Jordi Vives i Batlle Centre for Ecology and Hydrology, Lancaster, 27 April 2010.

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


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 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 (wr)


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 (DCC)


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: 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


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


-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


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


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)(


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)(


AF versus -energy

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


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


Calculation of DCCs: ERICA approach


Internal DCC formulae

int

intintint

4int

4int

4int

1077.5

1077.5

1077.5

DCCRWF

DCCRWFDCCRWFDCC

EpFDCC

EpFDCC

EpFDCC

lowlowtotal

iii

yiiiy

lowiiilowlow


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


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:


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)


Energy dependence of DCCs

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


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)

Allometric correlation with size


Comparing ERICA with other tools


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


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)


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”)


Special cases


)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


Inhomogeneous distribution ()Tadpole Earthworm Frog

Rat Crab Duck

Trout Flatfish Deer

Distributed source

Central point

Eccentric point

Data from Gómez-Ros et al. (2009)


Inhomogeneous distribution ()Central pointTadpole Earthworm Frog

Rat Crab Duck

Trout Flatfish Deer

Distributed sourceEccentric point

Data from Gómez-Ros et al. (2009)


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)


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:


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


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:


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


-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)


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

Jordi Vives i Batlle Centre for Ecology and Hydrology, Lancaster, 27 April 2010.

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Transcript of Jordi Vives i Batlle Centre for Ecology and Hydrology, Lancaster, 27 April 2010.