TOPIC: UNITS OF MEASUREMENT AND DOSIMETRY
🌟IMPORTANT POINTS TO REMEMBER
🔹Fundamental units: Fundamental units are units which are independent of each other and have their own standard (base).
🔹Derived units: Derived units are units which are not having their own standard (base) and are obtained from fundamental units.
🔹SI unit: In 1960, the new system of units – the SI units – was introduced.
🔹Units should be well-defined, reproducible, comparative, sensitive, objective, repeatable and linear.
🔹Air is used as the measurement material due to its availability, reproducibility, accuracy and sensitivity and its atomic number.
🔹Exposure: It is the measure of the quantity of radiation. Traditional unit is roentgen and SI unit is coulomb per kilogram.
🔹One roentgen is defined as an exposure that will produce 2.08 × 109 ion pairs in a cubic centimetre of air under standard temperature and pressure.
🔹RAD: It is a measure of the amount of energy absorbed from the radiation beam per unit mass of tissue or the deposition of energy in the patient by radiation exposure is called RAD. The SI unit is grey measured in joules/kg and the original unit is rad. It is the amount of radiation absorbed as a result of exposure.
🔹Equivalent dose: It is the measure that allows the different RBE of different types of radiation to be taken into account. It is the sum of product of absorbed dose (DT) and radiation weighing factor. SI unit is sievert and the original unit is rem.
🔹Effective dose: It is the measure that allows doses from different investigations of different parts of the body to be compared, by converting all doses to an equivalent whole-body dose. The SI unit is sievert.
🔹Collective effective dose or collective dose: This measure is used while considering the total effective dose to a population from a particular investigation or source of radiation. SI unit is man-sievert.
🔹Dose rate: It is a measure of the dose per unit time. SI unit is microsievert/hour.
🔹Kerma: It is defined as kinetic energy released per unit mass in the medium. SI unit is grey and the special unit is rad.
🔹Radioactivity: The term radioactivity refers to the disintegration rate of an isotope and it is measured by unit curie.
🔹One curie: One curie is the number of dps from 1 g of radium and it is found to be 3.7 × 1010 dps.
🔹Dosimetry: Dosimetry is the measurement of the quantity of radiation exposure or amount of energy absorbed per unit mass at an interest site.
🔹Frick dosimeter: It is based on the principle of chemical effects.
🔹Film badges: They consist of blue plastic frame containing a variety of different metal filters and a small radiographic film which reacts to radiation.
🔹Thermoluminescent dosimeters: These are used for personal monitoring of the whole body and the extremities as well as measuring the skin dose from particular investigation. They contain material, such as LiF which absorbs radiation.
🔹Ionisation chamber: Ionisation chamber is used for personal monitoring by physicist to measure radiation exposure.
🔹Rate meter: It is also called roentgen ray rate meter. It has an ion collection chamber that is continuously being charged by battery.
🔹Thimble ionisation chamber: It consists of outer shell of size of a thimble, conducting material that is earthed and central well-insulated electrodes connected to a suitable current measuring device and charged to a potential sufficient to insure saturation between it and the surrounding shell.
🔹Scintillation detector: Scintillation detector consists of luminescent material, optical device, optical coupling between the luminescent material and photomultiplier, photomultiplier tube and electrical circuit.
🔹Pocket dosimeter: It is of fountain pen size and can be kept conveniently in the pocket.
📌 REQUIREMENT OF UNITS
- Well defined: It should be well defined and should be of a suitable size.
- Reproducible: It should be easily and accurately reproducible.
- Changes to environment: It should not be affected by changes in temperature, pressure, time and other physical standards.
- Comparison: It should be easily comparable with the other similar units.
- Sensitive: Small amounts should be sufficient to obtain an acceptable reading.
- Linear: The response should be simply proportional to the quantity being measured.
- Objective: As far as possible personal subjective judgement should not be involved in obtaining an answer.
- Repeatable: Repeated measurements of the same amount of radiation should give the same reading whenever they are performed.
📌 MULTIPLE CHOICE QUESTIONS (MCQs)
1. The amount of energy absorbed from the radiation beam per unit mass of tissue is called
a. Effective dose
b. Absorbed dose
c. Equivalent dose
d. Exposure
2. If an ionising radiation imparts 1 J/kg mass to a body,
then absorbed dose is said to be
a. 2 Gy
b. 1.5 Gy
c. 1 Gy
d. 0.5 Gy
3. 1 Gy is equivalent to
a. 100 rad
b. 200 rad
c. 150 rad
d. 50 rad
4. The biological effect of different types of radiation to a
tissue or organ is measured by
a. Effective dose
b. Absorbed dose
c. Equivalent dose
d. None of the above
5. Sievert is the SI unit of
a. Equivalent dose
b. Effective unit
c. Absorbed dose
d. Both a and b
6. Effective dose is measured as a sum of product of
a. Equivalent dose × tissue weighing factor
b. Absorbed dose × radiation weighing factor
c. Absorbed dose × tissue weighing factor
d. Equivalent dose × absorbed dose
7. The size of radiographic film used in film badges is
a. 2 × 3 cm
b. 1 × 2 cm
c. 4 × 3 cm
d. 2 × 5 cm
8. The minimum dose that film badges can detect is about
a. 0.2 mSv
b. 1.0 mSv
c. 2.0 mSv
d. 0.5 mSv
9. The amount of dose that a person has received in
1 month which is considered as overexposure is
a. 15 mSv
b. 10 mSv
c. 8 mSv
d. 12 mSv
10. The material used in thermoluminescent dosimeter for
measuring radiation dose is
a. Lithium fluoride
b. Sodium fluoride
c. Aluminium fluoride
d. Silver fluoride
11. Which of the following statements is true regarding use
of Thermoluminescence dosimeter (TLD) badges?
a. Results can be reassessed.
b. Dose gradients are detectable.
c. Lithium fluoride is reusable.
d. They are cost-effective.
12. A pocket dosimeter is used to
a. Measure exposure time
b. Check film contrast
c. Measure radiation exposure
d. Check the timer
💡 Answers
1. b
2. c
3. a
4. c
5. d
6. a
7. c
8. a
9. b
10. a
11. c
12. c