FRCR II - Radioactivity Nick Harding Clinical Scientist Radiotherapy Department Castle Hill Hospital Hull & East Yorkshire Hospitals NHS Trust email: nicholas.harding@hey.nhs.uk

RADIOACTIVE DECAY Unstable nuclei / isotopes spontaneous disintegration transmutation emission of radiation These unstable isotopes are called radioactive isotopes or radionuclides The spontaneous nuclear transformation is called radioactivity or radioactive decay / disintegration

RADIOACTIVITY

RADIOACTIVITY α - particles β - particles γ - rays Helium nuclei electrons/positrons γ - rays electromagnetic

ACTIVITY Activity is defined as the number of radioactive atoms undergoing nuclear transformations per unit time The Système International unit is the becquerel (Bq) 1 Bq = 1 disintegration/second Traditionally, expressed in units of curies (Ci) 1 Ci = 3.7 × 1010 Bq = 37 GBq = 37,000 MBq

RADIOACTIVE DECAY Radioactive decay is a random process Number of atoms decaying per unit time is proportional to the number of unstable atoms

RADIONUCLIDE PARAMETERS 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 The decay constant (λ) is the probability that an atom will decay per unit time Both λ and τ1/2 are unique for each radionuclide

RADIOACTIVE DECAY LAW Number of Nuclei Half-life Parent Nuclei N 1/2 Daughter Nuclei D Number of Nuclei 1/4 1/8 1/16 1/32 Half-life

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 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) Internal conversion (IC) Isomeric transition (ΙΤ) Electron capture (ε or ec) Spontaneous fission (SF) Neutron emission (n)

ALPHA DECAY (α) 42Ηe Ζ = -2 Α = -4 p n n Parent Nucleus Daughter Nucleus 42Ηe p n Ζ = -2 Α = -4 n

ALPHA DECAY (α) Spontaneous emission of an alpha (α) particle from the nucleus An α particle is a Helium nucleus containing two protons and two neutrons Typically occurs  Heavy nuclides (A>150)

ΑLPHA PARTICLES Not used in medical imaging range in solids and liquids few micrometres (10-6m) range in air few centimetres (10-2m) Alpha particles cannot penetrate the dead layer of the skin Health hazard only when they enter the body eg. Po-210

Xofigo® THERAPY Used in medical therapy Ra-223 dichloride (Xofigo®) Used to treat prostate cancer once it has spreaded to the bones Radium acts like calcium

SMOKE DETECTOR Used in smoke detectors Smoke detector consists of 1 ion chamber 2 electrodes 1 Am-241 source (τ1/2 = 432y) Am-241 decays via alpha decay emits alpha particles ionise air within chamber electric current

SMOKE DETECTOR Used in smoke detectors Smoke detector consists of 1 ion chamber 2 electrodes 1 Am-241 source (τ1/2 = 432y) Am-241 decays via alpha decay emits alpha particles ionise air within chamber electric current

SMOKE DETECTOR Used in smoke detectors Smoke detector consists of 1 ion chamber 2 electrodes 1 Am-241 source (τ1/2 = 432y) Am-241 decays via alpha decay emits alpha particles ionise air within chamber electric current

ΒETA DECAY (β) Beta positive (β+) decay: Beta negative (β-) decay: Proton (p+) → neutron + positron (β+) Beta negative (β-) decay: Neutron (n0) → proton (p+) + electron (β-) Isobaric transitions – no change in A

β+ DECAY β+ Ζ = -1 Α = remains the same Parent Nucleus Increase N/Z ratio – occurs in neutron poor isotopes Parent Nucleus Daughter Nucleus n p β+ n p p Ζ = -1 Α = remains the same

β- DECAY β- Ζ = +1 Α = remains the same Parent Nucleus Decrease N/Z ratio – occurs in neutron rich isotopes Parent Nucleus Daughter Nucleus p β- n p Ζ = +1 Α = remains the same

positron annihilation β Particles positron annihilation photon (511 keV) β emission up to “a few mm”

TRITIUM 3H

TRITIUM ILLUMINATION Tritium decays by beta decay Electrons released Phosphor atoms excited/ionised Release of characteristic radiation Visible light!

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

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 particles no capture of particle by the nucleus

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

ISOMERIC TRANSITION

ELECTRON CAPTURE An alternative to positron decay for neutron poor radionuclides; Increase N/Z; Nucleus captures an orbital electron and converts a proton into a neutron;

ELECTRON CAPTURE e- Ζ = -1 Α = remains the same Parent Nucleus ` Parent Nucleus Daughter Nucleus p n e- p Ζ = -1 Α = remains the same

INTERNAL CONVERSION Nucleus in excited state (surplus of energy) De-excitation through ejection of a tightly bound electron (K- or L-shell) Ejected electron not a beta particle – no change of Z alternative mechanism to γ-decay

SPONTANEOUS FISSION Heavy nuclei decay by splitting into 2 or 3 nuclei release of neutrons release of energy

Neutron Emission Radioactive decay where neutron ejected from nucleus No change in Z so same element Becomes new isotope Generally short half life

Any questions?