The distribution of backscattered gamma ray photons in the scattering medium

Monte Carlo calculations were performed to simulate the behaviour of photons scattered back from a semi-infinite medium. The source considered was point isotropic on the surface of the medium, and the media simulated soils of various densities. The values of the number albedo obtained agreed well with those quoted in the literature. The radial distribution was found to fall off exponentially, with a dependence on distance from the source practically independent of source energy and material, provided the photo electric absorption cross section at the source energy is negligible. This conclusion is supported by calculation from results quoted in the literature. The distributions of the initial direction cosine relative to the radius joining source and point of exit, and of the number of scatters, were calculated and tabulated.     

Energy dependent 4-dimensional multiple scattering distributions

A complete analytic solution in Fourier space is presented of the four dimensional small angle, multiple scattering distribution MSD in angle and space, produced by an energy dependent single scattering cross section from an initial pencil beam of heavy particles. Independently, simple integrals are derived for the central moments of the energy dependent MSD in the continuous-slowing-down approximation. The distributions of the projections t and x of the scattering angle and displacement into a plane through the axis of propagation are evaluated numerically for a truncated Rutherford scattering cross section using a fast Fourier transform. The resulting MSDs for a wide range of particles, initial and final momenta, and scattering materials are found to be approximately represented by one-dimensional set of standard distributions symmetrized by a linear transformation in t−x-space.     

Energy distributions of charged and neutral hydrogen atoms backscattered from metal surfaces bombarded with 5 to 18 keV protons

Polycrystalline targets of Be, V, Nb, Mo and Ta have been bombarded with protons with primary energies in the range from 5 to 18 keV. The energy distributions of the charged and the neural particles backscattered at 135° with respect to the primary beam direction have been measured between 200 eV and 18 keV. The energy distribution of the neutrals has a pronounced maximum between 0.5 and 1 keV whose position does not depend on the primary energy or the target material, or the angle of emergence of the scattered particles. The energy distribution of the charged particles shows a less pronounced maximum between 1 and 1.5 keV. Only slight differences in the shape of the energy distributions have been observed for different target materials. The fractional number of charged backscattered particles increases from $\text{≈; 3\%}$ at 300 eV to $\text{≈; 40\%}$ at 18 keV.     

Ion implantation distributions in inhomogeneous materials

A simple, approximate, analytical technique for calculating implanted ion depth distributions in inhomogeneous materials is presented. The central concept in the procedure is the method of equivalent atomic stopping (MEAS) wherein an atom of type k in the target is replaced by ϵ_k equivalent atoms of a preselected standard species. The target, when viewed as a collection of “equivalent standard atoms” is therefore uniform and thus existing techniques for obtaining depth distributions in the equivalent medium can be applied. The constants ϵ_k are shown to be related to range parameters in uniform targets of atoms of type k. The approximation involved in MEAS is examined for the case of low energy implantation by inspection of the properties of Winterbon's generalization of the transport equation. Practical techniques for evaluating the accuracy at higher energies are presented. The method is applied to light and heavy ion implantation into a target consisting of alternating layers of amorphous silicon and germanium, and the results are compared with TRIM calculations. This method is particulary valuable for the initial estimation of appropriate parameters for implantation into complex, layered targets.     

Non-Maxwell velocity distributions in inhomogeneous materials

Determining velocity distribution functions for ensembles is one of the most fundamental problems in statistical mechanics. In this paper, based on the results of computer molecular dynamics (MD) methods, we point out that the velocity distribution of low-mass classical particles subjected to a strong force field in a heavy atom environment is generally noncanonical. When the system reaches equilibrium, a non-Maxwellian distribution persists. The effective temperature of some of the low-mass particles may be one or two orders of magnitude greater than the average temperature of the system. Cold fusion may thus be enhanced by as much as 30–50 orders of magnitude over that calculated on the basis of 300K Maxwell distributions.     

Monte Carlo模拟韧致辐射光子穿透闪烁体的能量和通量分布

在工业CT设计中,为了选择闪烁体的种类和几何尺寸,需要知道韧致辐射光子穿透闪烁体的能量和通量分布。根据Monte Carlo模拟光子输运过程的理论,模拟了2MeV、9MeV韧致辐射光子在NaI、CsI、CdWO4闪烁体内的输运过程,讨论了这类光子穿透闪烁体后光斑展宽问题和能量及通量的分布。研究表明,该方法可准确直观地定量分析韧致辐射光子在闪烁体内任意位置的能量和通量分布规律。     

Energy distributions of secondary ions from GaAs

Energy distributions have been measured for Ga⁻, As⁻, Ga⁺ and As⁺ secondary ions emitted from a GaAs crystal using Cs ⁺, O₂⁺, Ar⁺ and O⁻ primary ion bombardment. Dramatic differences among these distributions have been observed and provide new chemical information for the understanding of the ion formation mechanism.     

Energy distributions of atoms sputtered from polycrystalline surfaces

A new formula is presented for the energy and angular distributions of atoms ejected from polycrystalline solids due to keV particle bombardment. The formula predicts that the peak position in the energy distribution decreases with increasing polar angle from the surface normal. It also predicts that the angular distribution is ∼cos θ for low energy particles and is ∼cos²θ as the energy of the particles increases.     

Path dependence of the charge state distributions of low-energy F ions backscattered from RbI(1 0 0)

Projectile neutralization during backscattering from RbI(1 0 0) of F multicharged ions in the keV energy range was investigated utilizing a time-of-flight technique. The energy and charge state distributions of the scattered ions were measured as a function of the polar incidence angle and the target azimuthal orientation. We found significant variations in the neutralization degree for incident projectiles of different charge states. The charge state distribution of scattered ions, including negative charge states, was found to depend on both the polar incidence and azimuthal orientation angles. These variations are attributed to the particular hard and soft encounters with neighboring lattice sites at the target surface along the path of the ion. Sample data for few-keV F²⁺ and F⁷⁺ incident projectiles are presented to illustrate the underlying concepts.     

Investigations of multiple backscattering and albedos of 1.12 MeV gamma photons in elements and alloys

The energy and intensity distributions of multiple backscattering of 1.12 MeV gamma photons emerging from targets of elements and alloys are observed as a function of thickness and atomic number (Z) of the target. The numbers of these multiply backscattered events show an increase with increase in target thickness, and then saturate for a particular thickness of the target called saturation thickness (depth). The saturation thickness decreases with increasing atomic number and varies as e^−Z. The multiple backscattering, an interfering background noise in Compton profile, has been successfully used to assign the ‘‘effective atomic number’’ to alloys. Monte Carlo calculations also support the present experimental results. The number, energy and dose albedos are also found to be saturating for the same thickness where the numbers of multiply backscattered events saturate.     

Energy distributions of neutral atoms sputtered by very low energy heavy ions

Polycrystalline Cu was sputtered by normally incident, very low energy Ar⁺ ions (E₀ = 40–1000 eV). The kinetic energy (E) distributions of the neutral Cu atoms sputtered normally from the Cu surface were measured, using secondary neutral mass spectrometry. For values of E₀ above approximately 600 eV, the observed energy distributions agreed closely with the Thompson-Sigmund theory. For values of E₀ less than about 600 eV the distributions fell off faster than predicted by the Thompson-Sigmund theory, and the peak value of the distribution shifted to somewhat lower energies. Both these effects were exaggerated as E₀ was further lowered. The average kinetic energy of the sputtered neutral Cu atoms increased with increasing E₀. The rate of this increase was less at higher values of E₀.     

大面积塑料闪烁体中荧光光子收集研究

利用Geant－3软件包建立了一套计算塑料闪烁体中可见光产生和收集的程序，并与实验进行了对比，二者基本一致。