Dosimetry for biota

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Dosimetry for biota
GSF-National Research Centre for Environment and
Health
For the ERICA project
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Aim

• Quantify exposures to
• Biota from incorporated radionuclides
• Biota that live in a contaminated habitat (soil,
  water, sediment)
• Derive relationships of exposure and
• Radiation type
• Mass and shape of the organism
• Energy
• Geometry source-target

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Calculation of internal exposure to biota
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Calculation of external exposure to biota
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Road map to DCCs

• Definition of reference organisms
• Definition of exposure conditions
• Internal exposure
• External exposure
• Simulation of radiation transport for
  mono-energetic photons and electrons
• Link calculations for mono-energetic electrons
  and photons with nuclide-specific decay
  characteristics gt Dose conversion
  coefficients

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Reference organisms and habitats

• The enormous variability of biota requires the
  definition of reference organisms that represent
• Plants and animals
• Different mass ranges
• Different habitats
• Exposure conditions are defined
• Habitats
• In soil/on soil
• In water/on water
• In sediment/interface water sediment

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Simplifications

• Organisms are described by simple geometries as
  spheres, ellipsoids and cylinders
• The activity is homogeneously distributed in the
  body
• Organs are not considered
• The average dose for the whole body is calculated

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Exposure conditions for external exposure
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Dose concept

• Absorbed dose is the key quantity
• Deposited energy per unit mass
• Dose equivalent and effective doses as used for
  humans not applicable due to different endpoints
• Weighting factors to account for the
  effectiveness of a-radiation are under dicussion

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Key quantity Absorbed fraction (AF)

• Definition
• Fraction of energy emitted by a radiation source
  that is absorbed within the target tissue, organ
  or organism
• In a homogeneous medium (e.g. water), internal
  and external exposures of an organism are linked
  via the absorbed fraction
• Dint E AF(E)
• Dext E 1-AF(E)

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AF for electron in spheresin dependence on mass
and energy
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AF for photons in spheresin dependence on mass
and energy
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DCC for soil organisms at a depth of 25 cm in
soil, mono-energetic photons, uniformly
contaminated source in the upper 50 cm of soil
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DCC for an earthworm at various depths for
mono-energetic photons for a uniform source of
the upper 50 cm
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General dependencies of Dose Conversion
Coefficients (DCC) I

• The DCC for external exposure decrease with the
  size of the organism due to the increasing
  self-shielding effect.
• The differences in DCCs for external exposure
  among organisms are more pronounced for low
  energy g-emitters, since for such photons the
  effect of self-shielding is more important.
• The exposure to small organisms (e.g. mouse) from
  high-energy photons is higher for underground
  organisms, compared to aboveground organisms,
  whereas it is vice versa for larger organisms
  (e.g. fox).

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General dependencies of Dose Conversion
Coefficients (DCC) II

• The external exposure to low-energy photon
  emitters is in general higher for aboveground
  organisms, since the shielding effect of the soil
  is less pronounced.
• For internal exposure to g-emitters, DCCs
  increase in proportion to the mass of the
  organism due to the higher absorbed fractions.
  This dependence is more pronounced for
  high-energy photon emitters (e.g. 137Cs/137mBa).
• For a- and b-emitters, the DCCs for internal
  exposure are nearly size-independent.
• For internal exposure, the impact of the
  radiation quality is especially important for
  a-emitters.

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Application

• Only a small fraction of organisms can be
  simulated explicitely.
• The dose is the result of a complex interaction
  of exposure situation, energy and mass.
• For a given exposure situation, doses for
  organisms not explicitly considered can be
  determined with good accuracy by interpolation.

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Background exposures of terrestrial organisms

• External exposure is in the range of 0.10.4
  mGy/a, depending on size and habitat. The main
  contributor is 40K.
• Internal exposures are more variable.
• An important contributor is 40K with about 0.3
  mGy/a.
• Exposures to muscles and plant tissues due to U,
  Th, and Ra, Pb and Po are low.
• Liver, bone and kidney may be exposed at levels
  of 0.1 to 1 mGy/a unweighted absorbed dose.
  Weighted absorbed doses due to a-emitters are
  higher in proportion to the weighting factor
  assumed.
• Under specific environmental conditions, internal
  exposures may be much higher
• Burrowing mammals receive relatively high lung
  doses due to the inhalation of radon and its
  daughter nuclides.
• Animals that graze in Arctic regions may be
  exposed by 210Pb and 210Po that may be found in
  high levels in lichens.

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Background exposures in the aquatic environment

• Typical exposures are in the order of 1-10 µGy
  h-1
• The majority of the absorbed dose arises from
  internally incorporated a-emitters, with 210Po
  and 226Ra being the major contributors.
• The dose attributed is therefore closely
  proportional to the weighting factor assumed for
  a-radiation.
• Doses to freshwater biota are somewhat higher
  than for marine organisms
• The range of doses for freshwater organisms is
  also much greater due to the much greater
  variability of radionuclide concentrations in
  freshwater as compared to seawater