Post on 31-Mar-2015
Radiation dosimetry for animals and plants
Jordi Vives i Batlle
Centre for Ecology and Hydrology, Lancaster, 27 April 2010
www.ceh.ac.uk/PROTECT
Key concepts Kerma, absorbed dose, units, radiation weighting factor,
absorbed fraction, dose conversion coefficient (DCC)
ERICA approach to absorbed fraction calculation Reference habitats, organisms and shapes, Monte Carlo
approach, sphericity, dependence with energy / size
ERICA DCCs for internal and external exposure Internal and external DCC formulae, energy / size
dependency, allometric scaling
Comparing ERICA with other tools Special cases
Gases, inhomogeneous sources, non-equilibrium
Lecture plan
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Key concepts
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Kerma: sum of the initial kinetic energies of all the charged particles transferred to a target by non-charged ionising radiation, per unit mass
Absorbed dose: total energy deposited in a target by ionising radiation, including secondary electrons, per unit mass
Similar at low energy - Kerma am approximate upper limit to dose Different when calculating dose to a volume smaller than the range
of secondary electrons generated
Kerma and absorbed dose
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Units of absorbed dose (Grays) = Energy deposited (J kg -1)
Only small amounts of deposited energy from ionising radiation are required to produce biological harm - because of the means by which energy is deposited (ionisation and free radical formation)
For example - drinking a cup of hot coffee transfers about 700 Joules of heat energy per kg to the body.
To transfer the same amount of energy from ionising radiation would involve a dose of 700 Gy - but doses in the order of 1 Gy are fatal
I Gy = 1 J kg-1 = 6.24 1015keV ~ 1012 alphas
Units and significance
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Need to make allowance of such factors as LET or RBE in the description of absorbed dose
Equivalent dose = absorbed dose radiation weighting factor (wr)
Units of equivalent dose are Sieverts (Sv) No firm consensus - suggested values for wr:
1 for and high energy (> 10keV) radiation 3 for low energy ( 10keV) radiation 10 for (non stochastic effects in the species) vs. 20
for humans (to cover stochastic effects of radiation i.e. cancer in an individual)
Radiation weighting factor (wr)
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Fraction of energy E emitted by a source absorbed within the target tissue / organism
Internal and external exposures of an organism in a homogeneous medium:
Dint = k Aorg(Bq kg-1) E (MeV) AF(E) Dext = k Amedium(Bq kg-1) E [1-AF(E)] k = 5.76 10-4 Gy h-1 per MeV Bq kg-1
If the radiation is not mono-energetic, then the above need to be summed over all the decay energies (spectrum) of the radionuclide
Some models make simplifying assumptions: Infinitely large organism (internal exposure) Infinitely small organism (external exposure)
Absorbed fraction (AF)
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Defined as the ratio of dose rate per unit concentration in organism or the medium:
Dint = k Aorg E AF(E) = DCCint Aorg
Dext = k AmediumE[1-AF(E)] = DCCext Amedium
Units of Gy h-1 per Bq kg-1
Concentration in organisms is concentration in the medium times a transfer function: Aorg =Amedium (t)
In equilibrium, the transfer function is known as the “transfer factor”, TF
Dose conversion coefficient (DCC)
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The dose is the result of a complex interaction of energy, mass and the source - target geometry: Define organism mass and shape Consider exposure conditions (internal, external) Simulate radiation transport for mono-energetic
photons and electrons: absorbed fractions Link calculations with nuclide-specific decay
characteristics: Dose conversion coefficients Only a few organisms with simple geometry can be
simulated explicitly In all other cases interpolation gives good accuracy
Strategy for dose calculation
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Calculation of AFs: ERICA approach
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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 for different habitats: In soil/on soil In water/on water In sediment/interface water sediment
Reference habitats & organisms
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Reference organism shapes Organism shapes approximated by ellipsoids, spheres
or cylinders of stated dimensions
Homogeneous distribution of radionuclides within the organism: organs are not considered
Oganism immersed in uniformly contaminated medium Dose rate averaged over organism volume
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-0.6
-0.4
-0.2
0
0.2
0.4
0.6
-6 -4 -2 0 2 4 6
x coordinate
z co
ord
inat
e
Monte Carlo approach Monte Carlo simulations of photon and electron
transport through matter (ERICA uses MCNP code)
Includes all processes: photoelectric absorption, Compton scattering, pair creation, fluorescence
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Problems and limitations
Monte Carlo calculations are very time-consuming: Long range of high-energy photons in air, a large area around
the organism has to be considered A large contaminated area has to be considered as source Small targets get only relatively few hits Probability ~ 1/source-target distance2
Simulations require high number of photon tracks Therefore, a two-step method has been developed:
KERMA calculated in air from different sources on or in soil Dose to organism / dose in air ratio calculated for the
different organisms and energies
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Spherical AFs vs. mass & energy
10-5
10-4
10-3
10-2
10-1
100
10-1
10010-3
10-210-1
100101
102103
104105
106
Photon sources in spheresElectrons Photons
10-2
10-1
100
10-1
10010-3
10-210-1
100101
102103
104105
106
Electron sources in spheres
mE
nE
qaeeEF
EbEF
2)(1
1)(
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Represented by ellipsoidal shapes having the same mass as the spherical ones
AFs always less than those for spheres of equal mass. Non-sphericity parameter: = surface area of sphere of
equal mass (S0) / surface area (S) Derive re-scaling factors RF using the formula:
RF() can be approximated by a single-parameter curve:
RF() = 1 for large masses and low energies, and for very small masses and high energies.
Ulanovsky and Pröhl (2006)
Non-spherical bodies
)0,,(/),,(),,( MEAFMEAFMERF
ssRF
/111)(
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AF versus -energy
Absorbed fractions for electrons in different terrestrial organisms (Brown et al., 2003)
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For each radionuclide and reference organism energies and yields of all , and emissions are extracted from ICRP(1983) and overall and AF's calculated as:
where Ei is energy (MeV) and pi denotes the fractional yield of individual emissions
For each radionuclide and reference organism energies and yields of all , and emissions are extracted from ICRP(1983) and overall and AF's calculated as:
where Ei is energy (MeV) and pi denotes the fractional yield of individual emissions
Overall AF for radionuclide
1
)()(
iii
iii EpEFpF
1
)(
iii
iii EpEFpF
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Calculation of DCCs: ERICA approach
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Internal DCC formulae
int
intintint
4int
4int
4int
1077.5
1077.5
1077.5
DCCRWF
DCCRWFDCCRWFDCC
EpFDCC
EpFDCC
EpFDCC
lowlowtotal
iii
yiiiy
lowiiilowlow
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External DCC formulae
ext
extextlowlow
exttotal
iii
ext
iii
ext
lowiiilow
extlow
DCCRWF
DCCRWFDCCRWFDCC
EpFDCC
EpFDCC
EpFDCC
11077.5
11077.5
11077.5
4
4
4
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Calculation of dose rates
)kg Bq / sGy (
)/(
)()(
11-int
11
11int
org
mediumorgorganismmedium
mediumernal
DCC
kgBqkgBqCF
kgBqCsGyD
occupancymediumext
mediumexternal
fDCC
kgBqCsGyD
)kg Bq / sGy (
)()(11-
11
water
entsed
waterdsurfacesediment
surfacesoil
C
CK
fKff
ff
dimwhere
5.0:Aquatic
5.0:lTerrestria
Occupancy factor:
External exposure:
Internal exposure:
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0.01 0.1 1 1010-15
10-14
10-13
10-12
depth= 0,00 m depth= 0,05 m depth= 0,25 m depth= 0,50 m
DC
C (G
y pe
r pho
ton/
kg)
Photon source energy (MeV)
DCC for soil organisms
0.01 0.1 1 1010-15
10-14
10-13
10-12
woodlouse earthworm mouse mole snake rabbit fox
DC
C (G
y pe
r pho
ton/
kg)
Photon source energy (MeV)
DCCs for earthworm at various soil depths for monoenergetic photons. Assumes uniformly contaminated upper 50 cm of soil
DCCs for various soil organisms at a depth of 25 cm in soil for monoenergetic photons. Assumes uniformly contaminated upper 50 cm of soil (density: 1600 kg/m³)
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Energy dependence of DCCs
DCCs for mono-energetic photons for soil organisms as a function of photon energy (Brown et al., 2003)
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External DCCs decrease with size due to the increasing self-shielding, especially for low energy g-emitters
Small organism DCCs from high-energy photons higher for underground organisms, & vice versa for larger organisms
External exposure to low-energy emitters is higher for organisms above ground, due to lack of shielding by soil
DCCs for internal exposure to -emitters (esp. high-energy) increase with mass due to the higher absorbed fractions
For and -emitters, the DCCs for internal exposure are virtually size-independent
DCCs versus size and energy
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210Po: y = 2E-08x-0.8887, r2 = 1.00
125I: y = 4E-05x-0.2494, r2 = 0.97
134Cs: y = 0.0015x-0.4548; r2 = 0.93
1.E-13
1.E-12
1.E-11
1.E-10
1.E-09
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06
Area/volume (m-1)
Inte
rnal
D
PU
C (
Gy
h-1
per
Bq
kg-1
)
63Ni: y = 2E-11x0.9759, r2 = 0.99
14C: y = 5E-10x0.9087, R2 = 0.97
230Th: y = 7E-08x0.3688, R2 = 0.97
1.E-10
1.E-09
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06
Area/volume (m-1)E
xter
nal
DP
UC
(G
y h
-1 p
er B
q kg
-1)
Vives i Batlle et al. (2004)
Allometric correlation with size
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Comparing ERICA with other tools
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Intercomparison analysis
International comparison of 7 models performed under the EMRAS project: EDEN, EA R&D 128, ERICA, DosDimEco, EPIC-DOSES3D, RESRAD-BIOTA, SÚJB
5 ERICA runs by different users: default DCCs, ICRP, SCK-CEN, ANSTO, K-Biota
67 radionuclides and 5 ICRP RAP geometries Internal doses: mostly within 25% around mean External doses: mostly within 10% around mean There are exceptions e.g.α and soft β-emitters
reflecting variability in AF estimations (3H, 14C…) ERICA making predictions similar to other models
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Internal dosimetry comparison Estimate ratio of
average (ERICA) to average (rest of models)
Skewed distribution centered at 1.1
Fraction < 0.75 = 40% Fraction > 1.25 = 3% Fraction between 0.75
and 1.25 = 57%
0
10
20
30
40
50
60
0.05
0.19
0.33
0.48
0.62
0.76
0.91
1.05
1.19
1.34
BinF
req
ue
ncy
Worst offenders (< 0.25): 51Cr, 55Fe, 59Ni, 210Pb, 228Ra, 231Th and 241Pu
Worst offenders (>1.25): 14C, 228Th Conclude reasonably tight fit (most data < 25% off)
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0
10
20
30
40
50
60
70
0.00
0.33
0.66
0.99
1.32
1.65
1.98
BinF
req
ue
ncy
External dosimetry comparison
Same ratio method for external dose in water
Two data groups at < 0.02 and ~ 1.32
Fraction < 0.5 = 37% Fraction > 1.5 = 13% Fraction between 0.5 and
1.5 =50 % Worst offenders (< 0.02):
3H, 33P, 35S , 36Cl, 45Ca, 55Fe, 59,63Ni, 79Se, 135Cs, 210Po, 230Th, 234,238U, 238,239,241Pu, 242Cm
Worst offenders (>1.25): 32P, 54Mn, 58Co, 94,95Nb, 99Tc, 124Sb, 134,136Cs, 140Ba, 140La, 152,154Eu, 226Ra, 228Th
Still acceptable fit (main data < 50% “off”)
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Special cases
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)2(
)kgBqconc,Air ()(
factor) (reduction
2conc Soil
)(
)(dose) (External
concAir dose) Internal(
1.2)m Bq conc,Air ()kg Bq conc,Air (
)m Bq conc,Air (conc Soil
1external,
typeradiation
external,
internal,
organism,
3-1-
soil-3
organismorganism
nuclideorganismnuclide
organism
organismorganism
nuclide
organismnuclide
rganismnuclide, o
organismnuclidenuclidenuclideorganismnuclide
nuclidenuclide
nuclidenuclidenuclide
fsoilsurfair
DCCdoseImmersion
fair
fsoilsurfsoil
DCCdoseSoil
doseImmersiondoseSoil
DCCCF
CF
Approach for gases The following formulae can be used for radionuclides whose
concentration is referenced to air: 3H, 14C, 32P, 35S, 41Ar and 85Kr
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Inhomogeneous distribution ()Tadpole Earthworm Frog
Rat Crab Duck
Trout Flatfish Deer
Distributed source
Central point
Eccentric point
Data from Gómez-Ros et al. (2009)
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Inhomogeneous distribution ()Central pointTadpole Earthworm Frog
Rat Crab Duck
Trout Flatfish Deer
Distributed sourceEccentric point
Data from Gómez-Ros et al. (2009)
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Argon and krypton
Internal dose negligible: Ar and Kr CFs set to 0 No deposition but some migration into soil pores
Assume pore air is at the same concentration as ground level air concentrations
assume a free air space of 15%, density = 1500 kg m-3, so free air space = 10-4 m3 kg-1 & Bq m-3(air) * 10-4 = Bq kg-1 (wet)
Hence, a TF of 10-4 for air (Bq m-3) to soil (Bq kg-1 wet) For plants and fungi occupancy factors set to 1.0 soil, 0.5 air
(instead of 0) Biota in the subsurface soil and are exposed only to 41Ar and
85Kr in the air pore spaces External DCCs for fungi are those calculated for bacteria (i.e.
infinite medium DCCs)
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i
iRi ABI
0
- iN
L
R R+h
Conceptual representation of irradiated respiratory tissue
Simple respiratory model for 222Rn daughters
T
R
M
BDCC 91054.5
Radon dosimetry in biota
At equilibrium:
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Plants: breathing rate Use CO2 as analogue
entering plant, while water and O2 exit through the stomata
A full process dose model representing gas exchange through plant stomata is the next logical developmental step
Assume whole plant exchanges gases
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4
1
4
1
12
1
';'
MA
KDCCMKDCC
MS
SKDCC
LMWBL
RMTB
RMB
TB
WFPLWFwhole
tissuesensitive
RARDCC
aDCC31
1
1088.31099.1
1044.1
Radon DCC derivation
TB: Full tracheobronchial epithelium; L: Full lung; WB: Whole body; STBRM
and SBRM : Area of tracheobronchial tree or bronchial epithelium; a: Axis of
cylinder representing the plant; Rwfa:Weighting factor
Animal dose factors (Gy h-1 per Bq m-3):
Plant dose factors:
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Leaf interior
Radon_gas Unatt_daughters Att_daughters
Clustering Attachment
Leaf_exteriorStomataSurface
Interception_a
Interception_u Deposition_u Deposition_aDiffusion Respiration
Translocation
C1
Washout
Each sub-model contains the decay chain of radon: 222Rn 218Po 214Pb 214Bi 214Po
ICRP Radon model for plants
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
0 20000 40000 60000 80000
Time (days)
Do
se
ra
te (
mic
roG
y/h
)
InternalExternalSurface
Day Night
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-1.8
-1.6
-1.4
-1.2
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0 50 100 150 200 250 300
Time (h)
Ln (C
/C0 )
Phase 1
Phase 2
Environment(seawater)
Slow phaseFast phase Organism
FastUptake
SlowUptake
FastRelease
SlowRelease
Rad
ioac
tive
dec
ay
Radio
active decay
Non-equilibrium assessments Organisms can retain activity for a long time after it has been dispersed from
an environment.
Requires assessment tools based on a dynamic approach Time-dependent dose rates can be integrated over period following the intake
of radioactivity (lifetime)
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Brown J., Gomez-Ros J.-M., Jones, S.R., Pröhl, G., Taranenko, V., Thørring, H., Vives i Batlle, J. and Woodhead, D, (2003) Dosimetric models and data for assessing radiation exposures to biota. FASSET (Framework for Assessment of Environmental Impact) Deliverable 3 Report under Contract No FIGE-CT-2000-00102, G. Pröhl (Ed.).
Gómez-Ros, J.M., Pröhl, G., Ulanovsky, A. and Lis, M. (2008). Uncertainties of internal dose assessment for animals and plants due to non-homogeneously distributed radionuclides. Journal of Environmental Radioactivity 99(9): 1449-1455.
Ulanovsky, A. and Pröhl, G. (2006) A practical method for assessment of dose conversion coefficients for aquatic biota. Radiation and Environmental Biophysics 45: 20 -214.
Vives i Batlle, J., Jones, S.R. and Gómez-Ros, J.M. (2004) A method for calculation of dose per unit concentration values for aquatic biota. Journal of Radiological Protection 24(4A): A13-A34.
Vives i Batlle, J., Jones, S.R. and Copplestone, D. (2008) Dosimetric Model for Biota Exposure to Inhaled Radon Daughters. Environment Agency Science Report – SC060080, 34 pp.
References