8. Dosimetry of ionizing radiations Flashcards
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
What is the formula if someone is exposed to more than one type of radiation
What is effective dose? Formula? Unit?
The sum of weighting factors for various tissue multiplied by equivalent dose in the given tissue
Unit of E** : **Sievert (Sv)
What is the role of effective dose (E)?
- Consider sensitivity of different tissues
What does WT denote in this formula of effective dose (E)?
The probability of stochastic damage of the irradiated tissue or organ
→ estimation of the relative sensitivity of tissue
What is the sum of these weighting factors?
2 things that characterize the derived dose concepts
Collective dose
Dose rate
derived dose concepts
What is collective dose?
Sum of the doses received by a given number of people (Ni) in the course of a given time interval.
derived dose concepts
What is the formula for collective dose?
If Ni person was exposed to Ei effective dose, collective dose is given by the following formula.
What does this formula describe?
The collective dose by which the radiation burden of a given population is described.
What is dose rate?
Received dose over time.
Unit: varies with the type of radiation and the time period (pl. Gy/month, mSv/year etc.)
Examples of stochastic effects?
e.g.tumours, heriditary diseases
How to estimate the probability of stochastic effects?
- *HT (equivalent dose) and**
- *E (effective dose)** provide a basis for estimating the probability of stochastic effects for doses below the threshold of deterministic effects.
What are examples of deterministic effects?
- e.g. erythema, epilation, cataract
What are the 3 types of dose and dose rate measuring devices?
- Electronic detectors
- Chemical detectors
- Solid materials
Principle of operation of electronic detectors (for dose measurements)
absorbed energy generates free charges
3 examples of electronic detectors (for dose measurements)
- gas-ionization detectors
- scintillation detectors
- semi-conductor detectors
How does ionization chamber work?
As charges move towards the electrodes, they produce an electric current proportional to the dose rate.
The change in capacitor charge is proportional to the dose.
How to measure dose with ionization chamber?
As charges move towards the electrodes, they produce an electric current proportional to the dose rate.
The change in capacitor charge is proportional to the dose.
What does ionization
on voltage depend on?
Current impulses induced by incoming particles
What are the 3 types of scintillation detectors?
Liquid scintillator
Plastic scintillator
Inorganic crystals
3 things that characterize Liquid scintillator
– Solution of fluorescent compounds
– Primary excitation of solvent and follow-up excitation of diluted compound
– Light emission
Characterize plastic scintillator
Solid materials
Describe inorganic scintillator
– Primary excitation of crystal, follow-up excitation of luminescent atoms
Describe SEMICONDUCTOR DIODE DOSIMETE
It measures radiation dose from the quantity of electron-hole pairs generated in doped-semiconductor diodes (Si, Ge).
The current is proportional to the dose rate.
In basic setup, an ionization chamber can be used as( 1)___
→ the electric current is (2)___ to the particles generated per unit time and is related to the (3)___
- dose rate indicator
- proportional
- dose rate
Principle of operation of chemical detectors (for dose measurements). Give an example
based on radiochemical alterations
E.g, Film dosimeters follow-up evaluation
Principle of operation of solid materials (for dose measurements). Give an example
based on physical parameters of solid materials
→ thermoluminescent detector – TLD (LiF, CaF2, BeO, Al2O3)
thermoluminescent detector – TLD
The relationship between the number of emitted photons and absorbed dose
The number of emitted photons is proportional to the absorbed dose
