Presentation on theme: "Med Phys 3A03/3AB2 Practical Health & Medical Physics Communications D.R. Chettle, with D.F. Moscu TA: Helen Moise."— Presentation transcript:
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 7 th Air sampling for radioactivity using high volume air samplers Mon Jan 14 th practical Mon Jan 21 st practical Mon Jan 28 th report back
Scheduling It might work better to have: Mon Jan 7 th 13:30 – 14:20 Air sampling for radioactivity using high volume air samplers Mon Jan 14 th 13:30 – 15:20 practical group A Mon Jan 21 st 13:30 – 15:20 practical group B Mon Jan 28 th 13:30 – 14:20 report back Would this be possible?
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 125 I, 3 H, 222 Rn Powders or particulates released as aerosol: eg 137 Cs Nature of facility: eg 41 Ar from 40 Ar(n,γ) 41 Ar 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 Compare measured activity to DAC & ALI ? Derived Air Concentration ? Annual Limit on Intake ?
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) = A s (1-e -λEτ )Σ(AFxYxE) Mxλ E Where D(50) is the committed absorbed dose A s is the source activity λ E 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 (w R ) to get equivalent dose and tissue weighting factors (w T ) 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: 125 I inhalation, 5 m particles, e(50) = 7.3x10 -9, so ALI = 0.020/7.3x10 -9 = 2.74x10 6 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/m 3 A reference person is assumed to breathe 20 litre of air per minute, that is 0.020 m 3 /minute If (s)he works 2000 hours per year, then (s)he breathes 0.020x60x2000 = 2400 m 3 air per year So DAC[Bq/m 3 ] = ALI[Bq]/2400[m 3 ] For 125 I, 5 m inhaled, DAC = 2.74x10 6 /2400 = 1140[Bq/m 3 ]
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 125 I, e(50) inhaled is 7.3x10 -9 Sv/Bq for 5 m, but 5.3x10 -9 Sv/Bq for 1 m particles and e(50) ingested is 1.5x10 -8 Sv/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?