Radiative Neutron Capture Cross Sections Research Articles

Noniterative finite amplitude methods for giant resonances and the application to the neutron radiative capture cross sections

We calculate the electric dipole (E1) and the magnetic dipole (M1) giant resonances with noniterative finite amplitude methods and demonstrate how the fully microscopic density functional theory predicts the giant resonances without any phenomenological parameters. Then, we calculate neutron capture reactions based on the statistical Hauser-Feshbach theory with the result of E1 and M1 transitions and find that the capture cross sections for deformed nuclei are enhanced due to the contribution from the low energy M1 scissors mode.

Thermal neutron capture cross-section and resonance integral measurements of 186W(n,γ)187W reaction using the thermal column neutron source at the Dalat research reactor

The thermal neutron radiative capture cross-section (σ0) and the resonance integral (I0) of the reaction 186W(n,γ)187W were measured by the thin foil activation method using the well thermalized neutron source at the graphite thermal column facility of the Dalat research reactor. The activities of the irradiated target and monitor samples were measured using a high resolution HPGe gamma-ray spectrometer. The recent updated values of gamma-rays intensities and decay half-life of the 187W nucleus were applied in this experiment to substantially improve the cross-section results compared to the previous measurements. The thermal neutron capture cross- section and the resonance integral of the 186W(n,γ)187W reaction were determined relative to the standard reaction 197Au(n,γ)198Au. The values obtained are 31.76 ± 0.63 and 429.2 ± 25.6 barns for σ0 and I0 respectively. Our results and previous measured values are compared, and discrepancies are discussed.

Radiative neutron capture reaction rates for nucleosynthesis: The creation of the first r-process peak

About half of the elements beyond iron are synthesized in stars by rapid-neutron capture process (r-process). The stellar environment provides very high neutron flux in a short time ($\sim$ seconds) which is conducive for the creation of progressively neutron-rich nuclei till the waiting point is reached after which no further neutron capture reactions proceed. At this point such extremely neutron-rich nuclei become stable via $\beta^-$ decay. A detailed understanding of the r-process remains illusive. In the present work, we explore the radiative neutron-capture (n,$\gamma$) cross sections and reaction rates around the r-process peak near mass number eighty. The inherent uncertainties remain large in some cases, particularly in case of neutron-rich nuclei. When the low-energy enhancement exists, it results in significant increase in the reaction rate for neutron-capture.

Experimental evidence of large collective enhancement of nuclear level density and its significance in radiative neutron capture

The collective enhancement of nuclear level density and its fade out with excitation in the deformed 171Yb nucleus has been inferred through an exclusive measurement of neutron spectra. The statistical model analysis of neutron spectra demonstrated a large collective enhancement factor of 40±3, a value consistent with recent microscopic model predictions but is an anomalous result compared with the measurements in the nearby deformed nuclei. The complete picture of the energy dependent collective enhancement has been obtained by combining the present results with those obtained with the Oslo method below neutron binding energy. The signature of a large collective enhancement in radiative neutron capture cross section of astrophysical interest is highlighted.

Compton imaging for enhanced sensitivity (n,γ) cross section TOF experiments: Status and prospects

Radiative neutron-capture cross sections are of pivotal importance in many fields such as nucle-osynthesis studies or innovative reactor technologies. A large number of isotopes have been measured with high accuracy, but there are still a large number of relevant isotopes whose cross sections could not be experimentally determined yet, at least with sufficient accuracy and completeness, owing to limitations in detection techniques, sample production methods or in the facilities themselves. In the context of the HYMNS (High-sensitivitY Measurements of key stellar Nucleo-Synthesis reactions) project over the last six years we have developed a novel detection technique aimed at background suppression in radiative neutron-capture time-of-flight measurements. This new technique utilizes a complex detection set-up based on position-sensitive radiation-detectors deployed in a Compton-camera array configuration. The latter enables to implement gamma-ray imaging techniques, which help to disentangle true capture events arising from the sample under study and contaminant background events from the surroundings. A summary on the main developments is given in this contribution together with an update on recent experiments at CERN n_TOF and an outlook on future steps.

First 80Se(n,γ) cross section measurement with high resolution in the full stellar energy range 1 eV - 100 keV and its astrophysical implications for the s-process

Most elements heavier than iron have been generated in the stellar media by means of neutron capture reactions, approximately half are produced by the slow neutron capture or s-process. Radiative neutron capture cross section measurements are of fundamental importance for the study of this mechanism. In this contribution we present a brief summary on the measurement and results for the 80Se(n,γ) cross-section. The experiment was carried out at CERN n_TOF EAR1 via the time of flight (ToF) technique, using four C6D6 scintillation detectors with very fast response. More than a hundred new resonances have been analyzed for the first time with a high accuracy. The MACS obtained at kT = 8 keV is 36% smaller than the recommended value in KADo-NiS. Some of the astrophysical implications of this result are elucidated in this contribution.

The intermediate neutron capture process

Context. Carbon-enhanced metal-poor (CEMP) r/s-stars show surface-abundance distributions characteristic of the so-called intermediate neutron capture process (i-process) of nucleosynthesis. We previously showed that the ingestion of protons in the convective helium-burning region of a low-mass low-metallicity star can explain the surface abundance distribution observed in CEMP r/s stars relatively well. Such an i-process requires detailed reaction network calculations involving hundreds of nuclei for which reaction rates have not yet been determined experimentally. Aims. We investigate the nuclear physics uncertainties affecting the i-process during the asymptotic giant branch (AGB) phase of low-metallicity low-mass stars by propagating the theoretical uncertainties in the radiative neutron capture cross sections, as well as the 13C(α,n)16O reaction rate, and estimating their impact on the surface-abundance distribution. Methods. We used the STAREVOL code to follow the evolution of a 1 M⊙ [Fe/H] = − 2.5 model star during the proton ingestion event occurring at the beginning of the AGB phase. In the computation, we adopt a nuclear network of 1160 species coupled to the transport processes and different sets of radiative neutron capture cross sections consistently calculated with the TALYS reaction code. Results. It is found that considering systematic uncertainties on the various nuclear ingredients affecting the radiative neutron capture rates, surface elemental abundances are typically predicted within ±0.4 dex. The 56 ≲ Z ≲ 59 region of the spectroscopically relevant heavy-s elements of Ba-La-Ce-Pr as well as the r-dominated Eu element remain relatively unaffected by nuclear uncertainties. In contrast, the inclusion of the direct capture contribution impacts the rates in the neutron-rich A ≃ 45, 100, 160, and 200 regions, and the i-process production of the Z ≃ 45 and 65–70 elements. Uncertainties in the photon strength function also impact the overabundance factors by typically 0.2–0.4 dex. Nuclear level densities tend to affect abundance predictions mainly in the Z = 74 − 79 regions. The uncertainties associated with the neutron-producing reaction 13C(α,n)16O and the unknown β-decay rates are found to have a low impact on the overall surface enrichment. Conclusions. The i-process nucleosynthesis during the early AGB phase of low-metallicity low-mass stars remains sensitive to nuclear uncertainties, substantially affecting theoretical predictions of still unknown radiative neutron capture cross sections. Improved descriptions of direct neutron capture based on shell model calculations or experimental constraints from (d, p) reactions could help to decrease the uncertainties in the estimated rates. Similarly, constraints on the photon strength functions and nuclear level densities, for example through the Oslo method, in the neutron-rich region of A ≃ 100 and 160 would increase the predictive power of the present simulations.

Periodic System of Isotopes

With the help of a special algorithm being the principle of multilevel periodicity, the periodic change of properties at the nuclear level of chemical elements was discovered and the variant for the periodic system of isotopes was presented. The periodic change in the properties of isotopes, as well as the vertical symmetry of subgroups, was checked for consistency in accordance with the following ten types of the experimental data: mass ratio of fission fragments; quadrupole moment values; magnetic moment; lifetime of radioactive isotopes; neutron scattering; thermal neutron radiative capture cross-sections (n, γ); α-particle yield cross-sections (n, α); isotope abundance on Earth, in the Solar system and other stellar systems; features of ore formation and stellar evolution. For all the ten cases, the correspondences for the proposed periodic structure of the nucleus were obtained. The system was formed in the usual 2D table, similar to the periodic system of elements, and the mass series of isotopes was divided into 8 periods and 4 types of “nuclear” orbitals: sn, dn, pn, fn. The origin of "magic" numbers as a set of filled charge shells of the nucleus was explained. Due to the isotope system, the periodic structure is shown at a new level of the universe and the prospects of its practical use are opened up

Neutron radiative capture cross section for sodium with covariance analysis

The neutron radiative capture cross sections measurement has been carried out for the $$^{23}$$ Na nucleus in the neutron energy region from 0.6 to 3.2 MeV using the neutron activation technique followed by off-line $$\gamma $$ -ray spectrometry. The measurement was made relative to the $$^{115}$$ In(n,n $$\prime $$ $$\gamma $$ ) $$^{115}\hbox {In}^{m}$$ reference monitor reaction cross section. The neutrons were produced via the $$^{7}$$ Li(p,n) $$^{7}$$ Be reaction. Detailed uncertainty propagation has been performed using the covariance analysis, and the measured cross sections are being reported with their uncertainties, covariance, and correlation matrix. The necessary corrections have been made for the low background neutron energy contribution, $$\gamma $$ -ray true coincidence summing, and self-attenuation process. The obtained neutron spectrum averaged cross sections of $$^{23}$$ Na(n, $$\gamma $$ ) $$^{24}$$ Na are discussed and compared with the existing cross sections data retrieved from the EXFOR database. EMPIRE-3.2 and TALYS-1.9 calculations were performed in order to determine the radiative capture cross section in this energy region. The present results are also compared with the evaluated nuclear data from ENDF/B-VIII.0, TENDL-2019, IRDFF-1.05, JENDL-4.0, and JEFF-3.3. The obtained cross section results are in good agreement with existing experimental data, evaluated libraries, and reaction models for the highest energy points (2.11 and 3.13 MeV), while the lowest-energy point at 0.61 MeV underestimates them.

Cross sections and isomer ratios in the Rb(n,$$\gamma $$) and Sr(n,$$\gamma $$) reactions

The neutron radiative capture cross sections have been measured for the stable isotopes of Rb (mass numbers 85, 87) and Sr (mass numbers 84, 86, 88) using the activation technique, by observing the $$\gamma $$ rays emitted in the decays of the radionuclides produced in neutron irradiation. Values are reported for both the thermal cross sections and the resonance integrals. Internal consistency of the deduced values has been obtained where possible by measuring several radioisotopes from the same sample as well as by irradiating at several different reactor sites that have different mixtures of thermal and epithermal neutrons. Experimental values of the thermal cross sections are compared with values calculated from the known resonances. Isomer ratios are deduced and compared with values calculated from a phenomenological model. Also reported are spectroscopic studies of Rb and Sr radionuclides leading to improved values for the energy of $$\gamma $$ rays emitted in the decays of $$^{{86}}\hbox {Rb}$$ , $$^{{{85g}}} \hbox {Sr}$$ , and $$^{\mathrm {89}}\hbox {Sr}$$ . The half-life of $$^{{{87m}}}\hbox {Sr}$$ has been remeasured in order to resolve the conflict between two precise but discrepant values reported in the literature.

80Se(n,γ) cross-section measurement at CERN n TOF

Radiative neutron capture cross section measurements are of fundamental importance for the study of the slow neutron capture (s-) process of nucleosynthesis. This mechanism is responsible for the formation of most elements heavier than iron in the Universe. Particularly relevant are branching nuclei along the s-process path, which are sensitive to the physical conditions of the stellar environment. One such example is the branching at 79Se (3.27 × 105 y), which shows a thermally dependent β-decay rate. However, an astrophysically consistent interpretation requires also the knowledge of the closest neighbour isotopes involved. In particular, the 80Se(n,γ) cross section directly affects the stellar yield of the “cold” branch leading to the formation of the s-only 82Kr. Experimentally, there exists only one previous measurement on 80Se using the time of flight (TOF) technique. However, the latter suffers from some limitations that are described in this presentation. These drawbacks have been significantly improved in a recent measurement at CERN n TOF. This contribution presents a summary of the latter measurement and the status of the data analysis.

Introduction of a C6D6detector system on the Back-n of CSNS

Radiative neutron capture cross sections are very important in the field of basic physics research and nuclear device R&D. The Back-n white neutron beam line of China Spallation Neutron Source (CSNS) is the first spallation neutron beam line in China. On the purpose for radiative neutron capture cross section measurement, a C6D6detector system was built in the Back-n experimental station. The pulse height weighting technique (PHWT) was used to make the system’s detective efficiency independent of the cascade path and the energy of cascade gamma rays. The neutron energy spectrum was measured for the energy between 1eV and 80keV with a6Li loaded ZnS scintillation detector. Besides, a testing experiment with197Au and169Tm samples was carried out to examine this system. According to the preliminary results, this C6D6detector system can be used to perform neutron capture cross section measurement.

γ -ray strength function for barium isotopes

Photoneutron cross sections were measured for $^{137}\mathrm{Ba}$ and $^{138}\mathrm{Ba}$ at energies below two-neutron threshold using quasimonochromatic $\ensuremath{\gamma}$-ray beams produced in laser Compton scattering at the NewSUBARU synchrotron radiation facility. The photoneutron data are used to constrain the $\ensuremath{\gamma}$-ray strength function on the basis of the Hartree-Fock-Bogolyubov plus quasiparticle random phase approximation using the Gogny $D1M$ interaction. Supplementing the experimentally constrained $\ensuremath{\gamma}$-ray strength function with the zero-limit $E1$ and $M1$ contributions, which are unique to the deexcitation mode, we discuss radiative neutron capture cross sections relevant to the $s$-process nucleosynthesis of barium isotopes in the vicinity of the neutron magic number 82.

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.

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.

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.

Verification of detailed balance for γ absorption and emission in Dy isotopes

The photo-neutron cross sections of $^{162,163}\rm{Dy}$ have been measured for the first time in an energy region from the neutron threshold ($S_n$) up to $\approx$ $13$~MeV. The ($\gamma$,n) reaction was induced with quasi-monochromatic laser Compton-scattered $\gamma$ rays, produced at the NewSUBARU laboratory. The corresponding $\gamma$-ray strength functions ($\gamma$SF) have been calculated from the photo-neutron cross sections. The data are compared to reanalyzed $\gamma$SFs of $^{160-164}\rm{Dy}$, which are measured below $S_n$. The excellent agreement with the photo-neutron data at $S_n$ confirms the principle of detailed balance. Thus, a complete $\gamma$SF is established covering in total the energy region of 1 MeV $\leq$ E$_{\gamma}$ $\leq$ 13 MeV. These mid-shell well-deformed dysprosium isotopes all show scissors resonances with very similar structures. We find that our data predict the same integrated scissors strength as ($\gamma,\gamma^\prime$) data when integrated over the same energy range, which shows that the scissors mode very likely is consistent with the generalized Brink hypothesis. Finally, using the $\gamma$SFs as input in the reaction code TALYS, we have deduced radiative neutron-capture cross sections and compared them to direct measurements. We find a very good agreement within the uncertainties, which gives further support to the experimentally determined $\gamma$SFs.

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.

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.