Plutonium Biokinetics in Human Body A. Luciani - Kit-Bibliothek - FZK

techniques such as nose blow, nose swabs, sweat, hair and breath are used just for metabolic
studies of particular radionuclides or for specific investigations. Breath analysis is commonly
used for 226 Ra and 228 Th whose decay chains include gases that can be exhaled. Blood is
sampled in case of suspected high level contamination or dedicated metabolic studies [52].
However because of the lack of experimental studies and observations, biokinetic models
don’t describe how the human metabolism cumulates the radionuclides in these uncommon
samples. Therefore the interpretation of these measurements can be extremely difficult. For
research purposes the analysis of tissue samples is carried out post-mortem in specimens
collected at autopsy.
Urine sampling is the most common technique. The excreted activity is due to the
introduced activity that has reached the blood stream and depends on the transferability of the
compound (the capability of reaching the blood in case of contamination via inhalation or
ingestion). Generally a 24 h collection is preferred to limit the variability during the day, but a
collection over several days is carried out when adequate sensitivity must be achieved [52].
Fecal samples are also used but they are more difficult to analyze because of the large daily
fluctuations in fecal excretion. The activity excreted in the feces comes from two
contributions: The activity in the gastrointestinal tract, that was not absorbed into the blood,
and the activity absorbed into the blood that has reached the gastrointestinal tract through the
biliar secretion.
Whatever the bioassay technique, the sample must be prepared isolating the
radionuclide to be investigated from the matrix material. Generally different phases can be
identified[69]: Sample preparation (wet and/or dry ashing), chemical separation (by means of
ion-exchange resin or solvent extraction), source preparation (in function of the measuring
technique). The most common measuring techniques are α-spectrometry, β-counting, liquid
scintillation counting and γ-spectrometry. Fluorimetry, neutron activation analysis,
inductively coupled plasma - mass spectrometry (ICP-MS), and fission track analysis can be
also applied but they are not extensively used.
In the particular case of Plutonium, α-spectrometry is one of the most common
measuring techniques. Indicative values for the detection limit of indirect measurements of
239 Pu, based α-spectrometry, are provided Table 1.3.2 [52]. For good energy resolution, very
thin sources are needed. Therefore particular care is taken for source preparation, using
techniques such as direct evaporation or electrodeposition. Detection limits around one mBql -1
or fraction can be achieved [70]. Lower limits of detection can be accomplished by fission
track analysis [71, 72], but this measuring technique requires sophisticated infrastructures and
procedures. ICP-MS is also a very sensitive technique for Plutonium measurements [70, 73]
and its big advantages are that almost no treatment of samples is required and results can be
obtained in a few minutes of measurement.
The typical detection limits for in vivo measurements and in vitro measurements based
α-spectrometry techniques, provided in Table 1.3.2, can be compared with the expected
values of 239 Pu activity in lungs and bioassays, calculated by means of ICRP models: the
respiratory tract model from ICRP 66 [57] and the systemic model from ICRP 67 [74]. In the
evaluations an intake determining a reference committed effective dose of 20 mSv was
assumed. Such value is the annual average dose suggested by the ICRP as limit for the
professionally exposed workers [26]. The conditions of exposure (scenario of contamination)
were assumed according to the standard parameters (breathing rate, diameters of inhaled
particles, absorption from the gut tract into the blood) suggested by ICRP in case of inhalation
and ingestion. In comparison with in vivo measurements the analysis of excreta by means of
α-spectrometry is generally more adequate. In fact the detection limits for α-spectrometry
analysis of excreta allows detecting a Plutonium contamination in a large number of
contamination scenarios and for extended time intervals after intake.
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