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Decay Types

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  • Decay Types and Conversion of Nuclides in Radioactivity

    In general, radioactive decays can be divided into alpha, beta, and gamma decays. However, other process may also be considered as types of decay.

    • Alpha decay
      In an alpha, a, decay, a helium nucleus 4He is emitted from an atomic nucleus. Since 4He2 consists of 2 protons and 2 neutrons (mass number = 4), the parent nuclide MPZ becomes a daughter nuclide M-4DZ-2 after the emission of an alpha.

      MPZ ® M-4DZ-2 + 4He2

      Some typical examples of alpha decays are:

      235U92 ® 231Th90 + 4He2

      238U92 ® 234U90 + 4He2

      208Po84 ® 204Pb82 + 4He2

      Many isotopes of elements with atomic number greater than 83 are alpha emitters. Some rare earth elements (114Nd, 146Sm, ... 174Hf) and some light elements (8Li, 9Li etc) also emit alpha particles.

    • Beta decay The emission of a negative electron, a positive electron (positron), or the capture of an atomic orbital electron (EC) is called a beta decay process
      • Negative electron emission
        A free neutron (n) outside a nucleus is unstable, and it emits an electron and becomes a proton (p). The proton and the electron are the components of a hydrogen atom. A company particle in the emission of electron is the antineutrino v*.

        n ® p+ + e- + v*

        The antineutrino is the antiparticle of neutrino.

        When a nuclide MPZ emits an electron, we may consider one of the neutron in the nucleus being converted to a proton. For example,

        14C6 ® 14N7 + e- + v*

        40Ca19 ® 40Ca20 + e- + v*

        50V23 ® 50Cr24 + e- + v*

        87Rb37 ® 87Sr38 + e- + v*

      • Positive electron (positron) emission
        A positive electron is called positron, which is the antiparticle of electron. The company particle for positron emission is an neutrino (v). Some examples are given here to illustrate the process:

        22Na11 ® 22Ne10 + e+ + v

        21Na11 ® 21Ne10 + e+ + v

        30P15 ® 30Si14 + e+ + v

        34Cl17 ® 34S16 + e+ + v

        116Sb51 ® 116Sn50 + e+ + v

      • Electron capture (EC)
        The electron capture process is in competition with positron emission. Both processes reduce the number of proton in the nucleus. Examples of EC are:

        48V23 ® 48Ti22 + e+ + v     (50%)

        48V23 + e- ® 48Ti22 + v (+ X-ray)     (50%)

        40K19 + e- ® 40Ar18 + v (+ X-ray)

        65Zn30 + e- ® 65Cui29 + v (+ X-ray)

        7Be4 + e- ® 7Li3 + v (+ X-ray)

    • Gamma decay
      Gamma rays (g) are high-energy photons emitted from atomic nuclei.

      After alpha or beta emission, some daughter nuclei have excess energy, and they become stable after emission of gamma photons. Thus, gamma rays are emitted almost at the same time beta or alpha rays are emitted.

      60Co27 ® 60Ni28 + e- + v* + g

      24Na11 ® 24Mg12 + e- + v* + g

      Nuclei not releasing the excess energy immediately are called isomers, which are represented by a superscript m following the mass number. These isomers emit gamma rays.

      99mTc ® 99Tc + g

      99mTc ® 99Tc + g

      60Co27 ® 60mNi28 + e- + v*
      60mNi ® 60Ni + g

      This two-step process is in competition with the immediate release of gamma rays.

      An isomer may also undergo an internal conversion to release its energy. In this process, an electron from the inner atomic orbital is ejected, and the electron so ejected may be as energetic as beta particles. This is another mode of gamma decay.

    • Other decay processes
      Several recently discovered processes are given below:
      • Proton decay is rare, but an example is

        53mCo27 ® 52Fe26 + p     (1.5%)
        53mCo27 ® 53Fe26 + e+ + v     (98.5%)

      • Spontaneous fission is often overshadowed by alpha decay, but nuclides 235U, 238U, 231Th, 208Po, 258Fm100, 256Fm100, 254Fm100, 254Cf98, 252Fm100 and 250Cm96 undergo spontaneous fission. Alpha decays are much more dominant for these nuclides, however.

        As an example, the fission 256Fm may be represented by

        256Fm100 ® 140Xe54 + 112Pd46 + 4 n.

        The fission process splits the nucleus into two large fragments and some neutrons.

      • Beta-delayed Alpha and Proton Emissions have been observed. For example,

        8B ® 8mBe + b+ + n
        8Li ® 8mBe + b- + n
                  8mBe ® 2 a

        These processes can be represented as b+a, and b-a, respectively. Another example of b+a, is,

        20Na ® 20Ne + b+ + v
                    20Ne ® 16O + a