Pulsed Neutron Experiments Research Articles

Monte Carlo simulation of pulsed neutron experiments on samples of variable mass density

A method is presented to facilitate the interpretation of a pulsed neutron experiment (the variable geometric buckling experiment) when the mass densities of individual samples differ. A generalisation of the classic expression, which connects the fundamental mode decay constant to the thermal neutron diffusion parameters and to the geometrical buckling, is presented. The method has been tested (on polyethylene) by means of a computer simulation of the experiments. The simulation has been based on a Monte Carlo method, using the MCNP code. The described generalised buckling method is especially recommended for experiments with bulk materials.

Reduction of Delayed-Neutron Contribution to Variance-to-Mean Ratio by Application of Difference Filter Technique

The difference-filtering correlation analysis was applied to time-sequence neutron count data measured in a slightly subcritical assembly, where the Feynman-α analysis suffered from large contribution of delayed neutron to the variance-to-mean ratio of counts. The prompt-neutron decay constant inferred from the present filtering analysis agreed very closely with that by pulsed neutron experiment, and no dependence on the gate-time range specified could be observed. The 1st-order filtering was sufficient for the reduction of the delayed-neutron contribution. While the conventional method requires a choice of analysis formula appropriate to a gate-time range, the present method is applicable to a wide variety of gate-time ranges.

Reduction of Delayed-Neutron Contribution to Variance-to-Mean Ratio by Application of Difference Filter Technique.

The difference-filtering correlation analysis was applied to time-sequence neutron count data measured in a slightly subcritical assembly, where the Feynman-α analysis suffered from large contribution of delayed neutron to the variance-to-mean ratio of counts. The prompt-neutron decay constant inferred from the present filtering analysis agreed very closely with that by pulsed neutron experiment, and no dependence on the gate-time range specified could be observed. The 1st-order filtering was sufficient for the reduction of the delayed-neutron contribution. While the conventional method requires a choice of analysis formula appropriate to a gate-time range, the present method is applicable to a wide variety of gate-time ranges.

Effects of Spatial Harmonics on Thermal Neutron Spectra Measured by Time-of-Flight Method

Spatial effect—the variations in space of transient neutron flux noted in pulsed neutron experiments, and which is ascribable to the occurrence in moderator of spatial harmonics—is studied on the measured flight time spectra of neutrons reentering the atmosphere from parametrically varied depths of a graphite moderator. Measurements were made using moderator slabs of two different axial lengths, to examine the influence of differences in moderator size on the spatial effect. The master equation representing the scalar neutron flux as function of time and energy is solved by a semi-analytic method that takes account of spatial harmonics. Simulation of the solution thus obtained of the master equation proved to reproduce the measured flight time spectra with good accuracy. It was indicated that the spatial effect on the thermal neutron spectrum is stronger with a larger than with a smaller moderator slab. The neutron temperature in graphite moderator was calculated by formula fitted in Maxwellian distribution to the peaks of the thermal neutron spectra. It was indicated that in a graphite moderator of 120 cm axial length, spatial harmonics caused a variation of more than 40 K in neutron temperature between depths in moderator differing by 30 cm.

Extrapolation distance in critical and time-dependent two-region spherical systems / Extrapolierte Längen für kritische und zeitabhängige sphärische Systeme

Extrapolation distances for the neutron flux distribution in bounded media are usually defined in such a way that agreement is obtained between diffusion theory and transport theory. A typical application is the interpretation of pulsed neutron experiments. In this work we extend the conventional treatment of extrapolation distances to two-region spherical bodies. Assuming neutrons of one speed, extrapolation distances are calculated for two-region spheres of different neutron properties. Anisotropic scattering of the neutrons is also taken into account and both critical and time-dependent cases are studied.

A new idea of pulsed neutron experiments with bulk materials

Pulsed thermal neutron macroscopic parameters are density dependent. This leads to a serious difficulty in interpreting the experiments when bulk materials are investigated. A method of using the density-removed neutron diffusion parameters as well as the generalized time-decay constant and geometric buckling are presented.

Effects of Spatial Harmonics on Thermal Neutron Spectra Measured by Time-of-Flight Method.

Spatial effect—the variations in space of transient neutron flux noted in pulsed neutron experiments, and which is ascribable to the occurrence in moderator of spatial harmonics—is studied on the measured flight time spectra of neutrons reentering the atmosphere from parametrically varied depths of a graphite moderator. Measurements were made using moderator slabs of two different axial lengths, to examine the influence of differences in moderator size on the spatial effect. The master equation representing the scalar neutron flux as function of time and energy is solved by a semi-analytic method that takes account of spatial harmonics. Simulation of the solution thus obtained of the master equation proved to reproduce the measured flight time spectra with good accuracy. It was indicated that the spatial effect on the thermal neutron spectrum is stronger with a larger than with a smaller moderator slab. The neutron temperature in graphite moderator was calculated by formula fitted in Maxwellian distribution to the peaks of the thermal neutron spectra. It was indicated that in a graphite moderator of 120 cm axial length, spatial harmonics caused a variation of more than 40 K in neutron temperature between depths in moderator differing by 30 cm.

Multiple scattering and inelasticity corrections in thermal neutron scattering experiments on molecular systems

In this work we address the problem of multiple scattering and inelasticity corrections in neutron diffraction measurements for structural studies of molecular systems. A variety of pulsed neutron and reactor experiments was performed on hydrogenous samples under different experimental conditions. Monte Carlo simulations were carried out on the basis of a Synthetic Model to describe the neutron-molecule interaction, and they allowed a simultaneous evaluation of multiple, inelastic and beam attenuation processes into the samples. A very good agreement between measurements and simulations was obtained in all cases, for our demanding choice of samples and experimental conditions.

Calculation of local insulating structure of CaCuO2 with interplanar oxygen defects

The electronic band structures of CaCuO2 with various planar structures have been computed with the spin polarized, full potential pseudofunction method. The perfect tetragonal structure has metallic bands. If the CuO plane is puckered as proposed on the basis of pulsed neutron experiments, a semi-metallic-like behavior is found. When some of the oxygen atoms are moved from the basal plane into interplanar sites that are coplanar with Ca, an insulating ground state is obtained with antiferromagnetic moments of 0.21 μ B . Around this interplanar oxygen defect site, the charge and magnetic moment distributions resemble those of a CuO chain structure with strong coupling to Cu atoms from different basal planes. Small displacements of defect oxygens at interplanar sites along the CaO planar directions produces large changes in band shape near E F , suggesting strong electron-lattice coupling. The coupling is much weaker for displacements of O atoms within the CuO basal plane. Such interplanar defects could occur in superconducting cuprates and thus create small insulating regions for the pinning of magnetic flux lines.

Calculation of semiconductive properties of CaCuO 2-planar phase with interplanar oxygen defects

The electronic properties of CaCuO 2 in the tetragonal structure has been computed with the spin polarized, full potential pseudofunction energy band method. The bands are found to be metallic for the undistorted structure. An insulating antiferromagnetic ground state is obtained if some of the O atoms in the Cu-O plane are placed in the interplanar site between Cu-O planes so as to be co-layered with Ca. The magnetic moment on the Cu sites in this phase is 0.25 μ B. This structure has bond lengths similar to that in the insulating orthorhombic phase of CaCuO 2. In the case of the puckered Cu-O plane which was proposed on the basis of pulsed neutron experiments, semimetallic behavior is obtained. The microscopic origin of intrinsic flux pinning centers for the cuprates superconductivity is proposed based on our band structures for the O defect model. The density of states calculated for the orthorhombic phase agree well with photoemission spectra data for the CaCuO 2 film.

Utilization of intense neutron generators in science and technology

This review outlines the present status of conventional and intense neutron generators and their utilization in basic research and technology. The following topics are discussed: output characteristics of neutron generators; status of microscopic and integral neutron nuclear data; nuclear levels and decay schemes; miscellaneous applications of neutron generators (activation and prompt radiation analysis, pulsed-neutron experiments, fast-neutron irradiation effects).

High-precision thermal neutron diffusion parameters for Plexiglass

The thermal neutron diffusion parameters for Plexiglass have been determined with high accuracy. The classic pulsed neutron experiment based on the concept of variable geometric buckling has been performed (the buckling B2 being from 0.036 to 1.234 cm-2). Details on the treatment of data are given. The following values of the parameters have been obtained at the temperature of 20 degrees C: v Sigma a=(4120+or-30) s-1, D0=(37312+or-100) cm2 s-1, C=(6192+or-457) cm4 s-1, F=(1203+or-400) cm6 s-1. They are compared with the values reported by other authors. The influence of experimental conditions on the final results as well as some theoretical problems concerning diffusion cooling are discussed.

Glass scintillators for pulsed neutron detection

The paper demonstrates that lithium magnesium aluminosilicate glasses containing ∼ 8 wt% of 3L 6 as the neutron detector are the preferred detectors for high intensity pulsed neutron experiments. The Physics of the glasses is reviewed, and a novel fabrication method leading to significantly improved gamma ray rejection detectors is described. Measurements of luminescence decay times as a function of temperature over the range 4–700 k are shown to lead to a model for the glass performance. Fabrication of the glasses is outlined.

The Derivation of Neutron Generation Time for Reflected Systems and Its Physical Interpretation

For single-core reflected neutronic systems, generalized neutron generation time is derived and given physical interpretations in terms of importance. A system kinetic equation containing the moderator region response function previously introduced is reduced by a slow-variation approximation to the form of a conventional one-point kinetic equation, in which a parameter can be identified as generalized neutron generation time by analogy with a bare system. In such a mathematical expression for the parameter, one can further identify the amount of increase due to reflection over the bare system generation time. This amount is found to be the reflection time multiplied by the number of migrations that neutrons undergo between reflector and core in one generation. The theoretical generation time of the SHE assembly, a thermal-energy, graphite-moderated critical assembly, calculated by such a formation with cylindrical geometry, agreed well with that from pulsed neutron experiments.

Computer simulation of pulsed-neutron experiments performed on the Fort St. Vrain high-temperature gas-cooled reactor

Computer simulations were used to determine the optimum source location, detector location, and pulse rate prior to performing pulsed-neutron experiments on the 330-MW Fort St. Vrain high-temperature gas-cooled reactor (HTGR). The simulation procedure involved calculation of the amplitudes, decay constants, and modal shapes of the first few kinetic modes in the general expansion of the time response of the neutron flux following each pulse. The kinetic modes were calculated by the eigenfunction expansion method in two-dimensional geometry assuming two energy groups and six delayed-neutron precursors. The major limitation in the calculation is the use of two-dimensional core models, i.e., the assumption of separation of variables. For most power reactors on which pulsed-neutron experiments might be performed, this limitation should not be serious. (auth)

Analysis and results of pulsed-neutron experiments performed on the Fort St. Vrain high-temperature gas-cooled reactor

Pulsed-neutron experiments were performed on the Fort St. Vrain reactor to determine the reactivity of the core for various control rod configurations while the reactor was still subcritical. For all configurations the reactivity was inferred from the in-hour equation using the measured decay constant and a calculated generation time. For the configurations near critical, both the reactivity and generation time were determined using the extrapolated area-ratio method. The predicted reactivities agreed poorly with those inferred from the experiments. However, by adding 5 ppM of boron to the reflector calculational model, the calculated generation time was significantly reduced. This brought the inferred reactivity into good agreement with that calculated for all control rod configurations. This emphasizes the dependence of the interpretation of pulsed-neutron experiments on calculations and the importance of the reflector in a large HTGR. (auth)

Une méthode de détermination du temps de transfert dans un système de deux zones multiplicatrices de neutrons

With a simplified delay time distribution function in the kinetic equations of a binode system, the neutron evolution is found to follow a two mode exponential decay. In the non-excited core, the evolution curve presents a maximum which time of occurrence takes place in a relation giving the interzonal transfer time. Values obtained from pulsed neutron experiments are discussed.

Measurements of Time Dependent Energy Spectra of Neutrons in a Small Graphite Assembly

The time-dependent energy spectra of neutrons have been measured in a small 30×30×30 cm3 graphite assembly by means of the linac-chopper method, with a view to establishing experimental evidence that there is no asymptotic spectrum in such a small assembly, and in order to study the non-asymptotic behavior of neutrons. The arrangement of a polyethylene pre-moderator adjacent to the assembly made the measurements possible with the improvement obtained thereby of the neutron counting statistics. It was indicated from calculation that the presence of the pre-moderator had little effect—at least above the Bragg cut-off energy—on the evolution in time of the energy spectra of neutrons in the graphite assembly. The experimental results indicated very probable disappearance of asymptotic spectra, and revealed significant enhancement of trapping at Bragg energies with the lapse of time. This is consistent with the results of pulsed neutron experiments in small assemblies conducted by Takahashi et al., and falls in line with de Saussure's approximation. The spectra in the graphite assembly showed significant space dependence, the spectra becoming harder with increasing distance from the pre-moderator. This hardening may be attributed to the relatively faster propagation of higher energy neutrons.

A Theory for Pulsed-Neutron Experiments in Reactor Lattices

A Theory for Pulsed-Neutron Experiments in Reactor Lattices

Time dependent 237Np, 235U and 239Pu fission rates in a thorium assembly during the interval 0 to 200 ns using a pulsed 9Be( d,n) source—I. Experiment

This paper describes a series of integral pulsed neutron experiments performed in a 0·4 × 0·4 × 0·4 m 3 metallic thorium assembly in such a way as to allow direct comparison of space independent reaction rates with calculated reaction rates derived from a code which uses the asymptotic reactor theory approximation to describe leakage. The technique relies on the Fourier decomposition of measured space-time dependent reaction rates and the extraction from these of the reaction rate corresponding to the fundamental three-dimensional Fourier spatial mode. The reaction rates measured were the fission rates of 335U, 239Pu and 237Np following a short ∼ 10 ns) burst of neutrons with a mean energy of ∼ 2·7 MeV.