22) Dosimetry Flashcards
Dosimetry
Studies the quantities describing the effect of ionising radiation on matter and the methods of their measurement and calculation
Application in medicine
Radiography
Nuclear medicine
Radiobiology
Biological effects of ionising radiation
Due to the part of the radiation energy which is transferred to the medium
The energy leaving the medium with the radiation produces no effect
Absorbed dose D
The energy deposited by the ionising radiation per unit mass of the medium
D= 🔺E/🔺m
Units is gray Gy
I rad = 0.01 Gy
Biological effect increases with the absorbed dose
Absorbed dose rate Pd
The rate at which the absorbed rate is accumulated
P_D = 🔺D/🔺t
Biological effect increases with the absorbed dose rate
Equivalent dose rate H
The absorbed dose weight by the quality factor Q of the ionising radiation
H=QD
Realative biological effectiveness is Q
Quality factor Q
Radiation. Energy. Q
Photon
Beta
Proton Above 2 MeV
Neutron Below 10 Kev
100 Kev - 2 MeV
Alpha
BOOOOOK
Equivalent dose rate P_H
The rate at which the equivalent dose rate is accumulated
P_H= 🔺H/🔺T
Measurement is Sv/s
Biological effect increases as equivalent dose rate increases
Exposure x for x rays and gamma rays
Total charge for the ions of one polarity produced by the radiation in unit mass of the air
Exposure rate Px
The rate at which exposure is accumulated
PX = 🔺x/🔺T
Dose exposure relationship
Absorbed dose of a photonic radiation proportional to the exposure X
D=FX
F is exposure dose conversion factor
Conversion factor f
Measure of energy absorbed by the irradiated medium
It is proportional to the coefficient of the medium
Depends on the energy and the type of medium
Conversion factor f for tissues
Low energy is photoeffect
Conversion factor for bone is larger than soft tissue
Medium energy is Compton effect
The conversion factor is equal for all tissues
High energy is e- -e+
Conversion for bone is somewhat larger
Dosimetric instrument
Decide used to measure dosimetric quantities
Dosimeter
Measures dose exposure and rates
Radiometer
Measures activity of radioactive sources
Types of dosimetric detectors
Ionisation detector
Scintillation detector
Semiconductor detector
Chemical detector
Thermoluminescent detector
Ionisation detector
Gas filled tube with electrodes
.
Electrical conductivity of gas increases with ioniszing radiation
The disadvantage is that amount of ions produces in gases and tissues is different
Ionisation chamber
DRAWING
the raid iron ionises the gas and current flows in circuit
Current is proportional to exposure rate
Quartz fiber dosimeter
Small ionisation chamber
No power supply
Ionisation of the gas reduces the charge of the electrodes
Proportional counter
Average frequency of electric pulses is proportional to the activity of the radiation source
Geiger counter
Each ion produces a strong electric current and the voltage between the electrode drops
The average frequency of the electric pulse is proportional to the activity of the radiation source
Used as radiometer only
Scintillation detectors
Ionisation radiation produces fluorescence in the material of the detector
Disadvantage is that they’re not tissue equivalent
Frequency is proportional to dose rate
Magnitude depends on the type and energy of the ionising particle
Semiconductor detectors
Ionisation radiation increases the electrical conductance of the semiconductor material
Disadvantage is that they’re not tissue equivalent
Chemical detectors
Radiation initiates a chemical reaction
Advantage is that it’s tissue equivalent
Disadvantage is that it has low sensitivity
Photographic detectors
Photograaphic films are sensitive to radiation
Darkening of the film depends on the absorbed dose
Disadvantaged is low accuracy
Thermoluminescent detectors
The atoms excited by the radiation remain excited for long period of time
When crystal is heated it’s released as luminescent photons
Very sensitive
Natural background radiation
Cosmic radiation
Ionisizing radiaiton
Natural background radiation
Cosmic
Bulgaria is 300 usv/a
World is 400 usv/a
Terrestrial
Bulgaria 2000usv/a
World 2400usv/a
Technogenic radiation
Radiography 1400
Nuclear medicine 110
Global fallout 5
Occupation exposure 4
Fossil fuel power station 2
Nuclear power station 2
Background radiation
Natural
B is 2300
W is 2800
Technogenic
B is 1500
W is 800
Overall background rad
Add them up
Natural background radiation
Cosmic
Bulgaria is 300 usv/a
World is 400 usv/a
Terrestrial
Bulgaria 2000usv/a
World 2400usv/a
Technogenic radiation
Radiography 1400
Nuclear medicine 110
Global fallout 5
Occupation exposure 4
Fossil fuel power station 2
Nuclear power station 2
Background radiation
Natural
B is 2300
W is 2800
Technogenic
B is 1500
W is 800
Overall background rad
Add them up
Photographic detectors
Photograaphic films are sensitive to radiation
Darkening of the film depends on the absorbed dose
Disadvantaged is low accuracy
Thermoluminescent detectors
The atoms excited by the radiation remain excited for long period of time
When crystal is heated it’s released as luminescent photons
Very sensitive
Natural background radiation
Cosmic radiation
Ionisizing radiaiton
Radiation protection
Minimising exposure to ionising radiation of workers to the lowest reachable levels
ALARA principle
Compromise
The use of ionising radiation brings benefit
So you minimise the exposure not eliminate
Higher exposure to a small percentage of the population may be tolerated
Exposure limits
Occupational
50-100 nAv/a
Individual
5mSv/a
General public
1msv/a
Principles of radiation protection
Use ionising radiation if the results can’t be achieved by any other method
Establish that exposure limits are at the lowest reasonable achievable value
Methods of radiation protection
Ensure safe working conditions Time Amount Sheilding Distance Provide regular dosimetric screening and medical monitoring
