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Transcript of Www.ceh.ac.uk/PROTECT Jordi Vives i Batlle Centre for Ecology and Hydrology, Lancaster, 1 st – 3...
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Jordi Vives i Batlle
Centre for Ecology and Hydrology, Lancaster, 1st – 3rd April 2014
Radiation dosimetry for animals and plants
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Key concepts Radioactivity, 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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Introduction
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Role of dosimetry in assessment
Discharges to the environment
Aquatic environment Terrestrial environment
Water Sediments
Total (internal and external) exposure
Activity in soil
Total (internal and external) exposure
Uptake Uptake
Internal Dosimetry
External Dosimetry
Internal Dosimetry
External Dosimetry
Evaluation of exposures to biota
Relationship between dose and effects
TR
AN
SF
ER
AN
D D
OS
IME
TR
YE
FF
EC
TS
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ERICA exposure scenarios
Plant geometry: is it a root or is it a stem? Height above ground for grass & herbs - cm to m
5
6
Terrestrial
7
8
9
10
Freshwater
1
2
3
4
Marine
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Key concepts
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Atoms and atomic structure Atoms are the smallest
quantities of an element that preserve all of its chemical properties.
Essential components of all atoms: Proton (m = 1 unit, charge = +1 unit) Neutron (m = 1 unit, charge = 0) Electron (m = 5.48 × 10-4 units, charge
= -1 units) Mass unit: 1.67 x 10-27 kg - Charge: =1.6 × 10-19 C Electrons surround the nucleus, equal in number to
the protons (atomic number Z). Atoms have a small positively charged nucleus
comprised of protons (Z) plus neutrons (N)
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Radioactive decay Spontaneous process by
which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles (ionizing radiation).
Activity is the rate at which its atoms are undergoing transformation (rate at which individual emissions of radiation occur).
Expressed in units of Becquerels (Bq) where one Becquerel equates to one atom transformation per second.
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Henri A. Becquerel (1896) - radiation from U salts expose film.
Marie Curie (ca 1898) - radiation from thorium, polonium, radium – 2 Nobel prizes!
Ernest Rutherford (ca 1903) - alpha radiation as helium nuclei.
The great discoverers
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Time
teNN .0
Activity
Radioactive decay occurs as a statistical exponential rate process. The number of atoms likely to decay (dN/dt) is proportional to the number (N) of atoms present. The proportionality constant, l, is the decay constant.
Half-life = 0.693/l
Law of radioactive decay
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α rays - most massive, positive charge (helium nuclei)
rays - negative charge, same as electron, arise from weak interaction
rays - no electric charge, quanta of electromagnetic radiation
Radioactive isotopes found in nature emit three types of radiation:
All three types can excite and ionise atoms.
Marie Curie’s apparatus shows deflection of rays from Ra
Different types of radiation
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Biological effects result directly from energy loss as radiation passes through tissue.
Formation of ions and free radicals (radiolysis).Damage effect at sub-cellular level. Reaction with chromosomes and damage to DNA strands.
Biological effects of radiation
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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 an 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 how 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 their 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
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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 conservative 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
Where A = activity concentration, E = energy and AF(E) = absorbed fraction
Constant k adjusted to give dose units of Gy h-1 Concentration in organisms as a function of time,
c(t), is concentration in the medium times a transfer function:
Aorg =Amedium c(t) In equilibrium, the transfer function is known as
the ‘concentration ratio”, CR
Dose conversion coefficient
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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: the 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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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
Reference organism shapes
www.radioecology-exchange.orgImage from N. Semioschkina, Germany
So The world looks like this…
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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 simulations of photon and electron transport through matter (ERICA uses MCNP code)
Includes all processes: photoelectric absorption, Compton scattering, pair creation, fluorescence
Monte Carlo approach
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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
Problems and limitations
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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)(
Spherical AFs v. mass & energy
For alpha and beta <10 keV the absorbed fraction is ~1
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Absorbed fractions for electrons in different terrestrial organisms (Brown et al., 2003)
AF versus gamma energy
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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).
The absorbed fraction for the non-spherical body is the absorbed fraction of the “equivalent sphere” multiplied by a re-scaling factor.
Non-spherical bodies
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Calculation of DCCs: ERICA database
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For a radionuclide with various , or decay transitions we make the following groupings having the same radiation weighting factor:
Low energy (energy < 10 keV); High energy (> 10 keV) +; and
Then for each category we sum all transitions (represented by sub-index i) of probability pi:
The total DCC is:
orlowi
iiorlow EAFpDCC,
4int, 1077.5
int
intintint
DCCRWF
DCCRWFDCCRWFDCC lowlowtotal
Internal DCC formulas
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It’s nearly the same except we replace AF by 1 - AF:
The total DCC is:
orlowi
iiorlow EAFpDCC,
4int, 11077.5
int
intintint
DCCRWF
DCCRWFDCCRWFDCC lowlowtotal
External DCC formulas
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)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:Calculation of dose rates
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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 and 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)
Data from Vives i Batlle et al. (2004)
Data shows smooth dependency of DCC with area/volume
DCC correlation with size
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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)
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)
External DCCs for soil organisms
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DCCs for mono-energetic photons for soil organisms as a function of photon energy (Brown et al., 2003)
Energy dependence of DCCs
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Comparing ERICA with other tools
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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
Intercomparison analysis
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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)
Internal dosimetry comparison
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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
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”)
External dosimetry comparison
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Special cases outside the ERICA approach
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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
The following formulae can be used for radionuclides whose concentration is referenced to air: 3H, 14C, 32P, 35S, 41Ar and 85Kr
Approach for gases
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Inhomogeneous distributions Only a few nuclides homogeneously
distributed: 3H, 14C, 40K, 137Cs Many concentrate in specific organs
e.g. Green gland (99Tc), Thyroid (129,131I), Bone (90Sr, 226Ra), Liver (239Pu), Kidney (238U)
Data from Gómez-Ros et al. (2009) Shows moderate influence in organ position within ellipsoid for various animals
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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)
Argon and krypton
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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 At equilibrium:
Radon - a complex problem
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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
Incorporates internal, surface and external dose
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
ICRP radon model for plants
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ERICA makes many assumptions and simplifications Geometry greatly simplified by using ellipsoids Homogeneous distribution in uniformly contaminated
medium - organs not considered (some tests done) Only a few organisms with simple geometry can be defined Size interpolation works only within predefined mass
ranges: 0.0017 to 550 kg for animals above ground 0.0017 to 6.6 kg for animals in soil 0.035 to 2 kg for birds 1E-06 to 1000 kg for aquatic organisms
Otherwise use Table 10 in ERICA help file to estimate the uncertainty
Summary – ERICA key features
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There are some things ERICA cannot do Limitations on which reference organisms appear under
which ecosystems e.g. cannot calculate DCC for marine bird in air
Do conservative run for bird on water or sediment Plant geometries in ERICA are unrealistic - root versus
stem. Variable height above ground for grasses. They do not really represent whole-organisms The grass geometry is taken from the ICRP Wild Grass RAP - no ‘in
soil’ dose rates are estimated, but only dose above ground. If you are concerned create an organism to represent your plant
(e.g. leaf) and compare DCC values to the default grass. Gaseous radionuclides are beyond the scope of the tool
and require specialised models
Summary – what ERICA can’t do
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References 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 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: 203 -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.
Vives i Batlle, J., Barnett, C.L., Beaugelin-Seiller, K., Beresford, N.A., Copplestone, D., Horyna, J., Hosseini, A., Johansen, M., Kamboj, S., Keum, D-K., Newsome, L., Olyslaegers, G., Vandenhove, H., Vives Lynch, S. and Wood, M. (2011) Absorbed dose conversion coefficients for non-human biota: an extended inter-comparison of data. Radiation and Environmental Biophysics 50(2): 231-251.