8. Dosimetry of ionizing radiations Flashcards
(52 cards)
3 tasks for dosimetry
Estimation of health risk for prevention.
Estimation of biological damages.
Design of therapeutic procedures.
3 requirements for dose values
• proportional to the damagas and expected risk
- additive
- independent of other factors
What are the 3 dose concepts?
- physical dose concepts
- Biological dose concepts
- Derived dose cocepts
2 things that characterize physical dose concepts
- Absorbed dose
- Exposure
physical dose concept
What is validity of absorbed dose?
For any kind of material and any type of radiation without restriction
physical dose concept
Formula of absorbed dose?
Energy absorbed per unit mass
physical dose concept
Why isn’t the determination of absorbed energy a simple task?
Because it has a relatively small value
physical dose concept
It is difficult to measure the amount of charges generated by ionizing radiation in body tissues and in liquids generally
→ it is practical to take measurements in the ____ phase
gaseous
physical dose concept
What is exposure? Its formula and unit?
It measures the amount of positive or negative charges generated by x-ray or Gamma radiation in air of unit mass (during electron equilibrium)
Unit: C/kg
physical dose concept
What is validity of exposure?
in the air, only γ and X –rays, measured in electron equilibrium*
Unit: C/kg
physical dose concept
How are 2nd electrons generated?
During the course of measurement
→ irradiation generates secondary electrons are both inside and outside volume V (cavity filled with air)
physical dose concept
What is electron equilibrium? When is this condition most satisfied?
If the number of 2nd electrons leaving and entering volume of the cavity (V) are equal
→ It is most satisfied when air is surrounded by air
3 things to be considered about Electron-equilibrium
- composition of surrounding material (chamber wall) – air-equivalent wall!
- thickness of the wall
- Photon energy: E< 0.6 MeV
physical dose concept
What is the relationship between biological effects and absorbed dose?
They are proportional to each other
physical dose concept
How to calculate absorbed dose from exposure?
Take the amount of energy necessary to produce one ion pair multiplied by exposure
Dair =f0X (f0 = 34 J/C)
physical dose concept
Will the dose absorbed by tissue be relevant to degree of damage?
YES
In case of low energies (< 0.6 MeV), is there a difference between tissue and air in the process of absorption of X-ray and gamma radiation?
Yes
If intensities of radiation of volumes compared are equal, what is the relationship between absorbed energies and mass attenuation coefficient (µm)
The absorbed energies are in same proportion as the mass attenuation coefficient (µm)
2 things that characterize biological dose concepts
Equivalent dose
Effective dose
Is the absorbed dose enough to measure the possible biological consequences? Why?
No, because the biological consequences are influenced by
- The type of radiation → radiation weighting factor
- The sensitivity and biological function of target → tissue weighting factor
What is equivalent dose? Formula? Unit?
The dose of radiation R absorbed by tissue T (DT,R) through a “radiation” weighting factor wR
Units: J/Kg or Sievert (Sv)
Why are the fates of the rabbits different?
Because alpha radiation has a very significant weighting factor and high linear transfer energy
What is linear energy transfer?
the amount of energy that an ionizing particle transfers to the material traversed per unit distance.
-> the energy transferred to the material surrounding the particle track, by means of secondary electrons. (nEionpair/l)
Equivalent dose (H)
Is „Efficiency” of various forms uniform? Why?
No, because there could be Different in linear transfer energy and radiation weighting factor
