Fission Neutron Energy Research Articles

Development and Verification of Sampling Code NECP-SOUL for Evaluated Nuclear Data Files in ENDF-6 Format

A code called NECP-SOUL is developed to generate samples of evaluated nuclear data files in the ENDF-6 format. The motivation for the development is to obtain more accurate uncertainty quantification results based on the covariance data given in evaluated nuclear data files than the sampling method based on multigroup covariances. NECP-SOUL can handle all covariance data in ENDF-6 files, including average fission neutron multiplicities, resonance parameters, cross sections, angular distributions, energy distributions, and cross sections for the production of radioactive nuclides. In this work, samples of ENDF-6 files are input to the nuclear data processing code to generate A Compact ENDF (ACE)-formatted libraries, and these ACE-formatted libraries are used by the Monte Carlo code. Through statistical analysis, the uncertainty of the neutronics results can be obtained. NECP-SOUL is verified against another similar code, SANDY. In addition, the advantage of sampling ENDF-6 files is analyzed by comparing the uncertainty quantification results with those obtained by sampling multigroup covariance matrices. Moreover, the covariances of cross sections and angular distributions newly provided in ENDF/B-VIII.0 are used to analyze their effect on the uncertainty qualification results.

CGMF & FREYA Verification in Monte Carlo Code RMC

Monte Carlo simulation has become a crucial method internationally for simulating neutron multiplicity counting devices. This method requires sampling of fission neutron multiplicities, energies, and directions, making accurate simulation of fission events highly important. To meet this demand, the Fission Reaction Event Yield Algorithm (FREYA) and the Cascading Gammaray Multiplicity with Fission (CGMF) models have been integrated into RMC. These models can simulate individual fission events, preserving momentum, energy, and angular momentum, and then emit correlated particles. In addition to simulating fission neutrons, CGMF and FREYA models have been employed to compute fission photons, laying the foundation for subsequent photon multiplicity counting. Simultaneously, neutron multiplicity counting has been calculated using CGMF and FREYA, and point model equations have been used to invert Pu material, yielding favorable results.

Potassium-39-derived 36Ar production during fission-neutron irradiation and its effect on 40Ar/39Ar ages

Various interference reactions producing unwanted Ar isotopes from K, Ca, Cl and Ar require correction to satisfy the 40Ar/39Ar age equation. Using GEANT4, we design and build a model Cadmium Lined In Core Irradiation Tube (CLICIT) irradiation facility, as used in the Oregon State TRIGA Reactor (OSTR). We illustrate the complexity of the irradiation of geologic samples within this framework and determine an overlooked production channel of 36Ar. The production of 36Ar is fed from the 39K(n,α)36Cl nuclear channel, 36Cl subsequently decays to 36Ar (39K(n,α,β) 36Ar). Simulations in this work using a 235U fission neutron energy spectrum and modelled CLICIT facility, determine a production ratio for this reaction (36Cl/39Ar)K = 0.40 ± 0.01 (1σ); greater than an order of magnitude larger than any other K interference. The magnitude of the resulting age bias for an unknown sample will be a function of the integrated neutron flux, the length of irradiation (fluence), the time elapsed since irradiation, and the age relationship between the unknown and neutron fluence monitor. We show using the raw data of (Niespolo et al., 2017) that the age of Alder Creek sanidine can be modified to be ca. 0.1% older (1σ), at the 2σ level of current analytical precision for the Alder Creek age for this study. The 39K(n,α,β)36Ar inference should be incorporated into routine data analysis and may be especially important in the intercalibration of the 40Ar/39Ar system with other chronometers (e.g., 206Pb/238U).

Intense γ -Photon and High-Energy Electron Production by Neutron Irradiation: Effects of Nuclear Excitations on Reactor Materials

The effects of neutron irradiation on materials are often interpreted in terms of atomic recoils, initiated by neutron impacts and producing crystal lattice defects. In addition, there is a remarkable two-step process, strongly pronounced in the medium-weight and heavy elements. This process involves the generation of energetic {\gamma} photons in nonelastic collisions of neutrons with atomic nuclei, achieved via capture and inelastic reactions. Subsequently, high-energy electrons are excited through the scattering of {\gamma} photons by the atomic electrons. We derive and validate equations enabling a fast and robust evaluation of photon and electron fluxes produced by the neutrons in the bulk of materials. The two-step n-{\gamma}-e scattering creates a nonequilibrium dynamically fluctuating steady-state population of high-energy electrons, with the spectra of photon and electron energies extending well into the mega-electron-volt range. This stimulates vacancy diffusion through electron-triggered atomic recoils, primarily involving vacancy-impurity dissociation, even if thermal activation is ineffective. Tungsten converts the energy of fusion or fission neutrons into a flux of {\gamma} radiation at the conversion efficiency approaching 99%, with implications for structural materials, superconductors, and insulators, as well as phenomena like corrosion, and helium and hydrogen isotope retention.

Experimental Prompt Fission Neutron Spectra Comparisons for the 235,238U(n, f) and 239Pu(n, f) Reactions

The Chi-Nu project to measure prompt fission neutron energy spectra for the major actinides has now completed two measurements, for neutron-induced fission of 239Pu and 235U for incident neutron energies from 1 to 20 MeV, and has almost completed the prompt fission neutron spectrum measurement for 238U. In addition, similar data have been taken for spontaneous fission in some other isotopes, and a measurement of fission neutron spectra from 240Pu(n, f ) is in progress. These measurements are done at the same facility, with the same equipment and analyzed in a similar way. A useful way to look at such data is to examine ratios of the prompt fission neutron spectra among actinides, as some of the experimental uncertainties involved in these data are the same from isotope to isotope, making the ratios of fission neutron spectra less dependent on the experimental and analysis details. We discuss here the ratios of prompt fission neutron spectra between 239Pu(n, f ) and 235U(n, f ) , as well as the evolution of the mean fission neutron energy for 239Pu(n, f ) and 235,238U(n, f ) with increasing incident neutron energy.

Prompt neutrons in correlation with fission fragments from U235(n,f)

Improved knowledge of prompt emission in fission has significant implications for our understanding of the fission process. It is also important for improving nuclear data evaluation, with impact on applications. The correlation between prompt neutrons and fragments emitted in binary neutron induced fission of $^{235}\mathrm{U}$ has been studied at the GELINA facility. The experiment employs an array of proton recoil scintillators and a position-sensitive twin ionization chamber. This experimental arrangement permits measurement of the correlations between neutron emission, fragment angle, and mass and energy of the fission fragments. In this article, we present results on prompt fission neutron energy and multiplicity correlations with mass and total kinetic energy of the fission fragments. Results from the present measurement shows distinct differences compared to earlier studies of the correlated fission fragment and prompt neutron emission quantities. The differences with respect to earlier investigations are interpreted as improved fission fragment energy resolution in the present measurement. The present result supports several recent model calculations of prompt neutron and fission fragment correlations.

A comparison of Monte Carlo fission models for safeguards neutron coincidence counters

Monte Carlo simulation is a powerful tool used to model neutron coincidence detectors for international safeguards. The simulation has typically sampled properties such as the fission neutron multiplicity, energy, and direction, from independent probability density functions. However, multiplicity counters detect event-based neutron correlations and thus more accurate fission event modeling is needed. To respond to this need, the Fission Reaction Event Yield Algorithm (FREYA) and the Cascading Gamma-ray Multiplicity with Fission (CGMF) models were added in the newest version of MCNP, MCNP6.2. The models simulate individual fission events conserving momentum, energy, and angular momentum such that correlated particles are emitted.The effects of the new models on simulations of safeguards neutron coincidence counters were studied and compared to standard MCNP simulations. The MCNPX-PoliMi model was also included in the comparison. The properties of fission neutrons from safeguards relevant isotopes were compared to literature references. Then a hypothetical simplified detector was modeled to isolate the effects of specific differences between models. Experimental measurements from previous work were modeled and agreements were compared. Finally, the probabilities of correlated events occurring in the experimental measurements were calculated with the different models. For example, the probability was calculated of detecting neutrons from both induced fission in uranium and spontaneous fission of Cf-252 in the same fission chain.

Impact of fission neutron energies on reactor antineutrino spectra

Recent measurements of reactor-produced antineutrino fluxes and energy spectra are inconsistent with models based on measured thermal fission beta spectra. In this paper, we examine the dependence of antineutrino production on fission neutron energy. In particular, the variation of fission product yields with neutron energy has been considered as a possible source of the discrepancies between antineutrino observations and models. In simulations of low-enriched and highly-enriched reactor core designs, we find a substantial fraction of fissions (from 5% to more than 40%) are caused by non-thermal neutrons. Using tabulated evaluations of nuclear fission and decay, we estimate the variation in antineutrino emission by the prominent fission parents U-235, Pu-239, and Pu-241 versus neutron energy. The differences in fission neutron energy are found to produce less than 1% variation in detected antineutrino rate per fission of U-235, Pu-239, and Pu-241. Corresponding variations in the antineutrino spectrum are found to be less than 10% below 7 MeV antineutrino energy, smaller than current model uncertainties. We conclude that insufficient modeling of fission neutron energy is unlikely to be the cause of the various reactor anomalies. Our results also suggest that comparisons of antineutrino measurements at low-enriched and highly-enriched reactors can safely neglect the differences in the distributions of their fission neutron energies.

First Results on 238U(n,f) Prompt Fission Neutron Spectra from 1 to 200 MeV incident neutron energy

A new 238U(n,f) prompt fission neutron spectra (PFNS) measurement has been recently performed at the WNR facility of the Los Alamos National Laboratory. The measurement allows one to explore the dependence of the prompt fission neutron energy spectra on the incident neutron energy. The experimental setup couples the Chi-Nu scintillator array to a newly developed fission chamber, characterized by an improved alphafission discrimination and time resolution, a reduced amount of matter in the neutron beam and a higher actinide mass. The dedicated setup and the high statistics collected allow us to obtain a good precision on the measured fission neutron energy, as well as to explore the low energy region, down to 650keV, and the high energy region, above 5 MeV, of the emitted neutron spectrum. These are indeed the regions where discrepancies in the evaluated PFNS data are found. We present here the first preliminary results of the experiment.

What Is the Real PFNS of 235U at Thermal Point?

The traditional assumption of prompt fission neutron spectra (PFNS) integrated over emission angle applies for any calculation of the neutron interaction inside fissile material. Only these evaluated data are included in any neutron data library. But this is not correct. Prompt fission neutrons have very strong angular energy distribution relative to fission fragment (FF) direction. The FFs have anisotropy relative to direction of incident neutrons. What is the influence of this assumption or simplification? Results of Monte Carlo simulation are submitted in this paper. The incorporation of “real” angular energy distribution changes the yield of 238U fission, and this difference may be compensated by changing the average energy of PFNS in the traditional approach. This effect is connected with correlations between different characteristics of interacted neutrons inside the environment. An additional type of correlation between multiplicity and energy of fission neutrons, named ν-E correlation, is also discussed.

Prompt fission neutron energy spectrum of $^{252}Cf$ in center of mass and laboratory systems

In this work we discuss the influence of different forms of prompt fission neutron energy spectra in the fission fragment center of mass system on the laboratory spectrum of neutrons in spontaneous fission of $^{252}Cf$. We show that the Le Couteur spectrum, which takes into account multiple neutron emission of neutrons in the center of mass system, describes the observed neutron energy spectrum well when transformed to the laboratory system.

New features and improved uncertainty analysis in the NEA nuclear data sensitivity tool (NDaST)

Following the release and initial testing period of the NEA’s Nuclear Data Sensitivity Tool [1], new features have been designed and implemented in order to expand its uncertainty analysis capabilities. The aim is to provide a free online tool for integral benchmark testing, that is both efficient and comprehensive, meeting the needs of the nuclear data and benchmark testing communities. New features include access to P1 sensitivities for neutron scattering angular distribution [2] and constrained Chi sensitivities for the prompt fission neutron energy sampling. Both of these are compatible with covariance data accessed via the JANIS nuclear data software, enabling propagation of the resultant uncertainties in k eff to a large series of integral experiment benchmarks. These capabilities are available using a number of different covariance libraries e.g., ENDF/B, JEFF, JENDL and TENDL, allowing comparison of the broad range of results it is possible to obtain. The IRPhE database of reactor physics measurements is now also accessible within the tool in addition to the criticality benchmarks from ICSBEP. Other improvements include the ability to determine and visualise the energy dependence of a given calculated result in order to better identify specific regions of importance or high uncertainty contribution. Sorting and statistical analysis of the selected benchmark suite is now also provided. Examples of the plots generated by the software are included to illustrate such capabilities. Finally, a number of analytical expressions, for example Maxwellian and Watt fission spectra will be included. This will allow the analyst to determine the impact of varying such distributions within the data evaluation, either through adjustment of parameters within the expressions, or by comparison to a more general probability distribution fitted to measured data. The impact of such changes is verified through calculations which are compared to a ‘direct’ measurement found by adjustment of the original ENDF format file.

Improved fission neutron energy discrimination with 4He detectors through pulse filtering

This paper presents experimental and computational techniques implemented for 4He gas scintillation detectors for induced fission neutron detection. Fission neutrons are produced when natural uranium samples are actively interrogated by 2.45MeV deuterium-deuterium fusion reaction neutrons. Fission neutrons of energies greater than 2.45MeV can be distinguished by their different scintillation pulse height spectra since 4He detectors retain incident fast neutron energy information. To enable the preferential detection of fast neutrons up to 10MeV and suppress low-energy event counts, the detector photomultiplier gain is lowered and trigger threshold is increased. Pile-up and other unreliable events due to the interrogating neutron flux and background radiation are filtered out prior to the evaluation of pulse height spectra. With these problem-specific calibrations and data processing, the 4He detector's accuracy at discriminating fission neutrons up to 10MeV is improved and verified with 252Cf spontaneous fission neutrons. Given the 4He detector's ability to differentiate fast neutron sources, this proof-of-concept active-interrogation measurement demonstrates the potential of special nuclear materials detection using a 4He fast neutron detection system.

Prompt fission neutron spectra in fast-neutron-induced fission ofU238

Prompt fission neutron spectrum (PFNS) measurements for the neutron-induced fission of $^{238}\mathrm{U}$ are carried out at incident neutron energies of 2.0, 2.5, and 3.0 MeV, respectively. The time-of-flight technique is employed to determine the energy of fission neutrons. The prompt fission neutron energy spectra so obtained are analyzed using Watt parametrization to derive the neutron multiplicity and average prompt fission neutron energy. The present experimental PFNS data are compared with the evaluated spectra taken from the ENDF/B-VII.1 library and the predictive calculations carried out using the empire-3.2 (Malta) code with built-in Los Alamos (LA) and Kornilov PFNS models. The sensitivity of the empire-3.2 LA model--calculated PFNS to the nuclear level density parameter of the average fission fragment and to the total kinetic energy is investigated. empire-3.2 LA model PFNS calculations that use Madland 2006--recommended values [D. G. Madland, Nucl. Phys. A 772, 113 (2006)] of the total kinetic energy and the level density parameter $a=A/(10\ifmmode\pm\else\textpm\fi{}0.5)$ compare very well to measured data at all incident neutron incident energies.

A multiple parallel-plate avalanche counter for fission-fragment detection

A new low-mass multiple gas-filled parallel-plate avalanche counter for the fission-fragment detection has been developed to mark the fission occurrence in measurements of the prompt fission neutron energy spectrum as a function of incident neutron energy. It was used successfully for the neutron-induced fission of 235U and 239Pu with a total mass near 100mg each and the spontaneous fission of 252Cf. Both the incident neutron energy and the prompt fission neutron energy are measured by using the time-of-flight method. The design and performance of this avalanche counter are described.

Uranium and Plutonium Average Prompt-fission Neutron Energy Spectra (PFNS) from the Analysis of NTS NUEX Data

In neutron experiments (NUEX) conducted at the Nevada Test Site (NTS) by Los Alamos National Laboratory, the time-of-flight of fission-neutrons emitted from nuclear tests were observed by measuring the current generated by the collection of protons scattered from a thin CH2 foil many meters from the nuclear device into a Faraday cup. The time dependence of the Faraday cup current is a measure of the energy spectrum of the neutrons that leak from the device. With good device models and accurate neutron-transport codes, the leakage spectra can be converted into prompt fast-neutron-induced fission-neutron energy spectra. This has been done for two events containing plutonium, and for an earlier event containing uranium. The prompt-fission neutron spectra have been inferred for 1.5-MeV 239Pu(n,f) and 235U(n,f) reactions for outgoing neutron energies from 1.5 to ∼10.5 MeV, in 1-MeV steps. These spectra are in good agreement with the Los Alamos fission model.

The cross-section investigation of (n, α) reaction on chromium isotopes

Cross-sections of the (n, α) reaction on chromium isotopes are of great importance for determining the radiation hardness of stainless steel, which is widely used in nuclear reactors. The EXFOR library contains a limited amount of the relevant experimental data, mainly for the reactions induced by neutrons with an energy around 14 MeV. There are no experimental data on the probability of the (n, α) reaction at the fission neutron energies. In this work we present results of investigating the 50Cr(n, α) and 52Cr(n, α) reactions. It is shown that the ENDF/B VII library evaluations for the 50Cr(n, α) reaction are about 20 times as high as our experimental data. In addition, a structure that was predicted in none of the libraries is observed in the excitation function of the 50Cr(n, α) reaction. The 52Cr(n, α) reaction cross-sections at reactor neutron energies are obtained for the first time.

Neutron-induced235U fission spectrum measurements using liquid organic scintillation detectors

Measurements have been performed at Los Alamos Neutron Science Center to acquire neutron fission spectra of ${}^{235}$U in the energy range between 0.5 and 10 MeV. These new data complement the currently available experimental data, which are not well known in the energy range below 1 and above 5 MeV. Organic liquid scintillation detectors (EJ-309s) were used together with a digital data-acquisition system. The EJ-309 detectors show excellent pulse shape discrimination capabilities and this is vital for identifying only neutron pulses and rejecting $\ensuremath{\gamma}$-ray pulses. The measurement data show the dependence of average fission neutron energy as a function of the inducing neutron energy. The data agree well with previously published measurements and Watt spectra fits for energies up to tens of MeV. At high fission-inducing neutron energies the spectrum deviates from Watt-spectra fits. The increased energy deposited in higher energy neutron-induced fissions does not significantly increase emitted fission neutron energies for up to 10 MeV. A significant decrease in neutron energy around second-chance fission cannot be observed beyond the current measurement uncertainties.

Digital signal processing application to 252Cf(sf) reaction products spectroscopy

The emission of prompt fission neutrons in spontaneous fission of 252Cf was studied by the application of digital data processing for the first time. The study was aimed at comparing the obtained results with those from the pioneering work of Budtz-Jorgensen and Knitter. By using a twin Frisch grid ionization chamber for fission fragment detection and a neutron scintillator (NE213) for the detection of neutrons, a total of about 107 neutron coincidences were registered. The fission-fragment kinetic energies were measured using a fast (80 MHz) waveform digitizer that provided 10-bit amplitude resolution. Standard analog NIM modules were used for neutron time-of-flight and pulse shape measurements. A time-to-digital converter (TDC) with 1-ns resolution and a pair of gated charge-to-digital converters with 12-bit amplitude resolution were also used. The fission fragment signals were analyzed by means of digital signal processing algorithms. Data on prompt fission neutron energies were obtained by using a time-of-flight (TOF) spectrum unfolding procedure applicable to list-mode data.

An active drop counting device using condenser microphone for superheated emulsion detector

An active device for superheated emulsion detector is described. A capacitive diaphragm sensor or condenser microphone is used to convert the acoustic pulse of drop nucleation to electrical signal. An active peak detector is included in the circuit to avoid multiple triggering of the counter. The counts are finally recorded by a microprocessor based data acquisition system. Genuine triggers, missed by the sensor, were studied using a simulated clock pulse. The neutron energy spectrum of (252)Cf fission neutron source was measured using the device with R114 as the sensitive liquid and compared with the calculated fission neutron energy spectrum of (252)Cf. Frequency analysis of the detected signals was also carried out.