Phenomenological Description of Neutron Capture Cross Sections at 30 keV (original) (raw)
Related papers
2010
Cross sections for the capture of low-energy neutrons on unstable neutron-rich nuclei are important for nuclear science and their reliable knowledge is increasingly requested by nuclear astrophysics to test quantitatively the nucleosynthesis in the r-process, by nuclear engineering for the design of new reactors using novel fuel cycles, and by national security. It is difficult, and in many cases impossible, to make direct measurements for all the reactions for which good cross sections are needed.
Neutron capture cross sections: from theory to experiments and back
arXiv preprint nucl-th/0501082, 2005
The method for an experimental determination of the stellar enhancement factor for the cross section of the 151 Sm(n, γ) reaction process is proposed. This study offered the pretext for an excursus on the interconnections between capture and dissociation reactions and the interplay between theory and experiments in the determination of neutron capture cross sections.
Neutron capture cross sections of light neutron-rich nuclei relevant for r -process nucleosynthesis
Physical Review C, 2021
The measurements of neutron capture cross sections of neutron-rich nuclei are challenging but essential for understanding nucleosynthesis and stellar evolution processes in the explosive burning scenario. In the quest of r-process abundances, according to the neutrino-driven-wind model, light neutron-rich unstable nuclei may play a significant role as seed nuclei that influence the abundance pattern. Hence, experimental data for neutron capture cross sections of neutron-rich nuclei are needed. Coulomb dissociation of radioactive ion beams at intermediate energy is a powerful indirect method for inferring capture cross section. As a test case for validation of the indirect method, the neutron capture cross section (n, γ) for 14 C was inferred from the Coulomb dissociation of 15 C at intermediate energy (600A MeV). A comparison between different theoretical approaches and experimental results for the reaction is discussed. We report for the first time experimental reaction cross sections of 28 Na(n, γ) 29 Na, 29 Na(n, γ) 30 Na, 32 Mg(n, γ) 33 Mg, and 34 Al(n, γ) 35 Al. The reaction cross sections were inferred indirectly through Coulomb dissociation of 29,30 Na, 33 Mg, and 35 Al at incident projectile energies around 400-430 A MeV using the FRS-LAND setup at GSI, Darmstadt. The neutron capture cross sections were obtained from the photoabsorption cross sections with the aid of the detailed balance theorem. The reaction rates for the neutron-rich Na, Mg, Al nuclei at typical r-process temperatures were obtained from the measured (n, γ) capture cross sections. The measured neutron capture reaction rates of the neutron-rich nuclei, 28 Na, 29 Na, and 34 Al are significantly lower than those predicted by the Hauser-Feshbach decay model. A similar trend was observed earlier for 17 C and 19 N but in the case of 14 C(n, γ) 15 C the trend is opposite. The situation is more complicated when the ground state has a multi-particle-hole configuration. For 32 Mg, the measured cross section is about 40-90 % higher than the Hauser-Feshbach prediction.
Shell-model based study of the direct capture in neutron-rich nuclei
The European Physical Journal A, 2021
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.
Exotic Properties of Light Nuclei and their Neutron Capture Cross Sections
We have investigated the implications of the neutron halo configuration, observed in the ground-state of some neutron-rich light nuclei, on neutron radiative transition processes. In particular, we have studied the influence of the neutron halo on the direct radiative capture (DRC) process. The energy dependence as well as the strength of E1 emission due to incident p-wave neutrons is strongly influenced by the halo configuration of the residual nucleus capturing state. We have compared the calculated 10 Be(n, γ) 11 Be DRC cross section with that derived from the experiment in the inverse kinematics (Coulomb dissociation of 11 Be). We show from the comparison that some important information on the structure of the halo nucleus 11 Be can be derived.
Neutron-induced cross sections
The European Physical Journal Plus, 2018
Neutron capture cross sections are one of the most important nuclear inputs to models of stellar nucleosynthesis of the elements heavier than iron. The activation technique and the time-of-flight method are mostly used to determine the required data experimentally. Recent developments of experimental techniques allow for new experiments on radioactive isotopes. Monte-Carlo based analysis methods give new insights into the systematic uncertainties of previous measurements. We present an overview over the state-of-the-art experimental techniques, a detailed new evaluation of the 197 Au(n,γ) cross section in the keV-regime and the corresponding re-evaluation of 63 more isotopes, which have been measured in the past relative to the gold cross section.
Neutron Capture Nucleosynthesis
Heavy elements (beyond iron) are formed in neutron capture nucleosynthesis processes. A simple unified model is proposed to investigate the neutron capture nucleosynthesis in arbitrary neutron density environment. Neutron density required to reproduce the measured abundance of nuclei assuming equilibrium processes is investigated as well. Medium neutron density was found to play a particularly important role in neutron capture nucleosynthesis. Using these findings most of the nuclei can be formed in a medium neutron capture density environment e.g. in certain AGB stars. Besides these observations the proposed model suits educational purposes as well. INTRODUCTION Nearly sixty years after BBFH [1], it is possible and necessary to review and rethink our knowledge about the neutron capture nucleosynthesis. The result of the formation of the nuclei is shown in the various abundances. It is important to mention that the unstable nuclei decayed into stable nuclei and we are only able to observe the abundance of the remaining stable nuclei. "The success of any theory of nucleosynthesis has to be measured by comparison with the abundance patterns observed in nature." – say Käppeler, Beer and Wisshak [2], that is, we need to create such model that gives back the observed abundances.
Neutron-capture cross sections from indirect measurements
2012
Cross sections for compound-nuclear reactions reactions play an important role in models of astrophysical environments and simulations of the nuclear fuel cycle. Providing reliable cross section data remains a formidable task, and direct measurements have to be complemented by theoretical predictions and indirect methods. The surrogate nuclear reactions method provides an indirect approach for determining cross sections for reactions on unstable isotopes, which are difficult or impossible to measure otherwise. Current implementations of the method provide useful cross sections for (n,f) reactions, but need to be improved upon for applications to capture reactions.
Measurement of keV-neutron capture cross-sections for 164Dy
Annals of Nuclear Energy, 2001
The capture cross sections and capture-ray spectra of 209 Bi were measured in a neutron energy region from 5 to 80 keV and at 520 keV, using pulsed keV neutrons from the 7 Li(p; n) 7 Be reaction and a time-of-flight method. The capture rays from a bismuth or standard gold sample were detected with a large anti-Compton NaI(Tl) spectrometer. The capture yield of the bismuth or gold sample was obtained by applying a pulse-height weighting technique to the corresponding capture-ray pulse-height spectrum. The derived capture cross sections from 5 to 80 keV were in good agreement with recent measurements, but that at 520 keV was about half of previous measurements. This large discrepancy at 520 keV was ascribed to the incorrect background-subtraction in the previous measurements from a comparison between the present and previous capture-ray spectra. Strong transitions from the capture states to low lying states of 210 Bi were observed in the present-ray spectra. The multiplicities of observed rays were obtained from the-ray spectra.