14.77 MeV neutron-induced nuclear reaction cross sections for zinc,yttrium, and molybdenum targets

The cross sections for neutron-induced nuclear reactions on natural zinc, yttrium, and molybdenum targets were measured at a neutron energy of 14.77 ± 0.17 MeV using the activation technique, offline gamma-ray spectrometry, and a detailed covariance analysis. The uncertainty in the statistical model calculations of cross sections for the (n, 2n), (n, p), (n, α), and (n, γ) reactions with natural zinc, yttrium, and molybdenum at neutron energies from 13 to 17 MeV was calculated using the TALYS-1.96 nuclear code. The measured cross sections of the present study were compared with the experimental cross sections reported in the EXFOR database, the cross sections were calculated with the TALYS-1.96 and EMPIRE-3.2.3 nuclear codes and the evaluated nuclear data from the TENDL-2019, JENDL-5, and ENDF/B-VIII.0 libraries.     

The growth statistics of Zipfian ensembles: Beyond Heaps’ law

We consider an evolving ensemble assembled from a set of n different elements via a stochastic growth process in which independent and identically distributed copies of the elements arrive randomly in time, and their statistics are governed by Zipf’s law. The associated “Heaps process” is the stochastic process tracking the fraction of different element copies present in the evolving ensemble at any given time point. For example, the evolving ensemble is a text assembled from a stream of words, and the Heaps process keeps count of the number of different words in the evolving text. A detailed asymptotic statistical analysis of the Heaps process, in the limit n→∞, is conducted. This paper establishes a comprehensive “Heapsian analysis” of the growth statistics of Zipfian ensembles. The analysis presented far extends and generalizes Heaps’ law, which asserts that the number of different words in a text of length l follows a power law in the variable l.     

A test of the zero-range DWBA method at astrophysical energies

The DWBA method is tested through a comparison with a microscopic cluster model on the ¹³C (, n) ¹⁶O reaction in the energy range E = 0-5MeV, relevant for nuclear astrophysics. The conditions of the calculation are as close as possible to the reference model, i.e. the nucleus-nucleus potentials are phase equivalent, and the spectroscopic factors are identical. We find significant differences between both approaches, which means that antisymmetrization effects, missing in the DWBA, are important. Our work also points out the strong sensitivity of the DWBA with input parameters.     

Cumulative fission yield measurements with 14.7 MeV neutrons on 238U

The fission yield data in the 14 MeV energy neutron induced fission of ²³⁸U play an important role in decay heat calculations and generation-IV reactor designs. In order to accurately measure fission product yields (FPYs) of ²³⁸U induced by 14 MeV neutrons, the cumulative yields of fission products ranging from ⁹²Sr to ¹⁴⁷Nd in the ²³⁸U(n, f) reaction with a 14.7 MeV neutron were determined using an off-line γ-ray spectrometric technique. The 14.7 MeV quasi-monoenergetic neutron beam was provided by the K-400 D-T neutron generator at China Academy of Engineering Physics (CAEP). Fission products were measured by a low background high purity germanium gamma spectrometer. The neutron flux was obtained from the ⁹³Nb (n, 2n)^92mNb reaction, and the mean neutron energy was calculated using the cross-section ratios for the ⁹⁰Zr(n, 2n)⁸⁹Zr and ⁹³Nb(n, 2n)^92mNb reactions. With a series of corrections, high precision cumulative yields of 20 fission products were obtained. Our FPYs for the ²³⁸U(n, f) reaction at 14.7 MeV were compared with the existing experimental nuclear reaction data and evaluated nuclear data, respectively. The results will be helpful in the design of a generation-IV reactor and the construction of evaluated fission yield databases.     

Measurements of natCd(γ, x) reaction cross sections and isomer ratio of 115m,gCd with the bremsstrahlung end-point energies of 50 and 60 MeV

The flux-weighted average cross sections of ^natCd(γ, xn)^{115g,m,111m,109,107,105,104}Cd and ^natCd(γ, x)^{113g,112,111g,110m}Ag reactions were measured at the bremsstrahlung end-point energies of 50 and 60 MeV. The activation and off-line γ-ray spectrometric technique was carried out using the 100 MeV electron linear accelerator at the Pohang Accelerator Laboratory, Korea. The ^natCd(γ, xn) reaction cross sections as a function of photon energy were theoretically calculated using the TALYS-1.95 and the EMPIRE-3.2 Malta codes. Then, the flux-weighted average cross sections were obtained from the theoretical values of mono-energetic photons. These values were compared with the flux-weighted values from the present study and were found to be in general agreement. The measured experimental reaction cross-sections and integral yields were described for cadmium and silver isotopes in the ^natCd(γ, xn)^{115g,m,111m,109,107,105,104}Cd and ^natCd(γ, x)^{113g,112,111g,110m}Ag reactions. The isomeric yield ratio (IR) of ^115g,mCd in the ^natCd(γ, xn) reaction was determined for the two bremsstrahlung end-point energies. The measured isomeric yield ratios of ^115g,mCd in the ^natCd(γ, xn) reaction were also compared with the theoretical values of the nuclear model codes and previously published literature data of the ¹¹⁶Cd(γ, n) and ¹¹⁶Cd(n, 2n) reactions. It was found that the IR value increases with increasing projectile energy, which demonstrates the characteristic of excitation energy. However, the higher IR value of ^115g,mCd in the ¹¹⁶Cd(n, 2n) reaction compared to that in the ¹¹⁶Cd(γ, n) reaction indicates the role of compound nuclear spin alongside excitation energy.     

Cross section measurements on zirconium isotopes for ~14 MeV neutrons and their theoretical calculations of excitation functions

The cross sections for the ⁹⁴Zr(n,d*)^93m+gY, ⁹⁶Zr(n,γ)⁹⁷Z, ⁹⁶Zr(n,2n)⁹⁵Zr, ⁹⁰Zr(n,α)^87mSr, ⁹⁴Zr(n,α)⁹¹Sr,⁹⁰Zr(n,p)^90mY, ⁹²Zr(n,p)⁹²Y, and ⁹⁴Zr(n,p)⁹⁴Y reactions have been measured in the neutron energy range of 13.5-14.8 MeV by means of the activation technique. The neutrons were produced via the D-T reaction. A high-purity germanium detector with high energy resolution was used to measure the induced γ activities. In combination with the nuclear reaction theoretical models, the excitation curves of the above-mentioned eight nuclear reactions within the incident neutron energy range from the threshold to 20 MeV were obtained by adopting the nuclear theoretical model program system Talys-1.9. The resulting experimental cross sections were analyzed and compared with the experimental data from published studies. Calculations were performed using Talys-1.9 and are in agreement with our experimental results, previous experimental values, as well as results of the theoretical excitation curves at the corresponding energies. The theoretical excitation curves generally match the experimental data well.     

Manifestation of the nuclear viscosity effects in induced fission reactions at excitation energies below 30 MeV

A large set of experimental observables for the ²³²Th(α, xnf)reaction was analyzed theoretically within the dynamic-statistical approach, making it possible to interconsistently consider the manifestation of nuclear viscosity, the double-humped structure of the fission barrier, and the phenomenon of shell effect damping with temperature. Analyses were performed for the energy dependence of the finite lifetime effect in the investigated reaction, obtained using the crystal blocking technique; the fission probability isotopes produced in this reaction during the development of a neutron emission cascade; and the anisotropy of angular distributions of fission fragments. It is shown that this analysis allows us to obtain information regarding nuclear viscosity and its energy dependence at relatively low excitation energies (<30 MeV).     

Cross-section measurements of (n, 2n) and (n, p) reactions on 124,126,128,130,131,132Xe in the 14 MeV region and theoretical calculations of their excitation functions

The reaction cross-sections of ¹²⁴Xe(n, 2n)¹²³Xe, ¹²⁶Xe(n, 2n)¹²⁵Xe, ¹²⁸Xe(n, 2n)¹²⁷Xe, ¹³⁰Xe(n, 2n)^129mXe, ¹³²Xe(n, 2n)^131mXe, ¹³⁰Xe(n, p)¹³⁰I, ¹³¹Xe(n, p)¹³¹I, and ¹³²Xe(n, p)¹³²I were measured at the 13.5, 13.8, 14.1, 14.4, and 14.8 MeV neutron energies. The monoenergetic neutrons were generated via the ³H(d,n)⁴He reaction at the China Academy of Engineering Physics using the K-400 Neutron Generator with a solid ³H-Ti target. A high-purity germanium detector was employed to measure the activities of the product. The reactions ⁹³Nb(n, 2n)^92mNb and ²⁷Al(n, α)²⁴Na were adopted for neutron flux calibration. The cross sections of the (n, 2n) and (n, p) reactions of the xenon isotopes were obtained within the 13–15 MeV neutron energy range. These cross-sections were then compared with the IAEA-exchange format (EXFOR) database-derived experimental data, together with the evaluation results of the CENDL-3, ENDF/B-VIII.0, JENDL-4.0, RUSFOND, and JEFF-3.3 data libraries, as well as the theoretical excitation function obtained using the TALYS-1.95 code. The cross-sections of the reactions (except for the ¹²⁴Xe(n, 2n)¹²³Xe and ¹³²Xe(n, p)¹³²I) at 13.5, 13.8, and 14.1 MeV are reported for the first time in this study. The obtained results are beneficial in providing better cross-section constraints for the reactions in the 13–15 MeV region, thus improving the quality of the corresponding database. Meanwhile, these data can also be used for the verification of relevant nuclear reaction model parameters.     

Molecular dynamics simulation of the vapour → liquid transition of argon and the reaction coordinate of condensation

We have investigated the n-dependences of the rate constants of absorption and emission of monomers that are attached to and detached from the cluster of n monomers, and have determined n*, the number of monomers that form the critical nucleus of the homogeneous condensation of the Lennard-Jones?Ar vapour. The dynamics was clearly separated into two regions at the critical nucleus; n* did not, however, give the unique dividing point. The observed strong dependences of both the nucleus size and the barrier height of the nucleation on the induction time of the condensation suggest that a slowly changing variable instead of the cluster size is necessary as the reaction coordinate of the nucleation. Although the slow variable is not well characterised at the present stage of the investigation, it seems to be related to the local density of the gas atoms around the liquid-like clusters. Both the slow evolution of the radial distribution function of the gas atoms around the liquid-like atoms, and the correlation between n* and the onset of the condensation indicate that Gibbs energy curves represented in n-space change significantly with the activation of the slow variable.     

一维快装置中心反应率模拟计算的灵敏度和不确定度分析

采用广义微扰理论和离散纵标输运计算方法,推导了中心反应率比值对重要核反应参数的灵敏度的计算公式。以一维快基准装置Jezebel-Pu239为例,计算了其中心反应率比值对关键核数据的灵敏度,并结合多群协方差数据,分析了其中心反应率比值计算中由于核数据的不确定度所引入的不确定度及其来源。根据计算结果以及评价数据的协方差信息,建议对Pu-239的非弹截面数据进行更精细的评价,以期降低反应率比的模拟计算不确定度。On the base of the generalized perturbation theory and SN method, the theoretical model for calculating the sensitivity and uncertainty of reaction rates ratio (RRR) versus nuclear data is developed. Taking Jezebel-Pu239 critical benchmarking facility as an example, the sensitivity of F49/F25 ratio versus important nuclear data is calculated, as well as the uncertainty arising from the nuclear data. Basing on the sensitivity and uncertainty analysis as well as the covariance information of input evaluated data, refined evaluation of the inelastic data of Pu-239 is considered as a feasible measure to reduce the uncertainty of reaction rates ratios.     

Calorimetry

Methods for determining the heat content E ^*/A of hot nuclei formed in energetic nuclear reactions are discussed. The primary factors involved in converting raw data into thermal physics distributions include: 1) design of the detector array, 2) constraints imposed by the physics of the reaction mechanism, and 3) assumptions involved in converting the filtered data into E ^*/A. The two primary sources of uncertainty in the calorimetry are the elimination of nonequilibrium emissions from the event components and accounting for the contribution of neutron emission to the excitation energy sum.     

Evaluation of structural and electrochemical properties of the MnSb–Li system as anode for Li-ion batteries

The electrochemical reaction mechanisms between lithium and cystalline MnSb are investigated by X-ray diffraction, ¹²¹Sb Mössbauer spectroscopy, and X-ray absorption spectroscopy (XAS). The analysis of the experimental data at different depths of the electrochemical discharge process reveals a complex reaction mechanism comprising two steps. The main reaction of the first step corresponds to the dispersion of lithium in the MnSb matrix with formation of the intermediate compound LiMnSb. The second step corresponds to a Li–Sb alloying process with formation of Li₃Sb.     

Possibility of implementing a nondirect experiment aimed at determining the cross section for nucleon interaction with an excited nucleus

With the aid of experimental data on the level densities in the even-odd tungsten isotopes ^183,185,187W as determined in the respective (n, 2ρ) reactions, the ratio of the cross section for the interaction of an excited nucleus with a neutron to its counterpart calculated on the basis of the optical model of the nucleus was obtained from the spectra of evaporated neutrons in the reaction ¹⁸¹Ta(p, n)¹⁸¹W. A significant local increase in this ratio is qualitatively interpreted as that which is due to the possible increase in the penetrability of the nuclear surface for nucleon reaction products because of the effect of phonon-type excitations. Sources of possible nonstandard systematic errors in experimentally determining the level density in an arbitrary nucleus at energies below the nucleon binding energy B _n are analyzed. The extraction of information from the distribution of the intensities of cascades belonging to the nucleon and photon type is simulated. The resulting information may be highly reliable if use is made of the entire body of experimental data on the nucleus under analysis.     

Classical impact parameter approximation: Application to atomic collisions with n independent electrons and to recoil analysis

We show that a classical Impact Parameter Method may be derived when taking fully into account the smallness of the ratio between the electron and nuclear masses. It allows to calculate, exactly as in the quantum version, projectile scattering and therefore recoil momenta required for the interpretation of recent measurements. We prove an additivity theorem which allows, in particular, to reduce the n-non-interacting electron problem to a set of n one-electron problems. Consequences for the interpretation of target recoil measurements are discussed. Received 25 June 1999     

Theoretical studies on [2 + 2 + 2] reaction mechanisms of three ethynes. More accurate estimation of activation energy

The mechanisms of the [2 + 2 + 2] cycloaddition reaction of three ethyne molecules were studied by ab initio molecular orbital and density functional methods. The transition states range from that of the concerted mechanism with D_3h symmetry to that of the stepwise mechanism with C₂ symmetry. The transition state structure and the activation energy depend on the basis set and computational method employed in the analysis. The activation energy barrier was determined to be in the range of 36–44 kcal/mol. The activation energy determined by various methods corresponds to the interaction energy, which is related to the electron correlation energy. The best estimation of the activation energy barrier is 41.6 kcal/mol, achieved from the relation between the interaction energy and the activation energy. Copyright © 2013 John Wiley & Sons, Ltd.     

Influence of the medium on the electromagnetic radiation spectrum of highly excited atoms and molecules

The influence of neutral species in the E- and D-layers of the Earth’s upper atmosphere on the spectrum of the spontaneous emission (absorption) of Rydberg atoms and molecules for transitions that occur without changing the principal quantum number (Δn = 0) is examined. Along with the process of l-mixing, the splitting of orbitally degenerate states due to interaction with perturbing neutral species of the medium is taken into account. The possible types of radiative transitions between them are analyzed. It is demonstrated that, for principal quantum numbers of n = 10–30, decimeter-band radiation corresponds to transitions between the levels of split states, whereas meter-band radiation, to transitions between their individual components. It is established that, for these values of n, the ratios of the intensities of the decimeter and meter bands for Δn = 0 transitions to the intensity of IR radiation (Δn = 1) are 10⁻⁴ and 10⁻⁶, respectively. The issue of satellite signal phase shift because of multiple Raman scattering in the D-layer of the atmosphere is discussed.     

Determination of the half-life of 184gRe and the isomeric ratio of 184m,gRe induced by D-T neutrons

The isomeric ratio of ^{184m, g}Re and the half-life of ^184gRe were measured in the ¹⁸⁵Re (n, 2n) ¹⁸⁴Re reaction at 14.8 MeV, and the uncertainty was discussed in detail. The measurements were performed using the activation method implemented for a rhenium sample using the K-400 neutron generator at the Chinese Academy of Engineering Physics (CAEP). Isomeric state and ground state nuclei of ¹⁸⁴Re were identified by their γ-ray spectra. To eliminate the effect of the γ-ray emitted from the isomer on the counting of the ground state characteristic peaks, the isomeric ratio of ^{184m, g}Re was calculated to be 0.29 ± 0.11 according to the neutron activation cross-section formula. This result is consistent with previous data within the uncertainty and can be used to determine parameters that characterize the dependence of the level density on the excitation energy and angular momentum. Through exponential function fitting and a detailed discussion of the uncertainty evaluation, the half-life of ^184gRe was determined as 35.43 ± 0.16 d, which is consistent with the currently recommended value; however, the uncertainty assessment of the latter was barely documented. In addition, this study indicates that the half-life of the ground state can be obtained by eliminating the contamination of γ-rays emitted from the isomer, which provides the possibility of determining the half-lives of nuclides containing isomers.