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Radioactivity

Radioactivity. Manos Papadopoulos Nuclear Medicine Department Castle Hill Hospital Hull & East Yorkshire Hospitals NHS Trust. RADIOACTIVE DECAY. Only certain combinations of nucleons form a stable nucleus Unstable nuclei spontaneous nuclear transformation formation of new elements

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Radioactivity

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  1. Radioactivity Manos Papadopoulos Nuclear Medicine Department Castle Hill Hospital Hull & East Yorkshire Hospitals NHS Trust

  2. RADIOACTIVE DECAY • Only certain combinations of nucleons form a stable nucleus • Unstable nuclei • spontaneous nuclear transformation • formation of new elements • emission of radiation • These unstable isotopes are called • radioactive isotopes • The spontaneous nuclear transformation is called • radioactivity or radioactive decay / disintegration

  3. RADIOACTIVE DECAY • An unstable “parent” (P) nuclide is transformed into a more stable daughter (D) nuclide through various processes where d1 + d2 + … signify the emitted particles • The process is usually accompanied by the emission of gamma radiation

  4. RADIOACTIVITY

  5. ACTIVITY • Activity (A) is defined as: • the number of radioactive atoms (N) undergoing nuclear transformations per unit time (t)

  6. UNITS OF ACTIVITY • Traditionally, expressed in units of curies (Ci) • 1 Ci = 3.7 × 1010 disintegrations/second • Typical activities • for imaging: 0.1 to 30 mCi • for therapy: up to 300 mCi • The Système International (SI) unit is the becquerel (Bq) • 1 Bq = 1 disintegration/second

  7. DECAY CONSTANT • Radioactive decay is a random process • The number of atoms decaying per unit time (dN/dt) • is proportional to the number of unstable atoms (N) • where λ is the transformation constant (or decay constant) • being characteristic of each radionuclide

  8. HALF-LIFE • The half-life (τ1/2) is defined as: • the time required for the number of radioactive atoms in a sample to decrease by one half • λ and τ1/2 are related as follows: • where ln2 denotes the natural logarithm of 2 • Both λ and τ1/2 are • unique for each radionuclide

  9. RADIOACTIVE DECAY LAW • The rate at which a radioactive isotope disintegrates is defined by the following DECAY LAW: • Where • N(t): number of radioactive atoms at time t • N0: initial number of radioactive atoms (at time zero) • τ1/2: half-life • e: base of natural logarithm ( ≈ 2.718) • λ: decay constant

  10. RADIOACTIVE DECAY LAW N0 τ1/2 = 5730y 5730

  11. PROBLEM • A nuclear medicine technologist injects a patient with 800 MBq of [99Tcm]-SestaMIBI (τ1/2=6.02 hours). One hour later the patient is imaged. Assuming that none of the activity is excreted, how much activity remains at the time of imaging?

  12. SOLUTION • A0 = 800 MBq • λ=0.693/6.02 hours = 0.115 hours-1 • t = 1 hour

  13. RADIOACTIVE DECAY TYPES • Radioactive decays are classified by the types of particles that are emitted during the decay: • Alpha decay (α) • Beta decay (β) • Gamma decay (γ) • Isomeric transition (ΙΤ) • Electron capture (ε or ec) • Internal conversion (IC) • Spontaneous fission (SF) • Neutron emission (n)

  14. ALPHADECAY • Spontaneous emission of an alpha (α) particle • from the nucleus • An α particle is a Helium nucleus • containing two protons and two neutrons

  15. ALPHA DECAY • Typically occurs  Heavy nuclides (A>150) • Emission of gamma and characteristic X-Rays

  16. ALPHA DECAY • Alpha particle emitted from the atomic nucleus • Alpha particle and daughter nucleus have equal and opposite momentums

  17. ΑLPHA PARTICLES • Not used in medical imaging • range in solids and liquids • few micrometres • range in air • few centimetres • Alpha particles cannot penetrate the dead layer of the skin • Health hazard only when enter the body

  18. ΒETA DECAY • Beta positive (β+) decay: • Proton (p+) → neutron + positron (β+) + neutrino • Beta negative (β-) decay: • Neutron →proton (p+) + electron (β-) + antineutrino

  19. β-DECAY • converts one neutron into a proton and an electron • no change of A, but different element • occurs with nuclides with an excess number of neutrons

  20. β+DECAY • converts one proton into a neutron and a positron • no change of A, but different element • occurs with nuclides with an excess number of protons

  21. ΒΕΤΑ PARTICLES • Electron (β-) • Positron (β+) • As beta particles traverse  lose energy • Positron interacts with an electron • Annihilation radiation • two opposite directed 511 keV photons • threshold for positron decay  2×511 keV = 1.02 MeV • Used in Medical Imaging • Positron emitting radiopharmaceuticals • Positron Emission Tomography (PET) Anti-particles

  22. ΒΕΤΑ PARTICLES Positron Emission and Annihilation

  23. GAMMA DECAY • Nucleus in excited state (surplus of energy) • Release of excess energy  emission of γ-rays • nucleus returns to its ground state

  24. GAMMA DECAY • no change of A or Z – same element • release of photon • usually occurs in conjunction with other decay

  25. GAMMA DECAY Decay scheme of

  26. ISOMERIC TRANSITION • Half-lives from 10-12 sec – 600 years • These excited states are called • metastable or isomeric states • No change in • atomic number • mass number • neutron number

  27. ISOMERIC TRANSITION • Isomeric transition is a radioactive decay process • excited nucleus decays to lower energy state • gamma radiation emitted • no emission of corpuscular radiation (i.e. particles) • no capture of particle by the nucleus

  28. Mo-99 DECAY SCHEME

  29. Mo-99 DECAY SCHEME • 99Mo decays by β- decay • into 99Tcm (i.e. 99Tcm metastable state of 99Tc) • half-life = 66 hours • 99Tcm decays by isomeric transition • into 99Tc ground state with 6 hr half-life • half-life = 6.01 hours

  30. ELECTRON CAPTURE • Nucleus captures orbital electron (usually a K- or L-shell) • conversion of a proton into a neutron • simultaneous ejection of a neutrino • Emission of • characteristic X-rays • Auger electrons

  31. ELECTRON CAPTURE • converts one proton into a neutron • no change of A – but different element • occurs with nuclides with an excess number of protons

  32. Tl-201 DECAY SCHEME • 201Tl decays by electron capture • into 201Hg • half-life = 73.1 hours • 201Hg-characteristic X-Rays • 68.9-80.3 keV • Emission of characteristic X-Rays used in myocardial perfusion

  33. INTERNAL CONVERSION • Nucleus in excited state (surplus of energy) • De-excitation through • ejection of a tightly bound electron (K- or L-shell) • alternative mechanism to electron capture • No change of Z – same element

  34. SPONTANEOUS FISSION • Heavy nuclei decay by splitting into two daughter nuclei • release of neutrons • release of energy

  35. SUMMARY I • Half-Life (τ1/2) • the time required for radioactivity to decay to half its initial value • Decay Constant (λ) • the probability that an atom will decay/transform per unit time • Activity • rate of decay/transformation • At = A0 e-λt

  36. SUMMARY II • Radioactive Decay Modes • depending on the emitted radiation • Alpha particles • Helium nuclei – used in radionuclide therapies • Beta particles • used in imaging (e.g. positrons - PET) • used in therapy (e.g. 131I, 32P) • Gamma ray photons • used in imaging (e.g. 99Tcm, 201Tl)

  37. THE END Any questions ?

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