Radiative Capture Cross Sections Research Articles

New Results for Radiative 2H3H Capture at Astrophysical Energies

Within the framework of the modified potential cluster model with forbidden states which follow from the classification of orbital states of the clusters according to the Young tableaux, we have calculated the total cross section, astrophysical S-factor, and reaction rate of 2H3H radiative capture at low energies. We have shown that the implemented model and methods of classification of cluster states according to the Young tableaux allow one overall to correctly describe the available experimental data for the total cross section and astrophysical S-factor of this process.

Cross section for the neutron radiative capture on Lu173 nuclei

We report the measurement of a neutron radiative cross section on a target with a high gamma activity, at the Los Alamos DANCE detector (Detector for Advanced Neutron Capture Experiments) for the first time. The $(n,\ensuremath{\gamma})$ reaction properties of the unstable $^{173}\mathrm{Lu}$ isotope were studied. Two experimental campaigns were needed to determine the $^{173}\mathrm{Lu}(n,\ensuremath{\gamma})$ cross section. They were performed at the Los Alamos Neutron Science Center (LANSCE) spallation neutron source facility. To this end, two targets were produced from an Hf sample using successively proton irradiation and chemical separations. They were composed of $^{173}\mathrm{Lu},^{174}\mathrm{Lu}$, and $^{175}\mathrm{Lu}$ isotopes. The two measurements were conducted at two-year intervals, taking advantage of the difference of the isotopic decay lifetimes in order to identify resonances from the $^{173}\mathrm{Lu}(n,\ensuremath{\gamma})$ and $^{174}\mathrm{Lu}(n,\ensuremath{\gamma})$ reactions. Just over one hundred new resonances were observed, the majority of which come from the $^{173}\mathrm{Lu}(n,\ensuremath{\gamma})$ reaction. Only a few of them were assigned to the $^{174}\mathrm{Lu}(n,\ensuremath{\gamma})$ reaction. As regards the $^{173}\mathrm{Lu}(n,\ensuremath{\gamma})$ reaction, the radiative neutron capture cross section was determined over the energy range from thermal neutron up to 200 eV. The parameters of resonances in the resolved resonance region were extracted with the sammy code while calculations with talys determined the capture cross section in the unresolved resonances region. At the keV region, we performed standard talys calculations as well as other microscopic investigations by substituting the standard talys photon strength function by another one from QRPA models.

Neutron capture cross sections of Hf178 leading to Hfm2179

Cross sections for radiative neutron capture have been measured for the formation of 25-d $^{179}\mathrm{Hf}^{{m}_{2}}$ (${J}^{\ensuremath{\pi}}=25/{2}^{+}$) using targets of enriched $^{178}\mathrm{Hf}$. Based on irradiations using pure epithermal neutrons as well as mixed thermal + epithermal neutrons, the thermal cross section has been determined to be $\ensuremath{\sigma}=6(5)\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{b}$ and the resonance integral $I=1.63(5)\phantom{\rule{0.16em}{0ex}}\mathrm{mb}$. To enhance the determination of the cross sections, the energies and intensities of the $\ensuremath{\gamma}$ rays from the decay of $^{179}\mathrm{Hf}^{{m}_{2}}$ have been re-determined with improved precision. The half-life of $^{179}\mathrm{Hf}^{{m}_{2}}$ has been measured to be 24.91(27) d, in agreement with the previously accepted value. The analysis of the $^{179}\mathrm{Hf}^{{m}_{2}}$ decay required more precise spectrometry on competing $\ensuremath{\gamma}$ rays from the decay of $^{181}\mathrm{Hf}$, whose results are presented (along with that of the decay of $^{175}\mathrm{Hf}$) from irradiation of a source of natural Hf. The measured cross sections are compared with those of other high-spin neutron captures.

Improved 242Pu(n,$ \gamma$) thermal cross section combining activation and prompt gamma analysis

A good knowledge of the radiative capture cross section of 242Pu is required for innovative nuclear reactor studies, especially for MoX fuel reactors. However, the experimental data available show discrepancies in the energy regions of interest: the thermal point and the keV region. Previous experimental results of the thermal cross section deviate from each other by 20% and these discrepancies are reflected also in the evaluated libraries, each of them giving more credit to different data sets. A recent measurement by Genreith et al. did not succeed to solve the existing discrepancy due to the large uncertainties and correction factors in the analysis. This work presents a new measurement of the thermal capture cross section of 242Pu carried out in the Budapest Research Reactor using the same thin targets of a previous measurement at n_TOF-EAR1, each containing 30mg of 99.995% pure 242Pu . The combined analysis of the full prompt $ \gamma$-ray spectrum and the 243Pu decay has led to three compatible values for the thermal cross section. Their average value, 18.9(9)b, has an improved accuracy compared to recent measurements. Leaving aside the activation value of Genreith using an outdated intensity value for the 84 keV decay line of 243Pu , our average result is in very good agreement with the JEFF-3.2 evaluation and all the previous measurements, with the exception of the highest value 22.5(11)b reported by Marie et al., which has a strong influence in the ENDF evaluation.

Neutron capture cross sections of stable Cd isotopes

Cross sections for radiative neutron capture have been measured by activation for the stable isotopes of Cd with mass numbers 106, 108, 110, 112, 114, and 116 by observing the decays of the 8 radioisotopes produced by captures in samples of both natural and enriched abundances. Determination of the thermal cross sections requires correction for captures by epithermal neutrons, so careful attention has been paid to the measurement of the resonance integrals, which were found to vary for irradiations at different sites in the reactor.

Re-estimation of 180Ta nucleosynthesis in light of newly constrained reaction rates

Recent measurements of the nuclear level densities and γ-ray strength functions below the neutron thresholds in 180,181,182Ta are used as input in the nuclear reaction code TALYS. These experimental average quantities are utilized in the calculations of the 179,180,181Ta radiative neutron capture cross sections. From the latter, astrophysical Maxwellian-averaged (n,γ) cross sections (MACS) and reaction rates are extracted, which in turn are used in large astrophysical network calculations to probe the production mechanism of 180Ta. These calculations are performed for two scenarios, the s-process production of 180,181Ta in Asymptotic Giant Branch (AGB) stars and the p-process nucleosynthesis of Tam180 in Type-II supernovae. Based on the results from this work, the s-process in stellar evolution is considered negligible in the production of Tam180 whereas 181Ta is partially produced by AGB stars. The new measurements strongly constrain the production and destruction rates of Tam180 at p-process temperatures and confirm significant production of nature's rarest stable isotope Tam180 by the p-process.

Neutron capture rates of C18

Background: Small changes in reaction rates due to neutron capture, $\ensuremath{\alpha}$ capture, and $\ensuremath{\beta}$ decay by exotic nuclei in the low and medium mass region can direct the $r$-process path in different ways, thereby affecting the abundance pattern. In this context, it is important to find the most abundant carbon isotope closer to the neutron drip line. It was postulated that the $(n,\ensuremath{\gamma})$ reaction network will be broken at the $^{18}\mathrm{C}$ isotope and follow the $^{18}\mathrm{C}(\ensuremath{\alpha},n)^{21}\mathrm{O}$ reaction path [Terasawa et al., Astrophys. J. 562, 470 (2001)].Purpose: In this paper, we calculate the radiative capture cross section of $^{18}\mathrm{C}(n,\ensuremath{\gamma})^{19}\mathrm{C}$, taking into consideration the halo character of $^{19}\mathrm{C}$. Eventually, we calculate the reaction rate for the same reaction and compare it with Hauser-Feshbach estimates of $(n,\ensuremath{\gamma})$ and $(\ensuremath{\alpha},n)$ to determine the abundance pattern of carbon isotopes.Method: We compute the relative energy spectrum for elastic Coulomb breakup of $^{19}\mathrm{C}$ on a Pb target at a beam energy of 67 MeV/u using the finite-range distorted-wave Born approximation (FRDWBA) theory. We use the principle of detailed balance to calculate the radiative capture cross section from the photo disintegration cross section, and subsequently we calculate the reaction rate.Results: We report the $^{18}\mathrm{C}(n,\ensuremath{\gamma})^{19}\mathrm{C}$ capture cross section as a function of relative energy. Estimation of the energy range contributing to the reaction rates is done by calculating the integrand of the reaction rate expression. A comparison of the $^{18}\mathrm{C}(n,\ensuremath{\gamma})^{19}\mathrm{C}$ with $^{18}\mathrm{C}(\ensuremath{\alpha},n)^{21}\mathrm{O}$ (extracted from the Hauser-Feshbach estimates) shows the domination of the neutron radiative capture reaction in the relevant temperature range ${T}_{9}=0.1--4$.Conclusion: The radiative capture cross section of $^{18}\mathrm{C}$ calculated using FRDWBA theory agrees well with the experimental data, whereas the statistical model calculation lies orders of magnitude lower. We conclude that at equilibrium temperature of ${T}_{9}=0.62$, the $^{18}\mathrm{C}(n,\ensuremath{\gamma})^{19}\mathrm{C}$ reaction rate is orders of magnitude higher than that of the $^{18}\mathrm{C}(\ensuremath{\alpha},n)^{21}\mathrm{O}$ reaction, thereby pushing the carbon isotope abundance towards the neutron drip line.

Radiative-capture cross sections for the La139(n,γ) reaction using thermal neutrons and structural properties of La140

A set of prompt partial $\gamma$-ray production cross sections from thermal neutron-capture were measured for the $^{139}$La($n,\gamma$) reaction using a guided beam of subthermal (thermal and cold) neutrons incident upon a $^{\rm nat}$La$_{2}$O$_{3}$ target at the Prompt Gamma Activation Analysis facility of the Budapest Research Reactor. Absolute $^{140}$La cross sections were determined relative to the well-known comparator $^{35}$Cl($n,\gamma$) cross sections from the irradiation of a stoichiometric $^{\rm nat}$LaCl$_{3}$ sample. The total cross section for radiative thermal neutron-capture on $^{139}$La from the sum of experimentally measured cross sections observed to directly feed the $^{140}$La ground state was determined to be $\sigma_{0}^{\rm expt} = 8.58(50)$ b. To assess completeness of the decay scheme and as a consistency check, the measured cross sections for transitions feeding the ground state from levels below a critical energy of $E_{c} = 285$ keV were combined with a modeled contribution accounting for ground-state feeding from the quasicontinuum to arrive at a total cross section of $\sigma_{0} = 9.36(74)$ b. In addition, a neutron-separation energy of $S_{n} = 5161.005(21)$ keV was determined from a least-squares fit of the measured primary $\gamma$-ray energies to the low-lying levels of the $^{140}$La decay scheme. Furthermore, several nuclear structure improvements are proposed for the decay scheme. The measured cross-section and separation-energy results are comparable to earlier measurements of these quantities.

Simple empirical E1 and M1 strength functions for practical applications

Valuable theoretical predictions of nuclear dipole excitations in the whole chart are of great interest for different nuclear applications, including, in particular, nuclear astrophysics. Here on the basis of experimental and theoretical information on the $E1$ and $M1$ strength functions, inspired both from axially deformed quasiparticle random-phase approximation and shell-model calculations, we derive simple expressions to determine systematically the dipole strength in order to update former prescriptions with a special emphasis on new expressions for the $M1$ spin-flip and scissors modes. We compare our final prediction of the $E1$ and $M1$ strengths with available experimental data at low energies and show that a relatively good agreement is obtained. Its impact on the total radiative width as well as radiative neutron capture cross sections is also discussed. The new expressions are believed to represent an improvement with respect to the analytical systematics proposed in the past and used traditionally in reaction codes, such as the recommended ripl-3 prescriptions.

Photoneutron cross sections for Ni isotopes: Toward understanding (n,γ) cross sections relevant to weak s -process nucleosynthesis

Photoneutron cross sections were measured for $^{58}$Ni, $^{60}$Ni, $^{61}$Ni, and $^{64}$Ni at energies between the one-neutron and two-neutron thresholds using quasi-monochromatic $\gamma$-ray beams produced in laser Compton-scattering at the NewSUBARU synchrotron radiation facility. The new photoneutron data are used to extract the $\gamma$-ray strength function above the neutron threshold complementing the information obtained by the Oslo method below the threshold. We discuss radiative neutron capture cross sections and the Maxwellian-averaged cross sections for Ni isotopes including $^{63}$Ni, a branching point nucleus along the weak s-process path. The cross sections are calculated with the experimentally constrained $\gamma$-ray strength functions from the Hartree-Fock-Bogolyubov plus quasi-particle-random phase approximation based on the Gogny D1M interaction for both $E1$ and $M1$ components and supplemented with the $M1$ upbend.

Gogny-HFB+QRPA dipole strength function and its application to radiative nucleon capture cross section

Valuable theoretical predictions of nuclear dipole excitations in the whole nuclear chart are of great interest for different applications, including in particular nuclear astrophysics. Here we extend our large-scale calculations of the $E1$ and $M1$ absorption $\ensuremath{\gamma}$-ray strength function obtained in the framework of the axially symmetric deformed quasiparticle random-phase approximation (QRPA) based on the finite-range D1M Gogny force to the deexcitation strength function. To do so, shell-model calculations of the deexcitation dipole strength function are performed and their limit at low $\ensuremath{\gamma}$ energies used to complement phenomenologically the QRPA calculations. We compare our final prediction of the $E1$ and $M1$ strength with available experimental data at low energies and show that a fairly good agreement is obtained. Predictions of the dipole strength function for spherical and deformed nuclei within the valley of $\ensuremath{\beta}$ stability as well as in the neutron-rich region are discussed and compared with traditional Lorentzian-type prescriptions. Its impact on the total radiative width as well as radiative neutron and proton capture cross sections is studied.

Impact of the 7Be(α, γ)11C Reaction on the Primordial Abundance of 7Li

Abstract We calculate the radiative capture cross section for 7Be(α, γ)11C and its reaction rate of relevance for the Big Bang nucleosynthesis (BBN). The impact of this reaction on the primordial 7Li abundance is revised including narrow and broad resonances in the pertinent energy region. Our calculations show that it is unlikely that very low energy resonances in 11C of relevance for the BBN would emerge within a two-body potential model. Based on our results and a comparison with previous theoretical and experimental analyses, we conclude that the impact of this reaction on the so-called “cosmological lithium puzzle” is completely irrelevant.

Neutron radiative capture cross section of Cu63,65 between 0.4 and 7.5 MeV

Natural copper is commonly used as cooling and shielding medium in detector arrangements designed to search for neutrinoless double-$\ensuremath{\beta}$ decay. Neutron-induced background reactions on copper could potentially produce signals that are indistinguishable from the signals of interest. The present work focuses on radiative neutron capture experiments on $^{63,65}\mathrm{Cu}$ in the 0.4 to 7.5 MeV neutron energy range. The new data provide evaluations and model calculations with benchmark data needed to extend their applicability in predicting background rates in neutrinoless double-$\ensuremath{\beta}$ decay experiments.

The Gogny-HFB+QRPA dipole strength function and its application to radiative neutron capture cross section

Valuable theoretical predictions of nuclear dipole excitations in the whole chart are of great interest for different nuclear applications, including in particular nuclear astrophysics. Here we extend our large-scale calculations of the E1 and M1 absorption γ-ray strength function obtained in the framework of the axially-symmetric deformed quasiparticle random phase approximation (QRPA) based on the finite-range D1M Gogny force to the determination of the de-excitation strength function. To do so, shell-model calculations of the de-excitation dipole strength function as well as experimental data are considered to provide insight in the low-energy limit and to complement the QRPA estimate phenomenologically. We compare our final prediction of the E1 and M1 strengths with available experimental data at low energies and show that a relatively good agreement can be obtained. Its impact on the average radiative width as well as radiative neutron capture cross section is discussed.

ANC experiments for nuclear astrophysics in NPI CAS

Asymptotic Normalization Coefficients method is one of the indirect methods used in nuclear astrophysics. The method allows to deduce a direct part of a radiative capture cross section from the measurement of a two body reaction under specific conditions. The experimental works started in collaboration of NPI CAS, TAMU and INFN LNS from 90-ties and continue till the present. The introduction to this method is presented and experimental experiences from NPI CAS and the collaboration are shared.

Measurement of the U238(n,γ) cross section up to 80 keV with the Total Absorption Calorimeter at the CERN n_TOF facility

The radiative capture cross section of a highly pure (99.999%), 6.125(2) grams and 9.56(5) 10-4 atoms/barn areal density 238-U sample has been measured with the Total Absorption Calorimeter (TAC) in the 185 m flight path at the CERN neutron time-of-flight facility n_TOF. This measurement is in response to the NEA High Priority Request list, which demands an accuracy in this cross section of less than 3% below 25 keV. These data have undergone careful background subtraction, with special care being given to the background originating from neutrons scattered by the 238-U sample. Pileup and dead-time effects have been corrected for. The measured cross section covers an energy range between 0.2 eV and 80 keV, with an accuracy that varies with neutron energy, being better than 4% below 25 keV and reaching at most 6% at higher energies.

Neutron cross section sensitivity and uncertainty analysis of candidate accident tolerant fuel concepts

The aftermath of the Tōhoku earthquake and the Fukushima accident has led to a global push to improve the safety of existing light water reactors. A key component of this initiative is the development of nuclear fuel and cladding materials with potentially enhanced accident tolerance, also known as accident-tolerant fuels (ATF). These materials are intended to improve core fuel and cladding integrity under beyond design basis accident conditions while maintaining or enhancing reactor performance and safety characteristics during normal operation. To complement research that has already been carried out to characterize ATF neutronics, the present study provides an initial investigation of the sensitivity and uncertainty of ATF systems responses to nuclear cross section data. ATF concepts incorporate novel materials, including SiC and FeCrAl cladding and high density uranium silicide composite fuels, in turn introducing new cross section sensitivities and uncertainties which may behave differently from traditional fuel and cladding materials.In this paper, we conducted sensitivity and uncertainty analysis using the TSUNAMI-2D sequence of SCALE with infinite lattice models of ATF assemblies. Of all the ATF materials considered, it is found that radiative capture in 56Fe in FeCrAl cladding is the most significant contributor to eigenvalue uncertainty. 56Fe yields significant potential eigenvalue uncertainty associated with its radiative capture cross section; this is by far the largest ATF-specific uncertainty found in these cases, exceeding even those of uranium. We found that while significant new sensitivities indeed arise, the general sensitivity behavior of ATF assemblies does not markedly differ from traditional UO2/zirconium-based fuel/cladding systems, especially with regard to uncertainties associated with uranium.We assessed the similarity of the IPEN/MB-01 reactor benchmark model to application models with FeCrAl cladding. We used TSUNAMI-IP to calculate similarity indices of the application model and IPEN/MB-01 reactor benchmark model. This benchmark was selected for its use of SS304 as a cladding and structural material, with significant 56Fe content. The similarity indices suggest that while many differences in reactor physics arise from differences in design, sensitivity to and behavior of 56Fe absorption is comparable between systems, thus indicating the potential for this benchmark to reduce uncertainties in 56Fe radiative capture cross sections.

Neutron Capture on 11B and 12B at Astrophysical Energies

The possibility of describing the experimental data for the total cross sections of radiative n11B capture to the ground state of the 12B nucleus at thermal and astrophysical energies is considered within the framework of the modified potential cluster model with classification of orbital states according to Young tableaux. It is shown that it is possible to explain the magnitude of the experimental cross sections at energies from 25.3 meV to 100 keV on the basis of just the E1 transition from the S state of n11B scattering to the ground state of the 12B nucleus in the n11B channel. New theoretical results have been obtained for the total cross sections of n12B capture to the ground state of the 13B nucleus. The rates of both reactions are calculated in the temperature range from 0.01 to 5.0 Т9.

Reconciling Coulomb breakup and neutron radiative capture

The Coulomb-breakup method to extract the cross section for neutron radiative capture at astrophysical energies is analyzed in detail. In particular, its sensitivity to the description of the neutron-core continuum is ascertained. We consider the case of 14C(n, $\gamma$)15C for which both the radiative capture at low energy and the Coulomb breakup of 15C into 14C+n on Pb at 68 MeV/nucleon have been measured with accuracy. We confirm the direct proportionality of the cross section for both reactions to the square of the asymptotic normalization constant of 15C observed by Summers and Nunes [Phys. Rev. C 78, 011601 (2008)], but we also show that the 14C-n continuum plays a significant role in the calculations. Fortunately, the method proposed by Summers and Nunes can be improved to absorb that continuum dependence. We show that a more precise radiative-capture cross section can be extracted selecting the breakup data at forward angles and low 14C-n relative energies.

Cross section and astrophysical S-factor for [formula omitted] reaction with Halo Effective Field Theory at low-energies

We considered one of the proton halo nuclei candidates, 13N* nucleus, and calculated the cross section and astrophysical S-factor for 12C(p, γ)13N* reaction using halo effective field theory without pion (hEFT¬π). The halo effective field theory is used to examine the halo nucleus bound state with a large S-wave scattering length. We calculated the radiative proton capture cross section and the astrophysical S-factor from the fields of the core and the valence proton at the Leading-Order (LO). We showed that there is a good agreement among the our results for cross section and astrophysical S-factor of the 12C(p, γ)13N* reaction and the experimental data. The astrophysical S-factor that has been estimated at the zero energy (Ecm=0) by using a theoretical calculation of the cross section for direct radiative capture and an extrapolation of this calculation obtained S(0)=1.883×10−3 MeV-b.