Nucleus Spontaneously emit beta particles or capture A change in the orbit of an electron. The decay process of emitted electrons is called beta decay; The decay process of releasing positive electricity is called β+decay; The decay process in which the nucleus captures an orbital electron from the electron shell outside the nucleus is called Orbital electron capture The electron in the K layer captured is called K capture, the electron in the L layer captured is called L capture, and so on. Generally, the probability of K capture is large. In beta decay, the mass number of the nucleus remains unchanged, but the number of charges changes by one unit. External electron capture is also a kind of beta decay, called electron capture beta decay. Because beta particles are electrons, and the mass of electrons is much smaller than that of nuclei, the mass of a beta particle released from an atomic nucleus will only slightly decrease. The law of beta decay is that the mass number of the new nucleus remains unchanged while the charge number increases.
The new core is periodic table of ele ments The position in should be moved back one position. The electron energy released in beta decay is continuously distributed, but there is a maximum limit for each decay mode, up to several trillion electrons Volts Above, this part of energy is taken away by neutrinos. In 1957, Dr. Wu Jianxiong used the beta decay experiment of cobalt-60 to prove that Weak interaction Parity no in conservation 。 In beta decay, the following three types of nuclear changes occur:
Where X and Y represent the parent core and Daughter nucleus , A and Z are the mass number and proton number of the parent nucleus, e and e are electrons and positrons, and v and? Are neutrinos and anti neutrinos. β decay energy is expressed as β decay, where mx and my are the rest masses of parent and daughter atoms, and me is the electronic Static mass , Wi is the orbital electron binding energy, с is the speed of light. The subnuclear atom formed in the process of orbital electron capture is in an excited state due to the lack of an inner electron, which can be de excited in different ways. For K capture, when the L-layer electron jumps to the K-layer filling vacancy, it can emit marking X-ray, or characteristic X-ray. Its energy is the difference between the binding energy of K layer and L layer electrons hv=Wk WL; When the L-layer electron jumps to the K-layer vacancy, it can also not transmit the mark X-ray , and give the energy to another L-layer electron to overcome binding energy When flying out, this electron is called Auger electron, and its kinetic energy Ee=hv-WL=Wk-2WL. Orbital electron capture is always accompanied by the generation of labeled X-ray or Auger electrons. Electron neutrino theory of beta decay the energy of beta particles released in beta decay is from continuous distribution. To explain this phenomenon, in 1930, W Pauli Beta decay release is proposed neutral particulate Hypothesis. In 1933, E Fermi On this basis, it is proposed that Electron neutrino Theory. This theory holds that neutron and proton can be regarded as the same particle（ nucleon ）The mutual transformation between two different quantum states is equivalent to the transition of a nucleon from one quantum state to another quantum state , release electrons and neutrino 。 Beta particles are the products of transitions between different states of nucleons and do not exist in the nucleus in advance. Therefore, it is the interaction between the electron neutrino field and the atomic nucleus that causes the beta decay, which belongs to the weak interaction. This theory successfully explains the shape of β spectrum and gives a quantitative description of β decay. β transition probability According to the perturbation theory of quantum mechanics, Fermi's theory gives that the probability of β particles emitted momentum per unit time between p and p+dp is β decay, (1) where g is the weak interaction constant, Mif is the transition matrix element, 啚 is Planck's constant h divided by 2 π, F (Z, E) is the coulomb correction factor, which describes the influence of nuclear coulomb field on the emission of β particles, and is the charge number Z of the nucleus and the energy E of β particles function 。 The size of the transition probability is mainly determined by the size of the transition matrix element | Mif |. Classification of β transition According to the size of transition matrix elements, β transition can be divided into three types, which are described in the study of β decay. Formula (1) is often rewritten as allowable transition, first forbidden transition, second forbidden transition, etc. The higher the order, the smaller the transition probability; Between two adjacent levels, the probability can differ by several orders of magnitude. Fermi Theory Gives the Spin Sum of Beta Decay between Parent Nucleus and Child Nucleus Parity Variable selection rule: for allowable transitions, spin Change | Δ I |=0,1, parity change Δπ=+1; For Level I Forbidden transition ，|ΔI|=0，1，2，Δπ=－1； For the second or higher order forbidden transition, such as the n-order forbidden transition, | Δ I |=n, n+1, Δπ=(- 1). 
