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

2.3 ACTIVITY MEASUREMENTS IN ORGANS
In the following section basic principles relating to direct measurements of activity in
organs are presented. The presentation is limited to those principles and techniques adopted
specifically for carrying out measurements of Americium activity on the aforementioned case
of contamination. The relative results are presented in the last part of this section.
Particular care is taken in presenting those aspects characterizing a direct measurement
of transuranium elements in human body by low energy photon emissions in comparison to
indirect measurement of alpha particles emissions from bioassay samples, the technique
described in the last section. Therefore shieldings, detectors physics, detector-source
configuration and calibration procedures were emphasized, whereas for electronic devices
(preamplifier, amplifier, multichannel analyzer and signal elaboration), common to in vivo
and in vitro measurements and to other kind of radiation spectroscopy applications, one
should refer to the available scientific literature.
2.3.1 BASIC MEASUREMENT PRINCIPLES
2.3.1.1 Shielding
In order to perform direct measurements of low level x and gamma emitters in organs
the background radiation must be limited. The main radiation sources are:
• natural radioactivity in detectors materials;
• cosmic radiation;
• natural radioactivity in building materials;
• radioactive gas and dust naturally present in the area of measurements.
The detector and mounting materials normally contain natural radionuclides
sometimes in significant amount. For example NaI(Tl) detector contains also a homologue
element of Sodium, the Potassium, with its radioactive isotope K 40 [127]. The presence of
radionuclides and the amounts of their activity in detecting materials can be controlled and
limited only at the moment of detector design and production.
Primary cosmic radiation is mainly given by protons, helium nuclei and other nuclei.
The interaction with atmosphere elements produces secondary cosmic radiation composed by
charged particles, mesons and neutrons with fluxes of about 0.01 particles.cm -2 s -1 sr -1 . These
indicative values are strongly dependent on the atmospheric pressure. For instance a decrease
of 1 cm Hg of atmospheric pressure, decrease neutron flux of about 10 % [127]. The
interaction of secondary cosmic radiation with detectors and surrounding materials generates
further radiations with decreasing energies distribution towards low energy.
Radioactive elements too present in the building materials contribute to enhance
background radioactivity. Radionuclides as 40 K and others from natural radioactive series of
232 Th, 235 U and 238 U give the main contribution. Other elements from radioactive fall-out can
also be present. An example of activity concentration in building materials for few common
natural radionuclides is given in Table 2.3.1.
The most common way of decreasing the interference from cosmic radiation and
natural radioactivity in building material is based on shielding construction. The detection
system is surrounded by shielding barriers or often is directly located in shielded rooms. Lead
or steel is normally used because their high Z atomic number assures the absorption of mostly
high energetic radiations. When available pre-World War II naval plate steel is preferred
because much steel made since the war contains radionuclides from global fall-out and 60 Co
used in the steel manufacturing process [67]. After the interaction with such shielding
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