Available online at www.sciencedirect.com
Journal of Environmental Radioactivity 99 (2008) 1440e1448www.elsevier.com/locate/jenvrad
Methods for calculating dose conversion coefficientsfor terrestrial and aquatic biota
A. Ulanovsky a,*, G. Prohl a, J.M. Gomez-Ros b
a Helmholtz Zentrum Munchen, German Research Center for Environmental Health, Institute of Radiation Protection,Ingolstadter Landstraße 1, D-85764 Neuherberg, Germany
b CIEMAT, Av. Complutense 22, 28040 Madrid, Spain
Received 6 September 2007; received in revised form 16 January 2008; accepted 24 January 2008
Available online 10 March 2008
Abstract
Plants and animals may be exposed to ionizing radiation from radionuclides in the environment. This paper describes the underlying data andassumptions to assess doses to biota due to internal and external exposure for a wide range of masses and shapes living in various habitats.A dosimetric module is implemented which is a user-friendly and flexible possibility to assess dose conversion coefficients for aquatic and ter-restrial biota. The dose conversion coefficients have been derived for internal and various external exposure scenarios. The dosimetric model islinked to radionuclide decay and emission database, compatible with the ICRP Publication 38, thus providing a capability to compute dose con-version coefficients for any nuclide from the database and its daughter nuclides. The dosimetric module has been integrated into the ERICA Tool,but it can also be used as a stand-alone version.� 2008 Elsevier Ltd. All rights reserved.
Keywords: Internal exposure; External exposure; Dose conversion coefficients; Animals and plants; Aquatic; Terrestrial
1. Introduction
In the last years, the interest increased for assessment andevaluation of the radiological impact on wild living flora andfauna due to release of radionuclides to the environment.The consideration of the flora and fauna in the system of radi-ation protection requires reliable widely applicable models toassess doses to biota in different habitats from external and in-ternal radiation sources. Dosimetric approaches and modelshave been developed since the late 1970s. In view of the var-iability of shapes, sizes, habitats and sourceetarget relation-ships, the approaches being used have to compromisebetween the complexity of the modelling that is theoreticallypossible and the availability of relevant data and the practica-bility of the resulting model. The earlier studies (Amiro, 1997;Copplestone et al., 2001; DoE, 2002; IAEA, 1976, 1979, 1992;
* Corresponding author. Tel.: þ49 89 3187 4113; fax: þ49 89 3187 3363.
E-mail address: [email protected] (A. Ulanovsky).
0265-931X/$ - see front matter � 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jenvrad.2008.01.010
NCRP, 1991; Pentreath and Woodhead, 1988; Woodhead,1970) are based on analytical models with often simplifyingconservative assumptions that were developed for screeningpurposes.
More complex approaches, using Monte Carlo techniquesfor simulation of radiation transport in biota and the surround-ing media to avoid too conservative estimations, were devel-oped by Golikov and Brown (2003), Higley et al. (2003) andBeaugelin-Seiller et al. (2006). Within the FASSET project(Larsson, 2004), dosimetric models were developed fora wide range of aquatic and terrestrial organisms in differenthabitats (Vives i Battle et al., 2004; Taranenko et al., 2004).This dosimetric work was further developed in the ERICAproject (Larsson, 2008; Ulanovsky and Prohl, 2006) and sum-marized in a dosimetric module that has been integrated in theERICA Tool (Børretzen et al., 2005; Brown et al., 2008) thatenables estimations of internal and external exposures to biotathat cover a wide range of body masses and habitats for allradionuclides listed in the electronic version of ICRP 38(Eckerman et al., 1994; ICRP, 1983). This paper describes
Table 1
Reference organisms in ERICA project (Larsson, 2008) and Reference Ani-
1441A. Ulanovsky et al. / Journal of Environmental Radioactivity 99 (2008) 1440e1448
the underlying approaches and data that applied in the dosi-metric module of the ERICA Tool.
mals and Plants as defined by ICRP (2007)
ERICA reference organisms
(example)
ICRP Reference
Animals and Plants
Habitat Mass (kg)
2. Methodology and dosimetric approaches
Terrestrial environmentSoil invertebrate (earthworm) Earthworm In soil 5.24� 10�3
2.1. Dose concept Detritivorous invertebrate(woodlouse)
On and in
soil
1.70� 10�4
Bee In air 5.89� 10�4
Gastropod (snail) On soil 1.40� 10�3
Lichen and bryophytes
(Bryophite)
On soil 1.10� 10�4
Grasses and herbs Wild grass On soil 2.62� 10�3
Shrub On soil
Tree Pine tree On soil 4.71� 102
Burrowing mammal (rat) Rat In soil 3.14� 10�1
Small mammal (rat) Rat On soil 3.14� 10�1
Large mammal (deer) Deer On soil 2.45� 102
Bird Duck On soil 1.26
Bird Duck In air (3 m) 1.26
Bird egg Duck egg On soil 5.03� 10�2
Reptile (snake) On soil 7.44� 10�1
Amphibian (frog) Frog On and
in soil
3.14� 10�2
Marine environment
Phytoplankton In water 6.54� 10�11
Macroalgae Brown seaweed In water 6.54� 10�3
Vascular plant In water 2.62� 10�2
Zooplankton In water 6.14� 10�5
Polychaete worm In water 1.73� 10�2
Benthic mollusc In water 1.64� 10�2
Crustacean Crab In water 7.54� 10�1
The basic quantity for estimating exposures to ionizing radiation is the ab-
sorbed dose, which is defined as the amount of energy absorbed per unit mass
in given organ or the whole organism, and it is given in units of Gray (Gy).
Among various types of radiations that can contribute to the absorbed dose,
the most important are a-, b-, and g-radiation, whereas neutrons, heavy
ions, fission fragments are less relevant under environmental conditions. For
the same absorbed dose, different types of radiation are known to cause differ-
ent effects. A radiation weighting factor that compares the effectiveness of the
different types of radiation to the effectiveness of irradiation with 300 keV
photons has been introduced to account for this different biological effective-
ness for radiation protection of humans (ICRP, 2003). In the human dosimetric
system, the product of the quality factor and the absorbed dose results in the
equivalent dose with the unit Sievert (Sv); it has the advantage that allows
comparing exposures from different radiation types on the basis of the biolog-
ical effect.
The concept of equivalent dose for humans has to be modified before it can
be applied to biota. The radiation quality factors currently applied in human
dosimetry focus on stochastic effects. However, the investigation of effects
of ionizing radiation to biota primarily aims on deterministic ‘‘umbrella’’
effects as morbidity, mortality, reduced reproductive success, and mutations
induced in germ and somatic cells (Larsson, 2004). This means, the radiation
weighting factors used for the dose assessment to humans are not directly
applicable to assess doses and risks for biota; the discussion on appropriate
radiation quality factors for biota, especially for a-radiation, is ongoing
(Chambers et al., 2006).
Benthic fish Flat fish In water 1.31
Pelagic fish In water 5.65� 10�1
(Wading) bird Duck In water 1.26
2.2. Reference organismsMammal In water 1.82� 102
Reptile (marine turtle) In water 1.39� 102
Sea anemones/true corals In water 1.77� 10�3
Colony of sea anemones/
true corals
In water 1.96� 102
Freshwater environmentPhytoplankton In water 2.05� 10�12
Vascular plant In water 1.05� 10�3
Zooplankton In water 2.35� 10�6
Insect larvae In water 1.77� 10�5
Bivalve mollusk In water 7.07� 10�2
Gastropod In water 3.53� 10�3
Crustacean In water 1.57� 10�5
Benthic fish In water 1.47
Pelagic fish Salmonid/trout In water 1.26
Since it is impossible to consider all species of flora and fauna explicitly in
dose assessment, some reference organisms need to be selected as representa-
tive members of typical ecosystems. This approach allows reducing the assess-
ment efforts and illustrates the range of possible exposures to ionizing
radiation in typical ecosystems. The reference organism approach has been
introduced previously (e.g., Larsson, 2004; Pentreath and Woodhead, 2001).
The FASSET project defined the ‘‘reference organism’’ as a ‘‘series of entities
that provides a basis for the estimation of the radiation dose rate to a range of
organisms that are typical, or representative, of a contaminated environment’’.
A comparable concept is discussed by the ICRP (2007) where a set of 12 more
or less globally present reference animals and plants in different life stages
(e.g., fish egg, adult fish) has been selected that represent a wide range of
life forms (plants, animals), organisms’ shapes and masses, ecosystems (terres-
trial, aquatic) and habitats (air, soil, water, sediment). The reference organisms
are summarized in Table 1.
Bird Duck In water 1.26Mammal In water 3.90
Amphibian Frog In water 3.14� 10�2
2.3. General assumptions
Due to the enormous variability of biota in respect to size, shape and hab-
itats, the dosimetric models assume a number of simplifications in order to
cover a wide range of exposure situations. The most important simplifications
are as follows:
� The shapes of the organisms are approximated by spheres and ellipsoids.
� For internal exposure, organs are not considered explicitly and the radio-
activity assumed is homogeneously distributed in the whole body.
� An equilibrium concentration in the whole body is assumed, this means
the radionuclide kinetic in the organism is not taken into account.
� For calculating external exposures, a variety of sourceetarget relation-
ships are considered that represent typical situations, e.g., the external
exposure of a reference organisms that live on or in contaminated soil.
Dose coefficients for environmental biota are commonly expressed as
mean absorbed dose rates in the whole body per unit activity concentration
of the given radionuclide. Dose coefficients for internal exposure are defined
per average mass activity concentration in the whole body e mGy h�1 per
Bq kg�1. Dose coefficients for external exposure of aquatic species are
expressed per average volume activity concentration e mGy h�1 per Bq L�1.
External exposure of terrestrial organisms has been considered in different
1442 A. Ulanovsky et al. / Journal of Environmental Radioactivity 99 (2008) 1440e1448
geometries. For planar sources in soil, the dose coefficients are given per sur-
face source strength e mGy h�1 per Bq m�2. For volume sources, the coeffi-
cients are normalized per mass activity concentration e mGy h�1 per Bq kg�1.
2.4. Aquatic organisms
The key quantity for calculating internal doses is the absorbed fraction f. It
is defined as the fraction of energy emitted by a radiation source that is ab-
sorbed within the target tissue, organ or organism. A reference organism in
the aquatic environment represents the simplest case: the organism is within
a quasi-infinite homogeneous medium and the activity is assumed to be uni-
formly distributed throughout its body. The densities of the medium and the
organism’s body are the same.
Under these conditions, a uniform isotropic model according to Loevinger
and Berman (1976) can be applied. That is, both internal (Dint) and external
(Dext) dose conversion coefficients (defined as absorbed dose rate per activity
concentration in organism or medium) for mono-energetic radiation can be ex-
pressed as a function of the absorbed fraction, which depends on the energy E:
DintðEÞ ¼ EfðEÞ; ð1Þ
DextðEÞ ¼ Eð1�fðEÞÞ: ð2Þ
The second equation is an approximation that e in strict sense e only holds if
the organism and the surrounding medium are of the same density and elemen-
tal composition.
In an infinite homogeneous medium with uniform isotropic radiation sour-
ces, the dose per unit source strength cannot exceed the full absorption limit,
which equals the absorbed dose in uniform infinite media (NCRP, 1991). This
means that, for mono-energetic particles, the upper limit for the dose coeffi-
cients equals to:
DNðEÞ ¼ 5:76� 10�4E�mGy h�1 Bq�1 kg
�; ð3Þ
where E is the energy (MeV) of a mono-energetic source.
If the organism dimensions are much smaller than the radiation range in
the medium, the internal dose approximates zero due to the escape of radiation
from the body and the external dose approximates DN. Conversely, when the
size of the organism is much larger than the radiation range in the medium, the
whole body internal dose approaches DN, whereas the external dose tends to
zero: Dext/0. The ranges for a-particles and low-energy electrons are small
(less or equal to 50e100 mm), so, in many cases of practical relevance ab-
sorbed fractions are high, fz1. Then, according to Eqs. (1) and (2):
DintzDN and Dextz0. Contrary, for extremely small organisms and for longer
range radiations (high-energy electrons and photons) the absorbed fractions
are very small, fz0, thus Dintz0 and DextzDN.
In Ulanovsky and Prohl (2006), absorbed fractions for spheres and ellip-
soids in the mass range 10�6e103 kg have been computed by Monte Carlo
Fig. 1. Absorbed fraction for photons (a) and electrons (b)
simulation for photon and electron sources with energies in range from
10 keV to 5 MeV. The ellipsoids were defined by the ratios of the lengths of
their principal axes; the ratios varied from 1:1:1 (sphere) to 1:50:50 (flat ellip-
soid) and 1:1:50 (protracted ellipsoid).
The results for spheres are presented in Fig. 1. For photons, the absorbed frac-
tions vary with sphere mass and energy over more than four orders of magnitude.
The absorbed fractions for electrons are close or equal to 1 in a wide range of mass
and energy combinations. Only for small masses and high electron energies the
electron absorbed fractions are significantly less than 1. The variability for consid-
ered mass and energy ranges is less than two orders of magnitude.
Due to their short range, a-particles, spontaneous fission fragments, and
electrons with energies less than 10 keV have been considered as non-
penetrating radiations and their absorbed fractions have been set equal to 1.
For the absorbed fractions of arbitrary ellipsoids, an analytical approxima-
tion technique was developed in Ulanovsky and Prohl (2006) that is based on
the Monte Carlo simulation of the absorbed fractions for spheres and a set of
pre-defined ellipsoids covering above-mentioned range of non-sphericity. Non-
spherical organisms are defined by the body mass and the lengths of the minor
axes in terms of length of the major axis.
The set of absorbed fraction values for spherical bodies forms a basis for
approximation of those for non-spherical ellipsoidal bodies. Absorbed frac-
tions for non-spherical bodies are represented as
fðE;M;hÞ ¼ RFðE;M;hÞf0ðE;MÞ; ð4Þ
where f0ðE;MÞ and fðE;M;hÞ are the absorbed fractions for spherical and
non-spherical bodies, respectively, E is the energy of particle, M is the body
mass and h is a non-sphericity parameter defined as a ratio of surface area
of a sphere to that of a non-spherical body of equal mass. The parameters
of the approximation are determined for photons and electrons, separately.
The rescaling factors, RFðE;m;hÞ, are expressed as a function of the non-
sphericity parameter h for given mass and energy of the source particle:
RFðE;M;hÞ ¼�
1� j1� hj1=sðE;MÞ�sðE;MÞ
; ð5Þ
where sðE;MÞ is the fitting parameter, which depends on the type and energy
of the source particle as well as on the organism mass. Therefore, an additional
parameter, r0, was introduced, defined as the quotient of the radius of a sphere
with the same mass, R0, and the electron CSDA-range, LðEbÞ, or the photon
mean free path in water, lðEgÞ, i.e.
r0 ¼R0
LðEbÞðfor electronsÞ; ð6aÞ
r0 ¼R0
lðEgÞðfor photonsÞ: ð6bÞ
in spheres (according to Ulanovsky and Prohl, 2006).
1443A. Ulanovsky et al. / Journal of Environmental Radioactivity 99 (2008) 1440e1448
Both, the electron range and the photon mean free path depend on energy and
material density and composition. The radius R0 is simply calculated from the
organism’s mass.
The values of the parameter sðE;MÞ for electrons and photons are plotted
vs. scaled radius r0 in Fig. 2. For electrons and for a given value of r0, the
sðE;MÞ-values are in a narrow range, whereas for photons, the variations
are much more pronounced, especially for values of r0 < 1. This reflects
the fact that photons e unlike electrons e do not have a defined range in a me-
dium, due to the exponential attenuation.
Dependence of the parameter sðE;MÞ on r0 can be approximated by an em-
pirically derived relationship. The solid lines in Fig. 2 represent such approx-
imation derived by non-linear least square fits. Further details can be found in
Ulanovsky and Prohl (2006).
This method provides a possibility to estimate internal and external expo-
sure for aquatic organisms with different ellipsoidal body shape for a wide
range of masses, from 10�6 to 103 kg, and energies, from 10 keV to 5 MeV,
both for electrons and photons.
The uncertainties introduced by this approximation were estimated by Ula-
novsky and Prohl (2006). The mean absolute coefficient of variation does not
exceed 10% for electrons and 15% for photons. In many practically relevant
cases, the uncertainty is less than 3% for electrons and typically between 5
and 10% for photons.
2.5. Terrestrial environment
In the terrestrial environment, the situation is much more complex than in
the aquatic environment. Terrestrial organisms are surrounded by soil, air and
organic matter with different contamination, composition and density. The or-
ganisms live above, on and in soil, so the relative geometry of source and tar-
get vary considerably. Generally, these circumstances cannot be adequately
taken into account by the methods applied for aquatic organisms. Therefore,
the calculations of dose conversion coefficients are based on the simulation
of the radiation transport by means of Monte Carlo techniques as it was de-
scribed by Taranenko et al. (2004). Typical exposure situations concerning en-
ergies, contaminated media, and organism sizes were selected for detailed
considerations. Other exposure conditions for which detailed calculations
were not made can be approximated by interpolation between those cases.
The considered cases were the following (Taranenko et al., 2004):
� The terrestrial reference organisms have body masses in a range from
0.17 g to 550 kg.
� The Monte Carlo simulations were performed only for photons. External
exposure due to a- and b-radiation is less relevant due to their very short
range in soil and the shielding effects of skin and fur.
� For the calculations of external exposures for species living in the soil,
a uniformly contaminated volume source with a depth of 50 cm was
assumed.
� For reference organisms living on the ground, a planar radiation source
below soil layer with mass per unit area 0.5 g cm�2 is considered,1 which
approximates a fresh deposition and accounts for the effects of surface
roughness and initial migration of a radionuclide into the soil (Jacob
et al., 1986). A volume source with a depth of 10 cm was also simulated
as a representative source for aged radioactive deposits.
The main relationships between the mass of the organisms, the habitat and
the energy are shown in Figs. 3e5. In Fig. 3, the absorbed dose per photon
normalized to the photon energy is shown for several soil organisms as a func-
tion of the photon energy. A volumetric 50-cm-thick contaminated source has
been assumed with the organisms located in the middle at a depth of 25 cm.
Although the considered reference organisms vary in mass by a factor of
nearly 39 000, the impact of the mass on the normalized absorbed dose is
less than a factor of 5 for 30 keV photons and less than a factor of 1.5 for
1 Soil layer with mass per unit area 0.5 g cm�2 has thickness approximately
3 mm if the soil density is constant and equals to 1.6 g cm�3. For soil with
density 1.0 g cm�3 such layer would be 5-mm-thick.
high-energy photons. The exposure of an organism located on the surface of
this contaminated soil layer is about a factor of two less due to a change of
the irradiation geometry from 4p to 2p.
The correlation between mass, source energy and exposure for above-
ground organisms is depicted in Figs. 4 and 5. To estimate the external expo-
sure for reference organisms above the ground a two-step-approach has been
derived (Taranenko et al., 2004):
� In the first step, the air kerma, Kair, at the specific heights where the ref-
erence organism is located has been calculated by Monte Carlo simula-
tions. The heights vary from very close to the ground to 10 m for
birds. The dependence of air kerma on the mass of the reference organism
and the photon energy is shown in Fig. 4 for a planar source and a 10-cm-
thick volume source.
� In the second step, the ratio of the absorbed dose averaged over the whole
reference organism to the air kerma at the specific location of the organ-
ism, RðEg;MÞ, has been calculated by means of Monte Carlo simula-
tions. The dependence of dose to kerma ratios on mass and energy is
presented in Fig. 5. The ratios decrease with increasing mass of the organ-
ism due to the self-shielding. This relationship is more significant for
low-energy photons.
Then, the dose coefficients for external exposure to mono-energetic g-ra-
diation, DextðEgÞ, are computed as a product of the air kerma, KairðEgÞ, and the
dose to kerma ratio, RðEg;MÞ:
DextðEgÞ ¼ KairðEgÞRðEg;MÞ: ð7Þ
3. Calculation of nuclide-specific DCC
3.1. Radiation types
As it has been already mentioned in Section 2.1, the radia-tion weighting factors for non-human biota are still under dis-cussion. Therefore, the dosimetric module provides separatelythe contributions of the following radiation types to the totalDCC:
� a-particles;� low-energy electrons with discrete or continuous energy
Eb < 10 keV;� low-energy photons with energy Eg < 10 keV;� high-energy electrons with discrete or continuous energy
Eb � 10 keV;� high-energy photons with energy Eg � 10 keV;� spontaneous fission fragments.
Concerning the radiation weighting factors, default valuesof 10 for a-particles and fission fragments, 3 for low-energyelectrons and 1 for all other types of radiation have been ini-tially implemented.
3.2. Radionuclide decay chains
The decay of radionuclides often leads to an appearance ofnew radioactive daughter nuclides, which might contribute tothe organisms’ exposure. Therefore, an estimation of the dosecoefficients for any nuclide needs to consider also the radia-tions emitted by its daughter nuclides.
There are various possibilities to account for daughter nu-clides’ contribution. The default approach implemented in
Electrons
r0 (unitless)
10-2 10-1 100 101 102 103 104 105
s
0.0
0.2
0.4
0.6
0.8
1.0Photons
r0 (unitless)
10-3 10-2 10-1 100 101 102 103
s
0.0
0.2
0.4
0.6
0.8
1.0
Fig. 2. Parameter s of the approximation for the rescaling factors RFðhÞ for electrons (left) and photons (right) as a function r0. Points: parameter values derived
from results of Monte Carlo simulations. Solid line: least square fit. (From: Ulanovsky and Prohl, 2006.)
1444 A. Ulanovsky et al. / Journal of Environmental Radioactivity 99 (2008) 1440e1448
the ERICA Tool has been described by Taranenko et al.(2004). In this case, the decay chain is truncated at the firstradionuclide with a half-life greater than 10 days. All themembers in the decay chain are assumed to be in secular equi-librium and their activities are normalized to activity of theparent nuclide.
The other two options provide the relative activities of thedecay chain members either as an average within certain pe-riod (e.g., organism’s life span) or the activities at a giventime after the beginning of the parent nuclide decay.
Let consider the decay chain n0; n1;.; nn, where index ‘‘0’’denotes parent nuclide. At time T ¼ 0, the relative activities ofthe decay chain members are
Ajð0Þ ¼1; j ¼ 00; j ¼ 1;.;n
�ð8Þ
Using the decay data (half-lives and branching ratios), thechain is described by a system of linear ordinary differential
Fig. 3. Absorbed dose due to mono-energetic photons normalized to the pho-
ton energy for soil organisms that live at a depth of 25 cm of a homogeneously
contaminated 50-cm-thick soil layer (masses: 0.17, 35, 2000, 6600 g for wood-
louse, mouse, rabbit and fox, respectively).
equations that can be solved to obtain the activities of the de-cay chain members, AjðtÞ, at any time t. For the calculation ofthe DCCs, the activities of the decay chain members are ex-pressed relatively to the activity of the parent nuclide:
a0ðtÞh1; ajðtÞ ¼AjðtÞA0ðtÞ
; j ¼ 1;.;n: ð9Þ
Another possibility to account for daughter nuclides is to con-sider the time-averaged activities of the chain members:
a0h1; aj ¼
Z T
0
AjðtÞdtZ T
0
A0ðtÞdt
; j ¼ 1;.;n: ð10Þ
The nuclide-specific decay data on energies and yields of radi-ations emitted by the radionuclides are taken from the elec-tronic version of the ICRP Publication 38 (ICRP, 1983;Eckerman et al., 1994). Continuous energy b-spectra are inte-grated numerically.
3.3. Dose coefficients
The dosimetric model described in the Section 2 considersonly mono-energetic radiations. Nevertheless, in real dose as-sessment practice, dose coefficients for specific nuclides willbe required.
To calculate them, the DCC for mono-energetic radiationsare interpolated as described above. Then, for a given nuclidethese coefficients are summed to get the total DCC with con-tribution from daughter nuclides:
Dn0¼X
r
wr
Xn
an
Xi
Yn;iDðEn;iÞþZ
NnðEÞDðEÞdE
!ð11Þ
where r is the index for radiation type (a-, b-, g-radiations andspontaneous fission fragments), n is the index for decay chainmember, wr is the radiation weighting factor, an is the relativeactivity of the decay chain member (see Eq. (9)), En;i is the
10-21
10-20
10-19
10-18
10-17
10-16
10-15
10-14
10-13
10-2
10-1
100
101
100101102103104105
Kair (G
y p
ho
to
n-1 m
2)
E (M
eV)
Body mass (g)
10-20
10-19
10-18
10-17
10-16
10-15
10-2
10-1
100
101
100101102103104105
Kair (G
y p
ho
to
n-1 m
2)
E (M
eV)
Body mass (g)
Fig. 4. Air kerma as a function of photon energy and organ mass for a planar source in the soil (left) and a 10-cm-thick volume source in the soil (right).
1445A. Ulanovsky et al. / Journal of Environmental Radioactivity 99 (2008) 1440e1448
discrete ith energy emitted by nuclide n (MeV), Yn;i is theemission yield of the ith discrete energy radiation per decayof the radionuclide n (decay�1) and NnðEÞ is the energy spec-trum for continuous energy radiations of the nuclide n (here eonly b-particles) (decay�1 MeV�1).
3.4. Peculiarities for aquatic organisms
As it has been already mentioned in the previous section,the DCC for aquatic organisms were obtained using a uniformisotropic model, i.e. assuming the organism is within an infin-ite homogeneous medium and the activity is uniformly distrib-uted throughout its body. The densities of the medium and theorganism’s body are nearly equal. Then, the DCC for both ex-ternal and internal exposure can be expressed in terms of ab-sorbed fractions (see Eqs. (1) and (2)).
In the derivation of the DCCs, a-, b-, and g-radiation aretreated separately. Therefore, it is possible to apply specific
0.0
0.2
0.4
0.6
0.8
1.0
1.2
10-1
100
100
103
106
Do
se / A
ir kerm
a (rel.u
nits)
E (MeV)
Mass (gra
m)
Fig. 5. Dose to kerma ratio as a function of body mass and photon energy.
radiation weighting factors for the different radiation types.These radiation weighting factors are only applied for internalexposure because the weakly penetrating short range radiation(a-particles, low-energy electrons) do not contribute to exter-nal exposure.
3.5. Peculiarities for terrestrial organisms
The DCC for internal exposure of terrestrial reference or-ganisms as defined in FASSET (Taranenko et al., 2004) havebeen derived for a set of organisms with pre-defined shapeand with body mass ranging from 0.17 g to 550 kg. Comparedto the approach developed for the aquatic organisms (Ulanov-sky and Prohl, 2006), the FASSET data are limited both inmass scale and shape of the organism’s body. Therefore, in or-der to expand the functionality for terrestrial organisms, theapproach for aquatic species is applied to calculate DCC forinternal exposure of terrestrial ones. However, the internal ex-posure will be higher for an organism immersed in water thanone surrounded by air due to additional contribution of radia-tion backscattered from water to its body. This additional con-tribution is more significant as smaller mass of the body andhigher photon energy are considered. However, the overall ef-fect of the surrounding material on the absorbed fraction is rel-atively small and can be ignored for practically relevantproblems. For example, the DCC calculated by Monte Carlomethod for a spherical organism with a mass of 1 mg in waterand in air have shown that the overall effect of the backscat-tered radiation equals to 6% for 1.5-MeV-photons and lessthan 1% for 0.15-MeV-photons.
For terrestrial reference organisms, the estimation of exter-nal exposures is different from that in the aquatic environment.Since soil, air and organic matter differ considerably in com-position and density, the radiation transport was simulated formono-energetic photons by means of Monte Carlo techniques,as it is shown in Figs. 3e5. As it can be noted in these figures,the smooth dependence on mass and energy indicates that
1446 A. Ulanovsky et al. / Journal of Environmental Radioactivity 99 (2008) 1440e1448
intermediate values can be obtained with sufficient accuracyby interpolation.
4. Dosimetric module of the ERICA Tool
The dosimetric relationships described above have beenimplemented in a dosimetric module, which has been inte-grated into the ERICA Tool. The module is programmed inFORTRAN language and compiled as Windows DLL (dy-namic link library). The flowchart describing the module logicis shown in Fig. 6.
The main input parameters are the name of the radionuclideand the organism’s mass and shape. Other parameters describehabitat, type of biota, geometry of sources for external expo-sure of the terrestrial biota, etc. The list of input parameters,their meaning and values are given in Table 2.
For all categories of organisms, both external and internalexposure may be considered. A uniform isotropic model(Eqs. (1) and (2)) is applied to estimate both, external and in-ternal exposure of aquatic organisms as well as the internal ex-posure of terrestrial organisms. The calculations for organismswithin the mass range of 10�6e103 kg are based on pre-calculated values of the absorbed fraction. Although notvery likely, it may occur that the body mass falls outside therange from 10�6 to 103 kg, for which systematic calculations
Fig. 6. Flowchart of the dosimetric mod
were made. These cases correspond to spheres with radiiR0< 0.62 mm and R0> 0.62 m. For such organisms, internaland external exposures are determined by extrapolation fromthe 10�6 kg to zero mass, and from 103 kg to an infinitemass, respectively. According to Eqs. (1) and (2), D�int/0and Dext/DN, if mass approaches zero ðm/0Þ. If bodymasses are high ðm/NÞ, then Dint/DN and Dext/0. Thedetails of the extrapolation are given in Ulanovsky and Prohl(2006).
External exposures to terrestrial organisms are calculatedon the base of Monte Carlo simulation described above andthe results are integrated into the dosimetric module. Calcula-tion can be made for in soil and on soil organisms, for whicheither a planar or a volume source can be selected. The massrange is 0.17 g to 550 kg for on soil organisms, 0.17 g to6.6 kg for in soil organisms and 35 g to 2 kg for birds. TheDCC are interpolated over body mass and photon energy grids.External exposure in the terrestrial environment is the result ofa complex interaction of energy, material composition andsourceetarget geometry; therefore, a procedure for extrapola-tion to mass values out of the pre-defined mass range has notbeen implemented.
The dosimetric module is linked to the electronic databasewith the decay properties of 838 radionuclides (ICRP, 1983;Eckerman et al., 1994). That is, nearly any known radionuclidecan be considered.
ule integrated in the ERICA Tool.
Table 2
List of input and output parameters of the dosimetric module
Parameter Values Description
Input
Nuclide ‘‘AAe000a’’ Nuclide name
T-code �1, 0, 1 Options for construction of
decay chain (see text)
Time Days Meaning depends on the
T-code value (see text)
Habitat 1,2 1 e Aquatic, 2 e terrestrial
Pathway 1 Internal exposure (aquatic
and terrestrial)
2 External exposure in water
(aquatic) or external exposure
in soil (terrestrial)
3 External exposure on soil
(terrestrial)
4 External exposure above
soil (terrestrial)
Biota type 1, 2 1 e Animals and single
plants, 2 e vegetation layers
Body mass 10�6e103 kg Aquatic species (external
and internal exposure);
terrestrial (internal, only)
1.7� 10�4e550 kg Terrestrial animals (with two
exceptions, see next)
1.7� 10�4e6.6 kg External exposure of
terrestrial animals in soil
3.5� 10�2e2 kg External exposure of birds
and insects
Non-sphericity 0e1 Ellipsoid proportions (see text)
Vegetation 1, 2, 3 Vegetation type: 1 e herbs,
2 e shrub, 3 e trees
Source 1 Plane source in soil below
soil layer with mass per unit
area 0.5 g cm�2
2 Uniform volume 10-cm-thick
source in soil
Height 0e10 m Height above ground
(birds and insects only)
Output
dAlpha mGy h�1 Bq�1 kg or
mGy h�1 Bq�1 L or
mGy h�1 Bq�1 m2
Dose due to a-particles
dLowBet Dose due to b-particles with
energy � 10 keV
dHiBet Dose due to b-particles with
energy > 10 keV
dLowGam Dose due to photons with
energy � 10 keV
dHiGam Dose due to photons with
energy > 10 keV
dspFiss Dose due spontaneous fission
fragments
dTotal Sum of the above
1447A. Ulanovsky et al. / Journal of Environmental Radioactivity 99 (2008) 1440e1448
5. Conclusions
The dosimetric module developed during the ERICA pro-ject enables the assessment of exposures to biota for a widerange of organisms, habitats, sourceetarget geometries and ra-dionuclides. In view of the enormous variability of life forms,some simplifications are necessary, namely, an approximationof organisms by simple spherical and ellipsoidal shapes and anassumption that the radionuclides are uniformly distributed in
the organism. Moreover, the metabolism of radionuclides re-mains unconsidered.
From the assumed equilibrium concentrations in the envi-ronmental media or in the organisms, the dosimetric moduleprovides values of absorbed dose rates per unit activity con-centrations due to both external and internal exposure. How-ever, the impact of these simplifications on the resultsshould be clarified by further detailed calculations:
� Only a few radionuclides are relatively homogeneouslydistributed in organisms; examples are 3H, 14C, 40K and137Cs. Many radionuclides concentrate in specific organs,e.g., thyroid (131I, 129I), bone (90Sr, 226Ra), liver (239Pu)or kidney (238U). First results about the effect of such aninhomogeneous distribution are given by Gomez-Roset al. (2008). If a radionuclide accumulates in a specific or-gan, the ratio of the average dose rates in the organ and thewhole body can be approximated by the ratio of themasses of whole body and organ. This relationship is validwhen the absorbed fraction is 1 or close to 1 as it is thecase for a-particles and electrons due to their short range.For photons, the approximation is not so accurate due tothe penetration of the emitted photon (from the consideredorgan) into the surrounding tissue.� The existing data to estimate external exposures for birds
are limited. They depend on calculations of kerma in air at3 heights up to 10 m so, for intermediate heights, interpo-lation is necessary. Body mass range for birds is also lim-ited, spanning from 35 g to 2 kg.� In general, the accuracy of the calculated external expo-
sure in the aquatic environment is higher than for the or-ganisms in the terrestrial environment due to a simplergeometry. The DCC for external exposure in the aquaticenvironment are based on a much more comprehensivedata set than those for the terrestrial environment. For ex-ample, the mass range for aquatic organisms covers nineorders of magnitude from 10�6 to 103 kg, whereas the un-derlying mass range for terrestrial organisms is much less.Furthermore, the mass grid for terrestrial organisms isbased on FASSET data (Taranenko et al., 2004); therefore,it is coarser and less systematic thus leading to additionaluncertainties during interpolation.� The shapes of the organisms are very stylized. The reduc-
tion of the whole organism to simple shapes as spheres, el-lipsoids, and cylinders is an extreme simplification. This isunavoidable in view of the variability of life forms. How-ever, the confidence in the results would be increased, ifthe difference between a simplified and a realistic organ-ism models would be determined in some selected refer-ence organisms.� The radionuclide kinetic in the organism is not taken into
account.� The considered endpoint is in general the average whole
body dose rate per unit activity concentration in the organ-isms or the surrounding media. Doses to organs, as theyhave been calculated in human dosimetry, are not explic-itly considered. Again, a systematic consideration for all
1448 A. Ulanovsky et al. / Journal of Environmental Radioactivity 99 (2008) 1440e1448
reference organisms is not required, but a detailed analysisfor a small number of examples would elucidate thissource of uncertainty.� The external exposure due to radionuclides in a cloud is
not yet considered, since the work focused on the long-term pathways as internal exposure due to incorporationof radionuclides and the external exposure due to radionu-clides in soil and water. Nevertheless, in order to havea complete dosimetric system for biota that is reliably ap-plicable also in less typical exposure situations, this expo-sure pathway should be looked at in detail also for biota.� External exposure of the terrestrial species is the only bot-
tleneck of the present technique. It needs to be systemati-cally expanded in mass, height, and shape scales.
Acknowledgment
The work has been performed within EU-supported project‘‘ERICA e Environmental Risk from Ionizing Contaminants:Assessment and Management’’ under the Contract No. FI6R-CT-2003-508847.
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