Radiative Neutron Capture Research Articles

Photoneutron cross sections for samarium isotopes: Toward a unified understanding of(γ,n)and(n,γ)reactions in the rare earth region

Photoneutron cross sections were measured for the seven stable samarium isotopes ${}^{144,147,148,149,150,152,154}\mathrm{Sm}$ near the neutron threshold with quasi-monochromatic laser-Compton scattering $\ensuremath{\gamma}\phantom{\rule{0.28em}{0ex}}\mathrm{rays}$. Our photoneutron cross sections are found to be low by 20%--37% relative to existing data. The photoneutron data are analyzed with the talys reaction code by considering the Skyrme Hartree-Fock-Bogoliubov (HFB) plus quasiparticle random phase approximation (QRPA) model and the axially symmetric deformed Gogny HFB plus QRPA model of the $E1$ $\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{ray}$ strength. Using the $\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{ray}$ strength function constrained by the present photoneutron data, we made a thorough analysis of the reverse $(n,\ensuremath{\gamma})$ cross sections including the radioactive nucleus $^{151}\mathrm{Sm}$ with a half-life of 90 yr. The radiative neutron capture cross section for $^{153}\mathrm{Sm}$ with the half-life of 1.928 d is deduced with the $\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{ray}$ strength function method.

Photoneutron reactions in astrophysics

Among key problems in nuclear astrophysics, that of obtaining deeper insight into the mechanism of synthesis of chemical elements is of paramount importance. The majority of heavy elements existing in nature are produced in stars via radiative neutron capture in so-called s- and r processes, which are, respectively, slow and fast, in relation to competing β−-decay processes. At the same time, we know 35 neutron-deficient so-called bypassed p-nuclei that lie between 74Se and 196Hg and which cannot originate from the aforementioned s- and r-processes. Their production is possible in (γ, n), (γ, p), or (γ, α) photonuclear reactions. In view of this, data on photoneutron reactions play an important role in predicting and describing processes leading to the production of p-nuclei. Interest in determining cross sections for photoneutron reactions in the threshold energy region, which is of particular importance for astrophysics, has grown substantially in recent years. The use of modern sources of quasimonoenergetic photons obtained in processes of inverse Compton laser-radiation scattering on relativistic electronsmakes it possible to reveal rather interesting special features of respective cross sections, manifestations of pygmy E1 and M1 resonances, or the production of nuclei in isomeric states, on one hand, and to revisit the problem of systematic discrepancies between data on reaction cross sections from experiments of different types, on the other hand. Data obtained on the basis of our new experimental-theoretical approach to evaluating cross sections for partial photoneutron reactions are invoked in considering these problems.

Neutron capture cross sections of 194Hg and the decays of 195Hg

The thermal cross section and resonance integral have been determined for radiative neutron capture by radioactive 194Hg by observing the γ rays emitted in the decay of 195Hg. Captures leading to the low-spin isomer of 195Hg give σ=877b and I=10,270b, while captures leading to the high-spin isomer give σ=49b and I=608b. Energies and intensities of the γ rays emitted in the decays of 195Hg produced by neutron capture have been obtained with increased precision relative to previous studies with sources produced by other reactions, leading to correspondingly improved values for β-decay branching intensities and energy levels in 195Au. New values for the half-lives of 195gHg and 195mHg are also reported.

Improving Nuclear Safety of Fast Reactors by Slowing Down Fission Chain Reaction

Light materials with small atomic mass (light or heavy water, graphite, and so on) are usually used as a neutron reflector and moderator. The present paper proposes using a new, heavy element as neutron moderator and reflector, namely, “radiogenic lead” with dominant content of isotope 208Pb. Radiogenic lead is a stable natural lead. This isotope is characterized by extremely low micro cross-section of radiative neutron capture (~0.23 mb) for thermal neutrons, which is smaller than graphite and deuterium cross-sections. The reflector-converter for a fast reactor core is the structure capable of transforming some part of prompt neutrons leaked from the core into the reflected neutrons with properties similar to those of delayed neutrons, that is, sufficiently large contribution to reactivity at the level of effective fraction of delayed neutrons and relatively long lifetime, comparable with lifetimes of radionuclides-emitters of delayed neutrons. It is evaluated that the use of radiogenic lead makes it possible to slow down the chain fission reaction on prompt neutrons in the fast reactor. This can improve the fast reactor safety and reduce some requirements to the technologies used to fabricate fuel for the fast reactor.

Monte Carlo simulation of explosive detection system based on a Deuterium–Deuterium (D–D) neutron generator

An explosive detection system based on a Deuterium–Deuterium (D–D) neutron generator has been simulated using the Monte Carlo N-Particle Transport Code (MCNP5). Nuclear-based explosive detection methods can detect explosives by identifying their elemental components, especially nitrogen. Thermal neutron capture reactions have been used for detecting prompt gamma emission (10.82MeV) following radiative neutron capture by 14N nuclei. The explosive detection system was built based on a fully high-voltage-shielded, axial D–D neutron generator with a radio frequency (RF) driven ion source and nominal yield of about 1010 fast neutrons per second (E=2.5MeV). Polyethylene and paraffin were used as moderators with borated polyethylene and lead as neutron and gamma ray shielding, respectively. The shape and the thickness of the moderators and shields are optimized to produce the highest thermal neutron flux at the position of the explosive and the minimum total dose at the outer surfaces of the explosive detection system walls. In addition, simulation of the response functions of NaI, BGO, and LaBr3-based γ-ray detectors to different explosives is described.

Study of the neutron and proton capture reactions 10,11B(n, γ), 11B(p, γ), 14C(p, γ), and 15N(p, γ) at thermal and astrophysical energies

We have studied the neutron-capture reactions 10,11 B (n, γ) and the role of the 11 B (n, γ) reaction in seeding r-process nucleosynthesis. The possibility of the description of the available experimental data for cross-sections of the neutron capture reaction on 10 B at thermal and astrophysical energies, taking into account the resonance at 475 keV, was considered within the framework of the modified potential cluster model (MPCM) with forbidden states (FS) and accounting for the resonance behavior of the scattering phase shifts. In the framework of the same model, the possibility of describing the available experimental data for the total cross-sections of the neutron radiative capture on 11 B at thermal and astrophysical energies were considered with taking into account the 21 and 430 keV resonances. Description of the available experimental data on the total cross-sections and astrophysical S-factor of the radiative proton capture on 11 B to the GS of 12 C was treated at astrophysical energies. The possibility of description of the experimental data for the astrophysical S-factor of the radiative proton capture on 14 C to the GS of 15 N at astrophysical energies, and the radiative proton capture on 15 N at the energies from 50 to 1500 keV was considered in the framework of the MPCM with the classification of the orbital states according to Young tableaux. It was shown that, on the basis of the M1 and the E1 transitions from different states of the p15 N scattering to the GS of 16 O in the p15 N channel, it is quite succeed to explain general behavior of the S-factor in the considered energy range in the presence of two resonances.

Neutron capture by 94,96Zr and the decays of 97Zr and 97Nb

Cross sections for radiative neutron capture have been determined for 94Zr and 96Zr using the activation technique with samples of naturally occurring Zr metal. The sensitivity to the correction for epithermal neutrons in the determination of small thermal cross sections is discussed, particularly in view of the variation in the resonance integral at different sites in the reactor. Gamma-ray spectroscopic studies of the decays of 97Zr and its daughter 97Nb have been performed, leading to improved values of the energies and intensities of the emitted γ rays, and correspondingly improved values for the energy levels and β feedings of excited states populated in 97Nb and 97Mo.

New insight into thend→3Hγprocess at thermal energy with pionless effective field theory

We take a new look at the neutron radiative capture by a deuteron at thermal energy with the pionless effective field theory [EFT($\ensuremath{\pi}\phantom{\rule{-0.5em}{0ex}}/$)] approach. We present in detail the calculation of $nd\ensuremath{\rightarrow}$ ${}^{3}\mathrm{H}\ensuremath{\gamma}$ amplitudes for incoming doublet and quartet channels leading to the formation of a triton fully in the projection method based on the cluster-configuration space approach. In the present work, we consider all possible one-body and two-body photon interaction diagrams. In fact, additional diagrams that make significant changes in the results of the calculation of the total cross section in the $nd\ensuremath{\rightarrow}$ ${}^{3}\mathrm{H}\ensuremath{\gamma}$ process are included in this study. The properly normalized triton wave function is calculated and taken into consideration. We compare the cross section of the dominant magnetic $M1$ transition of $nd\ensuremath{\rightarrow}$${}^{3}\mathrm{H}\ensuremath{\gamma}$ up to $\text{next-to-next-to-leading}\phantom{\rule{4.pt}{0ex}}\text{order}\phantom{\rule{4.pt}{0ex}}{\text{(N}}^{2}\text{LO)}$ with the results of the previous model-dependent theoretical calculations and experimental data. The more acceptable results for cross section ${\ensuremath{\sigma}}_{\mathrm{tot}}^{(2)}=0.297\phantom{\rule{0.28em}{0ex}}(\text{LO})+0.124\phantom{\rule{0.28em}{0ex}}(\text{NLO})+0.048\phantom{\rule{0.28em}{0ex}}({\text{N}}^{2}\text{LO})=[0.469\ifmmode\pm\else\textpm\fi{}0.033]\phantom{\rule{0.222222em}{0ex}}\text{mb}$ show order by order convergence and cutoff independence. No three-body currents are needed to renormalize observables up to ${\text{N}}^{2}\text{LO}$ in this process.

Radiative Capture Cross Sections of 155,157Gd for Thermal Neutrons

Thermal neutron radiative capture cross sections σ0γ of 155,157Gd are determined by summing the transition cross sections feeding the ground states of the respective product nuclei. The transition cross sections feeding the ground states from the discrete states in the low-excitation region, where the decay schemes are known completely, were measured using a guided cold neutron beam at the Budapest Research Reactor. Transitions from the states at the higher excitation, the so-called quasi-continuum levels, are determined from simulations with the extreme statistical model normalized to the intensity balance through the low-lying discrete levels. A significant non-1/v correction was applied to 155,157Gd, leading to σ0γ(155Gd) = 56 700(2100) b and σ0γ(157Gd) = 239 000(6000) b.

Measurement of 173Lu(n,γ) Cross Sections at DANCE

A highly gamma-radioactive target, 3.7 GBq, of 173Lu isotope was placed inside the DANCE array (Detector for Advanced Neutron Capture Experiments) at Los Alamos to study the radiative neutron capture on an unstable isotope. The 173Lu element was produced by naturalHf(p,xn) reactions following by beta-decays at the Isotope Production Facility (IPF). Measurements of radiative neutron capture cross section on 173Lu were achieved at the Los Alamos Neutron Science Center (LANSCE) spallation neutron source facility over the neutron energy range from thermal up to 1 keV. A special configuration was necessary to perform the experiment using the DANCE [1] array due to the high gamma activity of the target. We will report on the target production, the experiment and the results obtained for the radiative neutron capture on 173Lu. The radiative capture cross section was obtained for the first time on this unstable nucleus. Some resonances have been characterized. A comparison with a recent data evaluation is presented.

Interaction of Fast Nucleons with Actinide Nuclei Studied with GEANT4

We model interactions of protons and neutrons with energies from 1 to 1000 MeV with 241Am and 243Am nuclei. The calculations are performed with the Monte Carlo model for Accelerator Driven Systems (MCADS) which we developed based on the GEANT4 toolkit of version 9.4. This toolkit is widely used to simulate the propagation of particles in various materials which contain nuclei up to uranium. After several extensions we apply this toolkit also to proton- and neutron-induced reactions on Am. The fission and radiative neutron capture cross sections, neutron multiplicities and distributions of fission fragments were calculated for 241Am and 243Am and compared with experimental data. As demonstrated, the fission of americium by energetic protons with energies above 20 MeV can be well described by the Intra-Nuclear Cascade Liège (INCL) model combined with the fission-evaporation model ABLA. The calculated average numbers of fission neutrons and mass distributions of fission products agree well with the corresponding data. However, the proton-induced fission below 20 MeV is described less accurately. This is attributed to the limitations of the Intra-Nuclear Cascade model at low projectile energies.

Study on the keV neutron capture reaction in 56Fe and 57Fe

The neutron capture cross-sections and the radiative capture gamma-ray spectra from the broad resonances of 56Fe and 57Fe in the neutron energy range from 10 to 90keV and 550keV have been measured with an anti-Compton NaI(Tl) detector. Pulsed keV neutrons were produced from the 7Li\( (p,n)\) 7Be reaction by bombarding the lithium target with the 1.5ns bunched proton beam from the 3MV Pelletron accelerator. The incident neutron spectrum on a capture sample was measured by means of a time-of-flight (TOF) method with a 6Li -glass detector. The number of weighted capture counts of the iron or gold sample was obtained by applying a pulse height weighting technique to the corresponding capture gamma-ray pulse height spectrum. The neutron capture gamma-ray spectra were obtained by unfolding the observed capture gamma-ray pulse height spectra. To achieve further understanding on the mechanism of neutron radiative capture reaction and study on physics models, theoretical calculations of the \( \gamma\) -ray spectra for 56Fe and 57Fe with the POD program have been performed by applying the Hauser-Feshbach statistical model. The dominant ingredients to perform the statistical calculation were the Optical Model Potential (OMP), the level densities described by the Mengoni-Nakajima approach, and the \( \gamma\) -ray transmission coefficients described by \( \gamma\) -ray strength functions. The comparison of the theoretical calculations, performed only for the 550keV point, show a good agreement with the present experimental results.

Photon strength functions in Gd isotopes studied from radiative capture of resonance neutrons

The experimental spectra of γ rays following radiative neutron capture on iso- lated resonances of stable 152,154−158 Gd targets were measured by the DANCE calorime- ter installed at the Los Alamos Neutron Scattering Center in New Mexico, USA. These spectra were analyzed within the extreme statistical model to get new information on the photon strength functions. Special emphasis was put on study of the scissors vibrational mode present in these isotopes. Our data show that the scissors-mode resonances are built not only on the ground states but also on the excited levels of all studied Gd iso- topes. The scissors mode strength observed in 157,159 Gd products is significantly higher than in neighboring even-even nuclei 156,158 Gd. Such a difference indicates the existence of an odd-even effect in the scissors mode strength. Moreover, there exists no universal parameter-free model of the electric dipole photon strength function describing the ex- perimental data in all of the Gd isotopes studied. The results for the scissors mode are compared with the (γ, γ � ) data for the ground-state transitions and with the results from 3 He-induced reactions.

Radiative Capture of Neutrons on 12C at Astrophysical Energies

Within the framework of the potential cluster model with classification of orbital state according to the Young schemes, the possibility of a description of experimental data for the total cross sections of radiative n 12C capture at energies from 25 meV (25⋅10-3 eV) to 550 keV to the ground state and three excited levels of the 13С nucleus is considered. It is shown that it is entirely possible to explain the magnitude of the cross sections in the considered energy range on the basis of only E1 transitions from various states of n 12C scattering to these bound states of the 13C nucleus in the n 12C channel.

RADIATIVE NEUTRON CAPTURE ON 9Be, 14C, 14N, 15N AND 16O AT THERMAL AND ASTROPHYSICAL ENERGIES

The total cross-sections of the radiative neutron capture processes on 9 Be , 14 C , 14 N , 15 N and 16 O are described in the framework of the modified potential cluster model with the classification of orbital states according to Young tableaux. The continued interest in the study of these reactions is due, on the one hand, to the important role played by this process in the analysis of many fundamental properties of nuclei and nuclear reactions, and, on the other hand, to the wide use of the capture cross-section data in the various applications of nuclear physics and nuclear astrophysics, and, also, to the importance of the analysis of primordial nucleosynthesis in the Universe. This article is devoted to the description of results for the processes of the radiative neutron capture on certain light atomic nuclei at thermal and astrophysical energies. The considered capture reactions are not part of stellar thermonuclear cycles, but involve in the reaction chains of inhomogeneous Big Bang models.

Strength of the scissors mode in odd-mass Gd isotopes from the radiative capture of resonance neutrons

Radiative neutron capture reaction was measured in the resolved resonance region for ${}^{152,154,156,158}$Gd targets with the Detector for Advanced Neutron Capture Experiments $\ensuremath{\gamma}$-ray calorimeter at the Los Alamos Neutron Science Center. The $\ensuremath{\gamma}$-ray energy spectra for different multiplicities were obtained for several $s$-wave resonances in each nucleus. These spectra were analyzed within the extreme statistical model to provide unique information on the photon strength functions and especially on the scissors mode in odd Gd isotopes. The main result of this analysis is that in all nuclei scissors-mode resonances are built on excited levels and not only on their ground states. The strength of the mode found in ${}^{153}$Gd and ${}^{155}$Gd is comparable to the strength observed in well-deformed even-even Gd isotopes while it is substantially higher in ${}^{157,159}$Gd. The difference in observed strengths of the scissors mode indicates an existence of an even-odd effect in this quantity.

Application of Radiogenic Lead with Dominant Content of 208Pb for Long Prompt Neutron Lifetime in Fast Reactor

As a rule, materials of small atomic weight (light and heavy water, graphite, and so on) are used as neutron moderators and reflectors. A new very heavy atomic weight moderator is proposed - radiogenic lead consisting mainly of isotope 208Pb. It is characterized by extremely small neutron radiative capture cross section ([approximately]0.23 mb for thermal neutrons, i.e., less than that for graphite and deuterium) and highest albedo of thermal neutrons.The use of radiogenic lead to make it possible to slow down the chain fission reaction on prompt neutrons in a fast reactor is evaluated. This can improve the safety of a fast reactor.It is noteworthy that radiogenic lead with high 208Pb content may be recovered from thorium (as well as thorium-uranium) ores without isotope separation. This has been confirmed experimentally by the investigations performed at the University of São Paulo in Brazil.

Neutron capture by light nuclei at astrophysical energies

Results obtained in the studies of radiative neutron capture on light nuclei at thermal and astrophysical energies are reviewed. The capture reactions in question are part of the primordial nucleosynthesis reaction chain underlying formation and development of the Universe. The reactions are treated within the potential cluster model with orbital states of clusters classified by Young diagrams. It is demonstrated that the intercluster potentials obtained from the phase shift analysis and description of the main characteristics of bound nuclear states in cluster channels can be used for the analysis of radiative capture characteristics.

Simultaneous and integrated neutron-based techniques for material analysis of a metallic ancient flute

A metallic 19th century flute was studied by means of integrated and simultaneous neutron-based techniques: neutron diffraction, neutron radiative capture analysis and neutron radiography. This experiment follows benchmark measurements devoted to assessing the effectiveness of a multitask beamline concept for neutron-based investigation on materials. The aim of this study is to show the potential application of the approach using multiple and integrated neutron-based techniques for musical instruments. Such samples, in the broad scenario of cultural heritage, represent an exciting research field. They may represent an interesting link between different disciplines such as nuclear physics, metallurgy and acoustics.

Radiative Capture Reactions in Lattice Effective Field Theory

We outline a general method for computing nuclear capture reactions on the lattice. The method consists of two major parts. In this study we detail the second part which consists of calculating an effective two-body capture reaction on the lattice at finite volume. We solve this problem by calculating the two-point Green's function using an infrared regulator and the capture amplitude to a two-body bound state. We demonstrate the details of this method by calculating on the lattice the leading M1 contribution to the radiative neutron capture on proton at low energies using pionless effective field theory. We find good agreement with exact continuum results. The approach we outline here can be used in a wide range of applications including few-body reactions in cold atomic systems and hadronic reactions in lattice quantum chromodynamics.