Neutron Angular Distributions Research Articles

Intensity measurements and shielding of a fast-neutron beam for biological and medical applications

The (d, n) reaction, on thick ( 9Be, 12C, natCu, natMo, 181Ta, 197Au) and thin ( 9Be) targets, has been performed at E d = 16, 33, 50 MeV; neutron spectra, angular distributions, and doses are compared with estimations. The 50 MeV neutron beam from the 9Be(d, n) reaction has been used to study the neutron transmission through some shielding materials, i.e. steel, lead, lucite, borated paraffine, and concrete.

Fast neutron yields and spectra from targets of varying atomic number bombarded with deuterons from 16 to 50 MeV (for radiobiology and radiotherapy)

Neutron production from targets of Be, C, Mo, Cu, Ta and Au has been studied at the isochronous cyclotron at Louvain-la-Neuve. Neutron spectra were measured by the time of flight method. The yields of neutrons and gamma rays were also measured, and the greatest ratio of neutrons to gamma rays in the forward direction was found to occur with 50 MeV deuterons on a Be target. The angular distribution of neutrons from Be was measured at 16, 33 and 50 MeV, and neutron spectra were measured as a function of angle with 50 MeV deuterons on Be.

Support for Crand Theory from measurements of Earth albedo neutrons between 70 and 250 MeV

The results of high energy neutron measurements during two balloon flights on May 1 and 25, 1971, from Palestine, Texas, are reported. At atmospheric depths of 8.6 and 4.7 g cm−2 we obtained the energy and angular distribution of atmospheric neutrons between 70 and 250 MeV. The upward moving part, in terms of the albedo neutron leakage current, can be described by J2π=2.53 · E−1.89 neutron/cm² s MeV in this energy range. The angular distribution of the neutron flux shows a pronounced peak in the horizontal direction (zenith angle, 90°) with a full width at half maximum of about 30°. The measured energy spectrum is in good agreement with results of recent Monte-Carlo calculations on atmospheric neutrons. Together with the angular distribution it also confirms theoretical estimates of the albedo neutron source strength required in a Crand (cosmic ray albedo neutron decay) theory where the high-energy protons in the earth's inner radiation belt are injected by decaying neutrons. The instrument used employs a double scattering technique and has been calibrated at the synchrocyclotron of the Atomic Energy Research Establishment, Harwell, England.

Neutron, proton, and α-particle emission from 3He induced reactions

A study of the three most important outgoing reaction channels has been undertaken in order to see if pre-equilibrium models can simultaneously explain spectra of different particles emitted in the same reaction. Measurements of energy and angular distributions of neutrons, protons and α-particles emitted from 3He induced reactions on 57Fe, 61Ni, 62Ni and 63Cu at an incident energy of 25.6 MeV were performed. The hybrid model succeeds in giving an overall description of particle spectra, when initial configurations of four excitons are involved in the case of nucleon emission. Besides the evaporative and pre-equilibrium processes, a contribution due to the break-up of the incident particle has also to be taken into account in the case of proton spectra.

Spin dependence of the nuclear level density from angular distributions of (α, n) reactions

The spin dependence of the nuclear level density of 49Cr, 57, 59Ni, 62Cu, 61, 63, 65Zn, 93Mo, 96Tc, 95Ru and 118Sb has been determined by deducing spin cut-off factors from measured angular distributions of neutrons from the 46Ti, 54, 56Fe, 59Co, 58, 60, 62Ni, 90Zr, 93Nb, 92Mo and 115In(α, n) reactions. For low energies of the emitted neutrons the angular distributions are symmetrical around 90° as expected for compound nuclear reactions. In most cases the anisotropy is of the order of 10%. The absolute cross sections agree well with the predictions of the statistical model of nuclear reactions including angular momentum. Experimental information on the energy dependence of relevant level densities given in recent compilations was used in the calculations. The spin cut-off factors were parametrized by effective moments of inertia for the final nuclei. Most of these are somewhat smaller than the moment of inertia of a rigid nuclear sphere assuming a radius parameter r 0 = 1.25 fm.

Estimation of Anisotropic Effect of Diffusion Coefficient in a Fast Critical Assembly

The effect of neutron streaming upon the neutron multiplication factor in a fast critical assembly FCA-VI is estimated by considering the anisotropy of the diffusion coefficient. In this paper, the Benoist formula is adopted to obtain the diffusion coefficient. In his original expression, many effects such as those of neutron angular distribution, multiple collision (correlation of different neutron passes) and finiteness of lattice system are included. In order to estimate these effects, the generalized first-flight collision probability method is adopted as in the previous paper, and an improvement is brought over the previous method so as to calculate the effective diffusion coefficient in a practical slab lattice cell which is asymmetric about the center of the cell. Using the method of calculation described above, the anisotropic effect of neutron streaming in the lattices TA, TB, TC and TD used in the FCA VI-1 assembly is estimated. The effect of anisotropy of the diffusion coefficient upon the mult...

Low lying states of 96Tc

Abstract The 96Mo(p, n) and 96Mo(p, nγ) reactions have been studied for proton energies between 3.8 and 5.5 MeV. Energy levels in 96Tc up to 632 keV excitation energy have been determined. Possible spin and parity assignments are given for several levels based on the neutron enhancement and angular distributions observed on and off resonance of the 5 2 >+ isobaric analog state in 97Tc, as well as the observed γ-yields. The first excited state reported at 34 keV was found to be a close doublet only 0.8 keV apart. The observation of this doublet in the (p, n) reaction was used to determine the ground state Q-value Q = − 3.760±0.010 MeV.

Angular distribution and altitude dependence of atmospheric neutrons from 10 to 100 MeV

The altitude dependence of atmospheric neutrons from ground level to 5 g/sq cm of residual atmosphere at neutron energies of 10 to 100 MeV is reported. Ground level measurements were taken at Cape Girardeau, Missouri, on Sept. 18, 1972. The other measurements were made during ascent and float on launch from Palestine, Texas, on Sept. 26, 1971. The intensity of both the downward- and the upward-moving neutrons is maximum at about 100 g/sq cm of residual atmosphere. Neutron angular distributions are reported from 20 to 80 deg and from 100 to 160 deg for 10- to 100-MeV neutrons. Omnidirectional fluxes at altitudes of 5, 50, 100, and 200 g/sq cm of residual atmosphere are in good agreement with recent theoretical calculations of Armstrong et al. (1973) in the three energy intervals of 10 to 30, 30 to 50, and 50 to 100 MeV.

Estimation of Anisotropic Effect of Diffusion Coefficient in a Fast Critical Assembly

The effect of neutron streaming upon the neutron multiplication factor in a fast critical assembly FCA-VI is estimated by considering the anisotropy of the diffusion coefficient. In this paper, the Benoist formula is adopted to obtain the diffusion coefficient. In his original expression, many effects such as those of neutron angular distribution, multiple collision (correlation of different neutron passes) and finiteness of lattice system are included. In order to estimate these effects, the generalized first-flight collision probability method is adopted as in the previous paper, and an improvement is brought over the previous method so as to calculate the effective diffusion coefficient in a practical slab lattice cell which is asymmetric about the center of the cell. Using the method of calculation described above, the anisotropic effect of neutron streaming in the lattices TA, TB, TC and TD used in the FCA VI-1 assembly is estimated. The effect of anisotropy of the diffusion coefficient upon the multiplication factor for the TA, TB, TC and TD lattices is found to be −0.156, −0.181, −0.242 and −0.330% δk/k, respectively. Usually, the effects of neutron distribution, multiple collision and anisotropic scattering are neglected in evaluating the diffusion coefficient when using the Benoist formula. Among these factors, the effect of multiple collision of neutrons has the largest effect upon the diffusion coefficient and upon the neutron multiplication factor. For the TA lattice, 30% of the overall effect of anisotropy is attributable to multiple collision.

Molecular flow of ultracold neutrons through long tubes

Monte Carlo calculations have been performed for the transmission probability of cylindrical neutron guides and for the angular distribution of neutrons leaving the guide, the directions of motion of entering neutrons being assumed to obey the cosine law. Specular reflections are supposed to occur at the guide walls. Analytical formulae suitable for long tubes have been obtained. It is shown that the directions of propagation of neutrons at the exit are distributed strongly in a direction parallel to the tube axis.

Phase-shift and optical-model analysis of the elastic scattering of neutrons from12C

A measurement of the angular distribution of neutrons elastically scattered from carbon-12 at 1.98 to 4.64 MeV, where the elastic scattering is the only energetically allowed reaction channel for the outgoing neutrons, is presented. Differential cross sections and resonances are also found. The purpose is to improve the experimental data and to present a phase shift and an optical model analysis of the results. (JFP)

Use of a Monte Carlo computer code to design a neutron therapy facility

A complete analogue Monte Carlo computer code is being used to optimize the collimator and shielding design for a proposed fast neutron therapy facility using a 30 inch cyclotron. The program uses as input the available neutron and gamma -ray cross-section data and the recently measured neutron energy and angular distributions from a thick beryllium target.

Precompound processes in (α, n) reactions

Energy and angular distributions of neutrons emitted in (α, n) reactions on 56Fe, 57Fe, 103Rh, 115In, 116Sn, 117Sn, 118Sn, 119Sn and 124Sn at E α = 20 MeV were measured by the time-of-flight method. Data were analysed in the framework of the pre-equilibrium model. Two alternative calculations, namely the Griffin-Blann and the hybrid models, were performed. Values of about 10% for the precompound fraction resulted from the analysis, in agreement with existing results. Different initial configurations consisting of three, four or five excitons were obtained, suggesting a systematic dependence on the neutron number of the target nucleus.

Calculations on the scattering of keV neutrons from the 7Li ( p, n) 7Be reaction in a typical target assembly

The angular distributions and energy spectra of keV neutrons scattered from a typical target assembly employing the 7Li ( p, n) 7Be reaction have been calculated by the Monte Carlo method for several proton energies, target backing and beam tube wall thicknesses and target radii. A brief discussion is given of possible errors due to scattering in the experimental measurements of neutron activation capture cross-sections, dose equivalents and angular distributions.

The Sensitivity of Neutron Transport Calculations to Nonelastic Neutron Angular Distributions

The Sensitivity of Neutron Transport Calculations to Nonelastic Neutron Angular Distributions

Angular distributions of neutrons behind an iron shield

Angular distributions of neutrons behind an iron shield

Excited States ofTc94Studied Through the Enhancement of Neutron and Gamma-Ray Yields from theMo94(p, n)Tc94Reaction at Isobaric Analog Resonances

A revised and extended level scheme for $^{94}\mathrm{Tc}$ results from the combined study of neutron and $\ensuremath{\gamma}$-ray spectra emitted at proton energies near isobaric analog resonances. The cross sections for the $^{94}\mathrm{Mo}(p, n)^{94}\mathrm{Tc}$ reactions and relative yields of $\ensuremath{\gamma}$ rays from the decay of the states of $^{94}\mathrm{Tc}$ have been measured on the energy interval between 5.35 to 6.1 MeV in the vicinity of three analog resonances. The energies of neutrons emitted were measured by the time-of-flight technique. Excitation energies of levels in $^{94}\mathrm{Tc}$ were determined from the neutron spectra and the energies of the $\ensuremath{\gamma}$ rays observed. The enhancement of the neutron and $\ensuremath{\gamma}$ ray yields with measurements of neutron angular distributions has been compared with statistical model calculations. The results provide spin-parity assignments to two levels and suggest others. Of particular interest is the location of the ${2}^{\ensuremath{-}}$ and ${3}^{\ensuremath{-}}$ states separated by an energy of 142 keV. The results illustrate the use of the statistical analysis at isobaric analog resonances according to the method of Robson for the extraction of spectroscopic information about nuclei. Comparison with shell-model calculations is given.

DWBA analysis of the reaction 12C(α, n) 15O

Angular distributions of neutrons from the reaction 12C(α, n) 15O(g.s.) have been measured at lab energies from 18.4 to 23.1 MeV and angles ranging from 0° to 130°, using a time-of-flight technique. The experimental curves generally show a forward peaking and a strong dependence on the incident energy. The data were compared with the angular distributions predicted by the distorted-wave theory of direct Nuclear reactions, and no agreement could be obtained when only a stripping mechanism was taken into account.

Space and angle dependent collision probability in cell problems

The integral transport equation is formulated in terms of the generalized collision probability in a cylindrical lattice cell in one velocity. The anisotropy of the neutron angular distribution and the scattering is considered by representing them with spherical harmonics series respectively. The space dependence of the neutron distribution is taken into account by representing it with a Legendre series in each annular region. The isotropic return condition or white boundary condition is adopted for the generalized collision probability. One group disadvantage factors are calculated for cylindrical lattices in both cases of isotropic and linearly anisotropic scattering using the generalized first-flight collision probability.

Measurements of neutron angular distributions from the 7Li(p, n) 7Be reaction

The absolute zero-degree differential cross sections of the 7Li(p, n 0) 7Be and the 7Li(p, n 1) 7Be reactions have been measured with a time-of-flight spectrometer at 0.1 MeV intervals from 2.20 to 5.50 MeV. Angular distributions of neutrons from both reactions were also measured at 0.2 MeV intervals from 2.60 to 5.40 MeV for the n 0 neutron group and from 3.20 to 5.40 MeV for the n 1 group. Absolute differential cross sections were obtained by normalization to the zero-degree measurement. The centre-of-mass distributions were fitted with a Legendre polynomial series. Total cross sections were obtained by integrating the angular distributions. The comparison between present and previous results has revealed some disagreement in the shapes of angular distributions and in the magnitude of total cross sections for the 7Li(p, n 1) 7Be reaction.