S-wave Neutron Resonance Research Articles

Indispensability of cross-shell contributions in neutron resonance spacing

Spin- and parity-dependent nuclear level densities (NLDs) are obtained for a configuration interaction shell model using a numerically efficient spectral distribution method. The calculations are performed for 24Na and 25,26,27Mg nuclei using full sd-pf model space that incorporates the cross-shell excitations from the sd to the pf-shell. The obtained NLDs are then employed to determine the s-wave neutron resonance spacing (D0), which is one of the crucial inputs for the predictions of astrophysical reaction rates. Although the considered nuclei are not neutron-rich, the contributions from cross-shell excitations to the pf-shell are indispensable for explaining the experimental data for D0, which otherwise are significantly overestimated.

Astrophysical reaction rates with realistic nuclear level densities

Realistic nuclear level densities (NLDs) obtained within the spectral distribution method (SDM) are employed to study nuclear processes of astrophysical interest. The merit of SDM lies in the fact that the NLDs corresponding to many body shell model Hamiltonian consisting of residual interaction can be obtained for the full configurational space without recourse to the exact diagnolization of huge matrices. We calculate NLDs and s-wave neutron resonance spacings which agree reasonably well with the available experimental data. By employing these NLDs, we compute reaction cross-sections and astrophysical reaction rates for radiative neutron capture in few Fe-group nuclei, and compare them with experimental data as well as with those obtained with NLDs from phenomenological and microscopic mean-field models. The results obtained for the NLDs from SDM are able to explain the experimental data quite well. These results are of particular importance since the configuration mixing through the residual interaction naturally accounts for the collective excitations. In the mean-field models, the collective effects are included through the vibrational and rotational enhancement factors and their NLDs are further normalized at low energies with neutron resonance data.

Relation of Experimental Features of P-Even, T-Odd Asymmetries in Ternary Nuclear Fission Induced by Cold Polarized Neutrons to Triple and Quintuple Scalar Correlations

In employing the concept of the isotropy of space, the coefficients D of P-even, T-odd asymmetry in the angular distributions of prescission alpha particles emitted as third particles in the ternary fission of nonoriented target nuclei that is induced by polarized cold neutrons are expressed in the lowest orders of perturbation theory in the neutron polarization vector in terms of two P-even scalar coefficients D3 and D5 associated with, respectively, triple and quintuple correlations depending on the unit vectors kα, kLF, and σn, which determine the features of the coefficients being studied. On the basis of the above representation, the experimental values of the coefficients D3 and D5 are determined by using the experimental values of D and the angular distributions of alpha particles emitted in the analogous reaction induced by unpolarized neutrons. The resulting coefficients D3 and D5 are compared with the analogous coefficients found by means of the classical method of trajectory calculations and by means of quantum-mechanical fission theory relying on the concept that it is the rotational mechanism that is responsible for the appearance of the asymmetries being studied. It turns out that the classical method, which disregards the interference between fission amplitudes for different S-wave neutron resonances, leads to an irremovable contradiction between the calculated coefficient D3 and the analogous experimental coefficient for the 233U target nucleus. In the case of employing the quantum-mechanical approach, it is concluded that three-body calculations of the Coriolis interaction—perturbed amplitude of the angular distributions of alpha particles moving in the Coulomb fields of fission fragments are required for the 233U target nucleus.

Phenomenological level density model with hybrid parameterization of deformed and spherical state densities

ABSTRACT A phenomenological level density model that has different level density parameter sets for the state densities of the deformed and the spherical states and the optimization of the parameters using experimental data of the average s-wave neutron resonance spacing are presented. The transition to the spherical state from the deformed one is described using the parameters derived from a microscopic nuclear structure calculation. The nuclear reaction calculation has been performed by the statistical model using the present level density. Resulting cross sections for various reactions with the spherical, deformed, and transitional target nuclei show a good agreement with the experimental data, which indicates the effectiveness of the present model. The role of the rotational collective enhancement in the calculations of those cross sections is also discussed.

The effect of deformation parameter of heavy nuclei on level density parameter

The possible effect of the collective motion in heavy nuclei has been investigated in the framework of Nilson model. This effect has been searched realistically by calculating the level density, which plays a significant role in the description of the reaction cross sections in the statistical nuclear theory. The nuclear level density parameter for some deformed radioisotopes of (even- even) target nuclei (Dy, W and Os) is calculated, by taking into consideration the collective motion for excitation modes for the observed nuclear spectra near the neutron binding energy. The method employed in the present work assumes equidistant spacing of the collective coupled state bands of the considered isotopes. The present calculated results for first excited rotational band have been compared with the accumulated values from the literature for s-wave neutron resonance data, and were in good agreement with those data.

Scissors Mode of162Dy Studied from Resonance Neutron Capture

Multi-step cascade γ-ray spectra from the neutron capture at isolated resonances of 161Dy nucleus were measured at the LANSCE/DANCE time-of-flight facility in Los Alamos National Laboratory. The objectives of this experiment were to confirm and possibly extend the spin assignment of s-wave neutron resonances and get new information on photon strength functions with emphasis on the role of the M1 scissors mode vibration. The preliminary results show that the scissors mode plays a significant role in all transitions between accessible states of the studied nucleus. The photon strength functions describing well our data are compared to results from 3He-induced reactions, (n,γ) experiments on Gd isotopes, and (γ,γ’) reactions.

Neutron transmission and capture cross section measurements for 241Am at the GELINA facility

Resonance parameters for neutron-induced reactions on 241Am below 110 eV have been determined. The parameters result from a resonance shape analysis of transmission and capture data measured at the time-of-flight facility GELINA, with the accelerator operating at a 50 Hz repetition rate. The transmission experiments were carried out at a 25 m station using a Li glass scintillator. The capture experiments were performed at a 12.5 m station by applying the total energy detection principle in combination with the pulse height weighting technique using a pair of C6D6 detectors. The normalization of the capture data was determined by a combined least squares adjustment of the transmission and capture data. From the adjusted resonance parameters a capture cross section of 749 ± 35 b for a neutron energy of 0.0253 eV and an average radiation width of 〈Γ γ 〉 = 42.0 meV for s-wave resonances were obtained. A missing-level analysis for s-wave neutron resonances within the statistical model results in compatible values with previous estimates. The neutron widths obtained in this work are approximately 22% larger compared to other experimental data and evaluated data libraries. Also the thermal capture cross section is larger than most of the recommended values. However, the resonance parameter file presented in this work is consistent with results of both integral experiments and of the experimentally determined resonance integrals.

Statistical nuclear properties (level densities, spin distributions)

A general overview is given on the phenomenological methods used to describe the level densities in nuclei. Two well-known two-parameter formulas of level densities, the Back-Shifted Fermi Gas (BSFG) model and the Constant Temperature (CT) model, were used. A common ingredient of both is the spin distribution function, which contains in Ericsons's parametrization the spin-cutoff parameter σ. A realistic description of the parameters of both spin distribution function and the two level density models has been obtained by fitting the experimental data of 310 nuclei between 18F and 251Cf, consisting of the complete level schemes at low excitation energies and the s-wave neutron resonance spacings at the neutron binding energy. We determine a simple formula for the spin-cutoff parameter as a function of mass number and excitation energy. Also, an even-odd spin staggering in the spin distribution of the even-even nuclei was observed, and described with a simple formula. Using this newly defined spin distribution function, an empirical set of parameters of the BSFG and CT models was determined by fitting both the low-energy levels and the neutron resonance spacings. For these parameters, simple formulas were proposed that involve only quantities available from the mass tables, and allow reasonable estimations of the level density parameters for nuclei far from stability. Both the BSFG and CT models describe equally well the level densities at energies up to at least the neutron binding energy. Finally, we discuss recent experimental evidence that the CT model is the more correct description of the nuclei in the low-excitation energy (pairing) regime.

Nuclear Level Density Parameters ofPb203-209andBi206-210Deformed Target Isotopes Used on Accelerator-Driven Systems in Collective Excitation Modes

The nuclear level density parameters of some deformed isotopes of target nuclei (Pb, Bi) used on the accelerator-driven subcritical systems (ADSs) have been calculated taking into consideration different collective excitation modes of observed nuclear spectra near the neutron binding energy. The method used in the present work assumes equidistant spacing of the collective coupled state bands of the considered isotopes. The present calculated results for different collective excitation bands have been compared with the compiled values from the literature for s-wave neutron resonance data, and good agreement was found.

Nuclear Level Densities with Microscopic Statistical Method Using a Consistent Residual Interaction

Nuclear level density (NLD) evaluated under the framework of the microscopic statistical method is examined. We calculate single-particle levels by the Skyrme-Hartree-Fock (SHF) + Bardeen-Cooper-Schrieffer (BCS) method, and the grand partition function is calculated with the same two-body interaction as used in the SHF+BCS method. The results are compared with preceding theoretical calculations using different residual interactions. Cumulative numbers and s-wave neutron resonance spacings are reproduced fairly well. The effect of a renormalization term neglected in conventional microscopic statistical methods is also investigated. In order to test a practical application, we calculate neutron capture cross section and compare it with experimental results.

The theoretical nuclear level density parameters of some deformed target radioisotopes [formula omitted] and [formula omitted] used on the accelerator-driven systems

In this study, the nuclear level density parameters of some deformed radioisotopes of target nuclei (W, Hg) used on the accelerator-driven subcritical systems (ADS) have been calculated taking into consideration different collective excitation modes of observed nuclear spectra near the neutron binding energy. The method used in the present work assumes equidistance spacing of the collective coupled state bands of the considered radioisotopes. The present calculated results for different collective excitation bands have been compared with the compiled values from the literature for s-wave neutron resonance data, and were found to be in good agreement with those data.

Optimized [formula omitted] spin assignments of s-wave neutron resonances

The multiplicity of γ ‐ ray emission following neutron capture at isolated resonances carries valuable information on the resonance spin. Several methods utilizing this information have been developed. The latest method was recently introduced for analyzing the data from time-of-flight measurements with 4 π γ ‐ calorimetric detection systems. The present paper describes a generalization of this method. The goal is the separation of the γ ‐ emission yields belonging to the two neutron capturing state spins of isolated (or even unresolved) s-wave neutron resonances on targets with non-zero spin. The formalism for performing this separation is described and then tested on artificially generated data. This new method was applied to the γ ‐ multiplicity data obtained for the Sm 147 ( n , γ ) Sm 148 reaction using the DANCE detector system at the LANSCE facility at Los Alamos National Laboratory. The analyzing power of the upgraded method is supported by combined dicebox and geant4 simulations of the fluctuation properties of the γ ‐ multiplicity distributions.

Cross section for 246Cm subbarrier fission

The cross section for 246Cm fission induced by neutrons of energy in the range 0.1 eV-20 keV was measured by the neutron lead slowing-down spectrometer (LSDS-100) of the Institute for Nuclear Research (INR, Russian Academy of Sciences, Moscow). The parameters of the resonance area and of the fission width were evaluated for several low-lying s-wave neutron resonances. The parameters of the intermediate structure in the cross section for the subbarrier fusion of 246Cm nuclei were found. The results obtained in this way were compared with available experimental data and with recommended evaluated data.

Cross section for the subbarrier fission of 244Cm

The cross section for 244Cm fission induced by neutrons of energy in the range between 0.07 eV and 20 keV was measured by using the lead slowing-down spectrometer (LSDS-100) of the Institute for Nuclear Research (Russian Academy of Sciences, Moscow). The parameters of the resonance areas were determined for the lowest eight s-wave neutron resonances, and the respective fission widths were evaluated. Also, the parameters of the intermediate structure in the cross section for the subbarrier fission of 244Cm nuclei were evaluated. The results were compared with available data and recommendations based on evaluations.

Nuclear level density parameters of some largely deformed nuclei of medical radionuclides in low-lying collective excitation modes

Abstract In this study, the nuclear energy level density based on nuclear collective excitation mechanism has been identified in terms of low-lying collective level bands near the neutron binding energy. Nuclear level density parameters of some large deformed nuclei of radionuclides widely used in medical applications have been calculated by using different collective excitation modes of observed nuclear spectra. The calculated parameters have been used successfully in estimation of the neutron-capture cross section basic data for the production of new medical radionuclides. The investigated radionuclides have been considered in the region of mass number 150 < A < 200. The method used in the present work assumes equidistance spacing of the collective coupled state bands of the interest radionuclides. The calculated results have been compared with the compiled values from the literatures for s-wave neutron resonance data.

Combinatorial level densities for practical applications

We review our calculated energy-, spin- and parity-dependent nuclear level densities based on the microscopic combinatorial model described in ref. [1]. We show that this model predicts the experimental sand p-wave neutron resonance spacings with a degree of accuracy comparable to that of the best global models available and also provides reasonable description of low energies cumulative number of levels as well as of the experimental data obtained by the Oslo group [2]. We also provide a renormalization recipe which enables to play with the tabulated results for practical applications. Finally, we study the impact of temperature dependent calculation on s-wave neutron resonance spacings.

CALCULATION OF NUCLEAR LEVEL DENSITY PARAMETERS OF SOME LIGHT DEFORMED MEDICAL RADIONUCLIDES USING COLLECTIVE EXCITATION MODES OF OBSERVED NUCLEAR SPECTRA

In this study the nuclear energy level density based on nuclear collective excitation mechanism has been identified in terms of the low-lying collective level bands near the neutron binding energy. Nuclear level density parameters of some light deformed medical radionuclides used widely in medical applications have been calculated by using different collective excitation modes of observed nuclear spectra. The calculated parameters have been used successfully in estimation of the neutron-capture cross section basic data for the production of new medical radionuclides. The investigated radionuclides have been considered in the region of mass number 40<A<100. The method used in the present work assumes equidistance spacing of the collective coupled state bands of the interest radionuclides. The present calculated results have been compared with the compiled values from the literatures for s-wave neutron resonance data.

Integer ratios ofSn/EninCa40+nresonances suggesting two-oscillator excitations in the target nucleus

In s-wave neutron resonances of $^{40}\mathrm{Ca}$ at ${E}_{n}\ensuremath{\leqslant}2.5 \mathrm{MeV}$, ${S}_{n}/{E}_{n}$ for many levels is found to be of the form $17(n/m)$ where $n$, $m$ are small integers. Statistical tests show small probabilities for the observed dispositions of many levels at ${E}_{n}=(j/k)(1/70)G$ ($j$, $k$; small integers). To meet the requirement of time periodicity of the compound nucleus at resonance, a breathing model is developed, where the excitation energies ${E}_{x}$ are written as a sum of inverse integers; ${E}_{x}={S}_{n}+{E}_{n}=G\ensuremath{\sum}(1/k)$ ($k$: integer). In $^{40}\mathrm{Ca}+n$, the separation energy ${S}_{n}=8362 \mathrm{keV}$ is written as ${S}_{n}=(17/70)G=(1/7+1/10)G$, where $G=34.4$ MeV. $G$ is almost equal to the Fermi energy of the nucleus. It is suggested that two oscillators of energy $(1/7)G$ and $(1/10)G$ are excited in $^{40}\mathrm{Ca}$ by neutron incidence, in which the recurrence energy $(1/70)G$ is resonat with neutrons of energies at $(j/k)(1/70)G$, forming a simple compound nucleus.

Analysis of the Nuclear Level-Density Parameters of Some Large Deformed Odd-A and Odd-Odd Nuclei in the Region of Rare Earth Elements

Nuclear level density plays an important role in estimation of nuclear reaction rates, statistical calculations of astrophysics, spallations neutrons measurements, and studies of intermediate-energy heavy-ion collisions. In particular, level densities have been used successfully in calculation of the neutron-capture cross-section basic data required for both design and nuclear model calculations in nuclear science and technologies.In this study the nuclear level density based on nuclear low-lying collective level bands at excitations near neutron binding energy is analyzed in terms of collective excitation modes. The nuclear level-density parameters of some large deformed odd-A and odd-odd nuclei in the region of rare earth elements have been calculated by considering different excitation bands of observed nuclear spectra. The method used assumes equidistant spacing of the collective coupled state bands of nuclei considered. The values of calculated nuclear level-density parameters have been compared with those of the compiled values for s-wave neutron resonance data.

Global microscopic nuclear level densities within the HFB plus combinatorial method for practical applications

New energy-, spin- and parity-dependent nuclear level densities based on the microscopic combinatorial model are proposed for practical applications. The combinatorial model includes a detailed microscopic calculation of the intrinsic state density and of the rotational enhancement factor, but a phenomenological treatment of the vibrational effect. The calculations make a coherent use of nuclear structure properties determined within the deformed Skyrme–Hartree–Fock–Bogolyubov framework. The present model predicts the experimental s-wave 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 reliable extrapolations to low energies where experimental data on the cumulative number of levels can be extracted. Level densities for more than 8500 nuclei are made available in a table format for practical applications.