10. Dosimetry Flashcards
What is ionizing radiation?
radiation with high enough energy to produce ion pairs directly or indirectly (+ ion, - free electron) while absorbed in the matter.
It includes corpuscular radiations and high-energy electromagnetic radiations (X-rays and γ-rays).
2 types of ionizing radiations
(1) Direct ionization (in case of charged particles)
2) Indirect ionization (in case of uncharged particles and high-energy electromagnetic radiation
2 Examples of direct ionization
Alpha
Beta
2 example of indirect ionization
Gamma
X-rays
The part of the ionizing radiation that is absorbed in a living organism leads, via different processes (physical, radiochemical, molecular biological), to macroscopic biological changes or damage, the severity of which depends on the parameters listed below:
List 4 parameters
(1) Type of radiation
(2) sensitivity of the organ or part of the body that received the radiation
(3) the received dose (D)
(4) the time course of the received dose
What is direct effect?
It occurs when the ionizing radiation is absorbed in biologically important macromolecules (e.g., DNA), and causes damage there directly.
What is indirect effect?
It occurs when the ionizing radiation is absorbed in water, the largest constituent, by quantity, of the organism, and induces the formation of free radicals (∙H, ∙OH, etc.).
=> These highly reactive free radicals then interact with the biologically important macromolecules.
What is deterministic effect?
the probability of radiation damage increases abruptly above the threshold dose.
Above this dose level, the severity of the damage is proportional to the dose.
Erythema and cataract (transparency loss of the eye lens) are typical deterministic effects.
What is stochastic effect?
the probability of radiation damage is proportional to the dose (a threshold dose cannot be defined).
Genetic disorders and some forms of cancer (which are a consequence of mutations) are caused by this “random” effect.
What is the role of radiation protection?
To prevent the deterministic effects
To decrease the risk of the stochastic effects to an acceptable level in the proximity of the radiation sources (therapeutic and diagnostic X-ray devices, closed and open radioactive preparations).
What is absorbed dose?
the amount of energy absorbed per unit mass
∆E - energy absorbed by mass ∆m
Unit - 1Gy = 1 J/kg
Formula for exposure
Where ∆q is the positive charged produced in air of mass ∆m by ionization
Unit - C/kg
2 types of rays that EXPOSURE formula is valid for
X-rays
Gamma rays
The role of effective dose equivalent (E)
It expresses the biological damage of the individual tissue or organ
formula for effective dose
the equivalent dose, which is multiplied by a tissue weighting factor (wT) characteristic for the tissue sensitivity towards radiation damage
Unit- sievert
The role of dose rate
It characterizes the strength of the ionizing radiation
What is dose rate in case of constant-intensity radiation?
The ratio of the dose and the duration of irradiation
According to their principle of operation, what are 2 main groups of dosimeters?
(1) Gas-ionization detectors
(2) Solid-state detectors
1 example of gas-ionization detectors
Ionization chamber
2 examples of solid-state detectors
Thermoluminescent dosimeter
Scintillation dosimeter
In case of dose measurement, the (1) Voltage U produced by the accumulated charge Q on _____ C is (3) proportional to the total amount of the _____
(2) Capacitor
(3) proportional
(4) separated charges
In case of dose rate measurement, instead of the capacitance, the potential drop is measured on a large ______ which is (2) Proportional to the (3)_____ that flows through per unit time, thus it is (4) Proportional to the (5)_____
(1) Resistance R
(2) Proportional
(3) Charge Q
(4) Proportional
(5) Exposure dose rate (X/t)
What is ionization chamber?
It measures radiation dose from the quantity of charges produced in a mass of air between the charged plates of a capacitor.
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
3 types of ionization chambers
(1) Thimble ionization chamber
(2) Pocket chamber dosimeter
(3) GM-tube
What is thimble ionization chamber?
The chamber (V < 1 cm3) which can be placed into body cavities is needed for the therapeutic irradiation
What is thimble ionization chamber?
The chamber (V < 1 cm3) which can be placed into body cavities is needed for the therapeutic irradiation
Pocket chamber dosimeter (aka. ______), is used for the purposes of personal dosimetry
The chamber has a (2)_______, and the (3) voltage between its (4)_____ is displayed on a scale of a built-in electroscope with quartz-fiber indicator.
When in use (during work), radiation that reaches the chamber creates (5)_____
=> the charge of the capacitor (6)_______ and the indicator displays a (7)_____ voltage.
The (8) charge and the (9)____ are proportional to the (10)______.
(1) capacitor ionization chamber
(2) capacitor
(3) voltage
(4) electrodes
(5) ions
(6) decreases
(7) smaller voltage
(8) charge
(9) voltage loss
(10) radiation dose received
GM-tube counter (Geiger-Müller counter) has a cylinder-shaped ionization chamber, where the (1)______ is the wall of the cylinder and the (2)______ is a thin metal thread in the axis of the cylinder.
The tube is filled with a (3)_____ and has a thin window (e.g. mica plate that passes -radiation), and it is possible to place absorbent plates to select the measured types of radiations (, ).
The (4)_____ of the chamber and the (5)______ on the electrodes accelerates the electrons so much that they induce an (6)______ in the gas. A single electron can cause as many as 108 avalanche electrons (gas-amplification), causing a current pulse that can be evaluated without an amplifier.
(1) cathode (-)
(2) anode (+)
(3) noble gas
(4) geometry
(5) high voltage
(6) avalanche of ionization
GM-tube counter
The principle of operation implies that a (1)_____ or (2)___ of any type of radiation produces (3)______, therefore the value shown by the GM counter is proportional to (4)______.
=> The GM-tube cannot provide information about the type or energy of the particle.
After calibration, however, it is possible to measure the dose rate of radiation of known type and energy.
Then, the total pulse number on the display is (5)______ to the dose. If the pulse frequency, thus the pulse number per unit time is displayed in rate-meter mode (pulse number/time), then the displayed value is proportional to the dose rate.
(1) single particle
(2) photon
(3) an equivalent current pulse
(4) the number of ionizing particles
(5) proportional
The GM tube is relatively insensitive to (1)_______, but it is extremely sensitive to (2) - and -(3)____ radiation, so it is widely used to (4)_____, e.g., (5)______.
Its time resolution is limited (approximately 1000 pulses/s), because it takes time to re-establish the noble gas atoms following the avalanche of ion-pair formation. Pulses can be followed through a built-in speaker as cracking sounds.
(1) gamma-radiation
(2) alpha
(3) beta
(4) detect low-activity radiation
(5) radioactive contaminations
The role of film dosimeter
It estimates radiation dose from the darkening of exposed and developed photographic film.
The role of thermoluminescent dosimeter (TLD)
It measures radiation dose from the quantity of electrons trapped on the dopant energy levels (e.g. Dy) of an insulator crystal (e.g. CaSO4).
(The necessary energy to lift an electron into the energy level of indium as a dopant, is only 1 % of the energy which is needed to raise a valence electron of silicon into the conduction band. With the inclusion of an electron, the dopant is negatively charged, such dopants are called acceptors (acceptare, lat.)
How does thermoluminescent dosimeter (TLD) work?
Readout is done by heating the crystal in the dark
=> forcing the electrons into the conduction band from where they can return to the valence band in a step accompanied by light emission.
The number of emitted photons is proportional to the absorbed dose.
Describe scintillation radiation detectors
In an insulator crystal (e.g., NaI) electrons are excited into the conduction band, then return, through the dopant (e.g., Tl) energy levels to the valence band while emitting light.
The number of emitted photons per unit time is proportional to the dose rate.
parts of ionization chamber
It consists of a gas-filled chamber with two electrodes; known as anode and cathode.
