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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. Course is in transition from: Communications in Medical Physics t o: Operational Health Physics Laboratory.

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Med Phys 3A03/3AB2

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  1. Med Phys 3A03/3AB2 Practical Health & Medical Physics Communications D.R. Chettle, with D.F. Moscu TA: Helen Moise

  2. Course is in transition from: • Communications in Medical Physics • to: • Operational Health Physics Laboratory

  3. 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

  4. 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?

  5. 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

  6. Evaluation • Practical performance 35 – 45% • Report communication 35 – 45% • Self-assessment 5 – 10% • Peer assessment 5 – 10% • Participation 10%

  7. 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

  8. 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

  9. 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

  10. 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

  11. How to interpret measured activity • Compare measured activity to DAC & ALI • ? • Derived Air Concentration • ? • Annual Limit on Intake • ?

  12. 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

  13. 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

  14. 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

  15. 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.

  16. 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

  17. 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]

  18. 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?

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