Neutron Capture Rates Research Articles

Reduction of the neutron imaginary potential off the stability line and its possible impact on neutron capture rates

The effect of the isovector imaginary optical potential has been studied experimentally by using the particle-evaporation technique for the $^{11}\mathrm{B}+^{48}\mathrm{Ca}$ reaction with a 21.8 MeV $^{11}\mathrm{B}$ beam. Spectra of neutron, proton, and $\ensuremath{\alpha}$ particles emitted from the neutron-rich compound nucleus $^{59}\mathrm{Mn}$ have been measured and analyzed with traditional optical model potentials with their original parametrizations as well as with adjusted isovector imaginary components. It is shown that the isovector component of the imaginary potential is indispensable in the reproduction of proton and $\ensuremath{\alpha}$-particle yields from this reaction and even needs to be enhanced compared with the suggestions of the original model parametrizations. This can lead to important consequences for astrophysical reaction-rate calculations.

Shell-model based study of the direct capture in neutron-rich nuclei

The radiative neutron capture rates for isotopes of astrophysical interest are commonly calculated within the statistical Hauser-Feshbach reaction model. Such an approach, assuming a high level density in the compound system, can be questioned in light and neutron-rich nuclei for which only a few or no resonant states are available. Therefore, in this work we focus on the direct neutron-capture process. We employ a shell-model approach in several model spaces with well-established effective interactions to calculate spectra and spectroscopic factors in a set of 50 neutron-rich target nuclei in different mass regions, including doubly-, semi-magic and deformed ones. Those theoretical energies and spectroscopic factors are used to evaluate direct neutron capture rates and to test global theoretical models using average spectroscopic factors and level densities based on the Hartree-Fock-Bogoliubov plus combinatorial method. The comparison of shell-model and global model results reveals several discrepancies that can be related to problems in level densities. All the results show however that the direct capture is non-negligible with respect to the by-default Hauser-Feshbach predictions and can be even 100 times more important for the most neutron-rich nuclei close to the neutron drip line.

Nuclear physics aspects of neutron capture and β-decay rates in stellar environment

Under stellar conditions, nuclear reaction rates are different from those measured in laboratory. A notable example of this situation is the stellar rate for neutron capture, when nuclear excited states can be thermally populated and the capture process can proceed not only for the ground state but also from nuclear excited levels. While the composite nucleus of the capture process is the same, different excited compound states can be populated from different target nuclear levels. In addition, inelastic scattering channels can be open for these states, in which the neutron is scattered out with kinetic energy higher than kinetic energy of the entrance channel (the so-called super-elastic scattering). An additional example of the modification of stellar rates with respect to laboratory rates is provided by the β-decay. Here, the different degree of ionization of the nuclei involved imply a modification of the phase-space factor in the decay probability. In addition, here again, excited parent decaying states can be populated in a stellar plasma and additional transitions, with different degrees of forbidness, become possible. A dramatic change in the β-decay rates under stellar conditions have been predicted and observed in a number of cases. A review of these two rates evaluations, from the theoretical point of view as well as from the perspectives of their experimental determination is presented with some relevant examples for both, neutron capture and β-decay processes.

Indirect, experimental constraints of (n,γ) reaction rates for the i- and r-process

Our Universe contains a large variety of chemical elements, for which only the lightest ones were produced during the Big Bang. To make elements heavier than iron, neutron-capture processes are called for, in particular the slow and the rapid neutron-capture processes. Recently, a so-called intermediate neutron-capture process has received a lot of attention as more and more evidence points towards its existence. Both the intermediate and rapid neutron-capture processes involve very neutron-rich nuclei, for which there exist little or no data on their neutron-capture cross sections. Here we present an experimental method to indirectly constrain neutron-capture reaction rates needed for calculating nucleosynthesis yields for the intermediate and rapid neutron-capture processes.

Extracting capture from transfer reactions

Indirect reaction techniques are very important in astrophysics as they provide information that is complementary to direct measurements or that otherwise cannot be obtained directly. It is then critical to have a reliable reaction theory that can connect the reaction measurement with the astrophysical information desired. This is a brief report on the progress made in the theory for transfer reactions when used to determine neutron capture rates for r-process and proton capture for rp-process nuclei. We will discuss the different types of experiments and their connection to astrophysics. An overview of the current status of the theory will be provided, with emphasis on several recent theory developments, including transfer to continuum, the improvement of the optical potential and uncertainty quantification. Applications to a couple of neutron rich and proton rich cases will be discussed.

Review and new concepts for neutron-capture measurements of astrophysical interest

The idea of slow-neutron capture nucleosynthesis formulated in 1957 triggered a tremendous experimental effort in different laboratories worldwide to measure the relevant nuclear physics input quantities, namely (n, γ) cross sections over the stellar temperature range (from few eV up to several hundred keV) for most of the isotopes involved from Fe up to Bi. A brief historical review focused on total energy detectors will be presented to illustrate how advances in instrumentation have led to the assessment of new aspects of s-process nucleosynthesis and to the progressive refinement of stellar models. A summary will be presented on current efforts to develop new detection concepts, such as the Total-Energy Detector with γ-ray imaging capability (i-TED). The latter is based on the simultaneous combination of Compton imaging with neutron time-of-flight (TOF) techniques, in order to achieve a superior level of sensitivity and selectivity in the measurement of stellar neutron capture rates.

Propagation of statistical uncertainties of Skyrme mass models to simulations of r -process nucleosynthesis

Uncertainties in nuclear models have a major impact on simulations that aim at understanding the origin of heavy elements in the universe through the rapid neutron capture process ($r$ process) of nucleosynthesis. Within the framework of the nuclear density functional theory, we use results of Bayesian statistical analysis to propagate uncertainties in the parameters of energy density functionals to the predicted $r$-process abundance pattern, by way not only of the nuclear masses but also through the influence of the masses on $\beta$-decay and neutron capture rates. We additionally make the first identifications of specific parameters of Skyrme-like energy density functionals which are correlated with particular aspects of the $r$-process abundance pattern. While previous studies have explored the reduction in the abundance pattern uncertainties due to anticipated new measurements of neutron-rich nuclei, here we point out that an even larger reduction will occur when these new measurements are used to reduce the uncertainty of model predictions of masses, which are then propagated through to the abundance pattern. We make a quantitative prediction for how large this reduction will be.

A direct measurement of the 17O(α,γ)21Ne reaction in inverse kinematics and its impact on heavy element production

During the slow neutron capture process in massive stars, reactions on light elements can both produce and absorb neutrons thereby influencing the final heavy element abundances. At low metallicities, the high neutron capture rate of 16O can inhibit s-process nucleosynthesis unless the neutrons are recycled via the 17O(α,n)20Ne reaction. The efficiency of this neutron recycling is determined by competition between the 17O(α,n)20Ne and 17O(α,γ)21Ne reactions. While some experimental data are available on the former reaction, no data exist for the radiative capture channel at the relevant astrophysical energies.The 17O(α,γ)21Ne reaction has been studied directly using the DRAGON recoil separator at the TRIUMF Laboratory. The reaction cross section has been determined at energies between 0.6 and 1.6 MeV Ecm, reaching into the Gamow window for core helium burning for the first time. Resonance strengths for resonances at 0.63, 0.721, 0.81 and 1.122 MeV Ecm have been extracted. The experimentally based reaction rate calculated represents a lower limit, but suggests that significant s-process nucleosynthesis occurs in low metallicity massive stars.

Influence of nuclear mass uncertainties on radiative neutron-capture rates

Neutron-capture rate plays a crucial role in the rapid neutron-capture process (r-process) and nuclear mass is one of the most important inputs for the estimations of neutron-capture rates. In this work, we employ ten nuclear mass models, including macroscopic, macroscopic-microscopic, and microscopic models, to calculate the radiative neutron-capture rates for the nuclei relevant to the r-process at various temperatures $T={10}^{5}--{10}^{10}\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. It is found that the differences in the predictions of neutron-capture rate using these mass models get larger when moving to the neutron-rich and superheavy regions, whereas they generally become smaller with the increase of temperature. Taking the nuclei on the $N=126$ r-process path as examples, we find that the uncertainties of neutron-capture rates at $T={10}^{9}\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ range from about one to four orders of magnitude. The influence of the experimental mass errors on neutron-capture rates is investigated as well.

Extreme brightness laser-based neutron pulses as a pathway for investigating nucleosynthesis in the laboratory

With the much-anticipated multi-petawatt (PW) laser facilities that are coming online, neutron sources with extreme fluxes could soon be in reach. Such sources would rely on spallation by protons accelerated by the high-intensity lasers. These high neutron fluxes would make possible not only direct measurements of neutron capture and β-decay rates related to the r-process of nucleosynthesis of heavy elements, but also such nuclear measurements in a hot plasma environment, which would be beneficial for s-process investigations in astrophysically relevant conditions. This could, in turn, finally allow possible reconciliation of the observed element abundances in stars and those derived from simulations, which at present show large discrepancies. Here, we review a possible pathway to reach unprecedented neutron fluxes using multi-PW lasers, as well as strategies to perform measurements to investigate the r- and s-processes of nucleosynthesis of heavy elements in cold matter, as well as in a hot plasma environment.

R-process 130Cd Waiting Point

Most of the elements heavier than Fe in the universe were produced by the so-called r-process. Its mechanism is based on a rapid neutron capture by the nuclei, so that the neutron capture rates are much faster than those of β -decay. When r-process reaches nuclei with magic neutron numbers, the neutron separation or binding energies increase and the process slows down, it has to wait for several β decays to produce heavier nuclei. These magic nuclei are the waiting points. Information about r-process are steel missing and it requires knowledge of the nuclear structure of the neutron rich nuclei. However, the nuclear properties of these nuclei are not sufficiently well understood due to the experimental difficulties in their production. The A=130 isobars with N=82 present one of the most interesting waiting points, because of their positions far from β stability and near the doubly magic 132Sn core, for which theoretical and experimental studies give important information about beta decay half-lives. In this context, we focus on the study of even-even 130 Cd waiting point nuclear properties. We have performed some spectroscopic calculations for energetic spectrum, β -decay half-life evolution in terms of temperature, using recent experimental data, by means of Oxbash nuclear structure code. The getting spectrum is in a reasonably agreement with the available experimental data. However, the calculated β-decay half-life for the studied waiting point is short in comparison with the experimental one.

ОБЛУЧАТЕЛЬНОЕ УСТРОЙСТВО ДЛЯ БЫСТРОГО РЕАКТОРА БОЛЬШОЙ МОЩНОСТИ

To address the growing demand for isotopes, it is planned to use irradiation devices in high-power fast sodium reactors. With the help of this type of irradiation devices, mass production of the necessary isotopes is assumed. The designs of irradiation devices for operating time of cobalt-60 isotope in BN reactors are presented. A new version of the irradiation device for use in a high-power sodium fast reactor is proposed. The comparison of different variants of irradiation devices according to the obtained specific activity is given. The new design of the irradiation device combines the retarder units and the steel protection developed for the BN-600 in a single assembly. It allows to achieve cobalt activity of 150 Ci/g for a campaign of fuel (5 years). The results of a study of the azimuthal and radial distribution of the cobalt-60 formation speed to ensure the effective production of the product in a high-power reactor are presented. According to the results of the azimuthal distribution of the neutron capture rate, there is an uneven propagation in the horizontal section of the irradiation device. The maximum capture rate is shifted to the periphery of the irradiation device facing the center of the core. The obtained spatial distribution of the capture rate indicates the possibility of further work to create a more efficient design of the irradiation device for the conditions of a large fast reactor.

Informing direct neutron capture on tin isotopes near the N=82 shell closure

Half of the elements heavier than iron are believed to be produced through the rapid neutron-capture process ($r$ process). The astrophysical environment(s) where the $r$ process occurs remains an open question, even after recent observations of neutron-star mergers and the associated kilonova. Features in the abundance pattern of $r$-process ashes may provide critical insight for distinguishing contributions from different possible sites, including neutron-star mergers and core-collapse supernovae. In particular, the largely unknown neutron-capture reaction rates on neutron-rich unstable nuclei near $^{132}\mathrm{Sn}$ could have a significant impact on the final $r$-process abundances. To better determine these neutron-capture rates, the $(d,p)$ reaction has been measured in inverse kinematics using radioactive ion beams of $^{126}\mathrm{Sn}$ and $^{128}\mathrm{Sn}$ and a stable beam of $^{124}\mathrm{Sn}$ interacting with a ${({\mathrm{CD}}_{2})}_{n}$ target. An array of position-sensitive silicon strip detectors, including the Super Oak Ridge Rutgers University Barrel Array, was used to detect light reaction products. In addition to the present measurements, previous measurements of $^{130,132}\mathrm{Sn}(d,p)$ were reanalyzed using state-of-the-art reaction theory to extract a consistent set of spectroscopic factors for $(d,p)$ reactions on even tin nuclei between the heaviest stable isotope $^{124}\mathrm{Sn}$ and doubly magic $^{132}\mathrm{Sn}$. The spectroscopic information was used to calculate direct-semidirect $(n,\ensuremath{\gamma})$ cross sections, which will serve as important input for $r$-process abundance calculations.

Publisher's Note: Experimental Neutron Capture Rate Constraint Far from Stability [Phys. Rev. Lett. 116, 242502 (2016)

This corrects the article DOI: 10.1103/PhysRevLett.116.242502.

Experimental constraints on the Zn73(n,γ)Zn74 reaction rate

Background: The recent observation of a neutron-star merger finally confirmed one astrophysical location of the rapid neutron-capture process (r-process). Evidence of the production of $Al140$ nuclei was seen, but there is still little detailed information about how those lighter elements are produced in such an environment. Many of the questions surrounding the $A\ensuremath{\approx}80$ nuclei are likely to be answered only when the nuclear physics involved in the production of r-process nuclei is well understood. Neutron-capture reactions are an important component of the r-process, and neutron-capture cross sections of r-process nuclei, which are very neutron rich, have large uncertainties.Purpose: Indirectly determine the neutron-capture cross section and reaction rate of $^{73}\mathrm{Zn}(n,\ensuremath{\gamma})^{74}\mathrm{Zn}$.Methods: The nuclear level density (NLD) and $\ensuremath{\gamma}$-ray strength function ($\ensuremath{\gamma}\mathrm{SF}$) of $^{74}\mathrm{Zn}$ were determined following a total absorption spectroscopy (TAS) experiment focused on the $\ensuremath{\beta}$ decay of $^{74}\mathrm{Cu}$ into $^{74}\mathrm{Zn}$ performed at the National Superconducting Cyclotron Laboratory. The NLD and $\ensuremath{\gamma}\mathrm{SF}$ were used as inputs in a Hauser-Feshbach statistical model to calculate the neutron-capture cross section and reaction rate.Results: The NLD and $\ensuremath{\gamma}\mathrm{SF}$ of $^{74}\mathrm{Zn}$ were experimentally constrained for the first time using $\ensuremath{\beta}$-delayed $\ensuremath{\gamma}$ rays measured with TAS and the $\ensuremath{\beta}$-Oslo method. The NLD and $\ensuremath{\gamma}\mathrm{SF}$ were then used to constrain the neutron-capture cross section and reaction rate for the $^{73}\mathrm{Zn}(n,\ensuremath{\gamma})^{74}\mathrm{Zn}$ reaction.Conclusions: The uncertainty in the neutron-capture cross section and reaction rate of $^{73}\mathrm{Zn}(n,\ensuremath{\gamma})^{74}\mathrm{Zn}$ calculated in TALYS was reduced to under a factor of 2 from a factor of 5 in the cross section and a factor of 11 in the reaction rate using the experimentally obtained NLD and $\ensuremath{\gamma}\mathrm{SF}$.

Influence of nuclear physics inputs and astrophysical conditions on r-process

The rapid neutron-capture process ($r$-process) is one of the main mechanisms to explain the origin of heavy elements in the universe. Although the past decades have seen great progress in understanding this process, the related nuclear physics inputs to $r$-process models include significant uncertainty. In this study, ten nuclear mass models, including macroscopic, macroscopic-microscopic, and microscopic models, are used to calculate the $\\beta$-decay rates and neutron-capture rates of the neutron-rich isotopes for the $r$-process simulations occurring in three classes of astrophysical conditions. The final $r$-process abundances include uncertainties introduced by the nuclear mass model mainly through the variation of neutron-capture rates, whereas the uncertainties of $\\beta$-decay rates make a relatively small contribution. The uncertainties in different astrophysical scenarios are also investigated, and are found to be connected to the diverse groups of nuclei produced during nucleosynthesis.

Computing Neutron Capture Rates in Neutron-Degenerate Matter

Neutron captures are likely to occur in the crust of accreting neutron stars (NSs). Their rate depends on the thermodynamic state of neutrons in the crust. At high densities, neutrons are degenerate. We find degeneracy corrections to neutron capture rates off nuclei, using cross sections evaluated with the reaction code TALYS. We numerically integrate the relevant cross sections over the statistical distribution functions of neutrons at thermodynamic conditions present in the NS crust. We compare our results to analytical calculations of these corrections based on a power-law behavior of the cross section. We find that although an analytical integration can simplify the calculation and incorporation of the results for nucleosynthesis networks, there are uncertainties caused by departures of the cross section from the power-law approach at energies close to the neutron chemical potential. These deviations produce non-negligible corrections that can be important in the NS crust.

Shell-model study of the M1 dipole strength at low energy in the A>100 nuclei

A low-energy enhancement of radiative strength functions was observed in experiment in several mass regions of nuclei, and it is believed to impact considerably the calculated neutron-capture rates. In the shell-model framework it was demonstrated to be possibly due to the magnetic dipole radiation. Up to now, the M1 deexcitation strength function was described within this framework in the mid-mass nuclei from the pf and pfg shells (e.g., Sc44,Cr50,Fe56,57,60,66,68,Ni59,60,70,Mo94–96,Zr90), indicating systematically the presence of an up-bend at low energy. Therefore, the enhancement was accepted as a robust feature of the radiative decay. However, its exact magnitude and regions of emergence in the nuclear chart are still to be characterized in microscopic calculations. In this work, for the first time, the low-energy behavior of the radiative M1 strength in heavier nuclei, which can be described in the g7/2d5/2,3/2s1/2h11/2 shell, is investigated. The enhancement of the M1 deexcitation strength around N=80 is confirmed, and is similar to that found previously in lighter nuclei. In contrast, no such up-bend is present on the proton drip line, in the deformed N=Z=54Xe108 nucleus. From analysis of calculated magnetic dipole strengths the orbital nature of the observed enhancement is suggested. Photoemission and photoabsorption distributions on selected states are compared and the impact of different uncertainties in the theoretical treatment on the magnitude of M1 strengths at low photon energies is discussed.

Weak processes in astrophysical nucleosynthesis

Neutron capture and neutrino absorption are of fundamental importance in the synthesis of elements. Both processes can be affected by the extreme conditions of the matter undergoing nucleosynthesis. In this work, we present results of neutron capture rates on 46Mg and several Sn isopotes. We find that the high degeneracy of neutron matter, a result of the large pressures found in a neutron star, changes the captures rates by orders of magnitude compared to those of non-degenerate neutron matter. We also study the neutrino opacity above the symmetry axis of a black hole accretion disk. We find that there is a region, where matter circulates around the black hole in stable orbits, with potentially low neutrino opacity. This motivates further studies of neutrino emission in the vicinity of a black hole.

Uncertainties in s-process nucleosynthesis in low-mass stars determined from Monte Carlo variations

The main s-process taking place in low mass stars produces about half of the elements heavier than iron. It is therefore very important to determine the importance and impact of nuclear physics uncertainties on this process. We have performed extensive nuclear reaction network calculations using individual and temperature-dependent uncertainties for reactions involving elements heavier than iron, within a Monte Carlo framework. Using this technique, we determined the uncertainty in the main s-process abundance predictions due to nuclear uncertainties link to weak interactions and neutron captures on elements heavier than iron. We also identified the key nuclear reactions dominating these uncertainties. We found that $\beta$-decay rate uncertainties affect only a few nuclides near s-process branchings, whereas most of the uncertainty in the final abundances is caused by uncertainties in neutron capture rates, either directly producing or destroying the nuclide of interest. Combined total nuclear uncertainties due to reactions on heavy elements are in general small (less than 50%). Three key reactions, nevertheless, stand out because they significantly affect the uncertainties of a large number of nuclides. These are $^{56}$Fe(n,$\gamma$), $^{64}$Ni(n,$\gamma$), and $^{138}$Ba(n,$\gamma$). We discuss the prospect of reducing uncertainties in the key reactions identified in this study with future experiments.