康普顿效应和对.PPT

* 第四讲 射线与物质的相互作用（3） Radiation Interactions with Matter 光子与物质的相互作用 A. Gamma Rays Following Beta Decay Gamma radiation is emitted by excited nuclei in their transition to lower lying nuclear levels. Notes: 1) For common examples widely used as gamma ray calibration sources. In each case, a form of beta decay leads to the population of excited state in the daughter nucleus. 2) The gamma rays from any one transition are nearly monoenergetic. 3) The common gamma ray source based in beta decay are generally limited to energies below about 2.8 MeV. 1 Source of Electromagnetic Radiation When the parent nucleus undergoes beta-plus decay, additional electromagnetic radiation is generated. The origin lie in the fate of the positrons emitted in the primary decay process. Annihilation Radiation: The original positron and electron disappear and are replaced by two oppositely directed 0.511MeV electromagnetic photons known as annihilation radiation. Example: the decay of 22Na: B. Annihilation Radiation C. Gamma Ray Following Nuclear Reactions If gamma rays with energies higher than those available from beta-active isotopes are needed, some other process must lead to the population of higher-lying nuclear states. Example 1: ?~4.44MeV Example 2: ?~6.130MeV Notes: 1) For monoenergetic electron that slow down and stop in a given material, the Bremsstrahlung energy spectrum is a continuum with photon energies that extend as high as the electron energy itself. 2) The shape of the energy spectrum from an x-ray tube can be beneficially alter by filtration or passage through appropriate absorber materials. 3) Bremsstrahlung is also produced by other source of fast electrons, including beta particles. I4) In addition to Bremsstrahlung, characteristic x-rays are also produced when fast electron pass through an absober. Therefore, the spectra from x-ray tubes or other Bremsstrahlung sources also show characteristic x-ray emission lines superimposed on the conti