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

The peak is located in the region B (length z b) and the other four regions (A 1, A 2, A 3 and A 4,
total length z 0) are set to evaluate the contribution of background in the B area of the peak. If
N i is the number of counts in the region A k (k from 1 to 4), two quantities are defined in the
time of measurement t M:
M o=N 1+N 2+N 3+N 4
that is the sum of the counts in the regions around the peak;
M 1= N 1-N 2-N 3+N 4
56
equation 2.3.4
equation 2.3.5
that is used for evaluating a non linear contribution of the background in the region of interest
B. The background contribution to the counts in the peak area is therefore calculated as:
N 0=qM 0-rM 1
equation 2.3.6
where:
q is z b/z 0, i.e. the ratio of the B area length by the total length of the four regions A k;
r is a factor that depends on whether the background can be considered linear in this region of
the spectrum; in case of linearity r=0. Otherwise r is defined as:
5 8
+ 4q +
r = q
4 3 ⎛
⎝
1 + 2q
q 2
⎞
⎠
equation 2.3.7
If N b is the gross number of counts in the B area of the peak, the net number of counts is
therefore calculated as:
N n=N b-N 0 equation 2.3.8
The efficiency factor ε is finally calculated by means of the certified activity (A) of the
radionuclides within the calibration phantom as:
ε = N n
At M y
equation 2.3.9
where y is the yield, i.e. the probability of emission of the considered radiation per nuclear
transformation and t m is the time of measurment.
For each full energy peak of one or more radionuclides an efficiency factor can be
calculated. A curve is normally fitted on these points to calculate the efficiency for all the
energy values of the spectrum. Yet the efficiency calibration curve is characterized by rapid
variation, particularly in the low energy range as for the transuranium elements emissions.
Therefore calibration phantoms containing the radionuclides that must be measured in the
actual situations are normally used. Thus, the efficiency factors are determined exactly for the
full energy peaks of these radionuclides instead of extrapolating them from an efficiency