Dosimetry 1 Flashcards
1
Q
What interactions are expected with charged particles?
A
- ionisation and excitation are most likely
- Bremsstrahlung is possible but more likely from small and light electrons
- Interactions are mediated by coulomb forces
2
Q
What are the laws of energy loss?
A
- Rate at which a charged particle loses energy is:
- proportional to the square of the particles charge
- inversely proportional to the square of its velocity
- Particles with a slow velocity loses energy quickly
- As a particle slows down, ionisation increases and the particle loses energy more quickly
3
Q
What differs between using electrons and using x-rays?
A
- start depositing energy straight away, no intermediate step (compton scatter etc)
- Electrons lose energy continually as depth increases and go deeper in the patient
- For electron beams Dmax occurs when max diffusion of electrons has occurred
4
Q
What is the practical range for electrons?
A
- Typically 0.5 x E
5
Q
What is the therapeutic range?
A
- Most useful treatment depth
- 0.3 x E
6
Q
What thickness is needed for electron cutouts?
A
- 0.5 x E + 1mm
7
Q
How is backscatter tackled?
A
- Use high charge materials for shielding which can lead to back scatter (can be good or bad)
8
Q
How do secondary electrons interact?
A
- Interact similarly to heavier charged particles
- Suffer multiple scattering and changes in direction
- Bragg peak still occurs but we cannot see it
- radiation losses are small in water/human tissue
- radiation losses are more prevalent when energy of the electron is higher and Z increases
9
Q
What happens to secondary electrons?
A
- deposit energy by collision events
- deposit energy by radiative events
- Electron will follow a path through the material losing energy by these means until it stops
10
Q
What is stopping power?
A
- A materials ability to stop an electron or charged particle
- Rate at which electron loses energy along a track
- Jm-1
11
Q
What is LET?
A
- Linear Energy Transfer
- Rate at which energy is deposited along a particle track
