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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 t o: Operational Health Physics Laboratory.

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med phys 3a03 3ab2

Med Phys 3A03/3AB2

Practical Health & Medical Physics Communications

D.R. Chettle, with D.F. Moscu

TA: Helen Moise

slide2

Course is in transition from:

  • Communications in Medical Physics
                  • to:
  • Operational Health Physics Laboratory
slide3

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 14thpractical
  • Mon Jan 21st practical
  • Mon Jan 28threport back
scheduling
Scheduling
  • It might work better to have:
  • Mon Jan 7th13:30 – 14:20 Air sampling for radioactivity using high volume air samplers
  • Mon Jan 14th13:30 – 15:20 practical group A
  • Mon Jan 21st13:30 – 15:20 practical group B
  • Mon Jan 28th13:30 – 14:20 report back
  • Would this be possible?
modules 5 6
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
Evaluation
  • Practical performance 35 – 45%
  • Report communication 35 – 45%
  • Self-assessment 5 – 10%
  • Peer assessment 5 – 10%
  • Participation 10%
why is radioactivity in the air an issue
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
How does radioactivity get into the air?
  • Gaseous or volatile compound: eg125I, 3H, 222Rn
  • Powders or particulates released as aerosol: eg137Cs
  • Nature of facility: eg41Ar from 40Ar(n,γ)41Ar in Reactor
air sampling vs monitoring
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
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
How to interpret measured activity
  • Compare measured activity to DAC & ALI
  • ?
  • Derived Air Concentration
  • ?
  • Annual Limit on Intake
  • ?
go back to the start
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
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
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
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
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 = 2.74x106Bq

dac again
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
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?