L3- Intro to Isotopes, Radioactivity, and Radioactive Decay

• nuclide


  A specific type of atom characterised by no. of protons/neutrons in nucleus
• Radioactive Decay Law

  (First order/exponential decay)

  Nt = N0 e^-λt (^)

  where:

  - Nt = no of radionuclides left

  - N0 = no of radionucleides at start

  - λ - decay constant

  - t = time elapsed

  
• Radiolabelling

  Using radioactive isotopes to track tracers in the body

  - Eg a drug can be labelled with an unstable radionuclide

  - PET = Positron Emission Tomography uses Fluorine-18

  - 18F goes into body by FDG (Fludeoxyglucose) which mimics glucose. Can track where glucose goes

  - 18F taken up by cells using lots of glucose eg brain/cancer cells

  - 20% FDG excreted renally, the rest decays via positron emission, converting to 18O

  
• Isotopic labelling

  Using stable isotopes to study drug metabolism
• Kinetic isotope effect

  - Heavier isotopes react slower than lighter isotopes

  - bond energies affected by mass of atoms that form the bond

  - most pronounced effect for hydrogen

  
• Stable isotopes

  Atoms with stable number of neutrons
• Isotopes

  Atoms with same atomic number but different atomic mass (same proton no, different neutron no)

  
• Half-life

  What's the equation?

  Time for half the radioactive nuclei to decay

  - t1/2 = 0.693/k

  where k = λ (decay constant)

  
• Decay constant λ

  What's the equation?

  probability that a given radioactive nuclei will decay per unit time

  λ

  
  

  λ = 0.693/t1/2

  
• Gamma particles


  Neutral EM radiation,

  - stopped by lead/concrete

  - uncharged particles

  - daughter nucleus in excited state, so decays to lower ground state by emitting a gamma ray photon (light particle)

  - most penetrating

  - least ionising

  
• Beta particles

  Electrons,

  - stopped by Al sheet

  - Beta decay: neutron turns into a proton, with emission of e- and a neutrino

  - mass no = same, proton no +1

  
• Alpha particles

  Example of alpha decay equation


  +ve He nuclei,

  - stopped by air so only observed in a vacuum

  - least penetrating

  - most ionising

  
• Radioactivity

  Discovered by Becquerel,

  - radioactive elements decay with exponential law

  - do NOT all decay at the same rate

  
• Neutrons:

  What is a neutron?

  What does it decay to when exposed to beta radiation?

  What does it do in the nucleus?


  - Uncharged particle with similar mass to proton

  - decays to a proton when exposed to beta radiation

  - stabilises nucleus by nuclear force, overcoming proton-proton repulsion, binding protons and neutrons together

  - mass no:proton ratio for light elements around 2. For heavier elements than 40C, always more neutrons than protons to stabilise the nucleus