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Topic – Physics 2a
Dose and Dose equivelent
• Prior learning
– Half life- [t ½ ]
• Key words
– Half life, ionising, dose, absorb, quality factor
Topic – Physics 2a
Detecting radiation
By the end of this lesson we should be able to:
• Describe term absorbed dose
• Explain dose equivalent in terms of body
damage
• Know and use absorbed dose and dose
equivalent equations
Topic – Physics 2a
Absorbed Dose
• The energy cells absorb from radiation is called
absorbed dose
• Radiation damage depends on 2 factors:
– The energy the cell absorbs
– The time over which the cell absorbs the energy
– Absorbed does = energy absorbed / mass of body part
• This is measured in joules per kg of tissue
• The unit for absorbed dose = Gray (Gy)
• 1 Gy = 1 joule per kg (JKg-1)
Topic – Physics 2a
Task 1: Read pg 86 STAWA
Answer following questions
1. What is absorbed dose?
2. What 2 factors affect
absorbed dose?
3. What is the equation for
absorbed dose?
4. What is a GRAY?
Topic – Physics 2a
Task 1: Answers
1.
What is absorbed dose? Energy cells
absorb from radiation
2. What 2 factors affect absorbed dose?
Energy and time
3. What is the equation for absorbed dose?
Energy / mass
4. What is a GRAY? Unit for absorbed dose
1 -J per kg
Topic – Physics 2a
Dose equivalent
•
Absorbed dose is only useful if comparing
different amounts of the same type of
radiation
•
Dose equivalent is much better as it
takes into account absorbed dose and a
weighting factor called the QUALITY
FACTOR. This factor is much bigger for
alpha as it reflects the amount of damage
done to living cells by the large alpha
particle.
Topic – Physics 2a
Dose equivalent
•
•
•
Dose equivalent = absorbed dose x Quality factor
Dose equivalent measured in Sieverts
1 Sievert(Sv) = 1 joule per kg (1Jkg-1)
Radiation type
Quality factor
Gamma
1
Beta
1
Slow neutrons
3
Fast neutrons
10
Alpha particles
20
Topic – Physics 2a
Task 2: Read pg 86 STAWA
Answer following questions
1. What is dose equivalent?
2. Why is it a better measure than
absorbed dose?
3. Draw the table for weighing factor
4. Why is weighting factor higher for
alpha particles?
5. What is lethal dose?
Topic – Physics 2a
Task 2: Answers
1.
2.
3.
4.
5.
What is dose equivalent? Absorbed dose x
quality factor
Why is it a better measure than absorbed dose?
Takes into account the damage done by alpha
particles
Draw the table for weighing factor
Why is weighting factor higher for alpha
particles? More ionising and larger – greater
damage done to living tissues
What is lethal dose? LD dose that results in
death LD-50 kills 50% of exposed people
Topic – Physics 2a
Task 3: body damage
Dose equivelent Sv
Body Damage
0.25
Reduction in t(WBC)
1
Possible nausea / vommiting
4
Nausea, diarrhoea, drop in blood count – 50%
die in weeks from failure of blood forming
organs
5
Loss of hair 50% die
6
Damage to stomach,intestines, loss of fluids,
radiation sickness, death within 3 weeks
10
Severe damage to nervous system – death in
days.
Topic – Physics 2a
Summary task
• Read summary 5.4 in text pg 178
• Try questions 5.4 task 1-4
Topic – Physics 2a
What have I learnt???
I can:
• Describe term absorbed dose
• Explain dose equivalent in terms of body
damage
• Know and use absorbed dose and dose
equivalent equations