3: Ionization Flashcards
What is ionization?
Creation of ion pairs - transfer of energy from photon or atom (electron that was forced out of atom)
Ionization can be caused when these types of particles interact with the atom.
Alpha/Beta, neutrons and free electrons.
HOW does ionization occur when the particles interact with the atom?
Energy transferred from moving particles to orbital electron and dislodges it.
energy received depends on mass, charge and speed
What is specific ionization?
Number of ion pairs produced per unit length of a particles path.
What is total ionization?
Total number of ion pairs produced along path.
Is ionization through electromagnetic radiation a result of direct collision?
No because electromagnetic radiation has no mass.
What is ionization through electromagnetic radiation a result of? (3)
- Photoelectric process
- Compton scattering
- Pair production
What is the photoelectric effect?
Photon (LOW energy) transfers its total energy (complete absorption) to an electron by colliding with it - photon completely absorbed.
*Excess energy transferred to electron in kinetic energy form.
What is Compton scattering?
Photon (MED energy) interacts with orbital electron and dislodges it - photon loses energy* (from interaction) but continues to travel in different direction room.
*Greater than energy required to eject.
What is pair production?
Interaction with nucleus.
Photon must have min energy (1.022 MeV) comparable to mass energy of 2 resting electrons.
Destroyed photon creates: electron (-1) and positron (+1)
Describe 3 characteristics of a positron:
- Short lived
- Disappears when collided with free electron
- Energy used to create 2 photons of 0.511 MeV each
What is the result of an electron and positron that was created through annihilation of a photon?
2 lower energy photons
What is radiation intensity directly proportional to?
Activity of radionuclide.
More rays = more ionization
Number of rays available is determined by what?
Dose rate
What is energy proportional to?
Frequency
What is energy inversely proportional to?
Wavelength.
What would be considered primary radiation?
Photons produced in an X-ray or following a disintegration of a radionuclide.
When primary radiation strikes an object, what does the gamma radiation and orbital electron produce?
A free electron AND a less energetic photon travelling different direction than incident — which is scattered radiation
Amount of an absorber depends on what 3 factors?
- Original energy of radiation
- Density/atomic structure of absorber
- Thickness of absorber
What is the absorbing material expressed in?
HVL and TVL
What happens to high energy radiation when passing through a gas?
Ionization
What does a survey meter measure?
Ion pairs
What is a scintillator?
Absorbs ionizing radiation and converts fraction of absorbed energy into visible/UV protons
What type of radiograph do you need when generating X-rays using a low kilo voltage?
Radiograph with high contrast.
Low radiation is more attenuated
X-ray films for general radiography consist of what?
- An emulsion made of gelatin
- Radiation sensitive silver halide crystals
- Emulsion costed on both sides of base in layers about 0.0005” thick
With exposure above 150 KV, where does the film need to go?
Between 2 thin sheets of lead compound in paper backing.
What type of screens are essential in basically ALL radiography with gamma rays?
Lead foil screens*
What are 3 benefits of lead foil screens?
- Shorter exposure time than without
- Absorb large portion of scattered radiation
- Intensifies primary radiation
When do you use fluorescent screens in NDT?
For very thick specimens.
Fluorescent screens = lower definition
What events occur during a photoelectric effect?
Incident photon with low energy transfers TOTAL energy to electron in a shell by colliding with it - excess energy transferred to electron in kinetic energy form.
What events occur in a Compton effect?
Med energy incident photon interacts with orbital electron and dislodges it - Compton electron ends up with kinetic energy, photon of less energy than incident travelling in different direction
What is radiographic contrast?
Degree of density difference between 2 areas on radiograph.
