Radiation Quantities and Radiation Quantities and Units Units - PDF document

Radiation Quantities and Radiation Quantities and Units Units George Starkschall, Ph.D. George Starkschall, Ph.D. Lecture Objectives Lecture Objectives • Define and identify units for the • Define and identify units for the following: following: – Exposure – Exposure – Kerma – Kerma – Absorbed dose – Absorbed dose – Dose equivalent – Dose equivalent – Relative biological effectiveness – Relative biological effectiveness – Activity – Activity Lecture Objectives Lecture Objectives • Define and identify units for the following: • Define and identify units for the following: – Particle number – Particle number – Radiation energy – Radiation energy – Particle flux – Particle flux – Energy flux – Energy flux – Particle fluence – Particle fluence – Energy fluence – Energy fluence – Planar fluence – Planar fluence 1

Lecture Objectives Lecture Objectives • Define and identify units for the following: • Define and identify units for the following: – Cross section – Cross section – Linear attenuation coefficient – Linear attenuation coefficient – Mass attenuation coefficient – Mass attenuation coefficient – Mass stopping power – Mass stopping power • Note: This lecture is an introduction to • Note: This lecture is an introduction to radiation quantities and units. This topic radiation quantities and units. This topic will be presented in significantly more will be presented in significantly more depth later in the course depth later in the course First things first First things first • What are we talking about in this • What are we talking about in this course? course? – Ionizing radiation: Sufficient energy to – Ionizing radiation: Sufficient energy to excite and ionize atoms of matter excite and ionize atoms of matter Types of ionizing radiation Types of ionizing radiation • Gamma rays • Gamma rays – Electromagnetic radiation emitted as a – Electromagnetic radiation emitted as a result of nuclear interactions result of nuclear interactions • Changes in nuclear energy levels • Changes in nuclear energy levels • Annihilation of positrons • Annihilation of positrons • Energy range: some keV to a few MeV • Energy range: some keV to a few MeV 2

Types of ionizing radiation Types of ionizing radiation • X-rays • X-rays – Electromagnetic radiation emitted as a result – Electromagnetic radiation emitted as a result of electronic interactions of electronic interactions • Changes in electronic energy levels – characteristic • Changes in electronic energy levels – characteristic x-rays x-rays • Deceleration of charged particles (usually electrons) • Deceleration of charged particles (usually electrons) – Bremsstrahlung (“braking radiation”) – Bremsstrahlung (“braking radiation”) • Energy ranges: • Energy ranges: – 0.1-20 kV – 0.1-20 kV Grenz rays Grenz rays – 20-120 kV – 20-120 kV diagnostic x-rays diagnostic x-rays – 120-300 kV – 120-300 kV orthovoltage x-rays orthovoltage x-rays – 300 kV-1 MV – 300 kV-1 MV intermediate-energy x-rays intermediate-energy x-rays – > 1 MV – > 1 MV megavoltage x-rays megavoltage x-rays Types of ionizing radiation Types of ionizing radiation • Electrons • Electrons – Charged particles emitted from a nucleus –  – Charged particles emitted from a nucleus –  rays (particles) rays (particles) – Fast electrons resulting from charged particle – Fast electrons resulting from charged particle collision –  rays collision –  rays – Continuous accelerated beams – Continuous accelerated beams • X-ray tube • X-ray tube • Van de Graaff generator • Van de Graaff generator – Pulsed accelerated beams – Pulsed accelerated beams • Linear accelerator (Linac) • Linear accelerator (Linac) • Betatron • Betatron • Microtron • Microtron Types of ionizing radiation Types of ionizing radiation • Heavy charged particles • Heavy charged particles – Protons – Protons – Deuterons – Deuterons – Alpha particles – Alpha particles – Heavy atom nuclei – Heavy atom nuclei – Pions – Pions • Neutrons • Neutrons – Obtained from nuclear interactions involving – Obtained from nuclear interactions involving high-energy charged particles or photons high-energy charged particles or photons 3

Types of ionizing radiation Types of ionizing radiation • Directly ionizing radiation • Directly ionizing radiation – Fast charged particles – Fast charged particles – Deliver energy to matter directly – Deliver energy to matter directly – Coulomb interactions – Coulomb interactions • Indirectly ionizing radiation • Indirectly ionizing radiation – X-rays,  -rays, neutrons – X-rays,  -rays, neutrons – Transfer energy to charged particles – Transfer energy to charged particles – Secondary charged particles deliver energy to – Secondary charged particles deliver energy to matter matter Exposure Exposure • Definition – Exposure is the absolute • Definition – Exposure is the absolute value of the total charge of ions of value of the total charge of ions of one sign produced in a small mass of one sign produced in a small mass of air, when all electrons liberated by air, when all electrons liberated by photons in air are completely photons in air are completely stopped in air, divided by the mass stopped in air, divided by the mass of air of air X = dQ/dm X = dQ/dm Some clarification needed Some clarification needed • “absolute value of the total charge of • “absolute value of the total charge of ions of one sign” ions of one sign” – Radiation causes ionization – Radiation causes ionization – Total charge produced is zero (positive – Total charge produced is zero (positive balances out negative) balances out negative) – Consequently, we only look at charge of – Consequently, we only look at charge of one sign or the other one sign or the other 4

Some clarification needed Some clarification needed • “produced in a small mass of air” • “produced in a small mass of air” – Ionization is a stochastic process – Ionization is a stochastic process – Need to have a large enough sample to – Need to have a large enough sample to determine a meaningful expectation determine a meaningful expectation value of charge production value of charge production The devil in the details The devil in the details • “all electrons liberated by photons in • “all electrons liberated by photons in air are completely stopped in air” air are completely stopped in air” – Aren’t we measuring photon exposure? – Aren’t we measuring photon exposure? – What do electrons have to do with this? – What do electrons have to do with this? – How does this make things – How does this make things complicated? complicated? Photon interactions Photon interactions • Photons interacting with absorber • Photons interacting with absorber (air molecules) give rise to (air molecules) give rise to secondary radiations (electrons) secondary radiations (electrons) which, in turn, interact further with which, in turn, interact further with absorber absorber – Single ionization (due to photon) yields – Single ionization (due to photon) yields many ionizations (due to electrons) many ionizations (due to electrons) downstream downstream 5

Path length of electrons produced Path length of electrons produced by photons by photons Photon Energy Maximum Electron Path Length in Air (MeV) (m) 0.3 0.3 1.0 3.0 3.0 12.2 10.0 40.9 Photon interactions Photon interactions • Photons interacting with absorber (air • Photons interacting with absorber (air molecules) give rise to secondary molecules) give rise to secondary radiations (electrons) which, in turn, radiations (electrons) which, in turn, interact further with absorber interact further with absorber – Not possible to track individual electrons – Not possible to track individual electrons producing ionizations downstream producing ionizations downstream – Introduces concept of electronic (charged – Introduces concept of electronic (charged particle) equilibrium particle) equilibrium Charged Particle Equilibrium Charged Particle Equilibrium • Energy deposited by charged • Energy deposited by charged particles produced inside a volume particles produced inside a volume and deposited outside the volume is and deposited outside the volume is equal to energy deposited by equal to energy deposited by charged particles produced outside charged particles produced outside the volume and deposited inside the the volume and deposited inside the volume volume 6