Dosimetry

1
Dosimetry Safety
2
Activity
The term 'Activity' of a source describes the
(in)stability of the atoms within a substance.
One atom decaying per second is called 1
Becquerel (Bq). Uranium is a very unstable
substance with an activity of 12,000 Bq. Sea
water is a stable substance with an activity of
11 Bq. Each atom that decays will release
potentially harmful radiation, in the form of
Alpha and Beta particles, Gamma rays (and
sometimes Protons, Neutrons and X-rays).
3
Activity Half-Life
You should already be aware that the Activity of
some substances will quickly decrease. The time
taken for the Activity to reduce by 50 is called
the Half-Life. Uranium 238 has a very long half
life of 4500000000 years while the half life of
Protactinium 238 is only 2 minutes. (More half
life information can be found here, or a quick
revision of half-life calculations can be found
here, otherwise continue to the next slide)
4
Biological Effect

• The radiation is harmful to living things, only
  if it is absorbed by the cells of that living
  thing. Absorption will cause ionisation in the
  cells.
• The risk of biological harm is affected by the
  following main factors
• The quantity of radiation energy absorbed by the
  body
• The type of radiation energy absorbed by the
  body.
• The type of tissue or organ etc that is receiving
  the energy.
• The amount (mass) of tissue that is receiving the
  energy.
• The time (exposure) period over which the energy
  was absorbed.

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Absorbed Dose
Absorbed Dose (D) accounts for the quantity of
energy absorbed per unit mass.

• i.e. How many Joules absorbed per Kilogram
•   1 JKg-1 absorbed is called 1 Gray.

• (Absorbed Dose Rate, is the Absorbed Dose over a
  period of time)
• Units are Gys-1 , Gyh-1, etc

6
Equivalent Dose
Equivalent Dose (H) accounts for the quantity and
type of energy absorbed.

• where wR is the radiation weighting factor.
•   Units are Sieverts (Sv)

A high wR value is more dangerous
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Equivalent Dose Rate
The Equivalent Dose Rate accounts for the
quantity and type of energy absorbed over a
period of time.

•   Units are Svh-1, Svyr-1

(This can also be expressed in two other ways)
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Effective Equivalent Dose
The Effective Equivalent Dose takes account of
all of these, and is used to indicate the risk to
health from ionising radiation.
The average Annual Effective Equivalent Dose for
people in the UK is about 2 milli-Sieverts (2 mSv)

• This is due to background radiation

In any one year, the public should not be exposed
to more than 5 mSv. On average over a longer
term, 1 mSvyr-1 should not be exceeded.
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Background Radiation
Cosmic radiation from the sun and outer space.
Radioactivity from rocks and soil on the Earths
surface.
Radioactive gases such as radon and thoron.
Radioactivity which is naturally present in the
human body.
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Example 1
By law, a radiation worker should not be exposed
to more than 50 mSv in any one year. A worker at
Hunterston Power Station receives 200 mGy of
alpha radiation during an average working month.
Will this worker exceed the annual effective
dose equivalent limit?
D 200x10-6 Gray wR 20 H ?
H D wR H (200x10-6) x 20 H 4x10-3 Sv
i.e. Equivalent Dose Rate is 4mSv per month, and
hence 48mSv over one year. The annual effective
equivalent dose limit is not exceeded.
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Example 2
A hospital radiographer receives 15mGy due to
alpha radiation and 400mGy due to X-Rays, in a
35-hour working week. Calculate her average
equivalent dose per working hour.
Due to alpha H D wR H (15x10-6) x 20 H
3.0x10-4 Sv
Due to X-Rays H DwR H (400x10-6) x 1 H
4.0x10-4 Sv

• Total equivalent dose, H 7x10-4 Sv over 35
  hours

Equivalent Dose Rate 7.0x10-4
35 2x10-5 Svhr-1
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Reducing Biological Risk
The 3 main ways in which we can reduce risk are

• By reducing our exposure time wherever possible,
• By increasing our distance from the source,
• By using shielding between source and ourselves.

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Half-Value Thickness
of shielding used to absorb gamma rays.

• Shielding cannot reduce the Activity of a source,
  nor can it affect the half-life time for a
  particular source.
• The half-life of Cobalt 60 is 5.3 years
• The half-life of Uranium 238 is 4.5 billion years

The purpose of shielding is to reduce the number
of counts per second, as received by the
Geiger-Muller tube (ie the radiation received by
the user)

• Common shielding materials include lead,
  aluminium, concrete and water.

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Experiment to find Half-Value Thickness
The count rate of a source is measured every
minute for 10 minutes using a Geiger - Müller
tube and counter. (The count rate is corrected
for background) The results are shown below
The results show that as a greater thickness of
lead shielding is used, the count rate detected
behind the lead shield decreases. What thickness
of lead shielding is required for the count rate
to fall by one half? We usually need to plot a
graph of results ..
15
The graph can be used to find the half-value
thickness
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480
t1
Select a point on the line which crosses grid
lines on both axes.
At t1 the count rate 480 counts/second
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480
240
t1
t2
Now find the point on the graph where the count
rate is half the previous value, (one half of 480
is 240). Call this t2
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480
240
hvt
t1
t2
The half-value thickness is the thickness of lead
shielding required to reduce the count rate by
half.
1 hvt t2 - t1 24 - 12 12 mm
19
It is sometimes possible to calculate the
half-value thickness of an absorber without using
a graph.
Example A source has a measured count rate of
12000 cps when no shielding is used. Using 24 cm
of concrete the activity has dropped to only 750
cps. What is one half-value thickness?
12000
6000
3000
1500
750
t2
t3
t4
t1
24 cm of concrete have halved the count rate 4
times. ie 24 cm is equal to 4 hvts. One hvt

Reducing Biological Risk
6 cm
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Revision of Half-Life Calculations
To say the half life of Protactinium 238 is 2
minutes means that in 2 minutes, 50 of the
nuclei within that Protactinium will have
decayed. (By emitting a Beta particle each
nucleus decays to become a Uranium nucleus)
By a further 2 minutes later, half of the
remaining 50 of the Protactinium nuclei will
also have decayed, and so on. For example
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The count rate of a source is measured every
minute for 10 minutes using a Geiger - Müller
tube and counter. (The count rate has been
corrected for background) The results are shown
below
The results show that as time passes the count
rate of the source is decreasing. How long does
it take for the count rate to fall by one
half? We usually need to plot a graph of results
..
22
The graph can be used to find the half-life of
the source.
23
480
t1
Select a point on the line which crosses grid
lines on both axes.
At t1 the count rate 480 counts/second
24
480
240
t1
t2
Now find the point on the graph where the count
rate is half the previous value, (one half of 480
is 240). Call this t2
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480
240
t1/2
t1
t2
The half-life is the time taken for count rate to
drop by half.
t1/2 t2 - t1 240 - 120 120 s 2 minutes
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It is sometimes possible to calculate the
half-life of a source without using a graph.
Example A source has an original activity of
12000 Bq. After 24 days the activity has dropped
to only 750 Bq. What is the half life of the
source?
12000
6000
3000
1500
750
t2
t3
t4
t1
4 half lives have passed in 24 days. One half
life
6 days
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