Statistical Decay Model Research Articles

Connection of four-dimensional Langevin model and Hauser-Feshbach theory to describe statistical decay of fission fragments

ABSTRACT We have developed a novel theoretical method to obtain independent fission product yields and prompt neutron observables by connecting mass and total kinetic energy (TKE) distributions calculated by a four-dimensional Langevin dynamical model to a Hauser-Feshbach statistical decay model. In the Langevin calculations, mass distributions corresponding to the standard I and II modes were obtained separately and superposed to reproduce the fission fragment yield of spontaneous fission of 238 , 240 , 242 Pu and thermal neutron-induced fission of 239 Pu . This was achieved by using different neck parameters for these two modes in the two-center shell model shape parametrization, and a systematics of the superposing ratio was obtained as a function of N − Z / A of the fissioning nuclei. The Hauser-Feshbach calculations were performed using a nuclear reaction code TALYS for 239 Pu n , f reaction in the incident energy range from thermal up to 5 MeV , and the calculated prompt fission observables were compared with experimental and evaluated data. Although further improvements are needed for the most important nuclides, it turned out that the present methodology has the capability to prepare fission-related nuclear data for nuclides for which measurements are difficult.

TALYS calculations of prompt fission observables and independent fission product yields for the neutron-induced fission of ^{235}U

The TALYS nuclear reaction code’s Hauser-Feshbach statistical decay model has been adapted in order to calculate prompt fission neutron and gamma -ray observables by iterating over deexciting fission fragments. Several fission fragment generators such as GEF, HF^{3}D, and SPY were employed to provide TALYS with databases. These databases contain standardized tables with fission fragment yields, mean excitation energies and their widths, and average total kinetic energy, as a function of charge and mass number of primary fission fragments. The resulting calculations, including prompt particle multiplicities, spectra, average energies, and independent fission product yields, were compared with experimental and evaluated data. This work first outlines the new methodology implemented in TALYS and examines the effects of three important parameters on the final evaporation data. Furthermore, the neutron-induced fission of ^{235}U is investigated in detail as a function of incident energy. The results from TALYS, with input from GEF and HF^{3}D, were compared with available experimental data and the results of the stand-alone GEF code. The proposed methodology contributes to an improved capability to model the fission process.

Influence of pre-scission neutron emission on high-energy U238 fission studied by the Langevin approach

Background: The information about the pre-scission emitted neutrons is crucial for the fission dynamics study such as fission time scale and dissipation mechanism. Purpose: The aim is to investigate the influence of the pre-scission neutron emission on the fragments mass, total kinetic energy, and the deformation distribution at the scission point in $^{238}\mathrm{U}(n,f)$ fissions at 110, 325, and 500 MeV. Method: A three-dimensional Langevin approach considering nucleus elongation, deformation, and mass asymmetry is applied to simulate fission dynamics. The Hauser-Feshbach statistical decay model is coupled to simulate the pre-scission neutron emission. Results: The properties of the pre-scission emitted neutron are first investigated. It is found that the multiplicity and the average kinetic energy increase with increasing excitation energy. The energy carried away by neutron emission is not negligible and it leads to the multichance fission, especially for the 110 MeV case. The fragments mass, total kinetic energy, and the deformation distribution at the scission point are then calculated with and without coupling the Hauser-Feshbach statistical decay model. It is found that the calculations partially reproduce the leftward shift of the experimental fragment mass distribution at high excitation energy. The pre-scission neutron emission also causes a reduction of the average total kinetic energy by about 2 MeV and a leftward shift of the distribution. The deformation distribution shows that about 75% of the fragments are produced in spherical and prolate ellipsoid shapes. No significant influence on deformation at the scission point was found. Conclusions: For high-energy fission, the energy reduction by pre-scission neutron emission should be considered, which will lead to the multichance fission. The pre-scission neutron emission is one of the reasons for the decrease of the average mass and the leftward shift of the fission fragments mass and total kinetic energy distribution.

Lagrangian Integro-Difference Equation Model for Precipitation Nowcasting

Abstract Delivering reliable nowcasts (short-range forecasts) of severe rainfall and the resulting flash floods is important in densely populated urban areas. The conventional method is advection-based extrapolation of radar echoes. However, during rapidly evolving convective rainfall this so-called Lagrangian persistence (LP) approach is limited to deterministic and very short-range nowcasts. To address these limitations in the 1-h time range, a novel extension of LP, called Lagrangian Integro-Difference equation model with Autoregression (LINDA), is proposed. The model consists of five components: 1) identification of rain cells, 2) advection, 3) autoregressive process describing growth and decay of the cells, 4) convolution describing loss of predictability at small scales, and 5) stochastic perturbations to simulate forecast uncertainty. Advection is separated from the other components that are applied in the Lagrangian coordinates. The reliability of LINDA is evaluated using the NEXRAD WSR-88D radar that covers the Dallas–Fort Worth metropolitan area, as well as the NEXRAD mosaic covering the continental United States. This is done with two different configurations: LINDA-D for deterministic and LINDA-P for probabilistic nowcasts. The validation dataset consists of 11 rainfall events during 2018–20. For predicting moderate to heavy rainfall (5–20 mm h−1), LINDA outperforms the previously proposed LP-based approaches. The most significant improvement is seen for the ETS and POD statistics with the 5 mm h−1 threshold. For 30-min nowcasts, they show 15% and 16% increases, respectively, to the second-best method and 48% and 34% increases compared to LP. For the 5 mm h−1 threshold, the increase in the relative operating characteristic (ROC) skill score of 30-min nowcasts from the second-best method is 10%. Significance Statement Delivering reliable forecasts of severe rainfall for the next few hours has a major societal importance. This is particularly true for densely populated urban areas, where flash floods can cause property damage and loss of lives. Such forecasts are conventionally produced by direct extrapolation of weather radar measurements. However, for intense localized rainfall this approach has low prediction ability beyond 30 min. To extend this limit, we propose a novel method that combines machine vision with a statistical model for growth and decay of rainfall. The method is designed for predicting highly localized rain cells and bands. In addition, a stochastic extension for producing probabilistic forecasts is developed. Using several verification metrics, we demonstrate that for predicting moderate to heavy rainfall (5–20 mm h−1), the proposed method has significantly improved forecast skill compared to the reference methods. The evaluation is done by using the NEXRAD WSR-88D that covers the Dallas–Fort Worth urban metroplex with a population of over 7 million. Demonstration of the applicability of LINDA in a larger domain is done by using the NEXRAD radar network that covers the continental United States.

Evolution of tropical cyclone forecasts of dynamical - Statistical Cyclone Prediction System (CPS) over the North Indian Ocean during the decade (2010-2019)

Tropical cyclone (TC) forecasting over the North Indian Ocean (NIO) has been improved significantly during past decade (2010-2019). The improvement is largely attributed to the numerical weather prediction (NWP) models and dynamical-statistical Cyclone Prediction System (CPS). The CPS has five components namely, (i) Genesis Potential Parameter (GPP), (ii) Multi-Model Ensemble (MME) technique for track prediction, (iii) Statistical Cyclone Intensity Prediction (SCIP), (iv) Rapid intensification (RI) and (v) Decay model for intensity forecast after landfall. Performance of CPS and NWP models is assessed to review the improvement during the decade. Forecast analysis revealed low false alarm ratio (0.13), high probability of detection (0.95) and high critical success index (0.84) for GPP. Mean track error of MME was ranged from 67 km at 24h to 246 km at 120 h and about 30% less than NWP models. The MME error has also reduced by 52% to 55% for 24 h to 48 h and 41% to 24% for 72 h to 96 h forecast during 2010-2019. Mean landfall point error was ranged from 31 km at 24 h to 127 km at 120 h and landfall time error was ranged from 2.2 h at 24 h to 8.1 h at 120 h. Mean intensity error of SCIP model was ranged from 8.3 kt at 24 h to 12.6 kt at 120 h. Probabilistic rapid intensification index (RII) achieved a good Brier score (BS) 0.079 for RI forecasting. Mean decaying intensity error (after landfall) was ranged from 6.6 kt at 6 h to 3.3 kt at 24 h. There has been improvement of forecasts for NWP models also but MME outperformed all models. Results demonstrate the role of CPS for improvement of cyclone forecast over the NIO in past decade.

The effects of pre-equilibrium emission and secondary decay on the determination of freeze-out volume at intermediate energies * *Supported by the National Natural Science Foundation of China (11905018) and Scientific and Technological Innovation Programs of Higher Education Institutions of Shanxi Province, China (2019L0908)

The effects of pre-equilibrium emission and secondary decay on the determination of the freeze-out volume are investigated using the isospin-dependent quantum molecular dynamics model accompanied by the statistical decay model GEMINI. Small-mass projectiles and large-mass targets with central collisions are studied at intermediate energies. It is revealed that the proton yields of pre-equilibrium emission are smaller than those of secondary decay. However, the determination of the freeze-out volume from the proton yields is more easily affected by pre-equilibrium emission. Moreover, the percentage of proton yields in the freeze-out stage is found to be approximately 50%.

Influence of nuclear thermometers and hot nuclei properties on nuclear temperature isospin effect measurement

The influence of nuclear thermometers and hot nuclei properties on nuclear temperature isospin dependence is studied. A systematic study of the nuclear temperature isospin effect is presented for heavy-ion collisions at moderate energy via the isospin-dependent quantum molecular dynamics model in the company of the statistical decay model (GEMINI). It is found that the isospin dependence of nuclear temperatures which is calculated by a momentum quadrupole fluctuation thermometer is more easily affected by hot nuclei mass range. However, the isospin dependence of nuclear temperatures which is calculated by an isotope yields ratio thermometer is more easily affected by neutron-proton ratio range of the hot nuclei. It is also found that the isospin dependence of nuclear temperatures can be changed by hot nuclei properties.

Influence of secondary decay on chemical freeze-out temperature isospin effect measurement

The influence of sequential decay on isospin dependence of isotope yield ratio temperatures is studied and a systematic study of isotope temperatures is presented for heavy-ion collisions at 600 MeV/nucleon via the isospin-dependent quantum molecular dynamics model in the company of the statistical decay model (GEMINI). Calculations show that the isospin dependence of the isotope yield ratio temperature is not affected by secondary decay.

Using CGMF to estimate corrections for fission yields measured via γ-ray spectroscopy

Fission product yields have been inferred using γ-ray spectroscopy for several decades. Typically, these efforts have focused on even-Z even-A fission products as their nuclear structure are less complicated. To further simplify the situation, it is often assumed that no side-feeding to the ground-state occurs and multiplicity cuts have a negligible effect on the inferred yields. Using CGMF, a Hauser-Feshbach statistical decay model for the primary fission fragments, we estimate the impact of these assumptions and determine corrections for specific fission product yields. We report on these corrections and investigate their sensitivity to various nuclear parameters, specifically the spin distribution of the fission fragments and the assumed nuclear structure. Our results indicate that even in the simplest of cases, say the 2+ → 0+ transitions in even-Z even-A fragments, average level corrections are on the order of 75%.

The odd–even effect of fragmentation cross sections produced in the de-excitation process

The odd–even effect of fragmentation cross sections is studied by improved quantum molecular dynamics model and the GEMINI statistical decay model for reactions 36Ar and 40Ar on C, Al, Cu and Pb targets at 400 A MeV. The odd–even effect is produced in the de-excitation process. The pairing energy plays an important role for the odd–even effect. The odd–even effect disappears without the pairing energy. The excitation energy of prefragments is researched. The odd–even effect for proton is produced obviously at low excitation energy for 36Ar projectile. The odd–even effect is stronger for the neutron fragmentation than the proton one.

Development of NWP Based Objective Cyclone Prediction System (CPS) for North Indian Ocean Tropical Cyclones — Evaluation of Performance

ABSTRACT An objective NWP based Cyclone Prediction System (CPS) has been developed and implemented at IMD for operational cyclone forecasting over the north Indian Ocean (NIO). The five forecast components of CPS are (a) Genesis Potential Parameter (GPP), (b) Multi-Model Ensemble (MME) technique for track prediction, (c) Statistical Cyclone Intensity Prediction (SCIP), (d) rapid intensification and (e) decay model to forecast intensity after the landfall. Verification shows GPP had higher probability of detection (0.98) and lower false alarm ratio (0.27) with higher critical success index (0.72). Mean track error of MME ranged from 74 km at 12 h to 200 km at 72 h and reduced by 27% to 52% for 36 h to 72 h forecast during 2009-2013. The mean forecast errors of landfall position ranged from 56 km at 24 h to 137 km at 72 h and landfall time error ranged from 3.6 h at 24 h to 6.1 h at 72 h. Mean intensity errors of SCIP ranged from 5.4 kt at 12 h to 16.9 kt at 72 h. The probabilistic rapid intensification forecast was skillful compared to climatology. The 6-hourly decaying intensity (after landfall) errors ranged from 3 kt to 4.9 kt. Results demonstrate the potential of CPS for operational cyclone forecast over the NIO.

High-resolution study of the113Cd(n,γ) spectrum by statistical decay model with discrete levels and transitions

The gamma-ray spectrum and the decay scheme of 114 Cd obtained from a radiative neutron capture experiment on 113 Cd samples are modelled in the framework of extreme statistical model. The unfolding of the experimental spectrum with proper normalization yields a total capture cross section of 21640 b and an average gamma-ray multiplicity of 4.1. Using the extreme statistical model the development of the low energy decay scheme of 114 Cd is in progress. In the model the constant-temperature level density is used, where the temperature parameter was very sensitive to the shape of the modelled gamma-ray spectrum. Using this sensitivity T = 0.62 MeV was obtained with the constraint of good description of low energy level density and the level density at the binding energy. This is in full agreement with our earlier publication. For the description of the continuum shape of the unfolded spectrum the inclusion of low energy enhancement for the photon strength function was an important new addition.

Measurement of aluminum activation cross section and gas production cross section for 0.4 and 3-GeV protons

To estimate the lifetime and the radiation dose of the proton beam window used in the spallation neutron source at J-PARC, it is necessary to understand the accuracy of the production cross section of 3-GeV protons. To obtain data on aluminum, the reaction cross section of aluminum was measured at the entrance of the beam dump placed in the 3-GeV proton synchrotron. Owing to the use of well-calibrated current transformers and a well-collimated beam, the present data has good accuracy. After irradiation, the cross sections of Al(p,x)7 Be, Al(p,x)22 Na-22 and Al(p,x)24 Na were obtained by gamma-ray spectroscopy using a Ge detector. It was found that the evaluated data of JENDL/HE-2007 agree well with the current experimental data, whereas intra-nuclear cascade models (Bertini, INCL-4.6, and JAM) with the GEM statistical decay model underestimate by about 30% in general. Moreover, gas production, such as T and He, and the cross sections were measured for carbon, which was utilized as the muon production target in J-PARC. The experiment was performed with 3-GeV proton having beam power of 0.5 MW, and the gasses emitted in the process were observed using a quadrupole mass spectrometer in the vacuum line for beam transport to the mercury target. It was found that the JENDL/HE-2007 data agree well with the present experimental data.

Investigating nuclear dissipation properties at large deformations via excitation energy at scission

Using the stochastic Langevin model coupled with a statistical decay model, we study nuclear dissipation properties at large deformations with excitation energy at scission (\(E_{\mathrm {sc}}^*\)) measured in experiments. It is found that the postsaddle dissipation strength required to fit \(E_{\mathrm {sc}}^*\) data is 12 \(\times 10^{21}\) s\(^{-1}\) for \(^{254,256} \)Fm and 6 \(\times 10^{21}\) s\(^{-1}\) for \(^{189}\)Au, which has a smaller postsaddle deformation than the former heavy nucleus, showing a rise of nuclear dissipation strength with increasing deformation.

Effect of pre-equilibrium emission on probing postsaddle nuclear dissipation with neutrons**Supported by National Nature Science Foundation of China (11575044)

Using the stochastic Langevin model coupled with a statistical decay model, we study the influence of pre-equilibrium (PE) emission on probing postsaddle friction (β) with neutrons. A postsaddle friction value of (14 − 16.5) × 1021 s−1 and (11 − 13) × 1021 s−1 is obtained from comparing calculated and measured prescission neutron multiplicities of heavy fissioning systems 248Fm and 256Fm in the absence and presence of the deformation factor. Moreover, it is found that a larger β is required to fit multiplicity data after the PE effect is accounted for, and that the effect becomes stronger when more energy is removed by PE particles. Our findings suggest that, to more accurately determine the postsaddle friction strength through the measurement of prescission neutrons, in addition to incorporating the contribution of PE evaporation source into the experimental multi-source analysis for particle energy spectra in coincidence with fission fragments, on the theoretical side, it is very important to make a precise evaluation of the energy that PE emission carries away from excited compound systems produced in heavy-ion fusion reactions.

Probing postsaddle nuclear dissipation with excitation energy at scission

Using the stochastic Langevin model coupled with a statistical decay model, we study postsaddle dissipation properties in fission by analyzing the excitation energy at scission (${E}_{\mathrm{sc}}^{*}$) measured in fissioning nuclei $^{179}\mathrm{Re}$ and $^{254,256}\mathrm{Fm}$. The postsaddle dissipation strength ($\ensuremath{\beta}$) required to fit ${E}_{\mathrm{sc}}^{*}$ data is found to be larger for $^{254,256}\mathrm{Fm}$ than light $^{179}\mathrm{Re}$ which has a smaller postsaddle deformation compared to heavy $^{254,256}\mathrm{Fm}$, showing a rise of nuclear dissipation strength at a greater deformation. Furthermore, we explore the influence of initial excitation energy of a fissioning system $^{246}\mathrm{Cf}$ on the sensitivity of its ${E}_{\mathrm{sc}}^{*}$ to $\ensuremath{\beta}$, and find that the sensitivity is significantly enhanced with increasing the initial excitation energy. Our finding suggests that, on the experimental side, to more accurately probe the postsaddle dissipation strength through the measurement of ${E}_{\mathrm{sc}}^{*}$, it is best to yield those fissioning systems with high energy.

Application of JAERI quantum molecular dynamics model for collisions of heavy nuclei

The quantum molecular dynamics (QMD) model incorporated into the general-purpose radiation transport code PHITS was revised for accurate prediction of fragment yields in peripheral collisions. For more accurate simulation of peripheral collisions, stability of the nuclei at their ground state was improved and the algorithm to reject invalid events was modified. In-medium correction on nucleon-nucleon cross sections was also considered. To clarify the effect of this improvement on fragmentation of heavy nuclei, the new QMD model coupled with a statistical decay model was used to calculate fragment production cross sections of Ag and Au targets and compared with the data of earlier measurement. It is shown that the revised version can predict cross section more accurately.

Analysis of angular distribution of fragments in relativistic heavy-ion collisions by quantum molecular dynamics

To predict angular distribution of fragments produced in nucleusnucleus collisions, JAERI quantum molecular dynamics model (JQMD) was improved. Because JQMD underestimated fragments in the forward angle, which were mainly produced by peripheral collisions, JQMD was revised so as to simulate peripheral collisions accurately. Density-dependent in-medium effect and relativistic effect on nucleonnucleon interactions were incorporated for this purpose. The revised version of JQMD coupled with a statistical decay model was used to calculate differential fragment production cross sections measured in earlier studies. Comparison of the measured data and calculation by the revised and old JQMD showed that the revised JQMD can predict fragment angular distribution better than old JQMD. Particularly, agreement of fragment yield in the forward angle is substantially improved.

Energy-dependent fragmentation cross sections of relativisticC12

The fragment production excitation functions of $^{12}\mathrm{C}$ interacting with carbon nuclei were measured in the energy range of 100--400 MeV/u. To measure cross sections as excitation functions, fragments produced from a thick carbon target bombarded with a monoenergetic $^{12}\mathrm{C}$ ion beam were detected by a telescope detector placed behind the target. By this measurement, eight isotopes---$^{11}\mathrm{B},^{10}\mathrm{B},^{10}\mathrm{Be},^{9}\mathrm{Be},^{7}\mathrm{Be},^{8}\mathrm{Li},^{7}\mathrm{Li}$, and $^{6}\mathrm{Li}$---were identified, and the energy dependence of their production cross sections in the few 100 MeV/u range was obtained. Based on the measured cross sections, the JAERI quantum molecular dynamics model was updated to incorporate the mechanisms particularly important for peripheral collisions. The improved version of the model coupled with a statistical decay model reproduces the measured data typically within a few 10% of difference.

Effect of fragment emission time on the temperature of momentum quadrupole fluctuations

A systematic study of a momentum quadrupole fluctuation thermometer has been presented for heavy-ion collisions at 35 MeV/nucleon via the isospin-dependent quantum molecular dynamics model accompanied by the statistical decay model GEMINI. It is determined that the fragment momentum fluctuation temperature indeed reflects the average temperature of the excitation source in the fragmentation process and the secondary decay. We find that the divergence between the initial temperature and the final measurement temperature is different for protons, tritons, and $^{3}\mathrm{He}$. The maximum divergence of the temperature is about $20%$ probed by protons. As an example of using protons as the probe particle, we find that the isospin dependence of the nuclear temperature is consistent between the initial temperature and the final measurement temperature.