Lecture 9

Question 1

Define equivalent dose

Answer 1

The dose that takes into account the net effect of the radiation on tissue (in the body) by using a radiation weighting factor. It is measured in sieverts (Sv).

Question 2

What is the equation for equivalent dose?

Answer 2

H = equivalent dose
D = dose
Wᵣ = radiation weighting factor

Question 3

How can the weighting factor for equivalent dose be found?

Answer 3

It can be found using physical evidence (data) of the effects of different sources of radiation rather than being based on calculations.

Question 4

What is the radiation weighting factor for beta?

Answer 4

1

Question 5

What is the radiation weighting factor for alpha?

Answer 5

20

Question 6

What is the radiation weighting factor for x-rays?

Answer 6

1

Question 7

What is the radiation weighting factor for gamma rays?

Answer 7

1

Question 8

Describe how the radiation weighting factor changes for neutrons of different energies

Answer 8

Question 9

Define effective dose

Answer 9

A description of the biological relevance of radiation exposure when different tissues/organs receive varying absorbed doses potentially from different radiation sources, measured is Sv.

Question 10

Effective dose is a quantification of _____.

Answer 10

Risk

Question 11

What is a tissue weighting factor?

Answer 11

A factor that describes the sensitivity of different tissues to radiation.

Question 12

What is the equation for effective dose in multiple organs?

Answer 12

E = effective dose
Wₜ = tissue weighting factor
H = equivalent dose
D = dose
Wᵣ = radiation weighting factor

Question 13

What is the total tissue weighting factor for the whole body?

Answer 13

1.00

Question 14

What is the risk factor per sievert of radiation in the whole body?

Answer 14

5.0%

Question 15

What sources of radiation have natural origin?

Answer 15

• Cosmic rays
• Terrestrial gamma
• Internal (K-40 and C-14)
• Inhaled (radon Rn-222 and decay products)

Question 16

What sources of radiation have man-made origin?

Answer 16

• Nuclear weapons, Chernobyl, leaks, etc.
• Average medical exposure

Question 17

What is the average annual equivalent dose ?

Answer 17

2.3 mSv (in UK)

Question 18

What is the average annual equivalent dose from cosmic rays?

Answer 18

0.3 mSv

Question 19

What is the average annual equivalent dose from terrestrial gamma?

Answer 19

0.35 mSv

Question 20

What is the average annual equivalent dose from internal sources (C-14 and K-40)?

Answer 20

0.38 mSv

Question 21

What is the average annual equivalent dose from inhalation (radon Rn-222 and decay sources)?

Answer 21

0.97 mSv

Question 22

What is the average annual equivalent dose from nuclear weapons, Chernobyl, leaks, etc.?

Answer 22

0.03 mSv

Question 23

What is the average annual equivalent dose from medical exposure?

Answer 23

0.25 mSv

Question 24

The average annual equivalent dose varies with ________ and ____________.

Answer 24

Geography
Geology

Question 25

Describe the pie chart for the distribution of annual equivalent dose

Answer 25

Question 26

What is the dose of a CT scan?

Answer 26

~10 mSv (> ~10 mGy)

Question 27

What is the lifetime risk of fatal cancer from a CT scan?

Answer 27

1:2000

Question 28

What is the normal risk of fatal cancer?

Answer 28

~1:5

Question 29

How can the lifetime dose be used to calculate the chances of fatal cancer in a lifetime?

Answer 29

1) Multiply the average annual equivalent dose by the average lifespan.
2) Find 5% of this value.

Question 30

Why is it risky to have many scans over a lifetime?

Answer 30

There is a high radiation dose administered to the patient (equal to 4 years worth of annual dose per scan).

Question 31

Why do children have a higher health risk from getting a CT scan?

Answer 31

• Smaller
• Growing tissue
• More stem cells
• More years left after scan than an adult (younger)
• Only used when the child is very unwell so they’re more likely to need multiple scans

Question 32

What is the dose of a paediatric CT scan?

Answer 32

5 mSv - 60 mSv

Question 33

Name 5 types of dosimeter technology

Answer 33

• Ionisation chamber (e.g. farmer-type chamber or parallel plate chamber)
• Geiger Mueller counter
• Film
• Thermoluminescence dosimetry
• Semiconductors

Question 34

What are parallel plate ionisation chambers used for?

Answer 34

• Low energy x-rays (< 60 kV)
• Electrons of any energy

These are used for radio-therapy and x-ray beam monitoring and calibration.

Question 35

What is Fricke gel?

Answer 35

A simple and cost-effective gel made of iron that can be used as a phantom tissue for parallel plate ionisation chambers. It changes colour when radiation is incident on it so is good for checking the position of beams and the dose of beams.

Question 36

What are BANG polymers?

Answer 36

Polymers that show a radiation pattern. The radiation causes polymerisation to take place so solid areas are areas of radiation. It is used as a phantom tissue for parallel plate ionisation chambers.

Question 37

How does thermoluminescence dosimetry work?

Answer 37

A type of radiation dosimeter, consisting of a piece of a thermoluminescent crystalline material. When the thermoluminescent crystal is exposed to ionising radiation, it absorbs and traps some of the energy in its crystal lattice. The level of this radiation can then be determined by a specialised detector that measures the intensity of the emitted light.

Question 38

What is a Geiger-Müller counter?

Answer 38

A very sensitive counter of radiation events (rather than a dosimeter) that can give a general alert of a radiation source. They are lightweight and portable.