Jordi Vives i Batlle Centre for Ecology and Hydrology, Lancaster, 25 th November 2010

www.ceh.ac.uk/PROTECT

Jordi Vives i Batlle

Centre for Ecology and Hydrology, Lancaster, 25th November 2010

Radiation dosimetry for animals and plants


Introduction


Role of dosimetry in assessmentDischarges 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

TRA

NSF

ER A

ND

DO

SIM

ETR

YEF

FEC

TS


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


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


Key concepts


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


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 their significance


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


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)


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


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


Calculation of AFs: the ERICA approach


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


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


-0.6

-0.4

-0.2

0

0.2

0.4

0.6

-6 -4 -2 0 2 4 6

x coordinate

z co

ordi

nate

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


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


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

q aeeEFEb

EF

2)(1

1)(

Spherical AFs v. mass & energy


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


Absorbed fractions for electrons in different terrestrial organisms (Brown et al., 2003)

AF versus gamma energy


Calculation of DCCs: ERICA database


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


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


)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

CCK

fKff

ff

dimwhere

5.0:Aquatic

5.0:lTerrestria

Occupancy factor:

External exposure:

Internal exposure:Calculation of dose rates


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

DCC

(Gy

per p

hoto

n/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)

DCCs for soil organisms


DCCs for mono-energetic photons for soil organisms as a function of photon energy (Brown et al., 2003)

Energy dependence of DCCs


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


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)

DCC correlation with size


Comparing ERICA with other tools


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


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

BinFr

eque

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


0

10

20

30

40

50

60

70

0.00

0.33

0.66

0.99

1.32

1.65

1.98

BinFr

eque

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


Special cases outside the ERICA approach


)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

organismorganismnuclide

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


Tadpole Earthworm Frog

Rat Crab Duck

Trout Flatfish Deer

Distributed source

Central point

Eccentric point

Data from Gómez-Ros et al. (2009)

Inhomogeneous distributions


Considering organ distribution of same activity as a distributed source has some influence:

No appreciable difference between assuming radioactivity distributed either: (a) within the first 50 cm of soil; or (b) to an infinite depth.

Under an assumption < 10 cm depth there would be an effect for high-energy photons.

Inhomogeneous distributionsTadpole Earthworm Frog

Rat Crab Duck

Trout Flatfish Deer

Distributed source

Central point

Eccentric point


Rat Crab Duck

Trout Flatfish Deer

Distributed source

Central point

Eccentric point


Rat Crab Duck

Trout Flatfish Deer

Distributed source

Central point

Eccentric point


Rat Crab Duck

Trout Flatfish Deer

Distributed source

Central point

Eccentric point


Rat Crab Duck

Trout Flatfish Deer

Distributed source

Eccentric point

Central point


Rat Crab Duck

Trout Flatfish Deer

Distributed source

Central point

Eccentric point


Rat Crab Duck

Trout Flatfish Deer

Distributed source

Central point

Eccentric point


Rat Crab Duck

Trout Flatfish Deer

Distributed source

Central point

Eccentric point


Rat Crab Duck

Trout Flatfish Deer

Distributed source

Central point

Eccentric point


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


i

iRi ABI

0

- iN

L

R R+h

Conceptual representation of irradiated respiratory tissue

Simple respiratory model for 222Rn daughters

T

R

MBDCC 91054.5 At equilibrium:

Radon - a complex problem


Leaf interior

Radon_gas Unatt_daughters Att_daughtersClustering 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)

Dos

e ra

te (m

icro

Gy/

h)

InternalExternalSurface

Day Night

ICRP radon model for plants


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

Conclusions


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 and is

a 'grass spike'; no in soil dose rates are estimated only 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

Conclusions


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

Documents

Jordi Vives i Batlle Centre for Ecology and Hydrology, Lancaster, 25 th November 2010