Transcript Med Phys 3A03/3AB2 - McMaster Faculty of Science

Med Phys 3A03/3AB2
Practical Health & Medical Physics
Communications
D.R. Chettle, with D.F. Moscu
TA: Helen Moise
• Course is in transition from:
• Communications in Medical Physics
• to:
• Operational Health Physics Laboratory
• 6 subsidiary objectives, or modules, each
taking 4 weeks (so 3 per term). So:
• Mon Jan 7th Air sampling for radioactivity using high
volume air samplers
• Mon Jan 14th practical
• Mon Jan 21st practical
• Mon Jan 28th report back
Scheduling
• It might work better to have:
• Mon Jan 7th 13:30 – 14:20 Air sampling for
radioactivity using high volume air samplers
• Mon Jan 14th 13:30 – 15:20 practical group A
• Mon Jan 21st 13:30 – 15:20 practical group B
• Mon Jan 28th 13:30 – 14:20 report back
• Would this be possible?
Modules 5 & 6
• Estimating doses & dosimetry
lecture: Monday Feb 4th, 13:30 – 14:20
labs: Mondays Feb 11th & 25th, 13:30 – 15:20
report back: Monday Mar 4th, 13:30 – 14:20
(Feb 18th – 22nd Reading week)
• Radiological incident response
lecture: Monday Mar 11th, 13:30 – 14:20
labs: Mondays Mar 11th & 25th, 13:30 – 15:20
report back: Monday Apr 1st, 13:30 – 14:20
Evaluation
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Practical performance
Report communication
Self-assessment
Peer assessment
Participation
35 – 45%
35 – 45%
5 – 10%
5 – 10%
10%
Why is radioactivity in the air an issue?
• Contamination: it drops out of the air onto
surfaces
• External exposure: walking through a
radioactive cloud
• Internal exposure: breathe it in and it decays
inside the body
How does radioactivity get into the
air?
• Gaseous or volatile compound: eg 125I, 3H,
222Rn
• Powders or particulates released as aerosol:
eg 137Cs
• Nature of facility: eg 41Ar from 40Ar(n,γ)41Ar in
Reactor
Air sampling vs Monitoring
• Sampling to establish whether or not there is
an issue
• Monitor continuously, repeatedly or
periodically, when there is an established
situation, which must be kept under control
Air sampling method
• Use a (vacuum) pump to draw a known
volume of air through a filter and/or a
cartridge.
– Filter will trap particles above a specified diameter
– Cartridge (eg charcoal, ion exchange, or similar)
will trap materials depending on their binding
properties
• Measure activity on filter or cartridge
How to interpret measured activity
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Compare measured activity to DAC & ALI
?
Derived Air Concentration
?
Annual Limit on Intake
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Go back to the start
• Set a limit on the dose that can be allowed
• For Nuclear Energy Workers, this is 100 mSv
over a 5 year period, or 20 mSv per year
(0.020 Sv/y)
• For members of the general public this is 20
times less, that is 0.001 Sv/y
Calculating a dose
• Use committed dose – attribute the eventual
dose to the year in which the person was
exposed
• Use (equivalent &) effective dose – take type
of radiation and organ/part of body exposed
into account
• So, use the committed effective dose
Committed dose
• D(50) = As(1-e-λEτ)Σ(AFxYxE)
MxλE
• Where D(50) is the committed absorbed dose
As is the source activity
λE is the effective elimination rate
τ is 50 years
AF is the absorbed fraction
Y is the branching ratio, or yield
E is the energy of an emission
M is the target mass
Simplification
• Take D(50) forward using radiation weighting
factors (wR) to get equivalent dose and tissue
weighting factors (wT) to get effective dose, so
have E(50)
• Then get this E(50) per unit activity, which is
termed the effective dose coefficient e(50)
– This e(50) will be small, because it is the
committed effective dose in sievert per bequerel.
Back to ALI
• Annual limit of intake (ALI) for nuclear energy
workers is therefore:
• 0.020[Sv]/e(50)[Sv/Bq] = [Bq]
• Example: 125I inhalation, 5 m particles,
e(50) = 7.3x10-9, so
ALI = 0.020/7.3x10-9 = 2.74x106 Bq
DAC again
• Derived air concentration (DAC) is the activity per unit
volume that a nuclear energy worker can breathe
throughout the working year and not exceed the ALI
and therefore not exceed the 0.020 Sv; units Bq/m3
• A “reference person” is assumed to breathe 20 litre of
air per minute, that is 0.020 m3/minute
• If (s)he works 2000 hours per year, then (s)he breathes
0.020x60x2000 = 2400 m3 air per year
• So DAC[Bq/m3] = ALI[Bq]/2400[m3]
• For 125I, 5 m inhaled,
DAC = 2.74x106/2400 = 1140[Bq/m3]
Not that simple
• Limit on committed effective dose is lower (0.001 Sv)
for general public than for nuclear energy workers
(0.020 Sv)
• Annual limit of intake (ALI) varies between ingested
and inhaled and, for inhaled, it varies with particle size
(For 125I, e(50)inhaled is 7.3x10-9Sv/Bq for 5 m, but
5.3x10-9Sv/Bq for 1 m particles and e(50)ingested is
1.5x10-8Sv/Bq.)
• Does a person really inhale 20 litres of air per minute
regardless of whether her/his work is strenuous or
sedentary?
• Is 2000 hours per year truly typical?