Radiative Capture Rates Research Articles

Calculation of Thermalized Component of Neutron Spectra in a D-D Neutron Generator

The results of the thermalized flux calculation that incorporate radiative capture reactions in the presence and absence of polyethylene blocks that form an enclosure for a deuteron-deuteron (D-D) neutron generator are presented. This method can be used to measure the moderated neutron flux component in a mixture of moderated and primary neutron spectra. Using 20-cm-thick polyethylene blocks to surround a D-D neutron generator, the moderation of primary neutrons was investigated using nine indium foils. In this paper, the relationship between the moderated neutron flux and the radiative capture rates in the presence and absence of polyethylene blocks is derived. This is compared to a MCNP simulation and a calculation of modulated flux that ignore the primary neutron components.

AbInitio Prediction of the ^{4}He(d,γ)^{6}Li Big Bang Radiative Capture.

The rate at which helium (^{4}He) and deuterium (d) fuse together to produce lithium-6 (^{6}Li) and a γ ray, ^{4}He(d,γ)^{6}Li, is a critical puzzle piece in resolving the discrepancy between big bang predictions and astronomical observations for the primordial abundance of ^{6}Li. The accurate determination of this radiative capture rate requires the quantitative and predictive description of the fusion probability across the big bang energy window (30 keV≲E≲400 keV), where measurements are hindered by low counting rates. We present first-principle (or, abinitio) predictions of the ^{4}He(d,γ)^{6}Li astrophysical S factor using validated nucleon-nucleon and three-nucleon interactions derived within the framework of chiral effective field theory. By employing the abinitio no-core shell model with continuum to describe ^{4}He-d scattering dynamics and bound ^{6}Li product on an equal footing, we accurately and consistently determine the contributions of the main electromagnetic transitions driving the radiative capture process. Our results reveal an enhancement of the capture probability below 100keV owing to previously neglected magnetic dipole (M1) transitions and reduce by an average factor of 7 the uncertainty of the thermonuclear capture rate between 0.002 and 2GK.

Re-analysis of radiative capture11C(p, γ)12N at low energy

Within the framework of the modified potential model, we computed the astrophysical S-factor of the proton radiative capture p + 11C → 12N + γ. At energies bearing astrophysical importance, radiative capture is a key process in the spectroscopic study of 12N. In this work, we consider the radiative capture cross-section for the proton capture by 11C within the framework of the potential model. For the possible electric and magnetic dipole transitions, we computed the partial components of the astrophysical S-factor within the energy range E = (0.01–0.8) MeV. The computed value of the S-factor at zero energy shows satisfactory agreement with the reported results. Furthermore, we computed the radiative capture rates for the selected reaction, which shows a better comparison with the reported data.

An implantation Diamond detector as a beam monitor for an intense radioactive ion beam

We present the characterization of a Diamond detector and its response as a beam rate monitor with full stopping of radioactive ion beams of high intensity. The detector has been implemented in the VAMOS focal plane at GANIL and utilised in conjunction with AGATA and MUGAST detector systems. In the present experiment, for the first time, the beam has been fully stopped, rather than being recorded by a transmission detector. The Diamond detector has been tested for use as a particle counter for monitoring a high intensity, radioactive ion beam in the study of the alpha transfer reaction 7Li(15O,t)19Ne. The present experiment, which took place in July 2019, has used a 15O radioactive beam with a high intensity of 107 particles per second due to the weak reaction population and it has been measured using the VAMOS spectrometer and the AGATA and MUGAST arrays. Detailed monitoring of beam intensities in the range of 106 – 107 particles per second is particularly challenging in radioactive ion beam experiments. Thus, the chosen method involves the diamond detector due to its sub-nanosecond response time as well as its radiation hardness. The study of the alpha transfer reaction 7Li(15O,t)19Ne will be performed to determine the radiative alpha capture rate on 15O which is a key breakout route from the Hot-CNO cycle which leads to a explosive nucleosynthesis in X-ray bursts.

Status of nuclear physics behind nucleosynthesis processes: The role of exotic neutron-rich nuclei

We give a brief overview of the current status of important nuclear physics inputs, like reaction rates, in hydrostatic and explosive nucleosynthesis. Recently, it has been proposed that exotic neutron-rich nuclei play an important role even in the formation of heavy elements via the r-process. The main problems here are identification, abundance estimation of seed nuclei in these processes, and their pathways. We will try to highlight how improved nuclear structure and reaction calculations can affect our present understanding of radiative capture rates of light-mass and medium-mass nuclei, which in turn can drastically influence the abundance of heavier-mass elements.

Re-examination of proton capture 13N(p, γ)14O in stellar matter

Radiative capture p + 13N → 14O + γ at energies bearing astrophysical importance is one of the key processes in the hot CNO cycle. The transition from normal CNO to hot CNO takes place when the proton capture rates on 13N are higher than the β decay rates of 13N. The hot CNO cycle initiates at stellar temperature T ≥ 0.11 ×109 K. Within the frame work of potential model, we calculate the nuclear reaction rates and astrophysical S-factor for the reaction 13N(p, γ)14O. It is to be noted that the nuclear cross-section is calculated for the first resonance of p–13N system within the proton energy range (0–1) MeV, where the E1 transition plays a dominant role. Our calculated radiative capture rates are in nice comparison with the existing data. Based on the radiative capture rates we further estimate the temperature at which the normal CNO turns into a hot CNO cycle

Radiative capture rates at deep defects from electronic structure calculations

We present a methodology to calculate radiative carrier capture coefficients at deep defects in semiconductors and insulators from first principles. Electronic structure and lattice relaxations are accurately described with hybrid density functional theory. Calculations of capture coefficients provide an additional validation of the accuracy of these functionals in dealing with localized defect states. We also discuss the validity of the Condon approximation, showing that even in the event of large lattice relaxations the approximation is accurate. We test the method on GaAs:${V}_{\text{Ga}}\text{\ensuremath{-}}{\text{Te}}_{\text{As}}$ and GaN:${\mathrm{C}}_{\text{N}}$, for which reliable experiments are available, and demonstrate very good agreement with measured capture coefficients.

Radiative capture reaction rate fromΛΛto H dibaryon in the imaginary time method

Radiative capture rates of thermal $\mathrm{\ensuremath{\Lambda}}\mathrm{\ensuremath{\Lambda}}+\mathrm{\ensuremath{\Xi}}N$ states into a H dibaryon are calculated in the novel imaginary time method. The H dibaryon is assumed to be a bound state of $\mathrm{\ensuremath{\Xi}}N$ with spin ${J}^{\ensuremath{\pi}}={0}^{+}$, isospin $I=0$ and strangeness $\ensuremath{-}2$. We consider the $E1$ transition to H from $\mathrm{\ensuremath{\Xi}}N (L=1)$ scattering states which mix with $\mathrm{\ensuremath{\Lambda}}\mathrm{\ensuremath{\Lambda}}(L=1)$. In order to calculate the transition rates, we formulate a coupled-channel imaginary time method by extending the one-channel formula originally proposed by Yabana and Funaki. The imaginary time method allows us to avoid the sum over all the excited thermal initial states, and thus to save computational time significantly. The transition rates are given as a function of temperature and the unknown binding energy of the H dibaryon, which we take as a parameter. It is found that the transition rate is not sensitive to the choices of the H binding energy or the strengths of the channel coupling for temperatures 3 MeV or higher.

Predictive Modeling of Coupled Systems: Uncertainty Reduction Using Multiple Reactor Physics Benchmarks

This work illustrates reactor physics applications of the predictive modeling of coupled multiphysics systems (PMCMPS), formulated by Cacuci (2014), by means of the benchmarks Godiva (a bare uranium sphere) and Jezebel-239 and Jezebel-240 (bare plutonium spheres). The PMCMPS methodology was ab initio developed in the response space, to reduce as much as possible the computational memory requirements for predictive modeling of very large systems involving not only many model parameters but also many model responses. The model parameters considered in this work include individual cross sections for each material, nuclide, reaction type, and energy group, giving the following totals: 2241 parameters for Jezebel-239, 1458 parameters for Jezebel-240, and 2916 parameters for Godiva. Eight responses were considered for Jezebel-239 (the effective multiplication factor; the center core fission rates for 233U, 238U, 237Np, and 239Pu; and the center core radiative capture rates for 55Mn, 93Nb, and 63Cu). Three responses (the effective multiplication factor and the center core fission rates for 233U and 237Np) were selected for Jezebel-240, and eleven responses were selected for Godiva (the reaction rate types listed for Jezebel-239, along with the radiative capture rates for 107Ag, 127I, and 81Br).The PMCMPS methodology ensures that increasing the amount of information yields more accurate predictions, with smaller predicted uncertainties, as long as the considered information is consistent. This fact is amply illustrated in this work, which shows that the interdependence of responses that were measured in more than one benchmark is stronger than for responses that were measured in a single benchmark. More generally, the consideration of the complete information, including couplings, provided jointly by all three benchmarks (as opposed to consideration of the benchmarks as separate systems) leads to more accurate predictions of nominal values for responses and model parameters, yielding larger reductions in the predicted uncertainties that accompany the predicted mean values of responses and model parameters.

Imaginary-time method for the radiative capture reaction rate

We propose a new computational method for astrophysical reaction rate of radiative capture process. In the method, an evolution of a wave function is calculated along the imaginary-time axis which is identified as the inverse temperature. It enables direct evaluation of reaction rate as a function of temperature without solving any scattering problem. The method is tested for two-body radiative capture reaction, ${^{16}{\rm O}}(\alpha,\gamma){^{20}{\rm Ne}}$, showing that it gives identical results to that calculated by the ordinary procedure. The new method will be suited for calculation of triple-alpha radiative capture rate for which an explicit construction of the scattering solution is difficult.

Muon capture constraints on sterile neutrino properties

We show that ordinary and radiative muon capture impose stringent constraints on sterile neutrino properties. In particular, we consider a sterile neutrino with a mass between 40 to $80 {\rm MeV}$ that has a large mixing with the muon neutrino and decays predominantly into a photon and light neutrinos due to a large transition magnetic moment. Such a model was suggested as a possible resolution to the puzzle presented by the results of the LSND, KARMEN, and MiniBooNE experiments. We find that the scenario with the radiative decay to massless neutrinos is ruled out by measurements of the radiative muon capture rates at TRIUMF in the relevant mass range by a factor of a few in the squared mixing angle. These constraints are complementary to those imposed by the process of electromagnetic upscattering and de-excitation of beam neutrinos inside the neutrino detectors induced by a large transition magnetic moment. The latter provide stringent constraints on the size of the transitional magnetic moment between muon, electron neutrinos and $N$. We also show that further extension of the model with another massive neutrino in the final state of the radiative decay may be used to bypass the constraints derived in this work.

Improved predictions of nuclear reaction rates with the TALYS reaction code for astrophysical applications

Nuclear reaction rates of astrophysical applications are traditionally determined on the basis of Hauser-Feshbach reaction codes. These codes adopt a number of approximations that have never been tested, such as a simplified width fluctuation correction, the neglect of delayed or multiple-particle emission during the electromagnetic decay cascade, or the absence of the pre-equilibrium contribution at increasing incident energies. The reaction code TALYS has been recently updated to estimate the Maxwellian-averaged reaction rates that are of astrophysical relevance. These new developments enable the reaction rates to be calculated with increased accuracy and reliability and the approximations of previous codes to be investigated. The TALYS predictions for the thermonuclear rates of relevance to astrophysics are detailed and compared with those derived by widely-used codes for the same nuclear ingredients. It is shown that TALYS predictions may differ significantly from those of previous codes, in particular for nuclei for which no or little nuclear data is available. The pre-equilibrium process is shown to influence the astrophysics rates of exotic neutron-rich nuclei significantly. For the first time, the Maxwellian-averaged (n,2n) reaction rate is calculated for all nuclei and its competition with the radiative capture rate is discussed. The TALYS code provides a new tool to estimate all nuclear reaction rates of relevance to astrophysics with improved accuracy and reliability.

A Modeling of BWR-MOX Assemblies Based on the Characteristics Method Combined with Advanced Self-Shielding Models

Calculations based on the characteristics method and different self-shielding models are presented for 9 × 9 boiling water reactor (BWR) assemblies fully loaded with mixed-oxide (MOX) fuel. The geometry of these assemblies was recovered from the BASALA experimental program. We have focused our study on three configurations simulating the different voiding conditions that an assembly can undergo in a BWR pressure vessel. A parametric study was carried out with respect to the spatial discretization, the tracking parameters, and the anisotropy order. Comparisons with Monte Carlo calculations in terms of keff, radiative capture, and fission rates were performed to validate the computational tools. The results are in good agreement between the stochastic and deterministic approaches. The mutual self-shielding model recently introduced within the framework of the Ribon extending self-shielding method appears to be useful for this type of assemblies. Indeed, in the calculation of these MOX benchmarks, the overlapping of resonances, especially between 238U and 240Pu, plays an important role due to the spectral strengthening of the flux as the voiding percentage is increased. The method of characteristics is shown to be adequate to perform accurate calculations handling a fine spatial discretization.

Constraints on theBe7(p,γ)B8radiative capture rate from charge symmetry

Cross sections for the dipole radiative capture reactions $^{7}\mathrm{Li}(n,\ensuremath{\gamma})^{8}\mathrm{Li}$ and $^{7}\mathrm{Be}(p,\ensuremath{\gamma})^{8}\mathrm{B}$ are calculated in a two-body model, which is based on a Woods-Saxon parametrization of the nuclear interaction. The well depth is adjusted for each reaction channel so that the measured separation energies and $s$-wave scattering lengths are reproduced. The calculations are repeated for a wide range of the radius and the diffuseness of the interaction. The predicted $S$ factor for the radiative proton capture on $^{7}\mathrm{Be}$ falls within a surprisingly narrow range of values when the model is calibrated to reproduce measurements of the mirror reaction $^{7}\mathrm{Li}(n,\ensuremath{\gamma})^{8}\mathrm{Li}$. The simplified model used here is consistent with the shell model approach for proton capture on $^{7}\mathrm{Be}$ but it gives a significantly smaller cross section for neutron capture on $^{7}\mathrm{Li}$.

Radiative muon capture by3He

The rate of the nuclear reaction $^3He+\mu^- \to ^3H+\gamma+\nu_{\mu}$ has been calculated using both the elementary particle model (EPM) approach and the impulse approximation (IA) approach. Using the EPM approach, the exclusive statistical radiative muon capture (RMC) rate for photon energy greater than 57 MeV is found to be $0.245 s^{-1}$ and the ordinary muon capture (OMC) rate to be $1503 s^{-1}$. The IA calculation exhibits a slight dependence on the type of trinucleon wave functions used. The difference between the IA and EPM calculation is larger for RMC than for OMC. To resolve the difference between the two approaches a more detailed investigation including meson exchange corrections will be required.

Asymptotic normalization coefficients, spectroscopic factors, and direct radiative capture rates

We compare the use of asymptotic normalization coefficients (ANC's) and spectroscopic factors determined from peripheral transfer reactions for determining the overall normalization of peripheral direct radiative capture reaction processes. We demonstrate that ANC's provide a natural way to parametrize the rates of both peripheral transfer and direct capture reactions. Furthermore, ANC's inferred from one reaction may be used in the analysis of a second reaction without further knowledge regarding their origin, and independent measurements of a given ANC may be combined to give an overall ``best value'' in a straightforward manner. In contrast, a spectroscopic factor derived from analysis of a peripheral transfer reaction can only be used in subsequent calculations if one has detailed knowledge of the single-particle bound state orbital that was assumed when the spectroscopic factor was obtained.

AstrophysicalSfactor for9Be(p,γ)10B

The [sup 9]Be(p,[gamma])[sup 10]B reaction plays an important role in primordial and stellar nucleosynthesis of light elements in the [ital p] shell, but the energy dependence of S(E) has not been well understood. We reanalyze the existing [sup 9]Be(p,[gamma])[sup 10]B experimental data within the framework of the [ital R]-matrix method. The direct capture part of the [ital S] factor is calculated using the experimentally measured asymptotic normalization coefficients for [sup 10]B[r arrow][sup 9]Be+p. The fitted parameters of the low-lying [sup 10]B resonances are also required to be consistent with previous measurements of [sup 6]Li([alpha],[gamma])[sup 10]B. A good simultaneous fit to both radiative capture reactions is found, in contrast to previous analyses. These results demonstrate that experimentally measured asymptotic normalization coefficients, coupled to the [ital R]-matrix method, can provide a reasonable determination of direct radiative capture rates, even when the captured proton is tightly bound in the final nucleus. [copyright] [ital 1999] [ital The American Physical Society]

Calculations of ordinary and radiative muon capture on 58,60,62Ni

The photon spectra and total radiative muon capture rate on heavy nuclei have been calculated for the first time using a microscopic description of the nuclear excitation function. The quasiparticle random phase approximation has been used to calculate the excitation spectra and transition amplitudes of ordinary and radiative muon capture in 58,60,62Ni. The parameters of the phenomenological nuclear spin-isospin residual interaction have been determined by comparison of the calculated and experimentally observed distributions of the transition strength over the excitation energies. The strength functions of Gamow-Teller ( σt ±) and spin-dipole ( r[ Y 1 σ] 0,1,2 t −) transitions are calculated and compared with the experimental data. The sum rule for the Gamow-Teller transitions is also considered. The nuclear amplitude of radiative muon capture has been treated in the framework of the standard impulse approximation and also in the modified impulse approximation, which takes into account the continuity equation for electromagnetic current. The calculated total rates of ordinary muon capture are close to the experimental data.

Radiative muon capture by a proton in chiral perturbation theory

The first measurement of the radiative muon capture (RMC) rate on a proton was recently carried out at TRIUMF. The TRIUMF group analyzed the RMC rate, Γ RMC exp , in terms of the theoretical formula of Beder and Fearing, and found the surprising result that g P ≡ f P ( q 2 =−0.88 m μ 2 ) is 1.5 times the value expected from PCAC. To assess the reliability of the theoretical framework used by the TRIUMF group to relate Γ RMC to the pseudoscalar form factor f P , we calculate Γ RMC in chiral perturbation theory, which provides a systematic framework to describe all the vertices involved in RMC, fulfilling gauge-invariance and chiral-symmetry requirements in a transparent manner. As a first step we present a chiral perturbation calculation at tree level which includes sub-leading order terms.

Direct radiative capture of electrons by average ions

We have derived direct radiative capture (RC) rate coefficients for average ions, which, in general, have a non-integer occupation number of orbital electrons, from the rate equations for detailed configuration accounting and developed a self-consistent-field theoretical method for calculating RC cross sections and rate coefficients for average ions. As an example, the RC cross sections for Mo, Ge and Ti ions have been calculated at a defined electron population, and have been compared with the results from the Kramers formula and the cross section average via detailed configuration accounting. This method is also applied to the calculations of RC cross sections and rate coefficients for the `real' and ions. The results are in good agreement with the experimental measurements.