PHYS C2

Radiation Def

process of emitting energy as waves or particles

Types:

Ionising - capable of ionisation (removing atom’s electrons) as they pass through matter

Non-Ionising

Types Ionising Radiation

Directly Ionising - Charged particles interact directly with atomic electrons through Coulombic forces (likes repel, opposites attract) - electrons, positrons, protons

Indirectly Ionising - Electrically neutral particles or rays do not interact strongly with matter. Ionisation mainly secondary (occurs after primary ionisation event) - neutrons, gamma rays, x-rays

Photon

A quantum of electromagnetic energy

Behaves as wave and particle

Fundamental particle

No mass or electric charge

Wave-Particle Duality

fundamental particles exhibit properties of both waves (like interference) when propagating and particles (like momentum) when interacting

α-particle

Helium nucleus (2 protons, 2 neutrons, +2 charge)

Highly ionizing - serious tissue damage

Slower, less penetrating, smaller range than β-particle

β-particle

Mass and charge of electron

Different to electron as it originates from nucleus

Less ionising, faster, more penetrating, larger range than α-particle

Lose large fraction of energy in collisions

easily deflected

neutrons

give rise to protons and α-particles via spontaneous radioactive decay and other nuclear reactions (fission, fusion)

Similar mass to proton

Stopping power

gradual loss of energy of charged particle or wave as it penetrates an absorbing medium

Range of a charged particle

total distance travelled through a medium until it comes to rest

Depends on:

-Particle charge

-Particle energy

-Particle mass (lighter = further)

-Medium density

Electron Excitation Formula

Emitted photon has energy: ΔE = E2 – E1

Nuclear binding energy

energy required for a stable atom to separate it into its constituent parts

negative as the separated condition is given an arbitrary energy of 0

different for isotopes

Electronic binding energy

energy needed to remove an electron from an atom

Radioactive Decay

Transition from the quantum state of the original nuclide (parent) to a quantum state of the product nuclide (daughter)

Daughter nuclei may decay further

Radioactive decay energy

release of nuclear energy from an unstable parent nucleus mostly in the form of particles and electromagnetic radiation

causes daughter nucleus to have less energy that parent

average decay rate formula

N - number radioactive atoms

λ - decay constant (probability of decay per unit time) unit: 1/s

e.g λ = 0.05 s^-1 an average of 5% percent of the atoms decay each second

Decay Rate

Expressed as negative as the number of atoms is decreasing

Constant, characteristic value for each radionuclide

Independent of the atom’s age, physical conditions

(temp, pressure), and chemical state of atom’s

environment

Activity

number of nuclei decays per unit time, denotes how quickly a source will shed mass or energy (to reach a more stable binding energy per nucleon)

depends on how many nuclei remain

Activity Formula

Activity (A, Bq)

N(t) - number of atoms at time t

Exponential Decay

models how the number of radioactive atoms in a sample decreases over time at a rate proportional to the amount present

Initially falls steeply then rate slows due to constant half life and decay constant

Exponential Decay with Decay Constant Formula

N(t) - number atoms remaining

N(0) - number initial atoms

Can be swapped for Activity (A(t) and A(0) )

Exponential Decay with Linear Attenuation Coefficient Formula

D(x) - Dose after passing through material

D(o) - initial dose

Can be swapped for Intensity (I(x) and I(o) )

(Physical) Half-Life

time required to reduce initial activity by half (50%, 25%, 12.5%)

if t = half life, N(t) = 0.5N(0)

Physical Half-Life Formula

Biological half-life def + formula

λ(b): time required for half of radiopharmaceutical to be lost from biological system

Effective rate of radioactivity loss def and formula

Radiopharmaceutical loss is due to biological half-life (via faecal and urinary excretion, perspiration) and physical decay (λ(p)) of radionuclide

Effective half-life formula and purpose

Useful assess radiation risk and planning safe doses and timing for nuclear medicine procedures

α decay

Nucleus emits α-particle and often some gamma rays

Alpha particle gets absorbed by matter quickly

E.g: radium-226 to radon-222

Electron Capture

Neutron deficient nucleus captures orbital electron which combines with a proton to form a neutron

Neutrino and x-ray emitted

Electrons from higher energy levels can drop down to the new vacancy, releasing photons in process

Daughter is often in excited state so may release gamma rays

Decay scheme is to the left since Z decreases

E.g: 125-I decay

β+ decay

Occurs in neutron-deficient radionuclide

Proton converts to neutron, positron, and neutrino

Additional gamma rays can be emitted

E.g: decay of 15-O

Neutrinos

most abundant particles that has mass (although very small) in the universe

Chargeless elementary particle

In lepton family

interacts through weak nuclear force and gravity

released in some nuclear reactions to conserve energy, momentum, and lepton number laws

Metastable State

Nucleus in a high energy state with an unusually long lifetime but will eventually decay into stabler, lower energy state

Isomeric Transition

Metastable daughter decays by emission of a γ-ray

E.g: 99m-Tc decays to 99-Mo

β – Decay

Occurs when neutron rich radionuclide converts a neutron into a proton, and emits a beta particle and an antineutrino

Decay scheme is to the right since Z increases

E.g: C-14 decays to C-12

Everyday Sources of Radiation

Cosmic rays

Radioactive substances in earth’s crust

Radioactive elements in air, food, and water

Radiation Indoors

uranium and thorium radioactive decay in bricks, tiles, concrete

Bore water and poor ventilation increases radon

High altitude = less shielding of atmosphere from cosmic rays

Artificial Radiation Sources

Nuclear medicine procedures

Industry pollution

Air travel

Australia average annual radiation dose from natural background radiation

1.5 to 2 mSv

Radiation Risk

Risk for developing cancer increases per Sv

Hereditary effects much more unlikely