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Modes of Radioactive Decay GE-PP-22502

Modes of Radioactive Decay GE-PP-22502. Author: Ken Jenkins Approved: Michael J. Kurtzman Date: 06/14/2003 Revision: 00. Nuclear Stability. Forces Acting Within the Nucleus. Nuclear Stability. The repulsive electrostatic forces between the protons have an impact on nuclear stability

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Modes of Radioactive Decay GE-PP-22502

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  1. Modes of Radioactive DecayGE-PP-22502 Author: Ken Jenkins Approved: Michael J. Kurtzman Date: 06/14/2003 Revision: 00

  2. Nuclear Stability Forces Acting Within the Nucleus GE-PP-22502-00

  3. Nuclear Stability • The repulsive electrostatic forces between the protons have an impact on nuclear stability • The number of neutrons must increase more rapidly than the number of protons to provide ‘dilution’ and to add additional nuclear forces • If the nuclear (attractive) and electrostatic (repulsive) forces do not balance, the atom will not be stable GE-PP-22502-00

  4. Nuclear Stability • An unstable nucleus will eventually achieve stability by changing its nuclear configuration • This includes changing neutrons to protons, or vice versa, and then ejecting the surplus mass or energy from the nucleus • This emitted mass or energy is called radiation GE-PP-22502-00

  5. Nuclear Stability • When an atom transforms to become more stable it is said to disintegrate or decay • The time required for half of a sample of atoms to decay is known as the half-life • The property of certain nuclides to spontaneously disintegrate and emit radiation is called radioactivity • The atom before the decay is the parent and the resulting atom is called the daughter GE-PP-22502-00

  6. 100 80 1 60 NUMBER OF PROTONS (Z) LINE OF STABILITY 40 20 0 0 40 60 80 20 100 120 140 NUMBER OF NEUTRONS (N=A-Z) Neutron / Proton Ratio GE-PP-22502-00

  7. Beta Decay • Betas are physically the same as electrons, but may be positively or negatively charged • Negative beta is a beta minus or negatron • Positive beta is a beta plus or positron • Betas are ejected from the nucleus, not from the electron orbitals • In all beta decays the atomic number changes by one while the atomic mass is unchanged GE-PP-22502-00

  8. Beta (β-) Minus Decay • Occurs in neutron-rich nuclides • The nucleus converts a neutron into a proton and a beta minus (which is ejected from the nucleus with an anti-neutrino) • Mass and charge are conserved GE-PP-22502-00

  9. Beta (β-) Minus Decay • For beta minus decays, GE-PP-22502-00

  10. Anti-neutrino Daughter Ca-40 Beta Particle Beta (β-) Minus Decay Parent K-40 GE-PP-22502-00

  11. Beta (β-) Minus Decay • During radioactive decay energy is released • Source of this energy is from the conversion of mass • Since energy is conserved, energy equivalent of the parent must equal energy equivalent of daughter, particles, and any energy released • Energy is released as kinetic energy of beta minus particle and an anti-neutrino GE-PP-22502-00

  12. Beta (β-) Minus Decay • For beta minus, energy of decay reaction (Q) is, • Mass of beta minus particle is not included since an additional electron is gained due to increase of Z GE-PP-22502-00

  13. Beta (β-) Minus Decay • Calculate Q for β- decay of Co-60. Mass of Co-60 is 59.933813 amu Mass of Ni-60 is 59.930787 amu GE-PP-22502-00

  14. Beta (β-) Minus Decay • The Q value for beta minus decay of Co-60, for example, is always the same • However, negatrons rarely are emitted with the same energies • Their energies can range from 0 MeV to the calculated maximum, Emax • The anti-neutrino carries energy difference between actual and calculated values GE-PP-22502-00

  15. Beta (β-) Minus Decay # of betas with energy E Energy GE-PP-22502-00

  16. 1.173 99+% 2.158 0.83 0.013% 1.332 0.12% Q Co-60 Decay Scheme GE-PP-22502-00

  17. Beta (β+) Plus Decay • Occurs in proton-rich nuclides • The nucleus converts a proton into a neutron and a beta plus (which is ejected from the nucleus with a neutrino) • As with negatrons, the positron can have a range of energies from 0 to EMax MeV • Positron is the negatron’s anti-particle • A positron and a negatron will annihilate one another and release two 0.511 MeV photons GE-PP-22502-00

  18. Beta (β+) Plus Decay • For beta plus decays, GE-PP-22502-00

  19. Neutrino Daughter O-18 Beta Particle Beta (β+) Plus Decay Parent F-18 GE-PP-22502-00

  20. Beta (β+) Plus Decay • For beta plus, energy of decay reaction (Q) is, • Since the energy equivalent of two electron masses is 1.022 MeV, the equation can be rewritten as, GE-PP-22502-00

  21. • • • • • • • Beta (β+) Plus Decay e- • • • • • • • • + GE-PP-22502-00

  22. Beta (β+) Plus Decay • Calculate Q for β+ decay of F-18. Mass of F-18 is 18.000937 amu Mass of O-18 is 17.999160 amu GE-PP-22502-00

  23. Electron Capture • Proton-rich nuclides may also decay via orbital electron capture (EC) • Usually an innermost K shell electron is captured and often referred to as K-capture • The electron and a proton are converted into a neutron and a neutrino is emitted • Electrons from higher orbitals will fill vacancy and usually emit characteristic x-rays GE-PP-22502-00

  24. Electron Capture • For electron capture decays, GE-PP-22502-00

  25. Electron Capture • For electron capture, energy of decay reaction (Q) is, • Since the electron was absorbed into the nucleus and not removed, there is no need to account for electron mass GE-PP-22502-00

  26. Auger Electrons • When electrons change shells, x-rays are usually emitted • In some instances, the excess energy is transferred to another orbital electron, which is then ejected from the atom • This ejected electron is known as an Auger electron • Another orbital vacancy now exists and x-rays may be emitted if they are filled GE-PP-22502-00

  27. Auger Electrons • • • • • • • • • • GE-PP-22502-00

  28. Beta Interactions • Excitation • The beta, via coulombic interaction, transfers enough energy to an orbital electron to move it to a higher energy level, but not to remove it from the atom • The atom remains electrically neutral • The excited electron will then return to its ground state and emit the excess energy as x-rays GE-PP-22502-00

  29. • x-ray Excitation • • • • • • - • GE-PP-22502-00

  30. Beta Interactions • Ionization • The beta, via coulombic interaction, transfers enough energy to an orbital electron to overcome its binding energy and remove it from the atom • With the loss of the negative electron, the remaining atom is now a positive ion • If the vacancy is filled, an x-ray will be emitted • The formation of each ion pair in air (gas) requires about 34 eV of energy from the beta GE-PP-22502-00

  31. • e- Ionization • • • • • • • - • GE-PP-22502-00

  32. Beta Interactions • Bremsstrahlung • German for ‘braking radiation’ • Occurs when beta is deflected by the positively charged nucleus • The kinetic energy lost by the beta is emitted as a photon (x-ray) • Bremsstrahlung increases with higher Z materials • For example, a lead blanket may shield betas, but generate higher levels of Bremsstrahlung (x-rays) GE-PP-22502-00

  33. x-ray Bremsstrahlung • • • • • • • - • GE-PP-22502-00

  34. Beta Interactions • Betas travel in zig-zag or tortuous paths • Collisions and deflections • Coulombic interactions • Not mono-energetic • Because of this, betas have a definite, predictable range (given in mg/cm2) • Basic thumb rule is that a 1.0 MeV beta will travel approximately 12 feet in air GE-PP-22502-00

  35. Beta Interactions GE-PP-22502-00

  36. Beta Interactions • All betas can be stopped, but Bremsstrahlung photons can be produced • Intensity is proportional to number of betas, their energy, and Z of the absorber • Shielding is designed to minimize and/or shield Bremsstrahlung • Low Z materials such as plastic (hydrocarbons) or aluminum are common GE-PP-22502-00

  37. Beta Interactions • The fraction of beta energy that appears as photon energy (Bremsstrahlung) can be estimated with the following equation: f = E x Z x 10-3 E = beta energy in MeV Z = atomic number of target (shield) material Average energy of the Bremsstrahlung photons is about 1/4 Emax GE-PP-22502-00

  38. Alpha Decay • Alphas are large particles ejected by the heavier nuclides • Alpha decay is primarily limited to nuclides with Z > 82 • Source is mainly from fuel-related materials • Alpha contains two protons and two neutrons (no electrons) and is, in effect, a helium nucleus • Thus, the atomic number decreases by two and the mass number decreases by four GE-PP-22502-00

  39. Alpha Decay • For alpha decays, GE-PP-22502-00

  40. Daughter Th-231 Alpha Decay Parent U-235 GE-PP-22502-00

  41. Alpha Decay • Since nothing else is emitted, all energy of decay goes to the alpha particle (except for a small amount towards recoil of nucleus) • Alphas, therefore, are mono-energetic • For alpha, energy of decay reaction (Q) is, GE-PP-22502-00

  42. Alpha Decay • Calculate Q for the  decay of Rn-222. Mass of Rn-222 is 222.017610 amu Mass of Po-218 is 218.009009 amu GE-PP-22502-00

  43. Alpha Interactions • Alphas interact primarily through Coulombic interactions due to their +2 charge • Energy transfer occurs through excitation and ionization • Orbital electronsmay receive enough energy to allow them to cause secondary ionizations of other atoms • Bremsstrahlung does not occur since the large alphas are not easily deflected GE-PP-22502-00

  44. Alpha Interactions • Because of their mass and charge, alphas travel in relatively straight paths over short distances (higher Z of absorber, less distance) • A 7 MeV alpha travels only about 0.0002 cm in lead • Alphas are considered internal hazards only • When an alpha slows enough, it captures two free electrons and converts to a helium atom GE-PP-22502-00

  45. Alpha Interactions GE-PP-22502-00

  46. Nuclear De-excitation • Daughter nuclei from radioactive decays are often ‘born’ with excess energy • Occasionally the excited nucleus will emit additional alphas or betas • Usually the excited nucleus reaches ground state via nuclear de-excitation • The excited nucleus and the final ground state nucleus have the same Z and A and are called isomers GE-PP-22502-00

  47. Nuclear De-excitation • If the excited state has a half-life >1 sec, it is said to be a metastable state • The metastable state is denoted by the use of a lowercase ‘m’, such as Ba-137m • The longest known excited state is Bi-210m with a half-life of 3.5 x 106 years • During de-excitation no nuclear transformation occurs, so no ‘new’ element is formed GE-PP-22502-00

  48. Nuclear De-excitation • Internal Conversion • The excess nuclear energy is transferred to an inner orbital (usually K or L) electron • This electron is then ejected from the atom with a distinct energy • X-ray emission may follow as electrons shift orbitals to fill vacancies GE-PP-22502-00

  49. Nuclear De-excitation • Gamma emission • Most frequently the excess energy is relieved via the emission of one or more gamma rays • Gammas have no mass or electric charge • If gammas are emitted by an isomer in the metastable state, the emission is known as an isomeric transition (IT) • Photon Energy (E) = hf • where h is Planck’s Constant (4.14 x 10-15 eV-sec) • f is frequency (sec-1) GE-PP-22502-00

  50. Gamma Rays Daughter Ni-60 Anti-neutrino Gamma Ray Radiation Parent Co-60 GE-PP-22502-00

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