10. Dosimetry

Question 1

What is ionizing radiation?

Answer 1

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).

Question 2

2 types of ionizing radiations

Answer 2

(1) Direct ionization (in case of charged particles)

2) Indirect ionization (in case of uncharged particles and high-energy electromagnetic radiation

Question 3

2 Examples of direct ionization

Answer 3

Alpha

Beta

Question 4

2 example of indirect ionization

Answer 4

Gamma

X-rays

Question 5

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

Answer 5

(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

Question 6

What is direct effect?

Answer 6

It occurs when the ionizing radiation is absorbed in biologically important macromolecules (e.g., DNA), and causes damage there directly.

Question 7

What is indirect effect?

Answer 7

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.

Question 8

What is deterministic effect?

Answer 8

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.

Question 9

What is stochastic effect?

Answer 9

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.

Question 10

What is the role of radiation protection?

Answer 10

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).

Question 11

What is absorbed dose?

Answer 11

the amount of energy absorbed per unit mass

∆E - energy absorbed by mass ∆m
Unit - 1Gy = 1 J/kg

Question 12

Formula for exposure

Answer 12

Where ∆q is the positive charged produced in air of mass ∆m by ionization

Unit - C/kg

Question 13

2 types of rays that EXPOSURE formula is valid for

Answer 13

X-rays

Gamma rays

Question 14

The role of effective dose equivalent (E)

Answer 14

It expresses the biological damage of the individual tissue or organ

Question 15

formula for effective dose

Answer 15

the equivalent dose, which is multiplied by a tissue weighting factor (wT) characteristic for the tissue sensitivity towards radiation damage

Unit- sievert

Question 16

The role of dose rate

Answer 16

It characterizes the strength of the ionizing radiation

Question 17

What is dose rate in case of constant-intensity radiation?

Answer 17

The ratio of the dose and the duration of irradiation

Question 18

According to their principle of operation, what are 2 main groups of dosimeters?

Answer 18

(1) Gas-ionization detectors

(2) Solid-state detectors

Question 19

1 example of gas-ionization detectors

Answer 19

Ionization chamber

Question 20

2 examples of solid-state detectors

Answer 20

Thermoluminescent dosimeter

Scintillation dosimeter

Question 21

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 _____

Answer 21

(2) Capacitor
(3) proportional
(4) separated charges

Question 22

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)_____

Answer 22

(1) Resistance R
(2) Proportional
(3) Charge Q
(4) Proportional
(5) Exposure dose rate (X/t)

Question 23

What is ionization chamber?

Answer 23

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

Question 24

3 types of ionization chambers

Answer 24

(1) Thimble ionization chamber
(2) Pocket chamber dosimeter
(3) GM-tube

Question 25

What is thimble ionization chamber?

Answer 25

The chamber (V < 1 cm3) which can be placed into body cavities is needed for the therapeutic irradiation

Question 26

What is thimble ionization chamber?

Answer 26

The chamber (V < 1 cm3) which can be placed into body cavities is needed for the therapeutic irradiation

Question 27

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)______.

Answer 27

(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

Question 28

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.

Answer 28

(1) cathode (-)
(2) anode (+)
(3) noble gas
(4) geometry
(5) high voltage
(6) avalanche of ionization

Question 29

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.

Answer 29

(1) single particle
(2) photon
(3) an equivalent current pulse
(4) the number of ionizing particles
(5) proportional

Question 30

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.

Answer 30

(1) gamma-radiation
(2) alpha􏰇
(3) beta
(4) detect low-activity radiation
(5) radioactive contaminations

Question 31

The role of film dosimeter

Answer 31

It estimates radiation dose from the darkening of exposed and developed photographic film.

Question 32

The role of thermoluminescent dosimeter (TLD)

Answer 32

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.)

Question 33

How does thermoluminescent dosimeter (TLD) work?

Answer 33

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.

Question 34

Describe scintillation radiation detectors

Answer 34

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.

Question 35

parts of ionization chamber

Answer 35

It consists of a gas-filled chamber with two electrodes; known as anode and cathode.