Radiative Neutron Capture Research Articles

Hadronic weak interaction in the two-nucleon system with effective field theories

Weak interactions in two-nucleon system at low energies are explored in the framework of effective field theory. We review our resent calculations of parity-violating observables in radiative neutron capture on a proton at threshold where both pionful and pionless theories are employed.

Impact of the phonon coupling on the radiative neutron capture

Inclusion of the coupling of quasiparticle degrees of freedom with phonon degrees is a natural extention of the standard QRPA approach. The paper presents the quantitative impact of this phonon coupling on the dipole strength and radiative neutron capture for the stable 124Sn and very exotic 150Sn isotopes, as an illustration, using the self-consistent version of the Extended Theory of Finite Fermi Systems. It was found that the phonon contribution to the pygmy-dipole resonance and radiative neutron capture cross section is increased with the (N − Z) difference growth. The results show that the self-consistent nuclear structure calculations are important for unstable nuclei, where phenomenological approaches do not work.

Calculation of Prompt Fission Product Average Cross Sections for Neutron-Induced Fission of 235U and 239Pu

Yield-weighted average cross sections of neutron radiative capture, (n,2n), and (n,3n) reactions over prompt fission products (FPs) from 235U and 239Pu are calculated. The prompt fission production yields are taken from the ENDF/B-VII.0 library. The FPs for each fissile material exist over a range of approximately 1000 neutron-rich nuclides. Several nuclear reaction codes are utilized for calculating the cross sections on each individual fission product—EMPIRE-2.19, TALYS-1.0, GNASH, and CoH. The influence of the FP isomers on the average cross sections is examined with TALYS. We investigate the dependence of the average cross sections on the number of FPs taken for averaging. It is shown that the average capture cross section is much more sensitive to the number of FPs included, compared with the (n,2n) and (n,3n) reactions. An intercomparison of the calculated cross sections with the different reaction codes is carried out. In the capture reaction, EMPIRE predicted lower cross section than TALYS and CoH owing to different default assumptions used in the γ-ray strength function modeling. Moreover, the preequilibrium models implemented in each code give different predictions for the neutron-emission reactions, although the differences are relatively small. We also discuss a difference between the macroscopic and microscopic calculation options in TALYS for the pre-equilibrium model, optical potential model, and γ-ray strength function. The predictive capability of the reaction codes for the capture reaction is examined by comparing their calculations with the ENDF data, which are based on measurements. Compared with the historic Foster and Arthur's evaluation, our new (n,2n) predictions are similar, although our capture predictions are almost an order of magnitude higher. Recommended cross sections for use in applications have been tabulated in ENDF-formatted files.

Neutron capture by Ru: Neutron cross sections ofRu96,102,104andγ-ray spectroscopy in the decays ofRu97,103,105

Cross sections for radiative capture of neutrons have been measured for stable isotopes of Ru with mass numbers $96,102$, and $104$. From separate irradiations using thermal and epithermal neutrons, independent values for the thermal cross section and effective resonance integral have been determined. Spectroscopic studies of the $\ensuremath{\gamma}$ rays emitted in the decays of $^{97,103,105}\mathrm{Ru}$ have enabled improvements in the precision of the energies and intensities of the radiations along with corresponding improvements in the $\ensuremath{\beta}$-decay feeding intensities and the energies of the levels in the respective daughter nuclei. Similar spectroscopic measurements of the decays of $^{105}\mathrm{Rh}$ (daughter of $^{105}\mathrm{Ru}$) and $^{96}\mathrm{Tc}$ (produced from $n,p$ reactions on $^{96}\mathrm{Ru}$) have resulted in improved $\ensuremath{\gamma}$-ray energies and intensities in those decays.

Photoneutron cross sections forZr96: A systematic experimental study of photoneutron and radiative neutron capture cross sections for zirconium isotopes

Photoneutron cross sections were measured for $^{96}\mathrm{Zr}$ near neutron threshold with quasimonochromatic laser-Compton-scattering $\ensuremath{\gamma}$-ray beams. A systematic analysis of photoneutron and radiative neutron capture data for zirconium isotopes within the statistical model calculation leads to a unified picture of low-energy $\ensuremath{\gamma}$-ray strengths for zirconium isotopes that is described by the HFB $+$ QRPA model of $E1$ strength supplemented with an extra $\ensuremath{\gamma}$ strength attributed to a giant $M1$ resonance. Results of the systematic analysis including radiative neutron capture cross sections for radioactive $^{95}\mathrm{Zr}$ and $^{93}\mathrm{Zr}$ nuclei are presented.

Calculation of Prompt Fission Product Average Cross Sections for Neutron-Induced Fission of 235U and 239Pu

Yield-weighted average cross sections of neutron radiative capture, (n,2n), and (n,3n) reactions over prompt fission products (FPs) from 235U and 239Pu are calculated. The prompt fission production yields are taken from the ENDF/B-VII.0 library. The FPs for each fissile material exist over a range of approximately 1000 neutron-rich nuclides. Several nuclear reaction codes are utilized for calculating the cross sections on each individual fission product—EMPIRE-2.19, TALYS-1.0, GNASH, and CoH. The influence of the FP isomers on the average cross sections is examined with TALYS. We investigate the dependence of the average cross sections on the number of FPs taken for averaging. It is shown that the average capture cross section is much more sensitive to the number of FPs included, compared with the (n,2n) and (n,3n) reactions. An intercomparison of the calculated cross sections with the different reaction codes is carried out. In the capture reaction, EMPIRE predicted lower cross section than TALYS and CoH owing to different default assumptions used in the γ-ray strength function modeling. Moreover, the preequilibrium models implemented in each code give different predictions for the neutron-emission reactions, although the differences are relatively small. We also discuss a difference between the macroscopic and microscopic calculation options in TALYS for the pre-equilibrium model, optical potential model, and γ-ray strength function. The predictive capability of the reaction codes for the capture reaction is examined by comparing their calculations with the ENDF data, which are based on measurements. Compared with the historic Foster and Arthur's evaluation, our new (n,2n) predictions are similar, although our capture predictions are almost an order of magnitude higher. Recommended cross sections for use in applications have been tabulated in ENDF-formatted files.

Photon strength in spherical and deformed heavy nuclei

Information on the photon strength in heavy nuclei with mass A > 150 will be given and compared to respective data. The photon strength function is a very important ingredient for statistical model calculations – especially when these are used to describe neutron capture. Several schemes for a transmutation of radioactive waste favor nuclear reactions with fast neutrons and these also influence the performance of various reactor types proposed to deliver nuclear energy together with only small quantities of such waste. Reactions with fast neutrons are less well studied as compared to those induced by thermal neutrons. As they are not easily accessible experimentally, reference is often made to calculations using the statistical model. Photon emission probabilities are needed as input to such calculations aiming for predictions on fission to capture ratios. Results of approximate calculations for radiative neutron capture will be presented.

Erratum: Coulomb dissociation ofC15and radiative neutron capture onC14[Phys. Rev. C80, 024608 (2009)

The semiclassical, dynamical description of diffraction dissociation of weakly bound nuclei is applied to analyze the decay-energy spectra of $^{15}$C that have been measured at 68 MeV/nucleon on a Pb target. The optical potentials that are used to describe the nuclear interaction of $^{15}$C with the target nucleus are realistic because the fits to the two measured spectra, one with a small and one with a very large acceptance angle, are consistent and of similar quality. The cross section for the radiative neutron capture on $^{14}$C to the 1/2$^+$ ground state of $^{15}$C is deduced from the analysis. When combined with an estimated contribution from the capture to the 5/2$^+$ excited state of $^{15}$C, an excellent agreement with a recent direct capture measurement is achieved.

Parametrization of low-energy cross sections for nonresonant neutron capture

The nonresonant component of radiative neutron capture reactions is parametrized at low energies by a polynomial of second degree. The potential model is first used to reproduce experimental data below 1 MeV with the help of spectroscopic factors. The fits are found sensitive to the scattering length of the initial s or p waves. The coefficients of a Taylor expansion are then calculated by resolution of the Schrodinger equation and its energy derivatives at energy zero. Such theory-guided parametrizations are derived for neutron capture by (7)Li, (12)C, (14)C, (16)O, and (18)O. When the capture proceeds from the s wave to a weakly bound state, a Pade-like parametrization better approximates the potential-model results.

Coulomb dissociation ofC15and radiative neutron capture onC14

The semiclassical, dynamical description of diffraction dissociation of weakly bound nuclei is applied to analyze the decay-energy spectra of $^{15}\mathrm{C}$ that have been measured at 68 MeV/nucleon on a Pb target. The optical potentials that are used to describe the nuclear interaction of $^{15}\mathrm{C}$ with the target nucleus are realistic because the fits to the two measured spectra, one with a small and one with a very large acceptance angle, are consistent and of similar quality. The cross section for the radiative neutron capture on $^{14}\mathrm{C}$ to the ${1/2}^{+}$ ground state of $^{15}\mathrm{C}$ is deduced from the analysis. When combined with an estimated contribution from the capture to the ${5/2}^{+}$ excited state of $^{15}\mathrm{C}$, an excellent agreement with a recent direct capture measurement is achieved.

Improved microscopic nuclear level densities within the Hartree-Fock-Bogoliubov plus combinatorial method

New developments have been brought to our energy-, spin-, and parity-dependent nuclear level densities based on the microscopic combinatorial model. As in our previous study, a detailed calculation of the intrinsic state density and of the rotational enhancement factor is included, but this time the vibrational contributions explicitly take the phonon excitations into account. The present model predicts the experimental $s$- and $p$-wave neutron resonance spacings with a degree of accuracy comparable to that of the best global models available. It is also shown that the model gives a reliable extrapolation at low energies where experimental data on the cumulative number of levels can be extracted. The predictions are also in good agreement with the experimental data extracted from the analysis of particle-$\ensuremath{\gamma}$ coincidence in the ($^{3}\mathrm{He}$, $\ensuremath{\alpha}\ensuremath{\gamma}$) and ($^{3}\mathrm{He}$, $^{3}\mathrm{He}$${}^{'}\ensuremath{\gamma}$) reactions. Total as well as partial level densities for more than 8500 nuclei are made available in a table format for practical applications. For the nuclei for which experimental $s$-wave spacings and enough low-lying states exist, renormalization factors are provided to reproduce simultaneously both observables. The same combinatorial method is used to estimate the nuclear level densities at the fission saddle points of actinides and at the shape isomer deformation. Finally, the new nuclear level densities are applied to the calculation of radiative neutron capture cross sections.

Measurements of neutrons produced by high-energy muons at the Boulby Underground Laboratory

We present the first measurements of the muon-induced neutron flux at the Boulby Underground Laboratory. The experiment was carried out with an 0.73 tonne liquid scintillator that also served as an anticoincidence system for the ZEPLIN-II direct dark matter search. The experimental method exploited the delayed coincidences between high-energy muon signals and gamma-rays from radiative neutron capture on hydrogen or other elements. The muon-induced neutron rate, defined as the average number of detected neutrons per detected muon, was measured as 0.079 ± 0.003 (stat.) neutrons/muon using neutron-capture signals above 0.55 MeV in a time window of 40–190 μs after the muon trigger. Accurate Monte Carlo simulations of the neutron production, transport and detection in a precisely modeled laboratory and experimental setup using the GEANT4 toolkit gave a result 1.8 times higher than the measured value. The difference greatly exceeds all statistical and systematic uncertainties. As the vast majority of neutrons detected in the current setup were produced in lead we evaluated from our measurements the neutron yield in lead as ( 1.31 ± 0.06 ) × 10 - 3 neutrons/muon/(g/cm 2) for a mean muon energy of about 260 GeV.

Current status of research on T invariance in neutron-nuclear reactions

In this work, the current state of research on T invariance in neutron-nuclear reactions is considered. The promising character of investigations in this field related to possible enhancement of T-invariance violation in compound states of medium and heavy nuclei is underlined. Progress in preparation of experimental tests of T invariance using three-and five-vector correlations in cross sections of interaction of polarized neutrons with aligned nuclei is described in detail. T-invariance tests in reactions of radiative neutron capture and in coherent scattering of polarized neutrons on crystals are also considered.

Measurement of the radiative neutron capture cross section ofPb206and its astrophysical implications

The (n, gamma) cross section of 206Pb has been measured at the CERN n_TOF facility with high resolution in the energy range from 1 eV to 600 keV by using two optimized C6D6 detectors. In the investigated energy interval about 130 resonances could be observed, from which 61 had enough statistics to be reliably analyzed via the R-matrix analysis code SAMMY. Experimental uncertainties were minimized, in particular with respect to (i) angular distribution effects of the prompt capture gamma-rays, and to (ii) the TOF-dependent background due to sample-scattered neutrons. Other background components were addressed by background measurements with an enriched 208Pb sample. The effect of the lower energy cutoff in the pulse height spectra of the C6D6 detectors was carefully corrected via Monte Carlo simulations. Compared to previous 206Pb values, the Maxwellian averaged capture cross sections derived from these data are about 20% and 9% lower at thermal energies of 5 keV and 30 keV, respectively. These new results have a direct impact on the s-process abundance of 206Pb, which represents an important test for the interpretation of the cosmic clock based on the decay of 238U.

The isovector imaginary neutron potential: A key ingredient for the r-process nucleosynthesis

The isovector contribution to the imaginary component of the microscopic optical model potential is adjusted on experimental neutron strength function data. Within the Brückner–Hartree–Fock approximation of Jeukenne–Lejeune–Mahaux, it is shown that experimental data favours a strong isovector component that can have a drastic impact on the radiative neutron capture cross section for neutron-rich nuclei. If confirmed, this result strongly inhibits the resonant capture by exotic nuclei, so that the traditional r-process picture of the fast neutron captures during the nucleosynthesis r-process needs to be revisited in depth.

Resonance capture cross section ofPb207

The radiative neutron capture cross section of Pb207 has been measured at the CERN neutron time of flight installation n_TOF using the pulse height weighting technique in the resolved energy region. The measurement has been performed with an optimized setup of two C6D6 scintillation detectors, which allowed us to reduce scattered neutron backgrounds down to a negligible level. Resonance parameters and radiative kernels have been determined for 16 resonances by means of an R-matrix analysis in the neutron energy range from 3 keV to 320 keV. Good agreement with previous measurements was found at low neutron energies, whereas substantial discrepancies appear beyond 45 keV. With the present results, we obtain an s-process contribution of 77±8% to the solar abundance of Pb207. This corresponds to an r-process component of 23±8%, which is important for deriving the U/Th ages of metal poor halo stars.Received 25 July 2006DOI:https://doi.org/10.1103/PhysRevC.74.055802©2006 American Physical Society

Neutron capture cross sections ofGd148and the decay ofGd149

The thermal cross section and resonance integral were measured for radiative neutron capture by radioactive {sup 148}Gd. The deduced values are {sigma}=9600{+-}900 b and I=28,200{+-}2300. We also deduced upper limits for the n,p and n, {alpha} cross sections, respectively, 0.25 b and 13 b. The {gamma}-ray spectrum from the decay of {sup 149}Gd was studied in singles mode at high resolution to verify the previously determined energies and intensities. From the latter measurements, new transitions are proposed and upper limits are deduced for previously reported transitions.

Radiative neutron capture on a proton at big-bang nucleosynthesis energies

The total cross section for radiative neutron capture on a proton, $np \to d \gamma$, is evaluated at big bang nucleosynthesis (BBN) energies. The electromagnetic transition amplitudes are calculated up to next-to leading order within the framework of pionless effective field theory with dibaryon fields. We also calculate the $d\gamma\to np$ cross section and the photon analyzing power for the $d\vec{\gamma}\to np$ process from the amplitudes. The values of low energy constants that appear in the amplitudes are estimated by a Markov Chain Monte Carlo analysis using the relevant low energy experimental data. Our result agrees well with those of other theoretical calculations except for the $np\to d\gamma$ cross section at some energies estimated by an R-matrix analysis. We also study the uncertainties in our estimation of the $np\to d\gamma$ cross section at relevant BBN energies and find that the estimated cross section is reliable to within $\sim$1% error.

Neutron capture cross sections ofSn112,116,122,124

The thermal cross sections and resonance integrals have been determined for radiative neutron capture by $^{112,116,122,124}\mathrm{Sn}$ leading to the ground and isomeric states of $^{113,123,125}\mathrm{Sn}$ and the isomeric state of $^{117}\mathrm{Sn}$. Using natural samples of Sn metal foils and high-resolution $\ensuremath{\gamma}$-ray spectroscopy, it was possible to determine a mutually consistent set of cross sections, particularly the small thermal cross sections leading to $11/{2}^{\ensuremath{-}}$ states. A detailed analysis of the $\ensuremath{\gamma}$-ray emissions from irradiated Sn samples enabled us to make new and more precise determinations of the energies and intensities of $\ensuremath{\gamma}$ rays emitted in the decays of 40-min $^{123m}\mathrm{Sn}$ and 9.5-min $^{125m}\mathrm{Sn}$ and corresponding adjustments to the energy levels of ${}^{123,125}$Sb. The present results support the recently adopted revised value for the branching ratio of the 255.1-keV $\ensuremath{\gamma}$ ray in the decay of $^{113}\mathrm{Sn}$. The presence of a $^{116m}\mathrm{In}$ impurity in our samples prompted a new determination of the energies and intensities of the $\ensuremath{\gamma}$ rays emitted in its decay and of the energy levels in $^{116}\mathrm{Sn}$.

Determination of thermal neutron radiative capture cross section of 6Li

The thermal neutron radiative capture cross section of 6Li was determined relative to the 1H(n,γ) reaction emitting the 2223.25 keV γ-ray. The sample used was a mixture of enriched 6Li 2CO 3 and melamine. Background correction was performed by considering the scattering effect of the sample. The determined thermal neutron radiative capture cross section of 6Li was 37.7 ± 3.0 mb.