18F reaction plays a crucial role in several stellar scenarios where the hydrogen burning phases takes place. In particular, in the temperature energy range of interest for AGB nucleosynthesis (20 MK< T <80 MK) the main contribution to the astrophysical reaction rate comes from the elusive 65 keV resonance. Indeed, this resonance strength is at the moment determined only through indirect measurements, with a reported value of ωγ = (1.6 ± 0.3) × 10−11 eV. With typical experimental quantities for beam current, isotopic enrichment and detection efficiency, this strength yields an expected count rate of less than one count per Coulomb, making the direct measurement of this resonance extremely challenging. The Laboratory for Underground Nuclear Astrophysics (LUNA) 400kV accelerator installed in Laboratori Nazionali del Gran Sasso (Italy) provides a unique possibility to directly measure this low resonance thanks to the reduction of cosmic ray background by six orders of magnitude with respect surface laboratories and thanks to an intense, narrow proton beam. To improve the experimental sensitivity, the environmental background was further reduced designing a lead and borated (5%) polyethylene shielding and the absorption of γ − rays emitted by the reaction was minimised by the installation of target chamber and holder made of aluminum.With about 400 Coulomb accumulated on Ta2O5 targets, with nominal 17O enrichment of 90%, the LUNA collaboration has performed the first direct measurement of the 65 keV resonance strength.

First Direct Measurement Constraining the ^{34}Ar(α,p)^{37}K Reaction Cross Section for Mixed Hydrogen and Helium Burning in Accreting Neutron Stars.

The rate of the final step in the astrophysical αp process, the ^{34}Ar(α,p)^{37}K 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 ^{34}Ar(α,p)^{37}K reaction cross section, using the Jet Experiments in Nuclear Structure and Astrophysics gas jet target. The combined cross section for the ^{34}Ar,Cl(α,p)^{37}K,Ar reaction is found to agree well with Hauser-Feshbach predictions. The ^{34}Ar(α,2p)^{36}Ar cross section, which can be exclusively attributed to the ^{34}Ar 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 of the αp process, in contrast to earlier findings from indirect reaction studies indicating orders-of-magnitude discrepancies. This removes a significant uncertainty in models of hydrogen and helium burning on accreting neutron stars.

Cross section measurement of the ^{12}C(p,gamma )^{13}N reaction with activation in a wide energy range

The CNO cycle is one of the fundamental processes of hydrogen burning in stars. The first reaction of the cycle is the radiative proton capture on ^{12}C and the rate of this ^{12}C(p,gamma )^{13}N reaction is related to the ^{12}C/^{13}C ratio observed e.g. in the Solar System. The low-energy cross section of this reaction was measured several times in the past, however, the experimental data are scarce in a wide energy range especially around the resonance at 1.7 MeV. In the present work the ^{12}C(p,gamma )^{13}N cross section was measured between 300 and 1900 keV using the activation method. This method was only used several decades ago in the low-energy region. As the activation method provides the total cross section and has uncertainties different from those of the in-beam gamma -spectroscopy technique, the present results provide a largely independent data set for future low-energy extrapolations and thus for astrophysical reaction rate calculations.

Experiments probing clustering effects in explosive nucleosynthesis

Nuclear clustering affects the nucleosynthesis occurring in a number of astrophysical environments. Highly-clusterized nuclear states typically occur near particle thresholds and therefore can produce dramatic impacts on the nuclear reaction rates. This is especially true for astrophysical explosions that are driven by the consumption of helium as fuel. Such burning can occur in X-ray bursts, supernovae type Ia, and core-collapse supernovae for instance. This article will focus on the explosive astrophysical events in which nuclear clustering is most important, will discuss the types of information and tools necessary to estimate the astrophysical reaction rates, and will discuss example experiments at Notre Dame and other facilities that have or will be performed to measure the critical nuclear data needed for such estimates.

Gamow shell model description of the Ca40(d,p) transfer reaction

Transfer reactions are essential to determine spectroscopic factors and astrophysical reaction rates. However, their theoretical evaluation is typically effected using standard reaction theory, from which structure degrees of freedom are absent. While reaction cross sections have been implemented in the frame of the no-core shell model with continuum, this model can be applied in practice only to the lightest nuclei. The use of the core + valence nucleon picture is then necessary to include inter-nucleon correlations in reaction cross sections involving medium nuclei. For this, we will use the recently developed coupled-channel Gamow Shell Model (GSM-CC) for direct reactions and extend it to the evaluation of transfer cross sections. As an example, we will study the ${ { }^{ 40 } }$Ca(d,p) transfer reaction with GSM-CC. Experimental data can be successfully reproduced, but at the price of the use of a very phenomenological Hamiltonian.

Towards a direct measurement of the 17O(p, γ)18F 65 keV resonance strength at LUNA

The 17O(p, γ)18F reaction plays a crucial role in the hydrogen burning phases of different stellar scenarios. At temperature of interest for AGB nucleosynthesis (20 MK &lt; T &lt; 80 MK) the main contribution to the astrophysical reaction rate comes from the poorly constrained 65 keV resonance. The strength of this resonance is presently determined only through indirect measurements, with a reported value of ωγ = (1.6 ± 0.3) 10−11 eV. With typical experimental quantities for beam current, isotopic enrichment and detection efficiency, this strength yields to an expected count rate of less than one count per Coulomb, making the direct measurement of this resonance extremely challenging. A new high sensitivity setup has been installed at LUNA (Laboratory for Underground Nuclear Astrophysics) of Laboratori Nazionali del Gran Sasso. The high performance LUNA 400kV accelerator underground location guarantees, indeed, a reduction of cosmic ray background by several orders of magnitude. The residual background was further reduced by a devoted shielding of lead and borated (5%) polyethylene. On the other hand, the 4π BGO detector efficiency was optimized installing aluminum target chamber and holder. With about 400 C accumulated on Ta2O5 targets, with nominal 17O enrichment of 90%, the LUNA collaboration has performed the first direct measurement of the 65 keV resonance strength.

The challenging direct measurement of the 65 keV resonance strength of the 17O(p, γ)18F reaction at LUNA

The 17O(p, γ)18F reaction plays a crucial role in AGB nucleosynthesis as well as in explosive hydrogen burning occurring in type Ia novae. At the temperatures of interest for the former scenario ( 20MK &lt; T &lt; 80MK) the main contribution to the astrophysical reaction rate comes from the poorly constrained ER = 65 keV resonance. The strength of this resonance is presently determined only through indirect measurements, with an adopted value ωγ =(16 ± 3) peV. A new high sensitivity setup was installed at LUNA, located at LNGS. The underground location of the LUNA 400kV accelerator guarantees a reduction of the cosmic ray background by several orders of magnitude. The residual background was further reduced installing a devoted shielding. On the other hand, to increase the efficiency, the 4π BGO detector was coupled with Al target chamber and holder. With more than 400C accumulated on Ta2O5 targets, nominal 17O enrichment of 90%, the LUNA collaboration has performed the first direct measurement of the 65 keV resonance strength.

The study of the 20Ne(p,γ)21Na reaction at LUNA

The NeNa and MgAl cycles have been the subject of much experimental activity during the last decade because of their relevance to the synthesis of Ne, Na, and Mg isotopes during the H burning in several astrophysical scenarios. Key reactions in these cycles are also believed to be the main agents of the observed anti-correlations in O-Na and Al-Mg abundances exhibited by the stars of Galactic globular clusters. The 20Ne(p,γ)21Na is the first reaction and the bottleneck of the NeNa cycle: having the slowest reaction rate, it controls the speed of the entire cycle. In order to better constrain the overall astrophysical reaction rate of this important reaction, the LUNA collaboration has started a new experimental effort to study the 366 keV resonance and to improve the knowledge of the cross section at proton energies below 400 keV. This contribution describes the experimental setup and preliminary results.

Stellar neutron capture reactions at low and high temperature

The determination of astrophysical reaction rates requires different approaches depending on the conditions in hydrostatic and explosive burning. The focus here is on astrophysical reaction rates for radiative neutron capture reactions. Relevant nucleosynthesis processes not only involve the s-process but also the i-, r- and gamma -processes, which from the nuclear perspective mainly differ in the relative interaction energies of neutrons and nuclei, and in the nuclear level densities of the involved nuclei. Emphasis is put on the difference between reactions at low and high temperature. Possible complications in the prediction and measurement of these reaction rates are illustrated and the connection between theory and experiment is addressed.

Measurements of the Zr96(α,n)Mo99 cross section for astrophysics and applications

The reaction $^{96}\mathrm{Zr}(\ensuremath{\alpha},\mathrm{n})^{99}\mathrm{Mo}$ plays an important role in $\ensuremath{\nu}$-driven wind nucleosynthesis in core-collapse supernovae and is a possible avenue for medical isotope production. Cross-section measurements were performed using the activation technique at the Edwards Accelerator Laboratory. Results were analyzed along with world data on the $^{96}\mathrm{Zr}(\ensuremath{\alpha},n)$ cross section and $^{96}\mathrm{Zr}(\ensuremath{\alpha},\ensuremath{\alpha})$ differential cross section using large-scale Hauser-Feshbach calculations. We compare our data, previous measurements, and a statistical description of the reaction. We find a larger cross section at low energies compared to prior experimental results, allowing for a larger astrophysical reaction rate. This may impact results of core-collapse supernova $\ensuremath{\nu}$-driven wind nucleosynthesis calculations but does not significantly alter prior conclusions about $^{99}\mathrm{Mo}$ production for medical physics applications. The results from our large-scale Hauser-Feshbach calculations demonstrate that phenomenological optical potentials may yet be adequate to describe $(\ensuremath{\alpha},n)$ reactions of interest for $\ensuremath{\nu}$-driven wind nucleosynthesis, albeit with regionally adjusted model parameters.

A Review on Bayesian Calculation of Nuclear Astrophysical Reaction Rates and Uncertainties

A Review on Bayesian Calculation of Nuclear Astrophysical Reaction Rates and Uncertainties

Improved Mo95 neutron resonance parameters and astrophysical reaction rates

Background: Improved $^{95}\mathrm{Mo}$ neutron resonance parameters and reaction rates are important for nuclear astrophysics, testing nuclear models, and nuclear criticality safety. However, despite many previous neutron-capture and total cross-section measurements on this nuclide, there still is much room for improvement as well as several discrepancies. For example, there are very few firm resonance spin and parity assignments; average resonance parameters are available only for each parity, the currently recommended astrophysical reaction rate results in disagreements between stellar models and meteoric isotopic anomalies, and there are substantial disagreements in the neutron-capture cross section at low energies important for nuclear criticality safety.Purpose: To obtain an improved set of neutron resonance parameters and astrophysical reaction rates for $^{95}\mathrm{Mo}$.Method: High-resolution neutron-capture and transmission data were measured at the Oak Ridge Electron Linear Accelerator (ORELA) using highly isotopically enriched $^{95}\mathrm{Mo}$ samples. The neutron-capture apparatus, data reduction, and analysis were improved so that information contained in the $\ensuremath{\gamma}$-ray cascade following neutron capture were used to assign resonance ${J}^{\ensuremath{\pi}}$ values. Following this, simultaneous analysis of the new neutron-capture and transmission data was used to obtain resonance energies, gamma widths, and neutron widths and their uncertainties to a maximum energy of 10 keV. Accurate neutron-capture cross sections also were obtained for the unresolved resonance region to a maximum energy of 500 keV and, together with the new resonance parameters, used to calculate the astrophysical reaction rates in the temperature range from 5 to 30 keV.Results: A vastly improved set of $^{95}\mathrm{Mo}$ neutron resonance parameters and an astrophysical reaction rate accurate to about 3% were obtained. Firm ${J}^{\ensuremath{\pi}}$ assignments were determined for 261 of the 314 observed resonances. This is a very large improvement over the previously published 32 firm ${J}^{\ensuremath{\pi}}$ assignments for 108 resonances. Also, the number of resonances having both firm ${J}^{\ensuremath{\pi}}$ assignments and ${\mathrm{\ensuremath{\Gamma}}}_{\ensuremath{\gamma}}$ values was increased by almost a factor of 24---from 11 to 261. Neutron- and total-radiation-width distributions and average resonance spacings, average total radiation widths, and neutron strength functions were obtained for the six different $s$- and $p$-wave possibilities. Parameters for the lowest $s$-wave resonance, which is most important for criticality benchmarks, were obtained with high accuracy.Conclusions: Simple modification of the neutron-capture apparatus and expansion and improvement of data analysis techniques led to a large increase in firm ${J}^{\ensuremath{\pi}}$ assignments for $^{95}\mathrm{Mo}$ neutron resonances. The resulting astrophysical reaction rate is 20%--30% larger than the currently recommended rate at $s$-process temperatures, which should lead to better agreement between stellar models and meteoric isotopic anomalies. The neutron-capture cross section at low energies is substantially larger than recommended in the latest evaluation, which is problematical for criticality benchmarks. The average resonance spacing as a function of spin and parity is significantly different from current models. The total-radiation-width distributions are significantly broader than predicted by theory and show significant departures from the expected Gaussian shapes.

Feasibility of studying astrophysically important charged-particle emission with the variable energy γ -ray system at the Extreme Light Infrastructure–Nuclear Physics facility

In the environment of a hot plasma, as achieved in stellar explosions, capture and photodisintegration reactions proceeding on excited states in the nucleus can considerably contribute to the astrophysical reaction rate. Such reaction rates including the excited-state contribution are obtained from theoretical calculations as the direct experimental determination of these astrophysical rates is currently unfeasible. In the present study, ($\gamma$,p) and ($\gamma$,$\alpha$) reactions in the mass and energy range relevant to the astrophysical $p$ process are considered and the feasibility of measuring them with the ELISSA detector system at the future Variable Energy $\gamma$-ray (VEGA) facility at ELI-NP is investigated. The simulation results reveal that, for the ($\gamma$,p) reaction on twelve targets of $^{29}$Si, $^{56}$Fe, $^{74}$Se, $^{84}$Sr, $^{91}$Zr, $^{96,98}$Ru, $^{102}$Pd, $^{106}$Cd, and $^{115, 117, 119}$Sn, and the ($\gamma$,$\alpha$) reaction on five targets of $^{50}$V, $^{87}$Sr, $^{123,125}$Te, and $^{149}$Sm, the yields of the reaction channels with the transitions to the excited states in the residual nucleus are relevant and even dominant. It is further found that for each considered reaction, the total yields of the charged-particle $X$ may be dominantly contributed from one, two or three ($\gamma$,$X_{i}$) channels within a specific, narrow energy range of the incident $\gamma$-beam. Furthermore, the energy spectra of the ($\gamma$,$X_{i}$) channels with $0\leq i\leq 10$ are simulated for each considered reaction, with the incident $\gamma$-beam energies in the respective energy range as derived before. It becomes evident that measurements of the photon-induced reactions with charged-particle emissions considered in this work are feasible with the VEGA+ELISSA system and will provide knowledge useful for nuclear astrophysics.

Astrophysical reaction rates with realistic nuclear level densities

Realistic nuclear level densities (NLDs) obtained within the spectral distribution method (SDM) are employed to study nuclear processes of astrophysical interest. The merit of SDM lies in the fact that the NLDs corresponding to many body shell model Hamiltonian consisting of residual interaction can be obtained for the full configurational space without recourse to the exact diagnolization of huge matrices. We calculate NLDs and s-wave neutron resonance spacings which agree reasonably well with the available experimental data. By employing these NLDs, we compute reaction cross-sections and astrophysical reaction rates for radiative neutron capture in few Fe-group nuclei, and compare them with experimental data as well as with those obtained with NLDs from phenomenological and microscopic mean-field models. The results obtained for the NLDs from SDM are able to explain the experimental data quite well. These results are of particular importance since the configuration mixing through the residual interaction naturally accounts for the collective excitations. In the mean-field models, the collective effects are included through the vibrational and rotational enhancement factors and their NLDs are further normalized at low energies with neutron resonance data.

The challenging direct measurement of the 65 keV resonance strength of the 17O(p,γ)18F reaction at LUNA

A precise determination of proton capture rates on oxygen is mandatory to predict the abundance ratios of the oxygen isotopes in a stellar environment where hydrogen burning is active. The 17O(p,γ)18F reaction, specifically, plays a crucial role in AGB nucleosynthesis as well as in explosive hydrogen burning occurring in type Ia novae. At temperatures of interest for the former scenario (20 MK ≤ T ≤ 80 MK) the main contribution to the astrophysical reaction rate comes from the Ec.m. = 65 keV resonance. The strength of this resonance is presently determined only through indirect measurements, with an adopted value of ωγ = (1.6 ± 0.3) × 10−11 eV. Thanks to the low background environment of the Laboratori Nazionali del Gran Sasso, the intense and stable beam provided by the LUNA 400 kV accelerator and the experience in oxygen target production, the LUNA collaboration is aiming the first direct measurement of the above mentioned resonance strength. In the present work details of challenging direct measurement planned at LUNA will be described.