Angular Neutron Research Articles

Experimental approach to measure thick target neutron yields induced by heavy ions for shielding

Double differential (angular and energy) neutron distributions were measured using an activation foil technique. Reactions were induced by impinging two low-energy heavy-ion beams accelerated with the GANIL CSS1 cyclotron: (36 S (12 MeV/u) and 208 Pb (6.25 MeV/u)) onto thick nat Cu targets. Results have been compared to Monte-Carlo calculations from two codes (PHITS and FLUKA) for the purpose of benchmarking radiation protection and shielding requirements. This comparison suggests a disagreement between calculations and experiment, particularly for high-energy neutrons.

On the calculation of angular neutron flux in MCNP

Modern Monte Carlo neutron transport codes offer many options for neutron flux and spectra calculations, however, they often lack the option to obtain the angular neutron flux in a region of the problem. The angular flux can also be obtained from deterministic programs, however, it includes biases due to discretization and other physical approximations. Therefore, a novel method for determining the angular neutron flux from the standard output of the MCNP is proposed in this paper. The method was also implemented as a set of Python libraries and tested in several examples. The results were then used to investigate the self-shielding effect in a realistic angular profile of the flux, i.e., the TRIGA research reactor.

On an analytical formulation for the mono-energetic neutron space-kinetic equation in full cylinder symmetry

In the present work we discuss a system of partial differential equations that model neutron space-kinetics in cylindrical geometry and are defined by two sectionally homogeneous cylinder cells, mono-energetic neutrons and one group of delayed neutron precursors. The solution is determined using the technique of variable separation. The associated complete spectra with respect to each variable separation are analysed and truncated such as to allow a parameterized global solution. For the obtained solution we present some numerical results for the scalar neutron flux and its time dependence and projection on the cylinder axis z and the radial and cylinder axis projection. As a case study we consider an insertion of an absorbing medium in the upper cylinder cell. Continuity of the scalar flux at the interface between the two cylinder elements and conserved current density is explained and related to scale invariance of the partial differential equation system together with the initial and boundary conditions. Some numerical results for the scalar angular neutron flux and associated current densities are shown.

Prospect for application of compact accelerator-based neutron source to neutron engineering diffraction

A compact accelerator-based neutron source has been lately discussed on engineering applications such as transmission imaging and small angle scattering as well as reflectometry. However, nobody considers using it for neutron diffraction experiment because of its low neutron flux. In this study, therefore, the neutron diffraction experiments are carried out using Riken Accelerator-driven Compact Neutron Source (RANS), to clarify the capability of the compact neutron source for neutron engineering diffraction. The diffraction pattern from a ferritic steel was successfully measured by suitable arrangement of the optical system to reduce the background noise, and it was confirmed that the recognizable diffraction pattern can be measured by a large sampling volume with 10mm in cubic for an acceptable measurement time, i.e. 10min. The minimum resolution of the 110 reflection for RANS is approximately 2.5% at 8μs of the proton pulse width, which is insufficient to perform the strain measurement by neutron diffraction. The moderation time width at the wavelength corresponding to the 110 reflection is estimated to be approximately 30μs, which is the most dominant factor to determine the resolution. Therefore, refinements of the moderator system to decrease the moderation time by decreasing a thickness of the moderator or by applying the decoupler system or application of the angular dispersive neutron diffraction technique are important to improve the resolution of the diffraction experiment using the compact neutron source. In contrast, the texture evolution due to plastic deformation was successfully observed by measuring a change in the diffraction peak intensity by RANS. Furthermore, the volume fraction of the austenitic phase in the dual phase mock specimen was also successfully evaluated by fitting the diffraction pattern using a Rietveld code. Consequently, RANS has been proved to be capable for neutron engineering diffraction aiming for the easy access measurement of the texture and the amount of retained austenite.

New derivation of Blackshow–Murrays formula for the Doppler-broadened scattering kernel and calculation of the angular moments via Lagrange interpolation

A new independent derivation of Blackshow–Murray’s classical formula for the Doppler-broadening elastic scattering kernel is given, including an extension to anisotropy of scattering in the center of mass. Also, the puzzling behavior of the secondary angular and energy neutron distribution about the lower resonances of heavy isotopes is explained by introducing an asymptotic ordering, which includes on the same foot thermal agitation and resonance effects, and by a detailed analysis for the case of collinear scattering. Finally, we use Lagrange polynomial interpolations for a fast and accurate calculations of the angular moments of the kernel. Results for 238U are presented, analyzed and compared to those in the existing literature, for both isotropic an anisotropic scattering in the center of mass.

Enhanced γ-Ray Emission from Neutron Unbound States Populated in β Decay.

Total absorption spectroscopy is used to investigate the β-decay intensity to states above the neutron separation energy followed by γ-ray emission in (87,88)Br and (94)Rb. Accurate results are obtained thanks to a careful control of systematic errors. An unexpectedly large γ intensity is observed in all three cases extending well beyond the excitation energy region where neutron penetration is hindered by low neutron energy. The γ branching as a function of excitation energy is compared to Hauser-Feshbach model calculations. For (87)Br and (88)Br the γ branching reaches 57% and 20%, respectively, and could be explained as a nuclear structure effect. Some of the states populated in the daughter can only decay through the emission of a large orbital angular momentum neutron with a strongly reduced barrier penetrability. In the case of neutron-rich (94)Rb the observed 4.5% branching is much larger than the calculations performed with standard nuclear statistical model parameters, even after proper correction for fluctuation effects on individual transition widths. The difference can be reconciled by introducing an enhancement of 1 order of magnitude in the photon strength to neutron strength ratio. An increase in the photon strength function of such magnitude for very neutron-rich nuclei, if it proves to be correct, leads to a similar increase in the (n,γ) cross section that would have an impact on r process abundance calculations.

Coupled fine-mesh neutronics and thermal-hydraulics – Modeling and implementation for PWR fuel assemblies

In this paper we present a fine-mesh solver aimed at resolving in a coupled manner and at the pin cell level the neutronic and thermal-hydraulic fields. Presently, the tool considers Pressurized Water Reactor (PWR) conditions. The methods and implementation strategy are such that the coupled neutronic and thermal-hydraulic problem is formulated in a fully three-dimensional (3D) and fine mesh manner, and for steady-state situations. The solver is built on finite volume discretization schemes, matrix solvers and capabilities for parallel computing that are available in the open source C++ library foam-extend-3.0. The angular neutron flux is determined with a multigroup discrete ordinates method (SN), solved by a sweeping algorithm. The thermal-hydraulics is based on Computational Fluid Dynamics (CFD) models for the moderator/coolant mass, momentum, and energy equations, together with the fuel pin energy equation. The multiphysics coupling is solved by making use of an iterative algorithm, and convergence is ensured for both the separate equations and the coupled scheme. Since all the equations are implemented in the same software, all fields can be directly accessed in such a manner that external transfer and external mapping are avoided. The parallelization relies on a domain decomposition which is shared between the neutronics and the thermal-hydraulics. The latter allows to exchange the coupled data locally on each CPU, thus minimizing the data transfer. The code is tested on a quarter of a 15×15 PWR fuel lattice. The results show that convergence is successfully reached, and correct physical behaviors of all fields can be achieved with a reasonable computational effort.

Neutron Cross Section Evaluation of Tungsten and Tantalum

The evaluations of neutron cross sections and covariances are presented for tungsten and tantalum which are considered as a prime candidate of plasma facing materials in fusion reactors and/or a structural material of the Korea Helium Cooled Ceramic Reflector (Ko HCCR) Test Blanket Module (TBM). The Fusion Evaluated Nuclear Data Library (FENDL-2.1) has been employed as the nuclear data for the neutronics analysis to obtain the optimal design parameters for Ko HCCR TBM. However, the neutron cross sections and energy and/or angular dependent neutron spectra from FENDL-2.1 showed some discrepancies with the differential and integral measured data available for tungsten and tantalum. In response to these situations, the neutron cross section for 181Ta and 180,182,183,184,186W have been newly evaluated using the up-to-date nuclear reaction code and the measured data available.

Application of a Bonner sphere spectrometer for the determination of the angular neutron energy spectrum of an accelerator-based BNCT facility

Experimental activities are underway at INFN Legnaro National Laboratories (LNL) (Padua, Italy) and Pisa University aimed at angular-dependent neutron energy spectra measurements produced by the 9Be(p,xn) reaction, under a 5MeV proton beam. This work has been performed in the framework of INFN TRASCO-BNCT project. Bonner Sphere Spectrometer (BSS), based on 6LiI (Eu) scintillator, was used with the shadow-cone technique. Proper unfolding codes, coupled to BSS response function calculated by Monte Carlo code, were finally used. The main results are reported here.

Benchmark test of TRIPOLI-4 code through simple model calculation and analysis of fusion neutronics experiments at JAEA/FNS

In order to examine a basic performance of the TRIPOLI code, two types of analyses were carried out with TRIPOLI-4.4 and MCNP5-1.40; one is a simple model calculation and the other is an analysis of iron fusion neutronics experiments with DT neutrons at the Fusion Neutronics Source (FNS) facility in Japan Atomic Energy Agency (JAEA). In the simple model calculation, we adopted a sphere of 0.5m in radius with a 20MeV neutron source in the center and calculated leakage neutron spectra from the sphere. We also analyzed in situ and Time-of-Flight (TOF) experiments for iron at JAEA/FNS. For the in situ experiment, neutron spectra and reaction rates for dosimetry reactions were calculated for several points inside the assembly. For the TOF experiment, angular neutron leakage spectra from the assembly were calculated. Results with TRIPOLI were comparable to those with MCNP in most calculations, but a difference between TRIPOLI and MCNP calculation results, probably caused by inadequate treatment of inelastic scattering data in TRIPOLI, appears in some calculations.

Calculations to support JET neutron yield calibration: Neutron scattering in source holder

After the coated CFC wall to ITER-Like Wall (Beryllium/Tungsten/Carbon) transition in 2010–11, confirmation of the neutron yield calibration will be ensured by direct measurements using a calibrated 252Cf neutron source deployed by the in-vessel remote handling boom and Mascot manipulator inside the JET vacuum vessel.The paper describes preliminary calculations and the results of numerical study of the effect of source holder on neutron detector response. The source baton was designed in such a way, that it does not significantly affect the neutron spectrum, angular neutron flux distribution or activation detector response. All effects are approximately equal to or less than 1%. The largest disturbance to the neutron flux angular distribution and to the neutron spectrum arises from the source capsule. Hence one should obtain as much information as possible about the capsule and the 252Cf source material in order to avoid additional systematic errors.

Development of methods for processing spectra of multiple small-angle neutron scattering

A method has been developed for processing spectra of multiple small-angle neutron scattering (MSANS) to obtain information about substance heterogeneities, namely, their size and concentration. A method has been proposed for constructing the dependence of the MSANS line width on the sample thickness starting from the angular neutron distribution measured for one sample. The standard method for processing this dependence has been improved for application at any scattering multiplicity and complete account for the instrumental line of the double-crystal spectrometer. The method has been tested for the MSANS spectra of the samples of the Fe-Ni ferromagnet, Al powders, and HTSC ceramics.

A new approach to prompt fission neutron TOF data treatment

Abstract The prompt neutron emission in spontaneous fission of 252Cf has been investigated applying digital signal electronics along with associated digital signal processing algorithms. A new mathematical approach, applicable to single events, was developed for prompt fission neutron (PFN) time-offlight distribution unfolding. The main goal was to understand the reasons of the long existing discrepancy between theoretical calculations and the measurements of prompt fission neutron (PFN) emission dependence on the total kinetic energy (TKE) of the fission fragments (FF). Since the 252 Cf (sf) reaction is one of the main references for nuclear data the understanding of the PFN emission mechanism is very important both for nuclear fission theory and nuclear data. The experimental data were taken with a twin Frisch-grid ionization chamber and a NE213-equivalent neutron detector in an experimental setup similar to the well known work of C. Budtz-Jorgensen and H.-H. Knitter. About 2.5 × 10 5 coincidences between fission fragment (FF) and neutron detector response to prompt fission neutron detection have been registered ( ∼ 1.6 × 10 7 of total recorded fission events). Fission fragment kinetic energy, mass and angular distribution, neutron time-of-flight and pulse shape have been investigated using a 12-bit waveform digitizer. The signal waveforms have been analyzed using digital signal processing algorithms. The main goal of this work was a detailed description of the prompt fission neutron treatment.

Application of a nodal collocation approximation for the multidimensional PL equations to the 3D Takeda benchmark problems

P L equations are classical approximations to the neutron transport equations, which are obtained expanding the angular neutron flux in terms of spherical harmonics. These approximations are useful to study the behavior of reactor cores with complex fuel assemblies, for the homogenization of nuclear cross-sections, etc., and most of these applications are in three-dimensional (3D) geometries. In this work, we review the multi-dimensional P L equations and describe a nodal collocation method for the spatial discretization of these equations for arbitrary odd order L, which is based on the expansion of the spatial dependence of the fields in terms of orthonormal Legendre polynomials. The performance of the nodal collocation method is studied by means of obtaining the k eff and the stationary power distribution of several 3D benchmark problems. The solutions are obtained are compared with a finite element method and a Monte Carlo method.

Investigation of the Neutron Angular Distribution and Neutron Yield on the APF Plasma Focus Device

This paper presents an experimental study of neutron yield as well as neutron angular distribution on the APF plasma focus device. The system operates with pure deuterium gas in varying the filling pressure and working voltages of 11.5 and 13.5 kV. The maximum average of neutron yield is (2.88 ± 0.29 × 108) neutrons per shot at the pressure of 7 torr. The neutron angular distribution is measured with housing an array of seven silver activation Geiger-Muller counters at the angles of 0°, ±30°, ±60°, and ±90° in a distance of 90 cm from the anode tip. The results of neutron angular distribution suggest that the neutron production mechanism may be predominantly beam target model.

Measurement of neutron elastic scattering cross sections for carbon at 134 MeV

Abstract Angular differential neutron elastic scattering cross sections have been measured by using a 7 Li(p,n) quasi-monoenergetic neutron beam. The neutron beam whose peak energy is 134 MeV was incident upon a natural carbon sample, and the scattering neutrons were measured by liquid organic scintillators with the time-of-flight (TOF) technique. The results were compared with the existing experimental data measured by a recoil-proton telescope, and the evaluated nuclear data of JENDL/HE-2007 and ENDF/B-VII.0.

Tensile Deformation Behaviors of Metastable Austenitic Stainless Steels Studied by Neutron Diffraction

Tensile deformation behaviors of three austenitic stainless steels, JIS-SUS310S, 304 and 301L, were studied by static tensile tests and in situ neutron diffraction. In the mechanical properties obtained by the static tensile tests, the 304 and 301L steels showed better balance of tensile strength and uniform elongation than the 310S one because of TRIP effect. The angular dispersion neutron diffractions with a wavelength of 0.16 or 0.182 nm were performed during stepwise tensile testing by using a neutron diffractometer for residual stress analysis (RESA) at the Japan Atomic Energy Agency. The lattice plane strain, stress-induced martensite volume fraction, dislocation density and so on were estimated by the profile analysis as a function of applied stress. The change in lattice plane spacing for austenite indicated four deformation stages. In the comparison of lattice plane strain among the tested steels, a phase stress caused by the stress-induced martensite seems to overlap the intergranular stress of austenite phase. Judging from the results of profile analysis, the strain partitioning of austenite phase in metastable austenitic steels became larger with increasing of the volume fraction of stress-induced martensite during tensile deformation.

Double-Differential Neutron Emission Cross-Section Measurements of 6Li and 7Li at Incident Neutron Energies of 8.17 and 10.27 MeV

Energy angular neutron emission double-differential cross sections (DDXs) of 6Li and 7Li were measured at incident neutron energies of 8.17 and 10.27 MeV, respectively, using normal and normal + abnormal fast neutron time-of-flight (TOF) spectrometers. The effects of breakup neutrons from a D(d,n) source and the influence of an aluminum container of Li samples, as well as 7Li in the 6Li sample and 6Li in the 7Li sample, on the secondary neutron spectra were eliminated. The data were derived by comparing the net TOF spectra with the calculated spectra using a realistic Monte Carlo simulation. The differential cross sections were determined by comparing the measured and simulated TOF spectra with respect to specific scattering fractions, i.e., with respect to the elastic lines and the resolved inelastic lines related to single levels or level groups, and normalized to n-p scattering. The angular distributions for the 6Li and 7Li elastic and inelastic neutron scattering were obtained also. The angle-integrated cross sections were derived for elastic scattering from 6Li and for the sum of elastic and 0.478-MeV state inelastic scattering from 7Li. Inelastic scattering cross sections were obtained for the 2.186-MeV state in 6Li and the 4.652-MeV state in 7Li. Meanwhile, based on the unified Hauser-Feshbach and exciton model, the calculated results of the DDXs for n + 6,7Li were compared with measurements.

Physics of ion acceleration in the solar flare on 2005 September 7 determines γ-ray and neutron production

Relativistic neutrons were observed by the neutron monitors at Mt. Chacaltaya and Mexico City and by the solar neutron telescopes at Chacaltaya and Mt. Sierra Negra in association with an X17.0 flare on 2005 September 7. The neutron signal continued for more than 20 min with high statistical significance. Intense emissions of γ -rays were also registered by INTEGRAL, and during the decay phase by RHESSI. We analyzed these data using the solar-flare magnetic-loop transport and interaction model of Hua et al. [Hua, X.-M., Kozlovsky, B., Lingenfelter, R.E. et al. Angular and energy-dependent neutron emission from solar flare magnetic loops, Astrophys. J. Suppl. Ser. 140, 563–579, 2002], and found that the model could successfully fit the data with intermediate values of loop magnetic convergence and pitch-angle scattering parameters. These results indicate that solar neutrons were produced at the same time as the γ -ray line emission and that ions were continuously accelerated at the emission site.

Optimizations in angular dispersive neutron powder diffraction using divergent beam geometries

Angular dispersive neutron powder diffractometers are usually built using beam divergencies defined by Soller type collimators. To account for the needs of resolution for crystal structure refinement a good in-pile collimation α 1 , a high take-off angle above 90 ∘ at the monochromator and a good collimation α 3 in front of the detector bank are chosen whereas the value of α 2 for the collimation between monochromator and sample is less crucial. During the last years new strategies were developed at our institute using wide divergent beam geometries defined by fan collimators or slit-type diaphragms which correlate ray direction and wavelength within the beam. Here we present the performance of a newly developed fan collimator, which enables one to adjust the opening of the collimator channels on both sides independently. This fan collimator is positioned in front of the monochromator at the instrument E6 at the Helmholtz Centre Berlin (formerly Hahn-Meitner-Institut Berlin). It will be shown that control of the beam divergency allows optimization of the resolution in a large angular diffraction range. Hence the resolution and intensity can be adapted to the needs of powder diffraction. Monte Carlo simulations using McStas are used to check and prove the optimal setting of the instrument. We obtain a very good agreement between experimental and simulated data and demonstrate the superior outcome of the new instrument configuration with respect to Soller type instruments.