Nuclear Medicine Flashcards
Structure of the atomic nucleus
- Protons + Neutrons
- Similar mass
- Mass number is P + N
- Atomic number is only P
Alpha decay
- Atomic nucleus emits an alpha particle (Helium)
- Decays into different atomic nucleus
- Mass number reduced by 4
- Atomic number reduced by 2
Energy spectra of alpha, beta and gamma radiations
- Alpha spectrum: Line, all excess energy goes to alpha particle that releases from unstable nucleus.
- Beta spectrum: Continuous, because emitted particles are electron/positron and neutrino/antineutrino.
- Gamma spectrum: Line, because gamma radiation is packed in quanta
Stability of the atomic nucleus
- Depends on Proton/Neutron ratio
- Isotopes can be stable or unstable
- Unstable atoms will decay until they are stable
Beta negative decay
Unstable nuclei convert a neutron into a proton, electron and antineutrino
Production of isotopes
- Some isotopes are unstable, so emit radiation
- Radiation can be harvested for medical use
- Isotopes produced in nuclear reactors by bombardment of stable nuclei with high energy particles.
- Tc generator can be used specifically when gamma radiation isotope of short half life is required.
Definition and types of isotopes
Two or more atoms that have the same atomic number but different atomic mass (dif. Number of neutrons)
Beta positive decay
- Positron and antineutrino are emitted.
- Proton in nucleus becomes a neutron
- Daughter nucleus will have a smaller atomic number but same mass number.
Types of radioactive decay
- Alpha
- Beta +/-
- Gamma
Gamma decay
- Product of excited nucleus that tried to achieve stable state.
- Products of either alpha/beta decay or of some other nuclear process.
Activity
Rate of decaying atoms per unit time. (Becquerel)
1Bq = 1 decay/second
Interaction of alpha radiation with matter
- Linear Ion density: Amount of ions produced over a certain length decreases after the particle loses its energy.
- Effective range: Distance covered by particle until energy is lost. (Mass and charge influence this)
- Beta particle is much smaller so it has a higher effective range, but much lower linear ion density.
Interaction of gamma radiation with matter I: photoeffect
- Gamma-photon removes an electron from the inner shell of an atom while being absorbed.
- K.E of electron = incident photon energy
Interaction of gamma radiation with matter II: Compton-scatter
Gamma-Photon removes electron from outer shell “Compton electron” and a photon is emitted.
Interaction of gamma radiation with matter III: pair production
High energy gamma-photon is absorbed near the nucleus and electron and positron are created.
Process called annihilation where 2 gamma photons will release.
Differential and integral forms of the decay law
Integral: N/T (1/s) (Bq)
Differential: N(t) = N^-(1/s)*t
Interaction of beta negative radiation with matter
- Directly ionizing atoms by coulomb’s force
- Scatter on electrons resulting in zigzag path
- Braking radiation (X-ray)
- More penetration compared to alpha (Lower mass and charge)
Half-life and average lifetime of an isotope
Average life: time required for number of undeclared nuclei to decrease to 1/e of initial amount.
Half life: Amount of time taken for undecayed nuclei to decrease to 1/2 of initial amount.
Interaction of beta positive radiation with matter
- Radiative particle is positron
- Collides with electron in annihilation, emitting 2 gamma rays
Neutron radiation, proton radiation, the Bragg-peak
- Neutron radiation: Excited nucleus expels a neutron. Neutron does not ionize directly so it collides and energy exchanges to atom. (Elastic scatter, inelastic collision, neutron capture)
- Proton radiation: Acts similar to alpha particle. (Large mass, shorter effective range)
- Bragg peak: Relationship between penetration depth and amount of radiation deposited.
Scintillation counter I.: the scintillation crystal
- NaI crystal (thallium activated by dropping)
- Has to be transparent to emission wavelength
- Absorbs energy of radiation
Scintillation counter II.: the photomultiplier tube
A tube with a photocathode, converting scintillations from crystal to electrons (photoeffect).
- Electrons multiplied by dynodes before reaching anode.
- Electrons accelerated by a voltage through the tube
-Every collision with dynode produces secondary electrons
- Multiplication factor corresponds to number of secondary electrons.
