Neutron Emission Spectra Research Articles

Microscopic evidence for scission neutrons

We discuss the neck rupture stage of fission and the emission of particles afterwards, two extremely rapid and highly non-equilibrium processes. Currently, the neck rupture cannot be directly probed by experiment, highlighting the importance of reliable theoretical predictions for this stage of fission. Time-dependent density functional theory (TDDFT) is used to simulate the spontaneous fission of 252Cf. In conjunction with statistical models, inputs from microscopic calculations can be used to make predictions for prompt neutron and gamma emission spectra, quantities which can be experimentally measured. Here we characterize the scission mechanism within TDDFT and estimate the number of scission neutrons and their kinetic energies.

Problem with gradual absorption in MSD/MSC calculations

Replacing classical exciton model with the better founded Multistep Direct (MSD) and Multistep Compound (MSC) mechanisms has been impeded by incapability of the latter models to describe central part of the neutron emission spectra at incident neutron energies of about 14 MeV and above. We have ascribed this deficiency to the decrease of absorption to the MSC mechanism resulting from the concept of gradual absorption. We were able to obtain very good reproduction of experimentally measured neutron spectra using MSD/MSC calculations when this option was turned off. Such treatment is, however, at odds with the fundamental distinction between MSD and MSC mechanisms that should proceed through the chain of open (P-space) and closed (Q-space) configurations respectively. By blocking gradual absorption we allow the first stage of MSC to be fully populated from the incident channel that at higher incident energies is impossible. We discuss various attempts of addressing the problem that, so far, remains open. In addition, we present an evidence for much tighter spin distribution of particle-hole states than normally assumed.

Microscopic modeling of direct pre-equilibrium emission: Impact on exclusive and inclusive (n,xn) and fission channels

We report on a microscopic modeling of the first order of multistep direct emission based on one phonon excitations, given by the QRPA model, and an effective in-medium nucleon nucleon interaction, described within the JLM folding model. The results of our coupled channels framework for deformed target are illustrated in the case of low energy discrete state excitations in 152Sm. Pre-equilibrium predictions and their impact on inclusive and exclusive (n,xn) cross sections and fission cross sections are discussed for actinides. We discuss the importance of i) collective excitations to describe the neutron emission spectra and of ii) spin distribution of the residual nucleus formed after the neutron pre-equilibrium emission, that is a key ingredient to model the residual nucleus decay.

Neutronic Characterization for a Pressurized Water Reactor Spent Fuel Assembly

The transfer of nuclear spent fuel from the reactor storage pool to dry storage or for reprocessing or final disposition requires information about its isotopic composition, decay heat, and other thermomechanical properties. The spent nuclear fuel assembly of a typical advanced pressurized water reactor, AP-1000, was characterized using the Monte Carlo MCNPX code and SCALE/ORIGEN code. The simulation of operational history started from the operation of the first fresh core for an average fuel assembly with certain physical isotopic parameters until 25 GWd/tonne U discharge burnup. The analysis considered the calculations of the radionuclide inventories, activity, neutron emission spectrum, gamma-ray emission spectrum, and decay power after 700 effective full power days and for post different time ranges until a 1 million–year cooling period. The comparison of some results of the two codes showed small differences due to the consideration of the continuous-energy variation for neutrons in the MCNPX code and the discrete energy assumption in the SCALE/ORIGEN code.

Simulations of neutral beam injection and ion cyclotron resonance heating synergy in high power EAST scenarios.

The EAST plasmas heated with deuterium neutral beam injection and ion cyclotron resonance heating (ICRH) have been simulated by the TRANSP code. The analysis has been conducted using the full wave solver TORIC5, the radio frequency (RF)-kick operator, and NUBEAM to model the RF heating effects on fast ion velocity distribution. In this work, we present several simulated results compared with experiments for high power EAST scenarios, indicating that the interactions between ICRH and fast ions can significantly accelerate fast ions, which are confirmed by the increased neutron yield and broadened neutron emission spectrum measurements.

Nuclear Reaction Mechanisms of Neutron Induced Fission of 237Np Nucleus

At FLNP JINR Dubna new prospects for developing new neutron facilities have already been started, that will replace the IBR-2 neutron pulsed research reactor which shut down in the year 2032. Several agreed projects use the fission processes induced by neutrons on neptunium-based fuels. An investigation of the neutron fission process on the 237Np nucleus was conducted in the present research. Several parameters were analyzed during neutron-induced fission of 237Np including the neutron cross-section, mass distributions, prompt neutron emission, isotope production, and neutron spectra. Evaluations of the above observables were conducted using TALYS – 9.1 Software, with incident neutron energies ranging from 0.4 to 25 MeV. The exact value of cross-section was obtained as 5 – 5.5 MeV in the present work, whereas from literature, it was found up to 10 MeV. Neutron spectra were analyzed and separated contributions from different nuclear reaction mechanisms were obtained. Our investigations have revealed some important yields and isotope production cross-sections that were in good agreement with literature data.

Quasi-differential neutron induced neutron emissions from 235U, and 239Pu

Uncertainty in nuclear reaction cross sections, angular distributions, and other nuclear data directly impact how well simulations of nuclear systems represent physical observations. To determine how well the nuclear data in ENDF/B-VIII.0, JEFF-3.3, and JENDL-4.0 evaluations describe the physical behaviour of 235U, and 239Pu when subjected to a neutron flux, the neutron emission spectrum was measured for carbon, 93.0% 235U, and 93.9% 239Pu samples, and compared against detailed MCNP6 simulations. The measurements were performed at the Los Alamos Neutron Science Center using a quasi-differential method previously developed at Rensselaer Polytechnic Institute. The measurement spanned 0.82–20 MeV and 30–150 degrees. The measurements show there are a significant number of discrepancies between library predictions of the neutron yield and physical observation. A few of the main discrepancies found are described in this paper. Based on these results a new evaluation utilizing these results for carbon, 235U, and 239Pu is recommended.

Forward modeling of pile-up events in liquid scintillator detectors for neutron emission spectroscopy.

When using liquid scintillator detectors to measure the neutron emission spectrum from fusion plasmas, the problem of pile-up distortion can be significant. Because of the large neutron rates encountered in many fusion experiments, some pile-up distortion can remain even after applying traditional pile-up elimination methods, which alters the shape of the measured light-yield spectrum and influences the spectroscopic analysis. Particularly, pile-up events appear as a high-energy tail in the measured light-yield spectrum, which obfuscates the contribution that supra-thermal ions make to the energy spectrum. It is important to understand the behavior of such "fast ions" in fusion plasmas, and it is hence desirable to be able to measure their contribution to the neutron spectrum as accurately as possible. This paper presents a technique for incorporating distortion from undetected pile-up events into the analysis of the light-yield spectrum, hence compensating for pile-up distortion. The spectral contribution from undetected pile-up events is determined using Monte Carlo methods and is included in the spectroscopic study as a pile-up component. The method is applied to data from an NE213 scintillator detector at JET and validated by comparing with results from the time-of-flight spectrometer TOFOR, which is not susceptible to pile-up distortion. Based on the results, we conclude that the suggested analysis method helps counteract the problem of pile-up effects and improves the possibilities for extracting accurate fast-ion information from the light-yield spectrum.

Cross Sections for Neutron-Production from 6- and 10-MeV Neutrons Incident on 10B and 11B

Using time-of-flight techniques, we have measured neutron emission spectra from 6- and 10-MeV incident neutron energies on 10B and 11B for laboratory angles between 20 and 145 deg using the unique neutron source reaction 1H(t,n). Double-differential cross sections and their integrated values have been extracted for elastic and inelastic processes, and their integrated values are summarized. In cases where discrete scattering peaks could be analyzed, differential cross-section angular distributions have been extracted and are compared to other results and to evaluated nuclear data.

Neutron emission and fast ion simulation for high performance long pulses at EAST.

Neutron emission spectroscopy and neutron yield measurements are important for high neutral beam injection (NBI) power heating at the Experimental Advanced Superconducting Tokamak (EAST). The neutron yields mainly depend on the deposition from NBI to the deuterium plasmas in the EAST. We have recently used TRANSP with time dependent diagnostic results to simulate the transport process of 30s long pulse deuterium plasma discharges in the EAST, obtaining the time dependent fast ion distribution, neutron emission spectrum, and total neutron emission rate. Combined with the time trace of the result measured by a standard 235U fission chamber, the effects of different configurations of NBI heating in EAST fusion plasmas have been evaluated.

Determination of 238U(n, γ) and 238U(n, f) reactions cross-section in the neutron emission spectrum of 241Am-Be source

The neutron radiative capture and neutron fission cross-sections were measured over the neutron emission spectrum of an 241Am-Be source with an effective energy range between 1 MeV and 11 MeV using the activation method. In foils were used as a neutron flux monitor based on 115In(n, n′)115mIn reaction. The sample was a Cd-covered natural UO2 pellet. Evaluations of the yield and reaction data in the energy range from 0.1 MeV to 11 MeV had been done to validate the integral experimental results. The neutron capture cross-section, using 239Np decay, was 32.9 ± 0.9stat. mb. The fission cross-section was probed using gamma lines from 143Ce, 133I, 105Ru, 134I, 91Sr, 135I, and 92Sr. The weighted average value of the fission cross-section was 570 ± 24 mb. 143Ce, 133I, 135I, and 92Sr had the highest merit among fission products for the validation, use upon application of long-lived Am-Be neutrons in quantification for nuclear safeguard and security applications.

Validation of neutron emission and neutron energy spectrum calculations on a Mega Ampere Spherical Tokamak with directional relativistic spectrum simulator

The recently developed directional relativistic spectrum simulator (DRESS) code has been validated for the first time against numerical calculations and experimental measurements performed on the Mega Ampere Spherical Tokamak. In this validation, the neutron emissivities and rates computed by DRESS are benchmarked against TRANSP/NUBEAM predictions while the neutron energy spectra provided by DRESS taking as input TRANSP/NUBEAM and ASCOT/BBNBI in Gyro-Orbit mode fast ion distributions are validated against proton pulse height spectra measured by the neutron flux monitor. Excellent agreement was found between DRESS and TRANSP/NUBEAM predictions of local and total neutron emission.

Modification of the DD Neutron Emission Spectrum at the 2.4 - 2.5 MeV Energy Range in Neutral-Beam-Injection-Heated Plasma and Its Application to Fuel Ion Ratio Diagnostics

Modification of the DD Neutron Emission Spectrum at the 2.4 - 2.5 MeV Energy Range in Neutral-Beam-Injection-Heated Plasma and Its Application to Fuel Ion Ratio Diagnostics

Characterization of 244Cm neutron sources

There is a need to find a replacement for the 252Cf sources currently used in Standards-based performance testing of neutron instrumentation. This is a result of its relatively short half-life, concerns about future availability and recent increases in cost. A potential replacement source, 244Cm is evaluated in this study where two commercially available sources have been acquired and their neutron emission spectra measured using a pair of spectrometers. Both instruments showed the 244Cm spontaneous neutron spectrum to be fully consistent with a MCNP6®-calculated spectrum using published Watt fission parameters for 244Cm.In addition, the emission rate of the weaker 244Cm source was established through a direct count rate comparison against a calibrated and similarly encapsulated 252Cf source. The 244Cm source emission rate was found to be in excellent agreement with value stated on the manufacturer's source certificate.It is concluded that 244Cm would be an ideal replacement for 252Cf based on its longer half-life (18.1 y) and its essentially identical neutron emission spectrum. In addition, 244Cm sources are commercially available at reasonable cost.

Calculation of Particle Emissions and Fusion Cross Sections After 8B(t,*), 9B(t,*), and 10B(t,*) Reactions by Monte Carlo Simulations

Boron nuclide and tritium projectile interactions are considerable in terms of nuclear energy systems. This study aims to investigate the realization of the nuclear fusion reactions of the bombardment of boron nuclei with tritium. In addition, the 8B(t,*), 9B(t,*), and 10B(t,*) reactions have focused on the use of the resulting product particles in nuclear technology applications, particularly in nuclear medicine applications, in terms of energy and number. Tritium-induced reactions from boron isotopes (8B, 9B, and 10B) at 50 and 100 MeV were modeled by the GEANT4 and EMPIRE Monte Carlo codes. Gamma, alpha, tritium, deuteron, proton, and neutron emission spectra (GEANT4-10.3) were obtained, and cross sections per energy (EMPIRE-3.2-MALTA) were calculated. Fusion cross sections and 6Li and 7Li production cross sections, which are critical in thermonuclear fusion reactors as basic fusion reactions, are discussed based on the 8B(t,*), 9B(t,*), and 10B(t,*) reactions.

A new reference database for beta-delayed neutrons

A new database containing all available experimental and evaluated β-delayed neutron data is presented in this paper. The database is the product of an international effort coordinated by the International Atomic Energy Agency. It comprises a microscopic section including all available experimental data on beta-decay half-lives, β-delayed neutron emission probabilities and spectra, as well as new systematics and global theoretical calculations for comparison. The beta-delayed neutron data for individual precursors have been benchmarked against available data on macroscopic properties such as total delayed-neutron yields and spectra, delayed-neutron decay curves and time-dependent group parameters using the summation method. All available measured macroscopic data have also been compiled in the macroscopic section of the database. The dedicated database is available online on the IAEA server (URL:http://www-nds.iaea.org/beta-delayed-neutron/database.html).

Neutron emission spectrum from gold excited with 16.6 MeV linearly polarized monoenergetic photons

ABSTRACT The neutron-emission spectrum from a 197Au target excited with a 16.6 ±0.2 MeV, monoenergetic, linearly polarized photon beam was measured using the time-of-flight method at the NewSUBARU-BL01. Two components were clearly detected in the neutron spectrum produced by photonuclear reactions. One component [component (A)] shows an evaporation-like spectrum with energies up to 4 MeV. The spectrum of the other component [component (B)] is shaped like a bump and energies ≥4 MeV. The intensity of the component (A) does not show any definite angular dependence, whereas, that of the component (B) follows the relation [a+ b cos(2Θ)] as a function of the angle Θ between the polarization and detector directions.

Detection of neutral beam deuterons acceleration by 2nd harmonic radio frequency heating in ASDEX Upgrade via neutron spectrometry

Second harmonic absorption of radio frequency in the ion cyclotron range of frequency (ICRF) by fast D is expected in the case of H minority heating at the fundamental cyclotron frequency, provided the ion Larmor radius is of the order of the RF-wavelength, as it is for neutral beam injection (NBI)-D ions. In tokamaks a strong increase of neutron production is observed when such an ICRF heating is applied on top of NBI, often referred to as ‘synergy effect’.On ASDEX Upgrade a scintillator-based compact neutron spectrometer (CNS) is available outside the torus hall, with perpendicular view through the plasma equatorial plane, measuring pulse height spectra (PHS), which allow to infer the ion distribution functions, in particular the suprathermal tail.The measured PHS exhibit clear energetic tails when ICRF is applied in the presence of NBI, proving there’s an overproportional tail in the fast-ion distribution at the high energy end. The PHS in the CNS line-of-sight are modelled coupling kinetic solvers with wave codes. Unfolding the PHS into neutron emission spectra allows to quantify the maximum fast ion energy with significant occurrence, showing a non-negligible population around 500 keV, significantly higher than the NBI energy. A sensitivity study assesses the maximum fast ion energy compatible with the measured PHS, confirming the presence of 500 keV D ions.

Modification of neutron emission spectrum by Alfvén eigenmodes in a deuterium–tritium plasma

The neutron emission spectrum can be modified from Gaussian due to excited Alfvén eigenmodes (AEs). We evaluate the neutron emission spectrum when the fuel-ion velocity distribution functions are distorted by AEs, assuming an ITER-like deuterium–tritium tokamak plasma. When shear AEs are excited by energetic particles in a plasma, the non-Maxwellian tails are created in the fuel-ion distribution functions in directions that are parallel and antiparallel to the magnetic field lines. Due to formation of these tails, the non-Gaussian components are formed in the neutron emission spectrum in the same and opposite directions of the toroidal axis. The anisotropic, non-Gaussian neutron emission spectrum can be used for experimental verification and diagnostics of the non-Maxwellian tails in the fuel-ion velocity distribution functions formed by AEs.

Dependence of spin induced structural transitions on level density and neutron emission spectra

The impact of spin induced deformation and shape phase transitions on nuclear level density and consequently on neutron emission spectra of the decay of compound nuclear systems 112Ru to 123Cs (N=68 isotones) is investigated in a microscopic framework of Statistical theory of superfluid nuclei. Our calculations are in good accord with experimental data for evaporation residue of 119Sb⁎ and 185Re⁎ and show a strong correlation between spin induced structural transitions and NLD. We find that the inverse level density parameter ‘K’ increases with increasing spin for all the systems, but it decreases with a deformation or a shape change that results in the enhancement of level density and emission probability. A sharp shape phase transition from oblate to uncommon prolate non-collective in well deformed nuclei leads to band crossing and enhancement of level density which fades away while approaching sphericity at or near shell closure manifesting shell effects.