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

LD = 1
2 k ( 1− + k1− ) 2
2 1
+ ( k1− + k1− ) N0 + s0 +
4 k ( 1− + k1− ) 2
59
equation 2.3.12
The minimum detectable activity (MDA) characterizes the system before performing any
measurements, therefore it is defined on the basis of the detection limit. It is calculated as:
equation 2.3. 13
where:
t M is the time of measurement;
y is the yield, i.e. the probability of emission of the considered radiation per nuclear
transformation;
ε is the efficiency factor.
In case of direct measurements of activity in organs or whole body as considered in
the aforementioned equations, “background” has a particular meaning because it is a “not
contaminated subject ”. However the person’s background is determined by the radionuclides
that are naturally present within body and that can significantly change from subject to
subject. The MDA calculated on the basis of its background spectrum will be characteristic
for such person. In order to have a more general description of MDA capability of a detecting
system, a general background spectrum is normally calculated as average of background
spectra of a reference group of not contaminated persons. The evaluations of MDA are then
carried out on this average spectrum.
DIN methodology is not applied to a phoswich detecting system. For such detector the
MDA is calculated by substituting the L D with 2 times the standard deviation of the net counts
rate in the region of interest in the spectrum [136]. The spectrum is evaluated by subtraction
of four reference person’s background spectra with similar body proportions and potassium
content. From the resulting differences the standard deviation is calculated.
2.3.2 MATERIALS AND METHODS
2.3.2.1 Description of the devices
MDA = L D
t Mεy
The direct measurements of activity in organs and tissues were performed with both
HPGe and phoswich detecting systems. Both systems are located in a shielded room with
approximately inner dimensions: height 2.25 m; width 2.25 m; lenght 4.20 m. The room is
accessible through a pneumatically moved steel door and shielded with four different layers.
In decreasing order of Z they are: Steel (15 cm), Lead (3 mm), Tin (1.5 mm) and Copper (0.5
mm). The room is also equipped with a double ventilation system operating at 292 cm 3 h -1 and