|
Internal conversion is a radioactive decay process wherein an excited nucleus interacts electromagnetically with one of the orbital electrons of the atom. This causes the electron to be emitted (ejected) from the atom.〔M.E. Rose: "Theory of Internal Conversion", in: Alpha-, Beta- and Gamma-Ray Spectroscopy, ed. by Kai Siegbahn, North-Holland Publishing, Amsterdam (1966), Vol. 2〕 Thus, in an internal conversion process, a high-energy electron is emitted from the radioactive atom, but not from the nucleus. For this reason, the high-speed electrons resulting from internal conversion are not beta particles, since the latter come from beta decay, where they are newly created in the nuclear decay process. Internal conversion is possible whenever gamma decay is possible, except in the case where the atom is fully ionised. During internal conversion, the atomic number does not change, and thus (as is the case with gamma decay) no transmutation of one element to another takes place. Since an electron is lost from the atom, a hole appears in an electron shell which is subsequently filled by other electrons. This process produces characteristic X-ray(s), Auger electron(s), or both. The atom thus emits high-energy electrons and X-ray photons, but none of these originate in the nucleus, which supplies only the needed energy. Since primary electrons from internal conversion carry a fixed (large) fraction of the characteristic decay energy, they have a discrete energy spectrum, rather than the spread (continuous) spectrum characteristic of beta particles. Whereas the energy spectrum of beta particles plots as a broad hump, the energy spectrum of internally converted electrons plots as a single sharp peak (see example below). ==Mechanism== In the quantum mechanical mathematical model for the internal conversion process, the wavefunction of an inner shell electron (usually an ''s'' electron) penetrates the volume of the atomic nucleus. This means that there is a finite probability of finding the electron within the nucleus. When this happens, the electron may couple to an excited energy state of the nucleus and take the energy of the nuclear transition directly, without an intermediate gamma ray being first produced. The kinetic energy of the emitted electron is equal to the transition energy in the nucleus, minus the binding energy of the electron to the atom. Most internal conversion (IC) electrons come from the K shell (the 1s state), as these two electrons have the highest probability of being within the nucleus. However, the s states in the L, M, and N shells (i.e., the 2s, 3s, and 4s states) are also able to couple to the nuclear fields and cause IC electron ejections from those shells (called L or M or N internal conversion). Ratios of K-shell to other L, M, or N shell internal conversion probabilities for various nuclides have been prepared.〔() Internal conversion branch tables]〕 The atomic binding energy of the s electron must at least be supplied to that electron in order to eject it from the atom to result in IC; that is to say, internal conversion cannot happen if the decay energy of the nucleus is insufficient to overcome the binding energy. There are a few radionuclides in which the decay energy is not sufficient to convert (eject) a 1s (K shell) electron, and these nuclides, to decay by internal conversion, must decay by ejecting electrons from the L or M or N shells (i.e., by ejecting 2s, 3s, or 4s electrons) as these binding energies are lower. Although s electrons are more likely for IC processes due to their superior nuclear penetration compared to electrons with orbital angular momentum, spectral studies show that p electrons (from shells L and higher) are occasionally ejected in the IC process. After the IC electron has been emitted, the atom is left with a vacancy in one of its electron shells, usually an inner one. This hole will be filled with an electron from one of the higher shells, and consequently one or more characteristic X-rays or Auger electrons will be emitted as the remaining electrons in the atom cascade down to fill the vacancy. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「internal conversion」の詳細全文を読む スポンサード リンク
|