散射与衍射read.ppt

第一篇 绪 论 吴小山 主要内容 衍射分析的一般方法 衍射分析的历史发展 中子、电子、X射线分析的各自特点 物质结构的一般阐述，结构测定的重要性 射线与物质相互作用过程 运动学理论与动力学理论的考虑范畴 衍射分析方法 中子、电子、X射线分析的各自特点 物质结构的一般阐述，结构测定的重要性 晶体(crystal) It is solid.The arrangement of atoms in the crystal is periodic. 格子（Lattice） An infinite array of points in space, in which each point has identical surroundings to all others. 晶体结构（Crystal Structure） It can be described by associating each lattice point with a group of atoms called the MOTIF (BASIS) 单位晶胞（Unit Cell） The smallest component of the crystal, which when stacked together with pure translational repetition reproduces the whole crystal 晶胞参数Unit Cell Dimensions a, b and c are the unit cell edge lengths. a, b and g are the angles (a between b and c, b between c and a , g between a and b c .) 运动学与动力学理论考虑的范畴 运动学理论的出发点： 1，原子的规则点阵排列； 2，原子只散射入射波，原子为散射点波源； 3，晶体的折射率为1； 4，晶体对入射波及衍射波无吸收； 5，不考虑全过程的能量守恒。 从另一个角度考虑X射线与物质的相互作用 三种相互作用: 1. 吸收; 2, 折射与反射; 3, 散射 衍射 1. X射线的吸收 通过光电子效应发生 Photon energy transferred to electron in a particular shell which is ejected, resulting in a fluorescent recombination characteristic of the shell Quantised shells give discontinuous jumps in absorption cross-section. Linear absorption coefficient Thickness Mass absorption Coefficient: Density Atomic Number Avogadro’s Number Absorption X-section Calculated mass absorption coefficient for Zr In between edges: the cross section varies as k-3. For fixed energy: the change with atomic number is ~Z4 中子的吸收 Occurs through nuclear reactions which causes fission or the expulsion of a particle such as a proton or an alpha particle or a gamma ray. No Quantised shells so no discontinuous jumps in absorption cross-section and the cross section varies inversely with wavevector. International Tables quote the cross-section for neutrons with k=k0=3.4947?-1 (v=2200ms-1) No systematic variation with atomic number: Gd - 49700 barn, Cd - 2520 barn but most 0.1-10 barn Also varies according to isotope because cross-section is related