Neutron streaming through conical ducts in a water shield

Total albedos, as well as albedos differentiated in terms of both energy and direction, were calculated by means of a Monte Carlo program for cylindrical water surfaces of different radii of curvature. It is shown that a cylindrical coordinate system in which the polar angle is measured relative to the normal on the scattering surface is preferable to one in which this angle is measured relative to the axial direction of the surface. For small polar angles in the preferred system, even when the surface radius approaches the mean free path of the incident neutrons, the effect of this radius remains small. With increasing polar angles the effect of the surface radius becomes more important, especially for those neutrons moving more or less perpendicular to the axial direction of the surface. In the latter case, even at radii as large as 10 n mean free paths, the radial effect at a large polar angles is significant.     

Theory of heterogeneous reactors with cylindrical lumps of a finite radius

This article presents a consistent theory of heterogeneous reactors with cylindrical lumps of a finite, but small, radius. The density of thermal neutrons inside a lump and in the moderator is described by taking into account the azimuthal dependence. The diffusion theory is applied to the entire reactor volume. Simple expressions for the diffusion length in the parallel and perpendicular directions with an accuracy to the first power of the lump surface to cell surface ratio are obtained. It is shown that the average scattering length for the perpendicular direction depends on the lump shape. A simple method for determining the diffusion tensor for the case of weak absorption and large spacing between the lumps is considered in section 4.The author is indebted to B. I. Il'ichev, N. I. Laletin, and Ya. V. Shevelev for the discussion of this paper.     

The Effect of Cylindrical Holes in a Moderator on the Passage of Virgin Neutrons from a Point Source

The mean free path and the mean square free path of 14 MeV neutrons, generated isotropically from a d-t point source located at the center of the moderator region of a square lattice composed of parallel circular cylindrical holes in water, are calculated for various void ratios. Very interesting inequalities between the axial and transversal components of the mean free path and the mean square free path are found, which contradict with the results of Behrens for homogeneous neutron sources. We may conclude that the effects on the free path lengths of the position of the point source located in an anisotropic and heterogeneous medium cannot be neglected.     

Calculation of the passage of fast neutrons through graphite

Using the Monte-Carlo method, the author calculates the results of passing neutrons from a plane unidirectional source, with E₀=3.3 or E₀=8.0 MeV, through graphite. The angle of incidence is taken as 0°. The graphite layer thicknesses considered are from 0.9 to 6 times the free-path length. The author calculates the dose, energy and numerical albedos of graphite and also the angular and energetic distributions of the reflected neutrons and the angular distribution of the energy of scattered neutrons. He plots the mean cosine of the angle of scattering vs. the albedo, and also vs. the value of the energy angulardistribution constant. The data obtained may be found useful in the design of shadow and labyrinth shielding.Translated from Atomnaya Énergiya, Vol. 22, No. 2, pp. 97–100, February, 1967.     

Porosity effects on the neutron total cross section of graphite

For subthermal neutron energies, polycrystalline graphite shows a larger total cross section than predicted by existing theoretical models. In order to investigate the origin of this discrepancy we measured the total cross section of graphite samples of three different origins, in the energy range from 0.001 to 10 eV. Different experimental arrangements and sample treatments were explored, to identify the effect of various experimental parameters on the total cross section measurement. The experiments showed that the increase in total cross section is due to neutrons scattered around the forward direction. We associate these small angle scattered neutrons (SANS) to the porous structure of graphite, and formulate a very simple model to compute its contribution to the total cross section of the material. This results in an analytic expression that explicitly depends on the density and mean size of the pores, which can be easily incorporated in nuclear library codes.     

Measurement of Cold Neutron Spectrum with Liquid Mirror Reflection

Making use of the wavelength dependency of the critical angle for the total reflection of neutrons, the possibility of determining the energy spectrum of cold neutrons by means of a liquid mirror is examined. A carbon tetrachloride mirror, combined with a pair of fine slits, are used for the measurement of the angular distribution of the reflection intensity of beryllium filtered neutrons. For determining the correction required to account for non-reflected neutrons detected by the counter, the direct beam component is measured by disturbing the liquid mirror surface with small ripples generated by an ultrasonic vibrator submerged in the liquid, and the value obtained with this clouded mirror is subtracted from that determined with the clear undisturbed liquid mirror. The experimental results qualitatively show good agreement with those obtained with a helical-slot neutron velocity selector, and further reveal even better resolution than possible with the latter method, in the cold neutron energy region.     

A New Target Material for the Production of I-131

The spacial distributions of neutrons and photons have been studied in a cylindrical duct through a simulated iron-water multi-layered shield, filled with an iron plug on which six helical grooves had been cut at regular intervals and with pitches of 22, 44, 88 or 176 cm.

The results obtained are discussed in relation to the three aspects of axial distribution, cross-sectional distribution and streaming. It is shown that the axial distributions of the fast neutrons and photons accord with an exponential law, while those of the thermal and epithermal neutrons are the sum of two exponential functions. It has been found that the cross-sectional distributions of neutrons and photons have quite a sharp peak on the beam axis, indicating that the transmission through the duct is more important than the scattering and diffusion in the duct. The streaming effect along the helical grooves is not very marked in any of the cases, because of the large volume fraction of the iron plug to the total duct volume (79–89%).     

Some remarks on the neutron elastic—And inelastic—Scattering cross sections of palladium

The cross sections for the elastic-scattering of 5.9, 7.1 and 8 MeV neutrons from elemental Pd were measured at 40 scattering angles distributed between ≈15 and 160°. The inelastic-scattering cross sections for the excitation of Pd levels at energies of 260–560 keV were measured with high resolution at the same energies, and at a scattering angle of 80°. The experimental results were combined with lower-energy values previously obtained by this group to provide a comprehensive data base extending from near the inelastic-scattering threshold to 8 MeV. That data base was interpreted in terms of a coupled-channels model, including the inelastic excitation of one- and two-phonon vibrational levels of the even isotopes of Pd. It was concluded that the Pd inelastic-scattering cross sections, at the low energies of interest in assessment of fast-fission-reactor performance are large (≈50% greater than given in widely-used evaluated fission-product data files). They primarily involve compound-nucleus processes, with only a small direct-reaction component attributable to the excitation of the one-phonon 2⁺ vibrational levels of the even isotopes of Pd.     

Point Scattered Function (PScF) for fast neutron radiography

Fast neutron radiography opened up a new range of possibilities to image extremely dense objects. The removal of the scattering effect is one of the most challenging problems in neutron imaging. Neutron scattering in fast neutron radiography did not receive much attention compared with X-ray and thermal neutron radiography. The purpose of this work is to investigate the behavior of the Point Scattered Function (PScF) as applied in fast neutron radiography.The PScF was calculated using MCNP as a spatial distribution of scattered neutrons over the detector surface for one emitting source element. Armament and explosives materials, namely, Rifle steel, brass, aluminum and trinitrotoluene (TNT) were simulated. Effect of various sample thickness and sample-to-detector distance were considered. Simulated sample geometries included a slab with varying thickness, a sphere with varying radii, and a cylinder with varying base radii. Different neutron sources, namely, Cf-252, DT as well as DD neutron sources were considered. Neutron beams with zero degree divergence angle; and beams with varying angles related to the normal to the source plane were simulated.Curve fitting of the obtained PScF, in the form of Gaussian function, were given to be used in future work using image restoration codes. Analytical representation of the height as well as the Full Width at Half Maximum (FWHM) of the obtained Gaussian functions eliminates the need to calculate the PScF for sample parameters that were not investigated in this study.     

Simulation of momentum flux ratios in the cascade in the sputtering of Mo---Mo

Momentum flux ratios ⁹²Mo/¹⁰⁰Mo towards the surface and the bulk through marker planes have been studied by a static Monte Carlo simulation program for an isotopic mixture ⁹²Mo_0.5¹⁰⁰Mo_0.5 bombarded with 5 keV Xe ions. The calculations show that the momentum flux ratios in the cascade deviate far from one at different surface depths. The calculations also show that the zero-fluence isotopic ratio in the near-normal direction and its difference between small and large polar emission angles are positive.     

Angular spectra of rainbow scattering at glancing keV He bombardment of NiAl(1 0 0) surface with transverse energies in the range 1-10 eV

Previous simulations of glancing incidence ion-surface interaction have demonstrated that classical dynamics using the row-model have successfully reproduced multimodal azimuthal and polar spectra. These studies have also shown considerable sensitivity to the form of the interatomic potential thus making it a strong test of the validity of such potentials and even allow deduction of the ion-surface potentials. In these simulations the individual pairwise interactions between the projectile and the target atoms have been replaced by cylindrical potentials.Comparison to numerous experimental studies have confirmed the existence of rainbow scattering phenomena and successfully tested the validity of the cylindrical potential used in these simulations. The use of cylindrical potentials avoids stochastic effects due to thermal displacements and allows faster computer simulations leading to reliable angular distributions.In the present work we extend the row-model to consider scattering from binary alloys. Using He⁺ scattered at glancing incidence from NiAl surfaces, Al or Ni terminated, a faster method has been developed to easily and accurately quantize not only the maximum deflection azimuthal angle but all the singular points in the angular distribution. It has been shown that the influence of the surface termination on the rainbow angle and the inelastic losses is small.     

Geometrical Interpretation of Mutual Relatonship between Ordinary Coherence and Noise Power Contribution Ratio

Angular neutron fluxes leaking from the surface of lithium-oxide and graphite slab assemblies have been measured with irradiation of D-T neutrons. The spectrum measurement was performed using the time-of-flight technique with an NE213 scintillation detector. The thicknesses of the slabs were 0.6 to 5 mean free path for 14.8 MeV neutrons, and the measured leaking angles of the angular fluxes were 0.0°, 12.2°, 24.9°, 41.8° and 66.8°. The experimental results have been compared with the results calculated by the continuous energy Monte Carlo transport code MCNP, using the data in the JENDL-3PR1, ?3PR2, and ENDF/B-V nuclear data files. The comparisons between the experimental and calculated results show that the data of ⁷Li in JENDL-3PR2 is improved for the secondary emission spectra of the 4.63 MeV level and (n, 2n) reactions; the angular distributions of 3rd-and 4th-level inelastic reactions of C in the JENDLs are questionable. The thickness dependences for high energy neutrons also suggest that the total cross section of ⁷Li and the elastic cross sections of C are slightly inadequate.     

Auger electron yields induced by channeled ions at grazing incidence

The spectroscopy of Auger electrons induced by channeled ions is discussed by experiments and computer simulations as regards its applicability in surface structure studies. A Ni(110) surface bombarded with 150 keV protons under axial surface channeling and planar channeling conditions is used as an example. It is shown that axial surface channeling provides an excellent surface sensitivity for the application to surface structure analysis and location of adsorbed atoms, whereas planar channeling due to larger grazing angles of incidence to the surface does not. The influence of the impact parameter dependence of the ionization probability on the channeling dips is investigated by means of two different Auger lines: nickel M₂₃VV and L₃VV. The results yield a criterium for the selection of the most suitable Auger transition for the application of this technique. The influence of the mean free path of the induced Auger electrons is examined and the effect of thermal vibrations and surface steps is discussed.     

The Characteristics of Columniform Surface Wave Plasma Excited Around a Quartz Rod by 2.45 GHz Microwaves

A novel surface wave plasma(SWP) source excited with cylindrical Teflon waveguide has been developed in our previous work. The plasma characteristics have been simply studied.In this work, our experimental device has been significantly improved by replacing the Teflon waveguide with a quartz rod, and then better microwave coupling and higher gas purity can be obtained during plasma discharge. The plasma spatial distributions, both in radial and axial directions, have been measured and the effect of gas pressure has been investigated. Plasma density profiles indicate that this plasma source can produce uniform plasma in an axial direction at low pressure, which shows its potential in plasma processing on a curved surface such as an inner tube wall. A simplified circular waveguide model has been used to explain the principle of plasma excitation. The distinguishing features and potential application of this kind of plasma source with a hardware improvement have been shown.     

Nucleus-nucleus bremsstrahlung from scattering angle selected Cl-Sn collisions

Continuum X-rays above the K lines of Sn were measured at 90° and 0° angle relative to the incident beam direction in coincidence with Cl projectiles scattered at very large laboratory angles between 8° and 164°. By varying the isotope mass of the projectile ions (³⁵Cl and ³⁷Cl) and of the target atoms (¹¹²Sn and ¹¹⁸Sn) the radiation at scattering angles > 10° could be clearly identified as bremsstrahlung from the elementary scattering process of the two nuclei. The intensity of the radiation emitted at 0° and 90° relative to the beam direction can be reasonably well described by a quantum mechanical calculation of nucleus-nucleus bremsstrahlung by Reinhardt et al. [7] which includes the interference between dipole and quadrupole radiation amplitude.     

Angle and energy distributions of fission neutrons

Using the statistical model of the nucleus the angle distribution of fission neutrons is determined taking into account the anisotropy in the angle distribution of the fission fragments. The latter quantity is approximated by a simple expression of the form 1 + kcos² α. It is assumed that the neutrons are emitted isotropically in the coordinate system in which the fragment is at rest. Using U²³⁸, a calculation is carried out to determine the values of P — the ratio of the intensity of fission neutrons emitted in the direction of the incident neutrons to that of neutrons emitted perpendicularly to the incident beam — at various energy values of the primary and secondary neutrons (in the region from 1 to 10 Mev). The mean value of P in fission in U²³⁸ by neutrons characteristic of a fission spectrum is found to be approximately 1.13. The energy spectra for fission neutrons at various primary-neutron energies are also obtained. The method of calculation can also be employed in investigations of the anisotropy of neutrons produced in fission of other nuclei.     

Surface Activation of Plane and Curved Automotive Polymer Surfaces by Using a Fittable Multi-Pin DBD Plasma Source

In this work, surface activation of automotive polymers using atmospheric pressure plasmas was investigated. The aim was to increase the polar fraction of the surface energy of both plane and convex polymer devices with a radius in the range of 30 mm. For this purpose, a fittable low temperature atmospheric pressure plasma source based on capacitively coupled multi-pin electrodes was set up and applied. Each single electrode generates a treatment spot of approximately 2 cm2 with a tunable power density of up to 1.4 W/cm2. The surface energy was evaluated by contact angle measurements. After treatment at a low energy density of 1.01 J/cm2, the polar fraction of the surface energy of the investigated polymers was increased by a factor of 3.3 to 132, depending on the polymer materials. It was shown that by applying the presented fittable plasma source, this effect is independent of the surface radius of the polymer sample.     

Reference Mean Temperature for Evaluation of Performance of Thermal Diffusion Column for Isotope Separation

In order to evaluate separative performance of a thermal diffusion column, a simplification is usually made in which the temperature dependence of the relevant properties such as thermal diffusion constant is ignored and some proper mean values evaluated at a specific “mean” temperature are used. Adoption of weighted average of temperature distribution is common for the “mean” temperature, but there exists no definite way of determining mean temperature. The present paper proposes a new reference mean temperature determined by the equation governing the free convection. It is based on the fact that the multiplication effect of free convection is essential to separation by thermal diffusion column. The reference mean temperature is related to pressure difference between top and bottom of column and is higher than a mass-averaged temperature (due to gravitational force) by a contribution of viscous force. The reference mean temperature was calculated, as a reference, for an Ar isotope separating column with an inner hot radius of 0.2 mm and an outer cold radius of 5 mm. The results confirmed the validity of an approximate formula expressing effects of temperature difference and ratio of inner and outer radii of column explicitly for the temperature. The reference mean temperature calculated from pressure difference given by axisymmetric solution of equations of change was in good agreement with the analytical solution.     

Bragg Cut-Off Effects on Neutron Wave Propagation in Graphite

Theoretical and experimental studies concerning the effects of Bragg cut-off on the neutron wave propagation in graphite are presented, as analyzed from the two following factors related to the Bragg cut-off.

First, the inordinately long mean free path of cold neutrons of energies below the Bragg cut-off causes in some cases serious problems due to the deep penetration and the strong leakage of the cold neutrons.

Secondly, the extremely low collision frequency of about 2,500 rad/sec for the cold neutrons produces a “singular frequency” at a point of even lower frequency, wherein the waves do not decay exponentially in the propagating direction.

The peculiar behavior of these cold neutrons severely restricts the conditions of experiment under which one can derive an exact value for the propagation parameter in graphite. In certain cases, however, one can obtain meaningful approximate information about wave propagation from experiments conducted beyond these limiting conditions, provided that certain precautions are taken in carrying out the experiment and in interpreting the results. Analysis to explain these phenomena and experiments to reveal the effects in graphite are described.     

CTOF measurements and Monte Carlo analyses of neutron spectra for the backward direction from a lead target irradiated with 200-1000 MeV protons

A calorimetric-time-of-flight technique was used for real-time, high-precision measurement of neutron spectra at an angle of 175^o from the initial proton beam direction, which hits a face plane of a cylindrical lead target of 20 cm in diameter and 25 cm thick. A comparison was performed between the neutron spectra predicted by the MARS, RTS&T, MCNP6, and the MCNPX 2.6.0 transport codes and that measured for 200, 400, 600, 800, and 1000 MeV protons. Neutron spectra were measured within the energy range from 0.7 to 250 MeV almost continuously. The transport codes tested here describe with different success the measured spectra, depending on the energy of the detected neutrons and on the incident proton energy, but all the models agree reasonably well with our data.