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Environmental dosimetry

Justin Brown

Assessing Risk to Humans and the Environment

NMBU, Ås, June 8th-9th 2017

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Lecture Contents

• Concepts and units

• Calculation of Absorbed fraction

• Internal and External DCCs

• ERICA Tool vs. Other tools

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Role of dosimetry in environmental assessment

The chart is reproduced from Jordi V. Batlle (2010).

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Key concepts: Kerma and absorbed dose

KERMA: Kinetic Energy Released per unit MAss

Kerma has the same units as absorbed dose but the absolute quantities

differ :

dm

dEK tr

Sum of the initial kinetic energies of all the charged particles liberated by

uncharged particles from ionizing radiation (e.g. protons, neutrons) in a unit

of mass:

especially for high energy photons and small volumes

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Absorbed dose

• Absorbed dose is the amount of energy deposited

by the ionising radiation to the material being

irradiated.

Louis Harold Gray

(1905 - 1965)

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)

• The unit of absorbed dose is the Gray (Gy),

named after a British physicist.

• One Gray is one joule of absorbed energy

per kg of material. It is a relatively large unit

• Sub-multiples such as milligray (mGy), and

microgray (µGy) are often used

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Absorbed dose rate

• Non-human species have great differences in their average

lifetimes, so the most appropriate quantity in any dose

assessment is the estimation of dose rates (dose per unit time).

• If required, one can switch to doses by integrating the dose rate

over the lifespan or some other relevant period of the life of the

organism (e.g. the period of embryonic development).

• ERICA uses µGy/h but other units usually used are µGy/day or

mGy/day.

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Density of ionisation

• The radiation absorbed dose is a useful physical concept, but not

a good indicator of the likely biological effect

• Biological damage is not only dependent on the energy deposited

but also the density of ionisation;

a greater density of ionisation leads to a higher probability of DSB in DNA.

• Thus, we have to differentiate between alpha, beta and gamma

radiation as they differ from each other by penetrating capacity,

particle size, energy and by their ability to produce ions in

biological tissues.

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Linear Energy Transfer (LET)

• The average energy deposited along the track of a particle is

called the linear energy transfer (LET);

dx

dELET

• Alpha-particles ( together with some other less common forms of

ionizing radiation) are referred to as high-LET radiation, whereas

• Beta-particles and gamma-radiation are referred to as low-LET

radiation.

Defined by Bethe-Bloch expression

Function of particle charge, velocity of particle, mean

excitation potential of the target, atomic number etc.

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Relative Biological Effectiveness (RBE)

The greater efficiency of high-LET radiation in causing damage to

cells is commonly expressed in terms of the relative biological

effectiveness (RBE).

The RBE is defined as the ratio of dose required to achieve a specific biological

effect from a standard radiation (typically gamma rays) to the dose required for

the same end point from different types of radiation:

LEThigh

radiationref

D

DRBE

_

_

Humans: emphasis are on individuals and stochastic effects.

Plants and animals: more emphasis placed upon ’endpoints’ that are relevant for the integrity of the population – mortality, morbidity, reproduction effects

Important to note:

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RBE and radiation weighting factor (wr)

• The radiation weighting factors are defined as multipliers of absorbed dose used to account for the relative effectiveness of different types of radiation in inducing health effects.

• For humans, a-rad ”weighting” factor = 20 but this value is specific to stochasticeffects and therefore cannot be directlyused for plants and animals whereemphasis placed on populations

• RBE is dependent upon dose-rate, species, endpoint studied etc.

• The values of RBE can be experimentally estimated for different types of radiation, but

– it is practically impossible to obtain experimental values of RBE for a great number of possible endpoints and every type of organisms.

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Wr : Some suggested values

• UNSCEAR (1996) has proposed a radiation wr of 5 for alpha particles,

based on the approach that deterministic effects are of greater importance for

wildlife than stochastic effects.

• Systematic treatment of data Chambers et al. (2006) – up to 10 for deterministic, population relevant endpoints.

• A value of 20 for alpha particles is suggested in a number of

publications (e.g. Woodhead, 1984; Blaylock et al., 1993; Environment Agency, 2001).

• Based on experimental data, Kocher and Trabalka (2000) suggested that the

wrs for deterministic effects of alpha radiation are within the range from

5 to 10.

• Regarding (low energy) beta a value of 3 has been proposed for tritium

(Environment Canada, 2000; Environment Agency, 2001); UNSCEAR (1996)

has made a general recommendation to use a value of 1 for all beta

emitters.

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Wrs in ERICA

• 3 for low energy (≤ 10 keV) β

radiation

• 1 for γ and high energy (> 10 keV) β radiation

• 10 for α (non stochastic effects in the species)

vs. 20 for humans (to cover stochastic effects of radiation i.e. cancer in an individual)

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Equivalent dose (human radiological

protection)

• The product of absorbed dose with radiation

weighting factor (wr) is called Equivalent dose

• Unit of equivalent dose is Sievert (Sv)

• Called after the Swedish medical

physicist Rolf Sievert (1896 – 1966)

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Absorbed Fraction (AF)

Energies in the range from 10 keV to 5 MeV

Masses in the range from 1 mg to 1 tonne

ERICA assumes that the radiation sources are

uniformly distributed in spheres/ellipsoids

immersed in infinite aquatic medium and makes use of Monte

Carlo simulations to calculate absorbed fractions for photon and

electron.

The calculations for ERICA default geometries cover:

sourceby emittedEnergy

by target absorbedEnergy

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Interpolation: Photon sources in spheres

10-5

10-4

10-3

10-2

10-1

100

10-1

100

10-3

10-210-1

100101

102103

104105

106

AF

E (MeV)

Mas

s (g

)

Photon sources in spheres is a function of

Energy

Mass

Large mass and low

energy

1

Small mass, and High

energy,

0

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Absorbed fraction: Monoenergetic electrons

FASSET (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.).

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Absorbed fraction: Monoenergetic γ-radiationW

ide ra

nge

FASSET, (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.).

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Internal dose ratesHomogeneous distribution in organism

Dose rates for internally incorporated radionuclides. The dose rates delivered

to organisms are evaluated from the concentration of each internally

incorporated radionuclide.

1

14

ii

i

iTorgint

kg.Bq.MeV

µGyh1076.5k

yE)E(AkD

where:

Aorg is the activity concentration in organism (Bq kg-1 w.w.);

Ei is the energy of component <i> of emitted radiation (MeV);

yi is the yield of emitted radiation of energy Ei (dis-1);

T(Ei) is the absorbed fraction in the target for energy Ei;

K is the factor to account for conversions of MeV to Joules and seconds to hours.

For each radionuclide data for the energy and yield of particle, photon and a particle

emissions have been extracted from the literature (ICRP, 1983).

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External dose ratesHomogeneous distribution in the environmental media

• Dose rates for external exposure from radionuclides present in soil or

in water column are calculated using a variant of the simple formula

for uniformly contaminated isotropic infinite absorbing medium:

1

14

ii

i

iTenvExt

kg.Bq.MeV

µGyh1076.5k

yE))E(1(AkD

Aenv is the activity concentration in the environment in Bq kg-1 or Bq m-3.

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Dose Conversion Coefficient (DCC)

• Defined as the ratio of dose rate per unit activity concentration in

organism or the medium:

org

intint

A

DDCC

env

extext

A

DDCC

Units of µGyh-1 per Bq kg-1

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A short summary

The dose is the result of a complex interaction of energy, mass and the

source – target configuration:

Only a few organisms with simple geometry can be simulated explicitly-

Reference organisms

Define organism mass and shape

Consider exposure conditions (internal, external)

Derive absorbed fractions: Simulate radiation transport for mono

energetic photons and electrons.

Calculate Dose Conversion Coefficients: Link calculations with

nuclide-specific decay characteristics

In all other cases interpolation has to be applied

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Reference organisms

The enormous variability of biota requires the definition of reference

organisms that represent:

Plants and animals

Different mass ranges

Different habitat

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Reference organism: Assumptions

• Homogeneous distribution of radionuclides within the organism

• Organism immersed in uniformly contaminated medium

• Organs are not considered

• Dose rate averaged over organism volume

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Interpolation method for ellipsoids

• In order to consider absorbed fractions for a wide range of different

ellipsoid shapes Rescaling Factors were used (Ulanovsky and Pröhl,

2006):

Ulanovsky, A., Pröhl, G., 2006. A practical method for assessment of dose conversion coefficients for aquatic biota. J.

Environ. Biophys. 45, 203–214.

),(

),,(),,(

0 ME

MEMERF

The differences between absorbed fractions for spherical and non-spherical

bodies have been found to depend on mass and shape of the body, as well as

on type and energy of the source particles.

• RF expressed as a function of a ”non-sphericity parameter” η

Defined as the ratio of the surface area of a sphere to that of a non-

spherical body of equal mass

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Representative situations

• The number of possible situations is enormous; therefore a

limited number of representative situations have been selected

for detailed calculations:

In soil/ on soil/ in air

In water/ interface water-air

In sediment/ interface water-sediment

Exposure target Radiation source

Air Soil Water Sediment

Air × ×

Soil surface × ×

Soil × ×

Water × ×

Sediment surface × ×

Sediment × ×

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Assumed compositions

Element

Material

Organism tissue Shielding layer Soil Air

H 10.2 7 2.1 0.064

C 14.3 50 1.6 0.014

N 3.4 16 75.09

O 71.0 24 57.7 23.56

Na 0.1

Al 5.0

Si 27.1

P 0.2

S 0.3 3

Cl 0.1

Ar 1.28

K 0.4 1.3

Ca 4.1

Fe 1.1

Density (g/cm3) 1.05 1.0 1.6 0.0012

The density of the materials involved and their elemental composition

have an important impact on the radiation transport.

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Photon mean free path vs. composition

The mean free path of photons is plotted for air, water, tissue and soil

(density = 1.6 g/cm³) as function of the energy

FASSET (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.).

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Monte Carlo approachSimulations of photon and electron transport through matter

• Materials differing in composition and density can be considered;

• Complex geometries of sources and targets can

be simulated;

• All relevant physical processes that

control radiation transport are

precisely treated;

• Self-shielding is implicitly considered.

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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 of a target being hit is inversely proportional to the square of the

distance between source and target

Simulations require high number of photon tracks

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Biota living on the ground

• The external DCCs have been calculated here as a product of

free-in-air kerma, Ka, in a place occupied by the animals’ body

and pre-computed dose-to-kerma ratios, R(Ei,M):

i

i

iiaext y)M,E(R)E(KDCF

Taranenko, V., Pröhl, G., Go´mez-Ros, J.M., 2004. Absorbed dose rate conversion coefficients for reference biota for

external photon and internal exposures. J. Radiol. Prot. 24, A35–A62.

• It is assumed that only photon sources contribute to external

exposure of the animals.

• A volume source uniformly distributed to a depth of 10 cm has

been assumed

Ei is the energy , yi is the

yield of specific photon (per

decay), and M is the mass of

the animal (kg).

The Monte Carlo calculations were performed for 19 energies in the range from

10 keV to10 MeV.

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Biota living in the ground

• It is assumed that the target organism is at a depth of 25 cm in a

uniformly contaminated soil layer of 50 cm

• Hence the Monte Carlo model geometry

consists of soil compartment which is the

isotropic source and the target in the

middle as receptor

• The Monte Carlo simulations have been

performed for seven initial energies in

the range from 10 keV to 3 MeV

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Locations within habitat

Terrestrial FreshwaterMarine

Fraction of time spent by organism at different locations

within its habitat

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Calculation of dose rates

• Internal exposure:

• External exposure:

intintint )( DCCCDCCCRCD orgnismorganismmediummedium

occupancyextmediumext fDCCCD

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Note on DCCs

We also normally interested in weighted total dose rates (in µGy/h)

Where:

wf = weighting factors for various components of radiation (low beta, + and alpha)

DCC = Dose Conversion Coefficient in µGy/day per Bq/L or kg

aa

,,

int,int,int,int

extlowextlowext

lowlow

DCCwfDCCwfDCC

DCCwfDCCwfDCCwfDCC

Need to apply : Radiation weighting factors (dimensionless):

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IAEA intercomparisons : dosimetric models (1)

• Models have been developed for specific situations in various countries

• International comparison of 7 models performed under the IAEA EMRAS I program: EDEN, EA R&D 128, DosDimEco, EPIC-Doses3D, RESRAD-BIOTA, SUJB and ERICA

• 5 ERICA runs by different users

• DCCs compared for 67 radionuclides and 5 ICRP RAP geometries

– Exploratory stats – central tendency, skewness, outliers etc.

– ‘‘Z-score’’, which is a measure of how many standard deviation units away from the mean a particular data value lies

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IAEA intercomparisons: dosimetric models (2)

• Greater variation for external exposure DCCs, explained by

Vives i Batlle J., Balonov M., Beaugelin-Seiller K., Beresford N. A., Brown J., Cheng J-J., Copplestone D., Doi M., Filistovic V., Golikov V.,

Horyna J., Hosseini A., Howard B. J., Jones S. R., Kamboj S., Kryshev A., Nedveckaite T., Olyslaegers G., Pröhl G., Sazykina T.,

Ulanovsky A., Vives Lynch S., Yankovich T. and Yu C. (2007). Inter-comparison of absorbed dose rates for non-human biota. Radiation and

Environmental Biophysics, 46, 349-373.

• DCCs for internal exposure similar for different approaches

– Number of daughter products being included in the DCC of the parent

– Differences in assumptions for media densities (particularly to low-energy -emitters)

ERICA made predictions similar to other models

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Inhomogeneous distributions

• In the absence of more detailed information, the whole body dose

rate due to internal exposure in reference organisms have been

calculated using:

The average whole body activity concentration and

• Realistic scenarios of internal exposure must account for some

radionuclides which tend to concentrate in specific organs or

tissues.

The DCCs values obtained assuming a homogeneous distribution.

• To study the effect of such inhomogeneous distributions, internal

DCCs have been calculated assuming both a central and an

eccentric point source (J.M. Gomez-Ros et al., 2008).

J.M. Gomez-Ros et al. (2008). Journal of Environmental Radioactivity, 99, 1449 -1455

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A note on combining DCCs in ERICA

• Any given decay chain is truncated

when the physical half-life of a given

daughter product exceeds 10 days.

The DCCs of all progeny up to that

point are then combined with the

parent radionuclide by assuming that

the entire group of radionuclides is in

secular equilibrium.

• The use of a 10 day cut-off is

somewhat arbitrary as is the

assumption that parent and daughter

will be at equilibrium

– More correct to consider decay and

ingrowth over environmentally relevant

integration periods

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Conclusions

• ERICA makes many assumptions and simplifications but it is well

elaborated and has been verified through international comparison

• There are some things ERICA cannot do:

• Limitations with regards to selection of habitat for reference

organisms e.g. cannot calculate DCC for marine bird in air

• Gaseous radionuclides are beyond the scope of the tool and

require specialised models

• Dosimetry of plants have to be improved; do not represent

whole-organism

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Future developments

Progression in human dosimetry towards voxel

phantoms provides a clear indication of the way

forward for improving the dosimetry for animals.

http://nsed.jaea.go.jp/ers/radiation/en/rpro/mouse-e.htm