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 emission of radiation These unstable isotopes are called radioactive isotopes The spontaneous nuclear transformation is called radioactivity or radioactive decay / disintegration

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

RADIOACTIVITY

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

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

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

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

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

RADIOACTIVE DECAY LAW N0 τ1/2 = 5730y 5730

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?

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

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)

ALPHA DECAY Spontaneous emission of an alpha (α) particle from the nucleus An α particle is a Helium nucleus containing two protons and two neutrons

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

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

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

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

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

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

ΒΕΤΑ PARTICLES Electron (β-) Positron (β+) Anti-particles 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

Positron Emission and Annihilation ΒΕΤΑ PARTICLES Positron Emission and Annihilation

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

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

GAMMA DECAY Decay scheme of

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

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

Mo-99 DECAY SCHEME

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

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

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

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

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

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

SUMMARY I Half-Life (τ1/2) Decay Constant (λ) Activity 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

SUMMARY II Radioactive Decay Modes Alpha particles Beta particles 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)

THE END Any questions ?