Attenuation And Radiation Protection Flashcards
What is photoelectric absorption and what increases the chances of it occurring
- When a photon matches the binding energy of an electron and transfers all this energy, ejecting the electron and the photon ceasing to exist
- this causes an electron from an outer shell to fall and fill the gap
- The energy given off is equal to the difference of binding energies involved
- It increases with atomic number (proportional to atomic number cubed)
- It decreases with photon energy ( inversely proportional to energy cubed)
- It increases with density (directly proportional)
What is Compton scatter and what alters the chance of it occurring?
- When the photon energy is much higher than the binding energy
- Some of the photon energy is transferred to the electron meaning it’s considered free and may cause further ionisations
- The photon is scattered and the greater the energy lost by the photon the larger the scattering angle will be (loosing the most will scatter it 180 degrees)
- It increases with electron density which is only impacted by hydrogen which is twice as high
- Up to to 100kVp it’s relatively constant and then decreases again
- It increases with density
Explain pair production and factors impacting it
- Photon with at least 1.02MeV is required for photo-nuclear interactions and any excess is given off as kinetic
- This photon interacts with the nuclear field causing the nucleus to recoil and the photon to cease to exist
- A positron electron pair is produced and as the positron can’t exist outside of the nucleus it is annihilated in an interaction with an electron. Two photon each with 0.51MeV are produced and fired 180% from each other
- Above 1.02MeV pair production increases with photon energy
- Pair production also increases with atomic number as a larger nucleus has a bigger nuclear field of
Why is lead a good absorber of X-rays?
- High atomic number of 82
- So a large range in binding energies meaning it’s an effective absorber of photons across a wide range of photon energies
- absorption is greatest when photon energies and binding energies are equal
- Less effective at absorbing MeV
What is a LINAC room
It’s a linear acceleration and these operate at MeV
Deterministic effects of radiation
- Not chance occurrences
- There is a threshold below where they don’t occur
- Severity proportional to dose
- Results in cell death
Stochastic effects of radiation
- Chance effects
- No set threshold
- likelihood increases with dose but not severity
- Cells mutate
Job of referrer
•Provides all relevant info like clinical details, previous exams and other test results that will allow the practitioner to decide whether it’s justified
Job of practitioner
To assess all the information and to make sure that all the risks are outweighed by benefits and to justify the procedure
Job of operator
- All the practical aspects of the procedure
* They can authorise but not justify
Difference between RPS and RPA
- RPS is day to day implementation of IRR 2017
* RPA advises on protection of staff and others and how to implement IRR 2017
What is the difference between thermoluminescent and optically stimulated (photostimuable) dosimeters
- They both work because of the presence of electron traps in the forbidden gap
- Both when exposed to radiation cause electrons to be excited and caught in traps
- TLD heating causes the electrons to be ejected from the trap and pass back to valence band with light emission
- OSL needs to be stimulated by lasers to cause the electrons to be ejected
- The amount of light emitted it proportional to radiation dose in both
