Function Of Incident Neutron Energy Research Articles

New operational quantities (RBE × D p lens/Φ) for eye lens neutron dosimetry as a function of energy and angle of incidence in the ICRU 95 formalism

This paper is a continuation of a study published recently by the authors. It presents and discusses computed personal absorbed dose in the lens of the eye (D p lens/Φ), and a relative biological effectiveness (RBE)-weighted absorbed dose (in terms of an newly proposed operational quantity RBE × D p lens/Φ), conversion coefficients for the lens of the eye for neutron exposure at incident energies from thermal to ∼20 MeV and at angles of incidence from 0° to 90° in 15° increments, at 180° and for rotational incidence irradiation geometry (from 0° to 360° in 5° increments). These conversion coefficients were obtained from a simulation model developed for this study that contains the stylised eye model, embedded in the adult UF-ORNL mathematical phantom, whereby the previously stated RBE-weighted absorbed dose was obtained using the proposed RBE versus neutron energy distribution compiled in a previous paper by the authors. The simulations carried out for this study using the Monte Carlo N-Particle transport code version 6.2, were conducted in a realistic human eye model, for the left and right sensitive and whole volume of the lens of the eye, considering the recent proposed redefinition of the operational quantities for external radiation exposure in International Commission on Radiation Units and Measurements (ICRU) report 95. A comprehensive set of tabulated data for neutron fluence-to-dose conversion coefficients (D p lens/Φ in pGy cm2) and RBE-weighted absorbed dose (RBE × D p lens/Φ in pGy cm2) conversion coefficients is included in this paper as a function of incident neutron energy and angle of incidence. Data for D p lens/Φ (pGy cm2) are compared to similar data from the literature for validation of our model. Data for RBE × D p lens/Φ (in pGy cm2), were also compared to the equivalent operational quantity H p(3,α)/Φ (in pSv cm2) conversion coefficients calculated at 3 mm depth in a cylindrical phantom for different incident neutron energies and angles of incidence from 0° to 75° in 15° increments to demonstrate the relevance of this newly proposed operational quantity for doses resulting in tissue reactions (deterministic effects) which should be quoted in Gray (RBE-weighted absorbed dose, RBE × D (Gy)), rather than Sievert (Sv) which is reserved for stochastic effects. The current neutron weighted absorbed dose (RBE × D p lens) is proposed for the tissue reactions in the eye-lens for neutron radiation as per National Council on Radiation Protection and Measurements report 180 and in line with the recent proposal for the review and revision of the System of Radiological Protection to Keeping the International Commission on Radiological Protection (ICRP) recommendations fit for purpose. This method would bring better alignment between the dose limits in ICRP 118 and the new operational quantity consistent with the units of the new eye-lens dose limits without being overly conservative. The utilization of the proposed new operational quantities, as outlined in ICRU 95, has the potential to address the ongoing challenge in enforcing regulatory limits for neutron eye dose, specifically the use of Gy instead of Sv. It should be noted that the applicability of this will vary from country to country as in many countries the legislation is likely to mandate the use of H p(3) until the regulation is amended. This approach can serve as an interim solution while awaiting the issuance of the new ICRP general recommendations, which is expected to take several years. Implementing the new operational quantities can contribute to enhancing the accuracy and effectiveness of neutron eye dose limit enforcement.

Advancements of the nSpec system

A new characterization of the detector response for the EJ301 liquid scintillator based nSpec system has been conducted at the Los Alamos Neutron Science Center. The new characterization was accomplished in two orders of magnitude less time and with better statistics than the previous characterization. The new and old characterization methods were compared by conducting both types of characterization on two detectors (one EJ301 and one EJ301D). The two methods show consistent results and compare well to results previously published in the literature; however, the results from the characterization of the original nSpec detector show discrepancies that have been identified as being caused by oxygen being absorbed into the detector due to a leak. The new characterized detector response shows improved spectrum unfolding performance than the previous characterization. The cross talk of the detector system was also characterized as a function of incident neutron energy and found to have a negligible impact on spectrum unfolding results.Alternative materials (EJ301D and stilbene) were also characterized in a first step towards an upgraded system. The different materials show little difference in unfolding performance, but the comparison is ongoing. A SiPM based stilbene detector was used to evaluate the use of SiPMs in a future system upgrade. The SiPM provides adequate performance, but the custom SiPM readout results in a much longer pulse width than PMT based systems. The measured light output of the SiPM based stilbene compares well with the results in literature. Future work will look at additional materials (deuterated stilbene and organic glass) and optimizing the SiPM readout to decrease the pulse width.

The correction of Inelastic Neutron Scattering data of organic samples using the Average Functional Group Approximation

The use of the Average Functional Group Approximation for self-shielding corrections at inelastic neutron spectrometers is discussed. By taking triptindane as a case study, we use the above-mentioned approximation to simulate a synthetic dynamic structure factor as measured on an indirect-geometry spectrometer, as well as the related total scattering cross section as a function of incident neutron energy and sample temperature, and the transmission spectra depending on the sample thickness. These quantities, obtained in a consistent way from the Average Functional Group Approximation, are used to calculate the energy-dependent self-shielding correction affecting the sample under investigation. The impact on the intensities of low-energy vibrational modes is discussed, showing that at typical experimental conditions the sample-dependent attenuation factor is about 15% higher compared to the correction at higher energies.

Empirical Neutron Cross Section Formulae for (n, p) and (n, d) Reactions of He-3 Target

In this work, new cross section formulae for (n, p) and (n, d) reactions of He-3 have been investigated as a function of incident neutron energy. The new Empirical formulae were produced by using the least squares method to the experimental cross sections data, which were taken from Experimental Nuclear Reaction Data EXFOR Database Version of 2021. Several functions were examined to choose the best one that fits the data. Statistical hypothesis testing was used to insure how well the suggested equations fit the set of data. Two statistical indicators were used for each case for goodness-of-fit. Very high compatibility was found between the empirical values and the experimental data for both reactions.

Prompt-fission-neutron spectra in the Pu239(n,f) reaction

Prompt-fission-neutron spectra from $^{239}\mathrm{Pu}$ ($\mathit{n},\phantom{\rule{0.16em}{0ex}}\mathit{f}$) were measured with respect to $^{252}\mathrm{Cf}$ spontaneous fission for incident neutron energies from 0.7 to $700\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$ at the Weapons Neutron Research facility (WNR) of the Los Alamos Neutron Science Center. A newly designed high-efficiency fission chamber was coupled to the highly segmented Chi-Nu neutron liquid scintillator array to detect neutrons emitted in fission events. The double time-of-flight technique was used to deduce the incident neutron energies from the spallation target and the outgoing-neutron energies from the fission chamber. Prompt-fission-neutron spectra (PFNS) were measured with respect to $^{252}\mathrm{Cf}$ spontaneous fission down to $200\phantom{\rule{0.16em}{0ex}}\mathrm{keV}$ and up to about $12\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$ for all the incident neutron energies with typical total uncertainties well below $2%$ up to about 7-MeV outgoing-neutron energy. The general trend of PFNS is well reproduced by JEFF3.3 and ENDF evaluations, although a better agreement is found with JEFF3.3. Discrepancies were observed for the low-energy part of the spectra, especially around the opening of the second-, third- and fourth-chance fission. Neutron average kinetic energies as a function of incident neutron energy are obtained experimentally with reported total uncertainties below $0.5%$. The measured values agree with the most recent data. The trend is fairly well reproduced by the JEFF3.3 evaluation, although it fails to reproduce the experimental values within their uncertainties.

Evaluation of the Prompt Fission Gamma Properties for Neutron Induced Fission of 235,238U and 239Pu

We present details of the prompt fission gamma property evaluations included in the released ENDF/B-VIII.0 files. The average prompt gamma multiplicity, the average prompt gamma spectrum, the average total prompt gamma energy released and the prompt gamma multiplicity distributions, included in the ENDF/B-VIII.0 files, are presented for 235U(n,f), 238U(n,f), and 239Pu(n,f) reactions, as a function of incident neutron energy up to 30 MeV. The evaluation is based on available experimental data and model calculations. Notable data include recent measurements by experimental groups at Los Alamos and Lawrence Livermore National Laboratories, at Geel and Budapest, at CEA labs, as well as seminal historical gamma production measurements by Drake at Los Alamos.

Angular distribution of γ rays from neutron-induced compound states of La140

Angular distribution of individual $\gamma$-rays, emitted from a neutron-induced compound nuclear state via radiative capture reaction of ${}^{139}$La(n,$\gamma$) has been studied as a function of incident neutron energy in the epithermal region by using germanium detectors. An asymmetry $A_{\mathrm{LH}}$ was defined as $(N_{\mathrm L}-N_{\mathrm H})/(N_{\mathrm L}+N_{\mathrm H})$, where $N_{\mathrm L}$ and $N_{\mathrm H}$ are integrals of low and high energy region of a neutron resonance respectively, and we found that $A_{\mathrm{LH}}$ has the angular dependence of $(A\cos\theta_\gamma+B)$, where $\theta_\gamma$ is emitted angle of $\gamma$-rays, with $A= -0.3881\pm0.0236$ and $B=-0.0747\pm0.0105$ in 0.74 eV p-wave resonance. This angular distribution was analyzed within the framework of interference between s- and p-wave amplitudes in the entrance channel to the compound nuclear state, and it is interpreted as the value of the partial p-wave neutron width corresponding to the total angular momentum of the incident neutron combined with the weak matrix element, in the context of the mechanism of enhanced parity-violating effects. Additionally we used the result to quantify the possible enhancement of the breaking of the time-reversal invariance in the vicinity of the p-wave resonance.

Evaluations of Energy Spectra of Neutrons Emitted Promptly in Neutron-induced Fission of 235U and 239Pu

The energy spectra of neutrons emitted promptly in the neutron-induced fission reactions of 235U and 239Pu were re-evaluated for ENDF/B-VIII.0. These evaluations are based on a careful modeling of all relevant physics processes, an extensive analysis of experimental data and a detailed quantification of pertinent uncertainties. Energy spectra of neutrons emitted in up to fourth chance fission are considered and both compound and pre-equilibrium processes are included. Also, important nuclear model parameters, such as the average total kinetic energy of the fission fragments and the multiple chance fission probabilities, and their uncertainties are estimated based on experimental knowledge, model information and evaluated data. In addition to experimental information already available for ENDF/B-VII.1, these new evaluations make use of recently published experimental data either of high precision or spanning a broad incident energy range, information on legacy measurements explaining discrepancies and recently measured data of the average total kinetic energy as a function of incident neutron energy. The resulting evaluated data and covariances agree well with the experimental database used for the evaluation. However, the evaluated spectra are softer than the 235U and 239Pu ENDF/B-VII.1, JENDL-4.0 and JEFF-3.2 evaluations for incident neutron energies Einc≤1.5 MeV and Einc≤5 MeV, respectively. For Einc>5 MeV, the evaluated spectra show structures due to the improved modeling which are not present in ENDF/B-VII.1 and JEFF-3.2 but can be observed in JENDL-4.0 evaluations. Part of these new evaluations were adopted for ENDF/B-VIII.0, while the ENDF/B-VII.1 239Pu PFNS was retained for Einc≤5 MeV awaiting more conclusive experimental evidence.

New prompt fission neutron spectra measurements in the238U(n,f) reaction with a dedicated setup at LANSCE/WNR

A new prompt fission neutron spectra (PFNS) measurement in the 238U(n,f) reaction was performed at LANSCE/WNR facility. Evaluated data show discrepancies on the low (below 1 MeV) and high (above 5 MeV) energy parts in the PFNS for different major and minor actinides. The goal is to improve these measurements in a wide range of incident energy. The energy of the incoming neutron, inducing the fission, and the prompt neutron energies, are measured by time-of-flight method. A dedicated fission chamber was developed, in order to improve alpha-fission discrimination, timing resolution, actinide mass, and to reduce the amount of neutron scattering. To detect prompt neutrons, the 54 Chi-Nu scintillator cells array were surrounding the fission chamber. High statistics were recorded during this experiment, allowing a precise study of PFNS behavior as a function of incident neutron energy, from 1 MeV to 200 MeV. This experiment also showed that all the new tools developed to improve PFNS measurements are performing. Therefore, measurements of PFNS with others actinides such as 239Pu are planned.

Prompt fission neutron and γ ray properties as a function of incident neutron energy

The CGMF code provides a complete description of the properties of the prompt neutrons and γ rays, emitted before beta decays. It is based on a Monte Carlo implementation of the Hauser-Feshbach statistical model, which provides an accurate phenomenological description of the de-excitation of the fission fragments toward stable configurations via neutron and γ-ray emissions. This approach allows a detailed description of a large number of observables, such as multiplicity probabilities and correlations between the emitted particles. In this contribution, we briefly review the approach and present selected examples of neutron and γ-ray observables for neutron incident energies from thermal to 20 MeV, and compare against available data for 235 U(n,f) reaction.

Prompt neutron emission and energy balance in 235U(n,f)

Investigations of prompt fission neutron (PFN) emission are of importance in understanding the fission process in general and the sharing of excitation energy among the fission fragments in particular. Experimental activities at JRC-Geel on PFN emission in response to OECD/NEA nuclear data requests is presented in this contribution. The focus lies on on-going investigations of PFN emission from the reaction 235 U(n,f) in the region of the resolved resonances taking place at the GELINA facility. For this reaction strong fluctuations of fission fragment mass distributions and mean total kinetic energy have been observed as a function of incident neutron energy in the resonance region. In addition, fluctuations of prompt neutron multiplicities have also been observed. The goal of the present study is to verify the current knowledge of PFN multiplicity fluctuations and to study correlations with fission fragment properties. The experiment employs a scintillation detector array for neutron detection, while fission fragment properties are determined via the double kinetic energy technique using a position sensitive twin ionization chamber. Results on PFN multiplicity correlations with fission fragment properties from the present study show significant differences compared to earlier studies on this reaction, induced by thermal neutrons. Specifically, the total kinetic energy dependence of the neutron multiplicity per fission shows an inverse slope FX 1TKE/FX 2ν approximately 35% weaker than observed in earlier studies of thermal neutron induced fission on 235 U. The inverse slope is related to the energy carried away per emitted neutron and is, thereby, closely connected to the energy balance of the fission reaction. The present result should have strong impact on the modeling of both prompt neutron and prompt γ-ray emission in fission of the 236 U compound nucleus.

Inspection of56Feγ-Ray angular distributions as a function of incident neutron energy using optical model approaches

Neutron inelastic scattering cross sections measured directly through (n,n) or deduced from γ-ray production cross sections following inelastic neutron scattering (n,n′ γ) are a focus of basic and applied research at the University of Kentucky Accelerator Laboratory (www.pa.uky.edu/accelerator). For nuclear data applications, angle-integrated cross sections are desired over a wide range of fast neutron energies. Several days of experimental beam time are required for a data set at each incident neutron energy, which limits the number of angular distributions that can be measured in a reasonable amount of time. Approximations can be employed to generate cross sections with a higher energy resolution, since at 125o , the a 2 P 2 term of the Legendre expansion is identically zero and the a 4 P 4 is assumed to be very small. Provided this assumption is true, a single measurement at 125o would produce the γ-ray production cross section. This project tests these assumptions and energy dependences using the codes CINDY/SCAT and TALYS/ECIS06/SCAT. It is found that care must be taken when interpreting γ-ray excitation functions as cross sections when the incident neutron energy is < 1000 keV above threshold or before the onset of feeding.

Fragment-mass, kinetic energy, and angular distributions forU234(n,f)at incident neutron energies fromEn=0.2MeV to 5.0 MeV

This work investigates the neutron-induced fission of $^{234}\mathrm{U}$ and the fission-fragment properties for neutron energies between ${E}_{\mathrm{n}}=0.2$ and 5.0 MeV with a special highlight on the prominent vibrational resonance at ${E}_{\mathrm{n}}=0.77$ MeV. Angular, energy, and mass distributions were determined based on the double-energy technique by means of a twin Frisch-grid ionization chamber. The experimental data are parametrized in terms of fission modes based on the multimodal random neck-rupture model. The main results are a verified strong angular anisotropy and fluctuations in the energy release as a function of incident-neutron energy.

Studies forγ-ray emission in the fission process with LICORNE

The LICORNE neutron source is a new device at the ALTO facility. Its use of inverse kinematics makes the production of naturally focused neutron beams possible with an energy range of 0.5 to 4 MeV. This is perfect for any studies concerning fast neutron induced reaction based on detection setup that require to be placed in a compact geometry - such as Ge based ? detection setup. In this paper, neutron production with LICORNE is described. The development of a gascell target and the extension of the energy range up to 7 MeV with the p(11B,n)11C are presented. An overview of the major research fields studied with LICORNE is given and two types of experiment are presented. The first one dedicated to prompt fission ?-ray emission in fission, as a function of incident neutron energy, is described. Some preliminary results are shown. The second, in the context of the MINORCA campaign, is detailed. The most recent outcomes in the data analysis process are also presented.

Investigating Prompt Fission Neutron Emission from235U(n,f) in the Resolved Resonance Region

Investigations of prompt emission in fission is of importance in understanding the fission process in general and the sharing of excitation energy among the fission fragments in particular. Experimental activities at IRMM on prompt neutron emission from fission in response to OECD/NEA nuclear data requests is presented in this contribution. Main focus lies on currently on-going investigations of prompt neutron emission from the reaction 235 U(n,f) in the region of the resolved resonances. For this reaction strong fluctuations of fission fragment mass distributions and mean total kinetic energy have been observed [Nucl. Phys. A 491, 56 (1989)] as a function of incident neutron energy in the resonance region. In addition fluctuations of prompt neutron multiplicities were also observed [Phys. Rev. C 13, 195 (1976)]. The goal of the present study is to verify the current knowledge of prompt neutron multiplicity fluctuations and to study correlations with fission fragment properties.

Measurement of theMo97(n,γ)reaction with the DANCEγcalorimeter array

Spectra of $\ensuremath{\gamma}$ rays following the $^{97}\mathrm{Mo}(n,\ensuremath{\gamma})$ reaction were measured as a function of incident neutron energy with the DANCE (Detector for Advanced Neutron Capture Experiments) array of $160 {\mathrm{BaF}}_{2}$ scintillation detectors at the Los Alamos Neutron Science Center using an enriched $^{97}\mathrm{Mo}$ target. These spectra were used for the assignment of spins of the $^{97}\mathrm{Mo}$ resonances up to neutron energy ${E}_{n}=1.7$ keV, as well as in the study of photon strength functions (PSFs) in $^{98}\mathrm{Mo}$. Analysis of the spectra with the nuclear statistical model showed that they can be well reproduced with the same PSF models which well described the $\ensuremath{\gamma}$ decay following slow neutron capture in $^{95}\mathrm{Mo}$. On the other hand, the spectra are inconsistent with PSFs describing some other experimental data in $^{98}\mathrm{Mo}$.

αand2p2nemission in fast neutron-induced reactions onNi60

Background: The cross sections for populating the residual nucleus in the reaction ${}_{Z}^{A}\text{X}{(n,x)}_{Z\ensuremath{-}2}^{A\ensuremath{-}4}\text{Y}$ exhibit peaks as a function of incident neutron energy corresponding to the ($n,{n}^{\ensuremath{'}}\ensuremath{\alpha}$) reaction and, at higher energy, to the ($n,2p3n$) reaction. The relative magnitudes of these peaks vary with the $Z$ of the target nucleus.Purpose: Study fast neutron-induced reactions on $^{60}\text{Ni}$. Locate experimentally the nuclear charge region along the line of stability where the cross sections for $\ensuremath{\alpha}$ emission and for $2p2n$ emission in fast neutron-induced reactions are comparable as a further test of reaction models.Methods: Data were taken by using the Germanium Array for Neutron-Induced Excitations. The broad-spectrum pulsed neutron beam of the Los Alamos Neutron Science Center's Weapons Neutron Research facility provided neutrons in the energy range from 1 to 250 MeV. The time-of-flight technique was used to determine the incident-neutron energies.Results: Absolute partial cross sections for production of seven discrete Fe $\ensuremath{\gamma}$ rays populated in $^{60}\text{Ni}(n,\ensuremath{\alpha}/2pxn\ensuremath{\gamma})$ reactions with $2\ensuremath{\le}x\ensuremath{\le}5$ were measured for neutron energies $1\phantom{\rule{4pt}{0ex}}\text{MeV}&lt;{E}_{n}&lt;250\phantom{\rule{4pt}{0ex}}\text{MeV}$. Hauser--Feshbach plus pre-equilibrium theoretical calculations are compared to the experimental results.Conclusions: There is good agreement between experimental results and theoretical predictions at lower neutron energies while discrepancies appear at higher neutron energies. The cross section for producing an isotope in fast neutron-induced reactions on stable targets via $\ensuremath{\alpha}$ emission at the peak of the ($n,\ensuremath{\alpha}$) and ($n,{n}^{\ensuremath{'}}\ensuremath{\alpha}$) reactions is comparable to that for $2p2n$ and $2p3n$ emission at higher incident energies in the nuclear charge region around Fe.

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.

Prompt Fission Neutron Experiments on 235U(n,f) and 252Cf(SF)

For nuclear modeling and improved evaluation of nuclear data knowledge about fluctuations of the prompt neutron multiplicity as a function of incident neutron energy is requested for the major actinides 235U and 239Pu. Experimental investigations of the prompt fission neutron emission in resonance neutron induced fission on 235U are taking place at the GELINA facility of the IRMM. The experiment employs an array of neutron scintillation-detectors (SCINTIA) in conjunction with a newly designed 3D position sensitive twin Frisch-grid ionization chamber. A preparatory experiment on prompt fission neutron emission in 252Cf(SF) was undertaken to verify analysis procedures relevant for the implementation of the SCINTIA neutron detector array. The available literature data on the TKE dependence of the multiplicity show strong deviations. Therefore, effort was focused on investigating experimental factors in low efficiency neutron counting experiments that may lead to faulty determination of this dependence. Taking these factors into account a result that agree well with data from high efficiency neutron counting experiments is obtained.

Neutron energy spectrum in a reactor exit channel with a single surface barrier sensor using GEANT4

A single surface barrier neutron spectrometer has been used to characterise the neutron energy spectrum at the neutron irradiation channel of a nuclear reactor. The response functions of a single surface barrier detector associated with a natural boron neutron-charged particle converter were calculated using the GEANT4 simulation toolkit. Calculations were performed for thermal, intermediate energy and fast neutrons and for four converter thicknesses from 50μgcm−2 to 3.26mgcm−2. The relationship between the response functions and the energy loss of the charged particles emitted by the 10B(n,α)7Li reaction in the converter as a function of incident neutron energy is discussed. Finally, the neutron energy spectrum at the neutronography irradiation site of the Algerian Research Reactor NUR was determined using the experimentally determined pulse height spectrum of the single surface barrier spectrometer with the help of the calculated response functions and an efficient unfolding code.